NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES MODULE 16- TEST EQUIPMENT

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-1 UNCLASSIFIED 2 MISCELLANEOUS MEASUREMENTS LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

1. Define and explain the use of the terms "dB" and "dBm" as they apply to power measurements. 2. Describe the use of resistive loads, bolometers, and thermocouples in power measurements. 3. Explain the measurement of mechanical rotation using the tachometer, stroboscope, and the strobotac. 4. Explain the measurement of frequency in various ranges using vibrating reeds, tuned circuits, heterodyne frequency meters, absorption wavemeters, cavity wavemeters, and frequency counters. 5. Describe the use of frequency-measurement devices, oscilloscopes, and spectrum analyzers in waveform analysis and maintenance. 6. Describe semiconductor testing and applicable terms in maintenance.

2.1 INTRODUCTION In chapter 1, you studied test equipment administration and the basic measurements that all technicians are responsible for performing. Chapter 2 presents miscellaneous measurements that are fairly common; keep in mind, however, that you may not routinely perform these measurements in your particular job. This chapter introduces you to several test instruments and components found in those test instruments. It will also serve as a review of some of the basics of electronic theory related to test equipment.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-2 UNCLASSIFIED 2.2 POWER MEASUREMENTS You may be required to check the power consumption and the input-signal power levels of electronic equipment. The determination of dc power is fairly simple; recall that the unit of power, the watt, is the product of the potential in volts and the current in amperes (P = E × I).

As discussed in NEETS, Module 2, Introduction to Alternating Current and Transformers, the phase angle of the voltage and current must be considered for accurate ac power measurements. The measurement of ac power is further complicated by the frequency limitations of various power meters. If there is no phase angle difference, you can compute ac power in the same manner as dc power; that is, by determining the effective value of the product of the voltage and current.

For equipments that operate in the audio-frequency (af) range, power levels have to be determined in the performance of routine checks and during corrective maintenance procedures.

Power measurements for af circuits are usually indicated in terms of decibels (dB) or decibels referenced to 1 milliwatt (dBm). Because the actual calculation of decibel measurements is seldom required, the following explanation is somewhat simplified. Most test equipment is designed to measure and indicate decibels directly. This eliminates the need for you to perform complicated calculations. Nevertheless, a basic explanation of the decibel measurement system is necessary for you to understand the significance of dB readings and amplifier-gain ratings that are expressed in decibels.

2.2.1 The Decibel System The basic unit of measurement in the system is not the decibel; it is the bel. The bel is a unit that expresses the logarithmic ratio between the input and the output of any given component, circuit, or system. It may be expressed in terms of voltage, current, or power. Most often, it is used to show the ratio between input and output power to figure gain. You can express the power gain of the amplifier (N) in bels by dividing the output (P 1) by the input (P2) and taking the base 10 logarithm of the resulting quotient. The formula for determining this gain is:

log10 = P1 P2

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-3 UNCLASSIFIED If an amplifier doubles the input power, the quotient of P1 to P2 will be 2. If you consult a logarithm table, you will find that the base 10 logarithm of 2 is 0.3, making the power gain of the amplifier 0.3 bel.

Q-1. What is the logarithmic ratio between the input and output of a given circuit called?

Experience has shown that because the bel is a rather large unit, it is difficult to apply. A more practical unit, and one that can be used more easily, is the decibel (1/10 bel). You can convert any figure expressed in bels to decibels by multiplying that figure by 10 or simply by moving the decimal point one place to the right. Applying this rule, we find that the above ratio of 0.3 bel is equal to 3 decibels.

The decibel (dB) cannot be used to represent actual power; only the ratio of one power compared to another. To say that an amplifier has a 3 dB gain means that the output power is twice the input power. This gives no indication of the actual power represented. You must be able to state the input power for it to be meaningful. In many applications, a mathematical expression represents the actual power, not a power ratio. One standard reference is the dBm.

The dBm is an abbreviation used to represent power levels above or below 1 milliwatt. Negative dBm (−dBm) represents power levels below 1 milliwatt, and positive dBm (+dBm) represents power levels above 1 milliwatt. In other words, a dBm value is a specific amount of power; 0 dBm is equal to 1 milliwatt. Briefly stated, the amount of power in a given value of dBm is the power which results if 1 milliwatt is amplified or attenuated by that dB value. For example, 40 dBm represents an actual power level (watts or milliwatts) that is 40 dB above 1 milliwatt, whereas −10 dBm represents a power level that is 10 dB below 1 milliwatt. The formula for finding dBm is a variation of the dB power formula:

dBm = 10 log actual power (P2) . 001 watt (P1)

Q-2. What term is used to represent power levels above or below a 1-milliwatt reference?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-4 UNCLASSIFIED You do not need to use the formula in most applications. The following shows conversions of dBm to mW:

+20dBm = 100mW +10dBm = 10mW +7dBm = 5mW +6dBm = 4mW +4dBm = 2.5mW +3dBm = 2mW 0dBm = 1mW -3dBm = .5mW -10dBm = .1mW

For a +10 dBm level, start with the 1 milliwatt reference and move the decimal point one place to the right (+10 dBm = 10 mW). Another 10 dB increment brings the power level to +20 dBm, thereby moving the decimal point another place to the right (+20 dBm = 100 mW). For a −10 dBm level, again start with 1 milliwatt, but this time move the decimal point one place to the left (−10 dBm = .1 mW). An additional 10 dB decrease results in another decimal point shift to the left (−20 dBm = .01 mW).

For a 3 dB increase, you double the power. For a 3 dB decrease, you reduce the power by one-half (+3 dBm = 2 mW and −3 dBm = .5 mW). A +6 dBm level is an additional 3 dB change from +3 dBm. In this case, you just double the power level of the +3 dBm (+6 dBm = 4 mW).

Q-3. What milliwatt value is equal to +6 dBm? The dB change can be made in either direction. For example, +7 dBm is a decrease from +10 dBm. Reducing the +10 dBm power by one-half, we have +7 dBm, or 5 mW. A +4 dBm power level is a 3 dB decrease from +7 dBm (+4 dBm − 2.5 mW). By using this simple method, you can quickly find any power level that corresponds to a given dBm.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-5 UNCLASSIFIED Some test instruments you will be using are calibrated in decibels and have a 1 milliwatt zero reference level. Figure 2-1 illustrates such an instrument. Notice that this is an ac voltmeter in which the upper scale of the meter indicates ac voltage and the lower scale indicates decibels. The zero power-level indicator on the decibel scale is located at, or near, center scale. If the power in the line being measured is more than the reference value, the meter will indicate a value to the right of the zero mark (+dB). If the power is less than the reference value, the meter will indicate a value to the left of the zero mark (−dB). Such meters are useful when recording measurements where a direct indication in decibels is desired. However, you must remember that this meter is still a voltmeter and that power measurements are not meaningful unless the circuit impedance is known. If you feel the need to review how to calculate power in ac circuits, refer to NEETS, Module 2.

Figure 2-1 Ac voltmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-6 UNCLASSIFIED 2.2.2 Measurement Methods At radio frequencies below the UHF range, power is usually determined by voltage, current, and impedance measurements. One common method used to determine the output power of radio-frequency (rf) oscillators and radio transmitters consists of connecting a known resistance to the equipment output terminals. Current flowing through this resistance is then measured and the power is calculated as the product of I2R.

Because power is proportional to the current squared, the meter scale can be calibrated to indicate power units directly. A THERMOCOUPLE AMMETER can be used in this manner for measuring rf power. The resistor used to replace the normal load is specially designed to have low reactance and the ability to dissipate the required amount of power. Such resistors are commonly called DUMMY LOADS or DUMMY ANTENNAS.

Q-4. What name is given to a resistor used to replace the normal load in a circuit? In the UHF and SHF frequency ranges, accurately measuring the voltage, current, and resistance is difficult. These basic measurements can vary greatly, depending on where in the circuit the measurements are made. They are also affected by small changes in parts placement in the vicinity of tuned circuits.

To measure the output of microwave radio or radar transmitters, you can use test instruments that convert rf power to another form of energy, such as light or heat. These instruments can be used to indirectly measure the power. A method used to measure the effect of a resistor load on a stream of passing air can also be used to indirectly measure power. Accurate measurement of large-magnitude power also can be achieved by measuring the temperature change of a water load. The most common type of power meter for use in this frequency range employs a BOLOMETER.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-7 UNCLASSIFIED 2.2.2.1 Bolometer The bolometer is a loading device that undergoes changes of resistance as changes in dissipated power occur. The two types of bolometers are the BARRETTER and the THERMISTOR. The barretter is characterized by an increase in resistance as the dissipated power rises. The thermistor decreases in resistance as the power increases. In either case, resistance is measured before and after the application of rf power. If the same change in resistance is then produced by a variable dc source of power, then the rf power is equal to the measured dc power. This relationship makes possible the direct calibration of a bridge circuit in units of power. In other words, one condition of balance exists when no rf power is applied; but in the presence of power, a second condition of balance exists because of the resistance changes of the bolometer. It is this change of resistance that is calibrated in power.

Q-5. What are the two types of bolometers? BARRETTER - The construction of a typical barretter is shown in figure 2-2. The fine wire (usually tungsten) is extremely small in diameter. This thin diameter allows the rf current to penetrate to the center of the wire. The wire is supported in an insulating capsule between two metallic ends, which act as connectors. Because of these physical characteristics, the barretter resembles a cartridge-type fuse. The enclosure is a quartz capsule made in two parts. One part is an insert cemented in place after the tungsten wire has been mounted. In operation, the barretter is matched to the rf line after power is applied.

Figure 2-2 Typical barretter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-8 UNCLASSIFIED THERMISTOR - A high degree of precision is made possible by the thermistor; therefore, it is widely used. Figure 2-3 shows the typical construction of a bead-type thermistor. The negative-temperature coefficient comes from the use of a semiconductor as the active material. Notice that the active material is shaped in the form of a bead. It is supported between two pigtail leads by connecting wires. The pigtail ends are embedded in the ends of the surrounding glass capsule.

The negative-resistance temperature coefficient of thermistors is desirable. This is because excessive power has the effect of changing the resistance of the thermistor to an extent that causes a pronounced rf mismatch. The resulting decrease in power transfer reduces the likelihood of burnout.

Figure 2-3 Bead-type thermistor

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-9 UNCLASSIFIED 2.2.2.2 Thermistor Bridge Figure 2-4, views A and B, is an example of a THERMISTOR BRIDGE used for rf power measurements. A thermistor bridge circuit includes other thermistor elements, referred to as compensating thermistors. These thermistors respond to fluctuations in ambient temperature so that the bridge balances and calibration are maintained over a wide temperature range. Compensating thermistors are usually in disc form so that they can be mounted on a flat metal surface, such as a chassis or a waveguide. The thermistor bridge in view B is located in the terminating section of a waveguide and contains RT-2, a bead thermistor, and two compensating thermistors, RT-1 and RT-3, on the outside of the waveguide (view A). RA-1 in view B, a calibrated attenuator, controls the amount of rf energy applied to RT-2.

Figure 2-4 Thermistor bridge

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-10 UNCLASSIFIED Before power is applied, R1 and R2 in view A of figure 2-4 are used to adjust the current through RT-2. When the resistance of RT-2 reaches the equivalent parallel resistance of R6 and RT1 (122.4 ohms), the bridge is balanced. Meter M-1 reads 0 at this time. The rf signal being measured is connected to the test set and applied via the calibrated attenuator to RT-2. This causes the temperature of RT-2 to increase, thus reducing its resistance. The bridge becomes unbalanced, causing meter M-1 to deflect an amount proportional to the decrease in resistance of RT-2. Meter M-1, because of the operation of RT-2, reads average power.

Q-6. As the dissipated power increases, what effect does this have on the resistance of a thermistor? If the ambient temperature rises, the resistance of RT-1 decreases. This shunts more current around the bridge network and allows RT-2 to cool. The resistance of RT-3 decreases, maintaining meter sensitivity independent of temperature changes. Cavity Z-1 in view B of figure 2-4 is an ABSORPTIONTYPE FREQUENCY METER. This type of meter will be discussed later.

2.3 FREQUENCY MEASUREMENTS Frequency measurements are an essential part of preventive and corrective maintenance for electric and electronic equipment. Some examples of the various frequency measurements follow:

• Rotation frequencies of some electro-mechanical devices, such as electric motors, must be determined. • The output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines. • Equipment that operates in the af range must be adjusted to operate at the correct frequencies. • Radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interference with radio circuits operating on other frequencies. • Radar sets must be properly tuned to obtain satisfactory performance.

As you can see from the above examples, frequency measurement does indeed play a valuable role in maintenance. These measurements can be divided into two broad categories: MECHANICAL-ROTATION FREQUENCY measurement and ELECTRICAL-OUTPUT FREQUENCY measurement. Depending upon your job and/or the type of command to which you are assigned, you may be tasked with performing one or both of these types of measurements.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-11 UNCLASSIFIED 2.3.1 Mechanical-Rotation Frequency Measurement The rotating frequency (speed in revolutions per minute) of armatures in electric motors and engine-driven generators, as well as the blade speed in turbines, is measured with devices called TACHOMETERS, STROBOSCOPES, and STROBOTACS.

2.3.1.1 Tachometer A tachometer is an instrument that measures the rate at which a shaft is turning. Although tachometers are installed on machinery, such as generators and engines, you may need to determine the speed of a rotating machine that is not equipped with a tachometer. In these instances, you will be required to use a PORTABLE TACHOMETER. Portable hand- held tachometers measure speed by direct contact with the shaft of the measured unit. Portable tachometers are for use only during testing and should not be used continuously. The common types of portable tachometers are the CENTRIFUGAL and the CHRONOMETRIC.

CENTRIFUGAL TACHOMETER - A centrifugal-type tachometer is illustrated in figure 2-5, view A. View B shows the internal arrangement of the centrifugal tachometer; refer to view B in this discussion. In the centrifugal tachometer, centrifugal force acts upon fly weights that are connected by links to upper and lower collars. The upper collar is affixed to a drive shaft; the lower collar is free to move up and down the shaft. A spring, which fits over the shaft, connects the upper and lower collars.

Figure 2-5 Centrifugal tachometer

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-12 UNCLASSIFIED Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it. Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the spring tension, causes an increase in the indication on the dial face.

When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in contact with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to 500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).

CHRONOMETRIC TACHOMETER - The chronometric tachometer (figure 2-6) is a combination watch and revolution counter. It measures the average number of revolutions of a shaft per minute. The chronometric tachometer also comes with hard rubber tips, which must be inserted over the drive shaft.

Figure 2-6 Chronometric tachometer

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-13 UNCLASSIFIED When applied to a rotating shaft, the outer drive shaft of this tachometer runs free until a starting button is depressed to start the timing element. In figure 2-6, note the starting button beneath the index finger. The chronometric tachometer retains readings on its dial after its drive shaft has been disengaged from a rotating shaft and until the pointers are returned to 0 by the reset button (usually the starting button). The range of a chronometric tachometer is usually from 0 to 10,000 rpm and from 0 to 3,000 feet per minute (fpm).

2.3.1.2 Stroboscope The rotation frequencies of recording devices and teletypewriter motors can be measured by the use of a STROBOSCOPE. The stroboscope is an instrument that allows you to view rotating or reciprocating objects intermittently and produces the optical effect of a slowing down or stopping motion. For example, electric fan blades revolving at 1,800 rpm will appear stationary if you look at them under a light that flashes uniformly 1,800 times per minute. At 1,799 flashes per minute, the blades will appear to rotate forward at 1 rpm; at 1,801 flashes per minute, they will appear to rotate backward at 1 rpm.

When the flashing rate of the light is adjustable, you can calibrate the control in flashes (or revolutions) per minute. The stationary image you see when the rate of the lamp and the rotational rate of a shaft are equal lets you record a very precise speed measurement.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-14 UNCLASSIFIED 2.3.1.3 Strobotac The STROBOTAC (figure 2-7) is an electronic flash device in which the flash duration is very short (a few millionths of a second). (Table 2-1 contains a description of the controls and indicators shown on the strobotac in figure 2-7.) Because of this short flash duration, the strobotac can measure very rapid motion. The box contains a swivel mount with a STROBOTRON LAMP in a reflector, an electronic pulse generator to control the flashing rate, and a power supply that operates from the ac power line. The flashing rate is controlled by the large knob; the corresponding speed (rpm) is indicated on an illuminated dial that is viewed through windows in the knob.

Figure 2-7 Electronic strobotac

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-15 UNCLASSIFIED

Table 2-1 Strobotac Controls and Indicators

NO CONTROLS AND INDICATORS (see figure 2-7) NAME USE 1. POWER switch Turns power on and off. 2. RPM control Controls the flashing rate of light as the fluted rim is rotated. Dial is calibrated directly in revolutions per minute (rpm). 3. Range switch Selects any of three rpm (internal oscillator) ranges, plus three external-input positions: Rpm ranges Intensity External input 110-690 rpm High 700 rpm max 170-4170 rpm Med 4000 rpm max* 4000-25,000 rpm Low 25,000 rpm max 4. CALibration indicator lamp Indicates the correct setting of CALibration adjustments for calibrating the RPM dial to power-line frequency. 5. HIGH CAL, LOW CAL Calibration adjustments used to calibrate the RPM dial. 6. OUTPUT TRIGGER jack A trigger pulse is available at this jack for strobotac types 1531, 1538, stroboslave type 1539, and strobolume type 1532. 7. INPUT jack Used for connecting the stroboscope to an external synchronizing signal from the electrical device or mechanical contactor. 8. Reflector-lamp assembly Produces and aims the flashing light 9. Power cord A permanently attached 6-foot power cord. For storage, the cord is wound clockwise around the range-switch knob and reflector. The plug is secured by sliding it onto the holding pin. 10. Holder pin Used to secure the plug-end of the power cord when unit is to be stored in its case. * Flashes at 3600 rpm until external signal is plugged in.

The normal speed range is from 110 to 25,000 rpm. At speeds below 600 rpm, "flicker" becomes a problem because the human eye cannot retain successive images long enough to create the illusion of continuous motion. The life of the strobotron lamp is approximately 250 hours if used at flashing speeds of less than 5,000 rpm, or 100 hours if used at higher speeds.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-16 UNCLASSIFIED 2.3.2 Electrical Output Frequency All alternating voltage sources are generated at a set frequency or range of frequencies. A FREQUENCY METER provides a means of measuring this frequency. The electrical output frequency of ac power generators can be measured by a vibrating reed, a tuned circuit, or by a crossed-coil, iron-vane type meter. The vibrating-reed device is the simplest type of frequency meter. It has the advantage of being rugged enough to be mounted on generator control panels.

A simplified diagram of a vibrating-reed frequency meter is shown in figure 2-8, views A through D. In view A, you can see that the current to be measured flows through the coil and exerts maximum attraction on the soft-iron armature twice during each cycle. The armature is attached to the bar, which is mounted on a flexible support. Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth, up to 130 hertz are mounted on the bar (view B). The reed with a frequency of 110 hertz is marked 55 hertz; the one with a frequency of 112 hertz is marked 56 hertz; the one with a frequency of 120 hertz is marked 60 hertz, and so forth.

When the coil is energized by a current with a frequency between 55 and 65 hertz, all the reeds are vibrated slightly; but, the reed having a natural frequency closest to that of the energizing current vibrates through a larger amplitude. The frequency is read from the scale value opposite the reed having the greatest amplitude of vibration.

In some instruments, the reeds are the same length; but they are weighted by different amounts at the top so they will have different natural rates of vibration. An end view of the reeds in the indicator is shown in view C. If the energizing current has a frequency of 60 hertz, the reed marked 60 will vibrate the greatest amount, as shown. View D shows a hand-held vibrating-reed frequency meter mounted on the casing of a motor-generator.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-17 UNCLASSIFIED

Figure 2-8 Vibrating-reed frequency meter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-18 UNCLASSIFIED 2.3.2.1 Tuned Circuits TUNED CIRCUITS are used as filters for the passage or rejection of specific frequencies. BANDPASS FILTERS and BAND-REJECT FILTERS are examples of this type. Tuned circuits have certain characteristics that make them ideal for certain types of filters, especially where a high degree of selectivity is desired. A series-tuned circuit offers a low impedance to currents of the particular frequency to which the circuit is tuned and a relatively high impedance to currents of all other frequencies. A parallel- tuned circuit, on the other hand, offers a very high impedance to currents of its natural, or resonant, frequency and a relatively low impedance to others. If you feel you need to review the subject of tuned circuits at this time, refer to NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, for more information on these circuits and their applications.

2.3.3 Audio Frequencies Frequency measurements in the af range can be made by the comparison method or the direct-reading frequency meter. Frequency comparisons can be made by the use of a calibrated af generator in conjunction with either an oscilloscope or a modulator and a zero-beat indicating device. Direct-reading frequency measurements can be made by instruments using series, frequency-selective electrical networks, bridge test sets having null indicators, or counting-type frequency meters.

2.3.3.1 Heterodyne Frequency Meters Heterodyne frequency meters are available in several varieties. They measure the frequency of the unknown signal by matching the unknown signal with a locally generated signal of the same frequency obtained from a calibrated, precision oscillator. This method is normally referred to as zero beating. When a perfect frequency match is obtained, it is indicated by the absence of a beat note (zero beat). The technician generally uses a set of headphones to detect a zero-beat condition in the equipment being tested.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-19 UNCLASSIFIED The basic heterodyne meter (figure 2-9) is a calibrated variable oscillator, which heterodynes against the frequency to be measured. Coupling is accomplished between the frequency meter and the output of the equipment under test. (NOTE: This coupling should be in accordance with the step-by-step procedures listed in the technical manual for the frequency meter.) The calibrated oscillator is then tuned so that the difference between the oscillator frequency and the unknown frequency is in the af range. This difference in frequency is known as the BEAT FREQUENCY. As the two frequencies are brought closer to the same value, the tone in the headset will decrease in pitch until it is replaced by a series of rapid clicks. As the process is continued, the clicks will decrease in rapidity until they stop altogether. This is the point of zero beat; that is, the point at which the frequency generated in the oscillator of the frequency meter is equal to the frequency of the unknown signal being measured.

Q-7. In a heterodyne-type frequency meter, what is the difference between the oscillator frequency and the unknown frequency?

Figure 2-9 Basic heterodyne meter (block diagram)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-20 UNCLASSIFIED For all practical purposes, the point of zero beat can be assumed when the clicks are heard at infrequent intervals. Figure 2-10 illustrates the zero-beat concept. Maintaining a condition of absolute silence in the earphones is extremely difficult when you are making this measurement. When the incoming signal is strong, the clicks are sharp and distinct. When the signal is weak, the zero-beat condition is evidenced by a slowly changing "swishing" or "rushing" sound in the headset. After the zero beat is obtained, the dial reading corresponds to the frequency measured.

The manufacturer’s calibration book is a very important part of the frequency meter package; in fact, the book is so important that it bears the same serial number as the heterodyne-type frequency meter itself. Contained in this book is a list of the dial settings and the corresponding frequencies produced by that meter at those dial settings. Operating instructions for the meter are also included.

Figure 2-10 Graph of sound heard in earphone when zero beating

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-21 UNCLASSIFIED 2.3.3.2 Absorption Wavemeter WAVEMETERS are calibrated resonant circuits used to measure frequency. The accuracy of wavemeters is not as high as that of heterodyne-type frequency meters; however, they have the advantage of being comparatively simple and can be easily carried.

Q-8. What equipment uses a calibrated resonant circuit to measure frequency? Any type of resonant circuit can be used in wavemeter applications. The exact kind of circuit used depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used for VLF through VHF wavemeters.

The simplified illustration of an absorption wavemeter, shown in figure 2-11, consists of a pickup coil, a fixed capacitor, a lamp, a variable capacitor, and a calibrated dial. When the wavemeter’s components are at resonance, maximum current flows in the loop, illuminating the lamp to maximum brilliance. The calibrated dial setting is converted to a frequency by means of a chart, or graph, in the instruction manual. If the lamp glows very brightly, the wavemeter should be coupled more loosely to the circuit. For greatest accuracy, the wavemeter should be coupled so that its indicator lamp provides only a faint glow when tuned to the resonant frequency.

Figure 2-11 Absorption wavemeter circuit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-22 UNCLASSIFIED 2.3.4 Frequencies above the Audio Range The signal frequencies of radio and radar equipments that operate in the UHF and SHF ranges can be measured by resonant, cavity-type wavemeters or resonant, coaxial-line- type wavemeters. When properly calibrated, resonant-cavity and resonant-coaxial line wavemeters are more accurate and have better stability than wavemeters used for measurements in the LF to VHF ranges. These frequency-measuring instruments are often furnished as part of the equipment. They are also available as general-purpose test sets.

Although many wavemeters are used in performing various functions, the cavity-type wavemeter is the type most commonly used. Only this type is discussed in some detail.

2.3.4.1 Cavity Wavemeter Figure 2-12 shows a typical CAVITY WAVEMETER. The wavemeter is of the type commonly used for the measurement of microwave frequencies. The device uses a resonant cavity. The resonant frequency of the cavity is varied by means of a plunger, which is mechanically connected to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment being tested is fed into the wavemeter through one of two inputs, A or B. The crystal rectifier then detects (rectifies) the signal. The rectified current is indicated on current meter M.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-23 UNCLASSIFIED

2.3.4.2 Electronic Frequency Counters Another device used to measure frequencies above the audio range is the ELECTRONIC FREQUENCY COUNTER. Since this instrument will be covered in detail in a later chapter, only a brief description is provided at this time.

The electronic frequency counter is a high-speed electronic counter with an accurate, crystal-controlled time base. This combination provides a frequency counter that automatically counts and displays the number of events occurring in a precise time interval. The frequency counter itself does not generate any signal; it merely counts the recurring pulses fed to it.

Figure 2-12 Typical cavity wavemeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-24 UNCLASSIFIED 2.4 WAVEFORM ANALYSIS WAVEFORM ANALYSIS can be made by observing displays of voltage and current variations with respect to time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and testing pulse-generating, pulse- forming, and pulse-amplifying circuits. The waveform visual display is also useful for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage.

Waveform analysis is used in various electrical and electronic equipment troubleshooting. This section will briefly discuss the oscilloscope and spectrum analyzer to provide you with basic knowledge of this test equipment.

Q-9. Name two instruments used to analyze waveforms. 2.4.1 Use of the Oscilloscope The CATHODE-RAY OSCILLOSCOPE (CRO or O-SCOPE) is commonly used for the analysis of waveforms generated by electronic equipment. Several types of cathode-ray oscilloscopes are available for making waveform analysis. The oscilloscope required for a particular test is determined by characteristics such as input-frequency response, input impedance, sensitivity, sweep rate, and the methods of sweep control. The SYNCHROSCOPE is an adaptation of the cathode-ray oscilloscope. It features a wide- band amplifier, triggered sweep, and retrace blanking circuits. These circuits are desirable for the analysis of pulse waveforms.

Oscilloscopes are also part of some harmonic analysis test equipments that display harmonic energy levels. To effectively analyze waveform displays, you must know the correct wave shape. The maintenance instructions manual for each piece of equipment illustrates what waveforms you should observe at the various test points throughout the equipment. Waveforms that will be observed at any one selected test point will differ; each waveform will depend on whether the operation of the equipment is normal or abnormal.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-25 UNCLASSIFIED The display observed on a cathode-ray oscilloscope is ordinarily one similar to those shown in figure 2-13. Views A and B show the instantaneous voltage of the wave plotted against time. Elapsed time (view A) is indicated by horizontal distance, from left to right, across the etched grid (graticule) placed over the face of the tube. The amplitude (view B) of the wave is measured vertically on the graph.

The oscilloscope is also used to picture changes in quantities other than simply the voltages in electric circuits. For example, if you need to see the changes in waveform of an electric current, you must first send the current through a small resistor. You can then use the oscilloscope to view the voltage wave across the resistor. Other quantities, such as temperatures, pressures, speeds, and accelerations, can be translated into voltages by means of suitable transducers and then viewed on the oscilloscope. A detailed discussion of the oscilloscope is presented in chapter 6 of this module.

Figure 2-13 Typical waveform displays

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-26 UNCLASSIFIED 2.4.2 Use of the Spectrum Analyzer The SPECTRUM ANALYZER is a device that sweeps over a band of frequencies to determine (1) what frequencies are being produced by a specific circuit under test and (2) the amplitude of each frequency component. To accomplish this, the spectrum analyzer first presents a pattern on a display. Then the relative amplitudes of the various frequencies of the spectrum of the pattern are plotted (see figure 2-14). On the vertical, or Y axis, the amplitudes are plotted; on the horizontal, or X axis, the frequencies (time base) are plotted. The overall pattern of this display indicates the proportion of power present at the various frequencies within the SPECTRUM (fundamental frequency with sideband frequencies).

Figure 2-14 Spectrum analyzer pattern

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-27 UNCLASSIFIED Q-10. What device sweeps a band of frequencies to determine frequencies and amplitudes of each frequency component? The spectrum analyzer is used to examine the frequency spectrum of radar transmissions, local oscillators, test sets, and other equipment operating within its frequency range. Proper interpretation of the displayed frequency spectrum enables you to determine the degree of efficiency of the equipment under test. With experience, you will be able to determine definite areas of malfunctioning components within equipment. In any event, successful spectrum analysis depends on the proper operation of a spectrum analyzer and your ability to correctly interpret the displayed frequencies. Later, in chapter 6, we will discuss the various controls, indicators, and connectors contained on the spectrum analyzer.

2.5 TESTING SEMICONDUCTOR DEVICES Because of the reliability of semiconductor devices, servicing techniques developed for transistorized equipment differ from those normally used for electron-tube circuits. Electron tubes are usually considered to be the circuit component most susceptible to failure and are normally the first components to be tested. Transistors, however, are capable of operating in excess of 30,000 hours at maximum rating without failure. They are often soldered in the circuit in much the same manner as resistors and capacitors. Therefore, they are NOT so quickly removed for testing as tubes.

Substitution of a semiconductor diode or transistor known to be in good condition is one method of determining the quality of a questionable semiconductor device. This method should be used only after you have made voltage and resistance measurements. This ensures the circuit has no defect that might damage the substitute semiconductor device. If more than one defective semiconductor is present in the equipment section where trouble has been localized, the semiconductor replacement method becomes cumbersome. Several semiconductors may have to be replaced before the trouble is corrected. To determine which stage(s) failed and which semiconductors are not defective, you must test all the removed semiconductors. You can do this by observing whether the equipment operates correctly as you reinsert each of the removed semiconductor devices into the equipment.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-28 UNCLASSIFIED 2.5.1 Testing Diodes Semiconductor diodes, such as general-purpose germanium and silicon diodes, power silicon diodes, and microwave silicon diodes, can be tested effectively under actual operating conditions. However, crystal-rectifier testers are available to determine dc characteristics that provide an indication of crystal-diode quality.

A common type of crystal-diode test set is a combination ohmmeter-ammeter. Measurements of forward resistance, back resistance, and reverse current can be made with this equipment. Using the results of these measurements, you can determine the relative condition of these components by comparing their measured values with typical values obtained from test information furnished with the test set or from the manufacturer’s data sheets. A check that provides a rough indication of the rectifying property of a diode is the comparison of the back-and-forward resistance of the diode at a specified voltage. A typical back-to-forward-resistance ratio is on the order of 10 to 1, and a forward-resistance value of 50 to 80 ohms is common.

Q-11. What is the typical back-to-forward resistance ratio of a good-quality diode? 2.5.1.1 Testing Diodes with an Ohmmeter A convenient test for a semiconductor diode requires only an ohmmeter. The back-and- forward resistance can be measured at a voltage determined by the battery potential of the ohmmeter and the resistance range at which the meter is set. When the test leads of the ohmmeter are connected to the diode, a resistance will be measured that is different from the resistance indicated if the leads are reversed. The smaller value is called the FORWARD RESISTANCE, and the larger value is called the BACK RESISTANCE. If the ratio of back-to-forward resistance is greater than 10 to 1, the diode should be capable of functioning as a rectifier. However, keep in mind that this is a very limited test that does not take into account the action of the diode at voltages of different magnitudes and frequencies. (NOTE: This test should never be used to test crystal mixer diodes in radars. It will destroy their sensitivity.)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-29 UNCLASSIFIED 2.5.1.2 Testing Diodes with Oscilloscopes An oscilloscope can be used to graphically display the back-and-forward resistance characteristics of a crystal diode. A circuit used in conjunction with an oscilloscope to make this test is shown in figure 2-15. This circuit uses the oscilloscope line-test voltage as the test signal. A series circuit (composed of resistor R1 and the internal resistance in the line-test circuit) decreases a 3-volt, open-circuit test voltage to a value of approximately 2 volts peak to peak.

Figure 2-15 Testing semiconductor diodes with an oscilloscope

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-30 UNCLASSIFIED The test signal applied to the crystal diode is also connected to the horizontal input of the oscilloscope. The horizontal sweep represents the voltage applied to the diode under test. The voltage developed across current-measuring resistor R2 is applied to the vertical input of the oscilloscope. Because this voltage is proportional to the current through the diode being tested, the vertical deflection will indicate crystal current. The resulting oscilloscope trace for a normal diode is similar to the curve shown in figure 2-16.

To test Zener diodes, you must use a higher voltage than the oscilloscope line-test signal. This test can be made with a diode test set or with the circuit shown in figure 2-17. In this circuit, rheostat R1 is used to adjust the input voltage to a suitable value for the Zener diode being tested. Resistor R2 limits the current through the diode. The signal voltage applied to the diode is also connected to the horizontal input of the oscilloscope. The voltage developed across current-measuring resistor R3 is applied to the vertical input of the oscilloscope. The horizontal sweep represents the applied voltage, and the vertical deflection indicates the current through the diode being tested. Figure 2-18 shows the characteristic pattern of a Zener diode. Note the sharp increase in current at the Zener voltage (avalanche) point. For the Zener diode to be usable, this voltage must be within limits specified by the manufacturer.

Figure 2-16 Characteristic curve of a semiconductor diode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-31 UNCLASSIFIED

Figure 2-17 Testing a Zener diode Figure 2-18 Zener diode characteristic curve

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-32 UNCLASSIFIED 2.5.2 Testing Transistors Most transistorized equipments use printed circuit boards on which components are neatly arranged. This arrangement makes the transistors and other components easy to reach while you are troubleshooting and servicing the equipment. While investigating with test probes, however, you must be careful to prevent damage to the printed wiring.

One of the outstanding advantages of transistors is their reliability. Tube failures account for over 90 percent of the failures in electron-tube equipments. Transistors, however, are long lived. This factor, among others, decreases maintenance required to keep transistorized equipment operating. The techniques used in testing transistorized equipment are similar to those for maintaining electron-tube circuits. Basically, these techniques include several checks and inspections.

2.5.2.1 Power Supply Checks When using test equipment to localize a trouble, you should check the power supply to see that its output voltages are present and of the correct values. Improper power supply voltages can cause odd effects. You will prevent many headaches by checking the power supply first.

2.5.2.2 Visual Inspection Visual inspection is a good maintenance technique. Occasionally, you will find loose wires or faulty connections, making extensive voltage checks unnecessary.

2.5.2.3 Transistor Checks Transistors can be checked by substitution. Transistors, however, have a characteristic known as leakage current, which may affect the results obtained when the substitution method is used.

The leakage current may influence the current gain or amplification factor of the transistor. Therefore, a particular transistor might operate properly in one circuit and not in another. This characteristic is more critical in certain applications than in others. As the transistor ages, the amount of leakage current tends to increase. One type of transistor checker used is the semiconductor test set. This test set can be used either for in-circuit or out-of-circuit tests or for collector leakage current or current gain. You should use extreme care when substituting transistors. More and more transistors have specific current and breakdown voltage requirements that may affect how they operate within a given circuit.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-33 UNCLASSIFIED Q-12. As a transistor ages, what happens to the leakage current? 2.5.2.4 Voltage Checks Voltage measurements provide a means of checking circuit conditions in a transistorized circuit just as they do in checking conditions in a tube circuit. The voltages, however, are much lower than in a tube circuit. The bias voltage between the base and emitter, for instance, is usually 0.05 to 0.20 volts. When making checks, observe polarity.

2.5.2.5 Resistance Checks Transistors have little tendency to burn or change value because of low voltage in their circuits. They can, however, be permanently damaged by high- voltage conditions that occur when the collector voltage is increased. They can also be permanently damaged when the ambient temperature increases and causes excessive collector current flow. Transistors are easily damaged by high current; therefore, resistance measurements must not be taken with an ohmmeter that provides a maximum current output in excess of 1 milliampere. If you are not sure that the range of ohmmeter you want to use is below the 1 milliampere level, connect the ohmmeter to a milliammeter and check it. See figure 2-19 for a method of measuring the current from an ohmmeter.

Resistance measurements usually are not made in transistorized circuits, except when you are checking for open windings in transformers and coils. When a resistance check is required, the transistors are usually removed from the circuit. Resistance checks cannot test all the characteristics of transistors, especially transistors designed for high frequencies or fast switching. The ohmmeter is capable of making simple transistor tests, such as open and short tests.

Refer to NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistor and semiconductor terms and theory.

Figure 2-19 Measuring current passed by an ohmmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-34 UNCLASSIFIED 2.6 SUMMARY The important points of this chapter are summarized in the following paragraphs:

The BEL is a unit that expresses the logarithmic ratio between the input and output of any given component, circuit, or system and can be expressed in terms of voltage, current, or power.

Any figure expressed in bels can be converted to DECIBELS by multiplying the figure by 10. The decibel cannot be used to represent actual power, only a ratio of one power to another.

The abbreviation dBm is used to represent power levels above or below a 1 milliwatt reference level.

A BOLOMETER is a device that undergoes changes in resistance as changes in dissipated power occur. The two types of bolometers most often used are the barretter and the thermistor.

FREQUENCY MEASUREMENTS can be divided into two broad categories: mechanical-rotation frequency and electrical-output frequency measurements.

MECHANICAL ROTATION frequency is measured using a device called a TACHOMETER. Three basic tachometers are used for measuring mechanical rotation frequency - the CENTRIFUGAL tachometer, the CHRONOMETRIC tachometer, and the STROBOSCOPIC tachometer.

ELECTRICAL-OUTPUT frequencies of ac generators can be measured by VIBRATING-REED devices or TUNED CIRCUITS.

AUDIO FREQUENCIES can be measured by a process known as ZERO BEATING. This is done by matching an unknown signal with a locally generated signal of the same frequency obtained from a calibrated high-precision oscillator. As the two frequencies are brought closer to the same value, they reach a point of zero beat. This is when the frequency generated in the oscillator is equal to the frequency of the unknown signal being measured. Another term for zero beating is HETERODYNING.

WAVEMETERS are calibrated resonant circuits used to measure frequency. Any type of resonant circuit can be used in wavemeter applications. The type used depends on the frequency range for which the meter is intended.

For measuring frequencies in the microwave range, the CAVITY WAVEMETER is the type most commonly used.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-35 UNCLASSIFIED The CATHODE-RAY OSCILLOSCOPE and the SPECTRUM ANALYZER are used to perform WAVEFORM ANALYSIS.

A typical BACK-TO-FORWARD-RESISTANCE ratio for a diode is 10 to 1.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

2-36 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q12.

A-1. Bel.

A-2. dBm.

A-3. 4 mW.

A-4. Dummy load or dummy antenna.

A-5. Barretter and thermistor.

A-6. It increases.

A-7. Beat frequency.

A-8. Wavemeter.

A-9. Oscilloscope and spectrum analyzer.

A-10. Spectrum analyzer.

A-11. 10-to-1 ratio.

A-12. It tends to increase.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-1 UNCLASSIFIED 3 BASIC METERS LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

1. Describe the basic theory of the galvanometer. 2. Describe the basic theory of the D’Arsonval meter movement. 3. State the proper procedure for connecting an ammeter to a circuit. 4. Define ammeter sensitivity. 5. State the proper procedure for connecting a voltmeter to a circuit. 6. Describe possible effects on a circuit caused by the connection of a voltmeter. 7. Define voltmeter sensitivity. 8. Describe the internal operation of an ohmmeter with the use of a block diagram. 9. Describe the operating procedure for using a megohmmeter. 10. Describe the use of the electrodynamometer-type meter as a voltmeter, ammeter, and wattmeter. 11. Describe the factors that limit wattmeter capability. 12. Describe an open circuit, a ground, a short, and the tests used to check for these conditions.

3.1 INTRODUCTION When troubleshooting, testing, or repairing electronic equipment, you will use various meters and other types of test equipment to check for proper circuit voltages, currents, resistances, and to determine if the wiring is defective. You may be able to connect these test instruments to a circuit and take readings without knowing just how the instruments operate. However, to be a competent technician, you need to be able to do more than merely read a test instrument. You need a basic knowledge of how test instruments operate. This chapter discusses the operating principles of some of the test instruments you will use in equipment troubleshooting.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-2 UNCLASSIFIED 3.2 METERS The best and most expensive measuring instrument is of no use to you unless you know what you are measuring and what each reading indicates. Remember that the purpose of a meter is to measure quantities existing within a circuit. For this reason, when the meter is connected to the circuit, it must not change the condition of the circuit.

3.2.1 Meter Power Source Meters are either SELF-EXCITED or EXTERNALLY EXCITED. Self-excited meters operate from their own power sources. Externally excited meters get their power from the circuit to which they are connected. Most common meters (voltmeters, ammeters, and ohmmeters) that you use in your work operate on the electromagnetic principle. All measuring instruments must have some form of indicating device, usually a meter, to be of any use to you. The most basic indicating device used in instruments that measure current and voltage operates by using the interaction between the magnetic fields associated with current flow in the circuit. Before continuing, you might want to review the properties of magnetism and electromagnetism in NEETS, Module 1, Introduction to Matter, Energy, and Direct Current.

Q-1. What meters operate from their own power sources? 3.2.2 Basic Meter Movement A stationary, permanent-magnet, moving-coil meter is the basic meter movement used in most measuring instruments used for servicing electrical equipment. When current flows through the coil, a resulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movable coil to rotate. The greater the intensity of current flow through the coil, the stronger the magnetic field produced; the stronger the magnetic field produced, the greater the rotation of the coil. The GALVANOMETER is an example of one type of stationary, permanent-magnet, moving-coil measuring instrument.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-3 UNCLASSIFIED 3.2.2.1 Galvanometer The galvanometer is used to measure very low currents, such as those in bridge circuits. In modified form, the galvanometer has the highest sensitivity of any of the various types of meters in use today. A simplified diagram of a galvanometer is shown in figure 3-1. It is different from other instruments used for the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaft and jewel-bearing arrangement often used in other instruments.

The movable coil is wrapped around the aluminum frame of the galvanometer. The coil is suspended between the poles of the magnet by means of thin, flat ribbons of phosphor bronze. These ribbons provide a conduction path for the current between the circuit being tested and the movable coil. The ribbons allow the coil to twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. They also provide the restoring force for the coil. Basically, the restoring force is that force necessary to return the movable frame to its resting position after a reading. The ribbons restrain or provide a counterforce to the magnetic force acting on the coil. When the driving force of the coil current is removed, the restoring force provided by the ribbons returns the coil to its zero position. Figure 3-1 Simplified galvanometer

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-4 UNCLASSIFIED Q-2. What physical component of a galvanometer provides the restoring force for the coil? To determine the amount of current flow, we must have a means to indicate the amount of coil rotation. Either of two methods may be used: (1) the POINTER arrangement or (2) the LIGHT AND MIRROR arrangement.

Q-3. In a galvanometer, what two methods are used to indicate the amount of coil rotation? In the pointer arrangement, one end of the pointer is mechanically connected to the rotating coil; as the coil moves, the pointer also moves. The other end of the pointer moves across a graduated scale and indicates the amount of current flow. The overall simplicity of this arrangement is its main advantage. However, a disadvantage of this arrangement is that it introduces a mechanical coil balancing problem, especially if the pointer is long.

Q-4. What is the primary disadvantage of the pointer arrangement for indicating coil rotation? In the light and mirror arrangement, the use of a mirror and a beam of light simplifies the problem of coil balance. When this arrangement is used to measure the turning of the coil, a small mirror is mounted on the supporting ribbon, as shown in figure 3-1. An internal light source is directed to the mirror and then reflected to the scale of the meter. As the movable coil turns, so does the mirror. This causes the light reflection to move across the graduated scale of the meter. The movement of the reflection is proportional to the movement of the coil; therefore, the intensity of the current being measured by the meter is accurately indicated.

If the beam of light and mirror arrangement is used, the beam of light is swept to the right or left across a translucent screen (scale). The translucent screen is divided uniformly with the zero reading located at center scale. If the pointer arrangement is used, the pointer is moved in a horizontal plane to the right or left across a scale that is divided uniformly with the zero reading at the center. The direction in which the beam of light or the pointer moves depends on the direction (polarity) of current through the coil.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-5 UNCLASSIFIED 3.2.2.2 D’Arsonval Meter Movement Most dc instruments use meters based on some form of the D’Arsonval meter movement. In D’Arsonval-type meters, the length of the conductor and the strength of the field between the poles of the magnet are fixed. Therefore, any change in current causes a proportional change in the force acting on the coil. Figure 3-2 is a simplified diagram showing the principle of the D’Arsonval movement.

In the figure, only one turn of wire is shown; however, in an actual meter movement, many turns of fine wire would be used, each turn adding more effective length to the coil. The coil is wound on an aluminum frame (bobbin) to which the pointer is attached. Oppositely wound hairsprings (only one is shown in the figure) are also attached to the bobbin, one at either end. The circuit to the coil is completed through the hairsprings. In addition to serving as conductors, the hairsprings serve as the restoring force that returns the pointer to the zero position when no current flows.

Q-5. What component of the D’Arsonval meter movement completes the circuit for current flow to the coil? Figure 3-2 D'Arsonval meter movement

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-6 UNCLASSIFIED COIL MOVEMENT - As we discussed previously, the deflecting (moving) force on the coil is proportional to the current flowing through the coil. This deflecting force tends to cause the coil to rotate against the restraining force of the hairsprings. When the deflecting force and the restraining force are equal, the coil and the pointer stop moving. As we have just stated, the deflecting force is proportional to the current in the coil, the angle (amount) of rotation is proportional to the deflecting force; therefore, the angle of rotation is proportional to the current through the coil. When current stops flowing through the coil, the deflecting force stops, and the restoring force of the springs returns the pointer to the zero position.

Q-6. What component supplies restoring force to the coil of the D’Arsonval meter movement? DIRECTION OF FORCE - The current through the single turn of wire is in the direction indicated in the figure (away from you on the right-hand side and toward you on the left-hand side). If we apply the right-hand motor rule, the direction of force is upward on the left-hand side and downward on the right-hand side; therefore, the direction of motion of the coil and pointer is clockwise. If the current were reversed in the wire, the direction of motion of the coil and pointer would be reversed. For a review of the right- hand rule for motors, refer to NEETS, Module 5, Introduction to Generators and Motors.

PRINCIPLE OF OPERATION - A more detailed view of the basic D'Arsonval movement, as it is used in ammeters and voltmeters, is shown in figure 3-3. The principle of operation is the same as that discussed in the simplified version. The iron core is rigidly supported between the pole pieces; it serves to concentrate the flux in the narrow space between the iron core and the pole piece. Current flows into one hairspring, through the coil, and out the other hairspring. The restoring forces of the spiral springs return the pointer to the normal zero position when the current through the coil is interrupted. Conductors connect the hairsprings with the outside terminals of the meter. If the instrument is not DAMPED to absorb the energy of the moving element, the pointer will oscillate (vibrate) for a period of time before coming to a stop in its final position. Damping is an energy-absorbing system that prevents this.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-7 UNCLASSIFIED

Figure 3-3 Detailed view of the basic D'Arsonval meter movement

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-8 UNCLASSIFIED DAMPING - This is accomplished in many D'Arsonval movements by means of the motion of the aluminum bobbin on which the coil is wound. As the bobbin rotates in the magnetic field, an electromotive force is induced into it as it cuts through the lines of force. Induced currents flow in the bobbin in a direction opposite to the motion; this causes the bobbin to go beyond its final position only once before stopping. The overall sensitivity of the meter can be increased by the use of a lightweight rotating assembly (bobbin, coil, and pointer) and by the use of jewel bearings, as shown in figure 3-3.

POLE CONSTRUCTION - Note that the pole pieces in figures 3-2 and 3-3 have curved faces. You can see the advantage of this type of construction if you remember that lines of force enter and leave a magnetic field in the air gap at right angles to the coil, regardless of the angular position of the coil. Because of this type of construction, a more linear scale is possible than if the pole faces were flat.

Q-7. What advantage is gained by using pole pieces with curved faces in the D’Arsonval meter movement? 3.2.3 DC Ammeter The movable coil of the D'Arsonval meter movement we have been discussing up to now uses small-size wire in its windings. This small-size wire places limits on the amount of current that can be safely passed through the coil. Therefore, the basic D'Arsonval movement discussed can be used to indicate or measure only very small currents. Certain circuit changes must be made to the basic D'Arsonval meter movement for it to be practical in everyday use. To measure large currents, you must use a SHUNT with the meter.

3.2.3.1 Shunts A shunt is a physically large, low-resistance conductor connected in parallel (shunt) with the meter terminals. It is used to carry the majority of the load current. Such a shunt is designed with the correct amount of resistance so that only a small portion of the total current flows through the meter coil. The meter current is proportional to the total load current. If the shunt is of such a value that the meter is calibrated in milliamperes, the instrument is called a MILLIAMMETER. If the shunt has such a value that the meter must be calibrated in terms of amperes, it is called an AMMETER.

Q-8. What structurally large, low-resistance conductor is connected in parallel with the meter movement to prevent damage?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-9 UNCLASSIFIED SHUNT RESISTANCE - A single, standardized meter movement is normally used in all ammeters, no matter what the range is for a particular meter. For example, meters with working ranges of 0 to 10 amperes, 0 to 5 amperes, or 0 to 1 ampere all use the same meter movement. The various ranges are achieved through the use of different values of shunt resistance with the same meter movement. The designer of the ammeter simply calculates the correct shunt resistance required to extend the range of the meter movement to measure any desired value of current. This shunt is then connected across the meter terminals. Shunts may be located inside the meter case (internal shunts) with the proper switching arrangements for changing them. They may also be located outside the meter case (external shunts) with the necessary leads to connect them to the meter.

EXTERNAL SHUNTS - An external-shunt circuit is shown in figure 3-4, view A. Typical external shunts are shown in view B. View C shows a meter movement mounted within the case. The case provides protection against breakage, magnetic shielding in some cases, and portability.

Figure 3-4 Dc ammeter using the D'Arsonval movement with external shunts

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-10 UNCLASSIFIED SHUNT CONSTRUCTION - The shunt strips (view B of figure 3-4) are usually made of the alloy Manganin. Manganin has a temperature coefficient of almost zero. The zero- temperature coefficient property is desirable because of the heavy currents that often flow through shunts producing heat. A zero-temperature coefficient material is not affected by this heat; therefore, it remains stable in temperature. Most other materials increase their resistance as they are heated. If shunts were made of these materials, they would carry less current. More and more current would flow through the meter movement, and the chances of damage would increase. Using shunts constructed with zero-temperature coefficient materials eliminates this problem.

Q-9. What type of temperature coefficient material does not produce increased heat in response to increased current flow? The ends of the shunt strips are embedded in heavy copper blocks. The blocks are attached to the meter coil leads and the line terminals. To ensure accurate readings, you should not interchangeably use the meter leads for a particular ammeter with those for a meter of a different range. Slight changes in lead length and size may vary the resistance of the meter circuit. If this happens, current will also change and cause incorrect meter readings. External shunts are generally used where currents greater than 50 amperes must be measured.

SHUNT SELECTION - When using an external-shunt ammeter, you should select a suitable shunt so that the scale deflection can be easily read. For example, if the scale has 150 divisions and the load current you want to measure is known to be between 50 and 100 amperes, a 150-ampere shunt would be the correct choice. Under these conditions, each division of the scale represents 1 ampere. In other words, a full-scale deflection of the pointer would rest on the 150th division mark, indicating that 150 amperes of load current is flowing. At half-scale deflection, the pointer would rest on the 75th division mark, indicating that 75 amperes of load current is flowing.

A shunt having exactly the same current rating as the expected normal load current should never be selected. If you were to select such a shunt, higher than normal load currents could possibly drive the pointer off scale and damage the meter movement. A good choice of shunt values will place the indicating needle somewhere near the midscale indication when the load current you are reading is normal. For example, assume that the meter scale is divided into 100 equal divisions and you want to measure a current of 60 amperes. The shunt to use would be a 100-ampere shunt. This would make each division of the scale equal to 1 ampere. The meter indication would fall on the 60th division showing that 60 amperes of load current is flowing. Therefore, an allowance (40 amperes) remains for unexpected surge currents.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-11 UNCLASSIFIED Q-10. A good choice of shunt resistance will place the indicating pointer near what part of the meter scale with a normal load? INTERNAL SHUNTS FOR METERS IN THE 0- TO 50-AMPERE RANGE - When measuring current ranges below 50 amperes, you will most often use internal shunts (Rshunt). In this way, you can easily change the range of the meter by means of a switching arrangement. A switch will select the correct internal shunt with the necessary current rating and resistance. Before you can calculate the required resistance of the shunt for each range, the total resistance of the meter movement must be known. For example, suppose you desire to use a 100-microampere D'Arsonval meter with an internal coil resistance of 100 ohms to measure line currents up to 1 ampere. The meter will deflect to its full-scale position when the current through the deflection coil is 100 microamperes.

Since the coil resistance is 100 ohms, you can calculate the coil's voltage (E coil) by using Ohm's law, as follows:

Ecoil = I × R

= 0.0001 amperes × 100 ohms

= 0.01 volt

When the pointer is deflected to full scale, 100 microamperes of current flows through the coil and 0.01 volt drops across it. Remember, 100 microamperes is the maximum safe current for this meter movement. Exceeding this value will damage the meter. The shunt must carry any additional load current.

The meter coil has a 0.01 volt drop across it, and, because the shunt and coil are in parallel, the shunt also has a voltage drop of 0.01 volt. The current that flows through the shunt is the difference between the full-scale meter current and the line current being fed into the shunt. In this case, meter current is 100 microamperes. Full-scale deflection is desired only when the total current is 1 ampere. Therefore, the shunt current must equal 1 ampere minus 100 microamperes, or 0.9999 ampere. Ohm's law is again used to provide the approximate value of required shunt resistance (R shunt), as follows:

Rshunt = E I

0.01 volt 0.999 ampere

= 0.01 ohm

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-12 UNCLASSIFIED To increase the range of the 100-microampere meter to 1 ampere (full-scale deflection), place a 0.01-ohm shunt in parallel with the meter movement.

You can convert the 100-microampere instrument to a 10-ampere meter by using a proper shunt. The voltage drop for a full-scale deflection is still 0.01 volt across the coil and the shunt. The meter current is still 100 microamperes. The shunt current must therefore be 9.9999 amperes under full-scale deflection. Again, this is an approximate figure found by the application of Ohm’s law.

You can also convert the same instrument to a 50-ampere meter by using the proper shunt resistance, as follows:

Rshunt = E I

0.01 volt 49.999 amperes

= 0.0002 ohm

INTERNAL SHUNTS FOR METERS IN THE MILLIAMPERE RANGE - The above method of computing the shunt resistance is satisfactory in most cases; however, it can only be used when the line current is in the ampere range and the meter current is relatively small compared to the load current. In such cases, you can use an approximate value of resistance for the shunt, as was done above. However, when the line current is in the milliampere range and the coil current becomes an appreciable percentage of the line current, a more accurate calculation must be made. For example, suppose you desire to use a meter movement that has a full-scale deflection of 1 milliampere and a coil resistance of 50 ohms to measure currents up to 10 milliamperes. Using Ohm's law, you can figure the voltage (E coil) across the meter coil (and the shunt) at full-scale deflection, as follows:

E coil = I × R

= 0.001 amperes × 50 ohms

= 50 millivolts

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-13 UNCLASSIFIED The current that flows through the shunt (Ishunt) is the difference between the line current and the meter current, as figured below:

Ishunt = Itotal − Imeter

= 10mA − 1mA

= 9 mA or 0.009 ampere

The shunt resistance (Rshunt) may then be figured, as follows:

Rshunt = E I

0.05 volt 0.009 amperes

= 5.55 ohms

Notice that, in this case, the exact value of shunt resistance has been used rather than an approximation.

The formula for determining the resistance of the shunt is given by Rs = I m /Is times Rm, where Rs is the shunt resistance in ohms; Im is the meter current at full-scale deflection; Is is the shunt current at full-scale deflection; and Rm is the resistance of the meter coil. If the values given in the previous example are used in this equation, it will yield 5.55 ohms, the value previously calculated.

SWITCHING SHUNT VALUES - Various values of shunt resistance can be used, by means of a suitable switching arrangement, to increase the number of current ranges that can be covered by the meter. Two switching arrangements are shown in figure 3-5. View A is the simpler of the two arrangements when a number of shunts are used to calculate the values of the shunt resistors. However, it has two disadvantages:

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-14 UNCLASSIFIED

1. When the switch is moved from one shunt resistor to another, the shunt is momentarily removed from the meter. The line current then flows through the meter coil. Even a momentary surge of current could easily damage the coil.

2. The contact resistance (resistance between the blades of the switch when they are in contact) is in series with the shunt, but not with the meter coil. In shunts that must pass high currents, this contact resistance becomes an appreciable part of the total shunt resistance. Because the contact resistance is of a variable nature, the ammeter indication may not be accurate.

The generally preferred method of range switching is shown in (figure 3-5, view B). Although only two ranges are shown, as many ranges as needed can be used. In this type of circuit, the contact resistance of the range-selector switch is external to the shunt and meter in each range position. The contact resistance in this case has no effect on the accuracy of the current measurement.

Figure 3-5 Ways of connecting internal meter shunts

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-15 UNCLASSIFIED 3.2.3.2 Ammeter Connections When you are using ammeters, a primary rule of safety is that such current-measuring instruments must always be connected in series with a circuit, never in parallel with it. When an ammeter is connected across a constant-potential source of appreciable voltage, the low internal resistance of the meter bypasses the circuit resistance. This results in the application of the source voltage (or a good portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil and renders the meter useless until repaired.

Q-11. In what manner are current-measuring instruments connected to a circuit? If you do not know the approximate value of current in the circuit, you should take a reading at the highest range of the ammeter; then you should switch progressively to lower ranges until a suitable reading is obtained. Most ammeter scales indicate the current being measured in increasing values from left to right. If you connect the meter without observing proper polarity, the pointer may be deflected backwards (from right to left). This action often damages the meter movement. You should ensure that the ammeter is always connected so that the current will flow into the negative terminal and out the positive terminal. Figure 3-6 shows various circuit arrangements and the proper ammeter connection methods to measure current in various portions of the circuit.

Q-13. (True or False) The larger the current required to produce full-scale deflection of the meter coil, the better the sensitivity of the meter.

Figure 3-6 Proper ammeter connection

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-16 UNCLASSIFIED Good sensitivity is especially important in ammeters to be used in circuits in which small currents flow. As the meter is connected in series with the load, the current flows through the meter. If the internal resistance of the meter is a large portion of the load resistance, an effect known as METER-LOADING will occur. Meter-loading is the condition that exists when the insertion of a meter into a circuit changes the operation of that circuit. This condition is not desirable. The purpose of inserting a meter into a circuit is to allow the measurement of circuit current in the normal operating condition. If the meter changes the circuit operation and changes the amount of current flow, the reading you obtain will be in error. An example of this is shown in figure 3-7.

Q-14. What condition exists when the insertion of a meter into a circuit changes the operation of the circuit? In view A of figure 3-7, the circuit to be tested has an applied voltage of 100 millivolts and a resistance of 100 ohms. The current normally flowing in this circuit is 1 milliampere. In view B, an ammeter that requires 1 milliampere for full-scale deflection and that has an internal resistance of 100 ohms has been inserted. Since 1 milliampere of current flow is shown in view A, you might naturally assume that with the meter inserted into the circuit, a full-scale deflection will occur. You might also assume that the 1 milliampere of circuit current will be measured. However, neither of these assumptions is correct. With the ammeter inserted into the circuit, as shown in view B, the total resistance of the circuit is 200 ohms. With an applied voltage of 100 millivolts, applying Ohm’s law shows the actual current (I circuit) to be 0.5 milliampere.

Figure 3-7 Ammeter loading effect

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-17 UNCLASSIFIED I = E R

Icircuit = 100 × 10−3 volts 200 ohms

= 0.5 × 10−3 ampere

= 0.0005 ampere or 0.5 milliampere

Since the meter reads 0.5 milliampere instead of the normal value of current, the meter reveals that a definite loading effect has taken place. In cases such as this, the use of ammeters, which have a lower internal resistance and a better current sensitivity, is desirable.

3.2.4 DC Voltmeter Up to this point, we have been discussing the 100-microampere D’Arsonval movement and its use as an ammeter. However, it can also be used to measure voltage if a MULTIPLIER (high resistance) is placed in series with the moving coil of the meter. For low-voltage instruments, this resistance is physically mounted inside the meter case with the D’Arsonval movement. The series resistance is constructed of a wire-wound resistance that has a low temperature coefficient wound on either a spool or card frame. For high-voltage ranges, the series resistance can be connected externally. A simplified diagram of a voltmeter is shown in figure 3-8.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-18 UNCLASSIFIED

Q-15. What modification is made to the D’Arsonval meter movement to enable the meter to measure voltage? Keep in mind that the D’Arsonval meter movement uses current flow to produce a magnetic field that is proportional to the current. The meter movement is, therefore, an indicator of current flow rather than voltage. The addition of the series resistance is what allows the meter to be calibrated in terms of voltage; that is, the meter movement of a voltmeter operates because of the current flow through the meter, but the scale is marked in volts. For example, the meter movement shown in figure 3-9 has an internal resistance of 100 ohms, requires 100 microamperes for full-scale deflection, and has a voltage drop of 10 millivolts when full-scale deflection is reached. If you were to place this meter directly across a 10-volt source, an excessive current (in milliamperes) would flow. The meter would be destroyed because of the excessive current flowing through the meter movement. This can be seen in the following Ohm’s law application:

Figure 3-8 Internal construction and circuit of a simplified voltmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-19 UNCLASSIFIED

Using this equation, you can see that a current through the meter of 100 milliamperes is excessive and will cause damage.

I = E R

= 10 volts 100 ohms

= 100 milliamperes

Since the normal voltage drop for the meter is 10 millivolts at full-scale deflection, some means must be supplied to drop the extra 9.99 volts without applying it directly to the meter. This is done by the addition of a multiplier resistor, as shown in figure 3-9.

Figure 3-9 Use of multiplier resistors with D'Arsonval meter movement

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-20 UNCLASSIFIED 3.2.4.1 Extending Voltmeter Ranges The value of series resistance is determined by the current required for full-scale deflection and by the range of the voltages to be measured. Since the current through the meter circuit is directly proportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed value of series resistance. For example, let’s assume that the basic meter is to be made into a voltmeter with a full-scale deflection of 1 volt. The coil resistance of the basic meter is 100 ohms, and 100 microamperes of current causes full-scale deflection. The resistance (R meter) required to limit the total current in the circuit to 100 microamperes can be found as follows:

Rmeter = E I

1 volt 100 microamperes

= 10 kilohms

Because the meter coil already measures 100 ohms, the series resistance required is equal to 10 kilohms minus 100 ohms, or 9.9 kilohms.

Q-16. What factors determine the value of the multiplier resistor? Multi-range voltmeters use one meter movement. The required resistances are connected in series with the meter by a switching arrangement. A schematic diagram of a multi- range voltmeter with three ranges is shown in figure 3-10. The total meter resistance (R meter) for each of the three ranges, beginning with the 1-volt range, is figured by the application of Ohm’s law, as follows:

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-21 UNCLASSIFIED

1-volt range

Rmeter = 1 . 0001 = 10 kilohms

100-volt range

Rmeter = 100 . 0001 = 10 megohm

1,000-volt range

Rmeter = 1000 . 0001 = 10 megohms

The actual value of the multiplying series resistor ( Rseries) for each of these circuits is 100 ohms less than the total resistance. This allows for the resistance of the meter coil (Rcoil).

Figure 3-10 Multi-range voltmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-22 UNCLASSIFIED 3.2.4.2 Voltmeter Circuit Connections When voltmeters are used, a primary rule of safety is that such voltage-measuring instruments must always be connected in parallel with (across) a circuit. If you are unsure of the level of the voltage to be measured, take a reading at the highest range of the voltmeter and progressively (step by step) lower the range until a suitable reading is obtained. In many cases, the voltmeter you will be using will not be a center-zero- (0 reading is in the center) indicating instrument. Observing the correct polarity is important when connecting the instrument to the circuit. Voltmeter polarity is the same as for the dc ammeter; that is, current flows from negative to positive.

Q-17. In what manner are voltage-measuring instruments connected to the circuit to be measured? 3.2.4.3 Influence of a Voltmeter in a Circuit The purpose of a voltmeter is to indicate the potential difference between two points in a circuit. When a voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has a low resistance, it will draw a substantial amount of current. This action lowers the effective resistance of the circuit and changes the voltage reading. When you are making voltage measurements in high-resistance circuits, use a HIGH-RESISTANCE VOLTMETER to prevent the shunting action of the voltmeter. The effect is less noticeable in low-resistance circuits because the shunting effect is less. The problem of voltmeter shunting (sometimes called circuit loading) is illustrated in figure 3-11.

Figure 3-11 Shunting action caused by a voltmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-23 UNCLASSIFIED Q-18. When making voltage measurements in a high-resistance circuit, you should always use a voltmeter with what relative value of resistance? In view A of figure 3-11, a source of 150 volts is applied to a series circuit consisting of two 10-kilohm resistors. View A shows the voltage drop across each resistor to be 75 volts. In the 150-volt range, the voltmeter to be used has a total internal resistance of 10 kilohms. View B shows the voltmeter connected across the circuit. The parallel combination of R2 and the meter now present a total resistance of 5 kilohms. Because of the addition of the voltmeter, the voltage drops change to 100 volts across R1 and 50 volts across R2. Notice that this is not the normal voltage drop across R2. Actual circuit conditions have been altered because of the voltmeter.

3.2.4.4 Voltmeter Sensitivity The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R meter), plus the series resistance (Rseries), by the full- scale reading in volts. In equation form, sensitivity is expressed as follows:

sensitivity = Rm + Rs E

This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current. In equation form, this is expressed as follows:

sensitivity = ohms volt

= 1 volt ohms

= 1 ampere

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-24 UNCLASSIFIED Therefore, the sensitivity of a 100-microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.

sensitivity = 1 ampere

= 1 . 0001

10,000 ohms per volt

Q-19. What term is used to express the sensitivity of a voltmeter? 3.2.5 Meters Used For Measuring Resistance The two instruments you will use most often to check continuity, or to measure the resistance of a circuit or circuit component, are the OHMMETER and the MEGGER (MEGOHMMETER). The ohmmeter is widely used to measure resistance and to check the continuity of electrical circuits and devices. Its range usually extends to only a few megohms. The megger is widely used for measuring insulation resistance, such as that between a wire and the outer surface of its insulation, and the insulation resistance of cables and insulators. The range of a megger can be extended to more than 1,000 megohms.

Q-20. What instrument is used for measuring the insulation resistance of cables?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-25 UNCLASSIFIED 3.2.5.1 The Ohmmeter A simple ohmmeter circuit is shown in figure 3-12. The ohmmeter consists of the dc milliammeter, discussed earlier in this chapter, and the added features shown below:

• A source of dc potential; and • One or more resistors (one of which is variable).

Q-21. What added features enable a dc milliammeter to function as an ohmmeter?

Figure 3-12 Simple ohmmeter circuit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-26 UNCLASSIFIED The deflection of the pointer of an ohmmeter is controlled by the amount of battery current passing through the moving coil. Before you can measure the resistance of an unknown resistor or electrical circuit, you must calibrate the ohmmeter to be used. If the value of resistance to be measured can be estimated within reasonable limits, select a range on the ohmmeter that will give approximately half-scale deflection when the resistance is inserted between the probes. If you cannot estimate the resistance to be measured, then set the range switch on the highest scale. Whatever range you select, the meter must be calibrated to read zero before the unknown resistance is measured.

To calibrate the meter, you first short the test leads together, as shown in figure 3-12. With the test leads shorted, a complete series circuit exists. The complete series circuit consists of the 3-volt source, the resistance of the meter coil (R meter), the resistance of the zero-adjust rheostat, and the series multiplying resistor (Rseries). The shorted test leads cause current to flow and the meter pointer to deflect.

Notice that the zero point on the ohmmeter scale (as opposed to the zero points for voltage and current) is located at the extreme right side of the scale. With the test leads shorted, the zero-adjust potentiometer is set so that the pointer rests on the zero mark. Therefore, a full-scale deflection indicates zero resistance between the leads.

Q-22. A full-scale deflection on an ohmmeter scale indicates what resistance between the leads? If you change the range on the meter, you must "zero" (calibrate) the meter again to obtain an accurate reading. When you separate the test leads, the pointer of the meter will return to the left side of the scale. This action, as explained earlier, is caused by the restoring force of the spring tension acting on the movable coil assembly. The reading at the left side of the scale indicates an infinite resistance.

After you have adjusted the ohmmeter for zero reading, it is ready to be connected to a circuit to measure resistance. A typical circuit and ohmmeter arrangement is shown in figure 3-13. You must ensure that the power switch of the circuit to be measured is in the de-energized (OFF) position. This prevents the source voltage of the circuit from being applied to the meter, a condition that could cause severe damage to the meter movement.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-27 UNCLASSIFIED

Remember that the ohmmeter is an open circuit when the test leads are separated. To take a resistance reading with a meter, you must provide a path for current flow produced by the meter’s battery. In view A of figure 3-13, the meter is connected at points A and B to produce this path. Connecting these test leads places resistors R1 and R2 in series with the resistance of the meter coil, the zero-adjust potentiometer, and the series multiplying resistor. Since you previously calibrated the meter, the amount of coil movement now depends only on the resistances of R1 and R2.

The addition of R1 and R2 into the meter circuit raises the total series resistance and decreases the current. This decreases the amount of pointer deflection. The pointer comes to rest at a scale reading that indicates the combined resistance of R1 and R2. If you were to replace either R1 or R2, or both, with a resistor having a larger ohmic value, the current flow in the moving coil of the meter would be decreased even more. This would further decrease the pointer deflection, and the scale indication would read a still higher circuit resistance. View B is a simplified version of the circuitry in view A.

From our ohmmeter discussion, two facts should be apparent: (1) Movement of the moving coil is proportional to the amount of current flow, and (2) the scale reading of the ohmmeter is inversely proportional to current flow in the moving coil.

Figure 3-13 Measuring circuit resistance with an ohmmeter.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-28 UNCLASSIFIED The amount of circuit resistance to be measured may vary over a wide range. In some cases, it may only be a few ohms; in other cases, it may be as great as 1 megohm. Scale multiplication features are built into most ohmmeters so that they will indicate any ohmic value being measured and offer the least amount of error. Most ohmmeters are equipped with a selector switch for selecting the multiplication scale desired. For example, view A of figure 3-14 shows a typical meter that has a six-position switch. The positions are marked on the meter in multiples of 10, from R × 1 through R × 100K.

Figure 3-14 Ohmmeter with multiplication switch

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-29 UNCLASSIFIED The range used to measure any particular unknown resistance (Rx in view A of figure 3- 14) depends on the approximate ohmic value of the unknown resistance. For instance, the ohmmeter scale of the figure is calibrated in divisions from 0 to infinity. Note that the divisions are easier to read on the right-hand portion of the scale than on the left. For this reason, if Rx is greater than 1,000 ohms and if you are using the R × 1 range, you will be unable to accurately read the indicated resistance. This happens because the combined series resistance of resistors Rx is too large for range R × 1 to allow enough battery current to flow to deflect the pointer away from infinity. You need to turn the range switch to the R × 10 position to obtain the 1,000-ohm reading.

Let’s assume that you have changed the range switch to the R × 10 position and the pointer now deflects to a reading of 375 ohms, as shown in view B of figure 3-14. This would indicate to you that unknown resistance R x has 3,750 (375 times 10) ohms of resistance. The change of range caused the deflection because resistor R × 10 has only 1/10 the resistance of resistor R × 1. Therefore, selecting the smaller series resistance allowed a battery current of sufficient value to cause a readable pointer deflection. If the R × 100 range were used to measure the same 3,750 ohm resistor, the pointer would deflect still further to the 37.5-ohm position, as shown in view C. This increased deflection would occur because resistor R × 100 has only 1/10 the resistance of resistor R × 10.

Q-23. The R × 100 resistance selection on an ohmmeter has what amount of resistance compared to the R × 10 selection? The circuit arrangement in view A of figure 3-14 allows the same amount of current to flow through the moving meter coil. The same amount is allowed to flow whether the meter measures 10,000 ohms on the R × 1 scale, 100,000 ohms on the R × 10 scale, or 1,000,000 ohms on the R × 100 scale.

The same amount of current must always be used to deflect the pointer to a certain position on the scale (midscale position, for example), regardless of the multiplication factor being used. Since the multiplier resistors are of different values, you must always "zero" the meter for each multiplication scale selected. When selecting a range on the ohmmeter, select the one that will result in the pointer coming to rest as close to the midpoint of the scale as possible. This will enable you to read the resistance more accurately because scale readings are more easily interpreted at or near midpoint.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-30 UNCLASSIFIED 3.2.5.2 The Megohmmeter An ordinary ohmmeter cannot be used for measuring multimillion ohm values of resistances, such as those in conductor insulation. To test for such insulation breakdown, you need to use a much higher potential than that supplied by the battery of an ohmmeter. This potential is placed between the conductor and the outside of the insulation. A megger (megohmmeter) is used for these tests. The megger, shown in figure 3-15, is a portable instrument consisting of two main elements: (1) a hand-driven dc generator, which supplies the necessary voltage for making the measurement, and (2) the instrument portion, which indicates the value of the resistance you are measuring. The instrument portion is of the opposed-coil type, as shown in view A. Coils a and b are mounted on movable member c. A fixed angular relationship exists between coils, and they are free to turn as a unit in a magnetic field. Coil b tends to move the pointer counterclockwise, and coil a tends to move it clockwise.

Figure 3-15 Megger internal circuit and external view

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-31 UNCLASSIFIED Coil a is connected in series with R3 and unknown resistance Rx. The combination of coil a, R3, and Rx forms a direct series path between the + and − brushes of the dc generator. Coil b is connected in series with R2, and this combination is also connected across the generator. Notice that the movable member (pointer) of the instrument portion of the megger has no restoring springs. Therefore, when the generator is not being operated, the pointer will float freely and may come to rest at any position on the scale.

The guard ring, shown in view A of figure 3-15, shunts any leakage currents to the negative side of the generator. This prevents such current from flowing through coil a and affecting the meter reading.

Q-24. What is the purpose of the guard ring in a megohmmeter? If the test leads are open, no current will flow in coil a. However, current will flow internally through coil b and deflect the pointer to infinity. This reading indicates a resistance too large to measure. When a resistance, such as R x, is connected between the test leads, current also flows in coil a; the pointer tends to move clockwise. At the same time, coil b still tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position in which the two forces are balanced. This position depends upon the value of Rx, which controls the amount of the current in coil a. Because changes in voltage affect both coils in the same proportion, the position of the moving element is independent of the voltage. If you short the test leads together, the pointer will come to rest at zero because the current in coil a is relatively large. Since R3 limits the current, the instrument will not be damaged under these circumstances. The external appearance of one type of megger is shown in view B of figure 3-15.

Most meggers you will use are rated at 500 volts; however, there are other types. Meggers are usually equipped with friction clutches, which are designed to slip if the generator is cranked faster than its rated speed. This prevents the generator speed and output voltage from exceeding rated values. A 1,000-volt generator is available for extended ranges. When an extremely high resistance, such as 10,000 megohms or more, is to be measured, a high voltage is needed to cause enough current flow to actuate the meter movement.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-32 UNCLASSIFIED CAUTION

When using a megger, you can easily be injured or damage equipment if you do not observe the following MINIMUM safety precautions:

• Use meggers on high-resistance measurements only (such as insulation measurements or to check two separate conductors on a cable). • Never touch the test leads while the handle is being cranked. • De-energize and discharge the circuit completely before connecting a megger. • Whenever possible, disconnect the component being checked from other circuitry before using a megger

Q-25. Most meggers you will use are rated at what voltage? Q-26. The development of excessive test voltages is avoided by the use of meggers equipped with what device? 3.3 ELECTRODYNAMOMETER-TYPE METERS The electrodynamometer-type meter differs from the galvanometer types we have just studied in that two fixed coils are used to produce the magnetic field instead of a permanent magnet. Two movable coils are also used in the electrodynamometer meter. The electrodynamometer meter is most commonly found in various types of power meters.

Q-27. What components in an electrodynamometer-type meter movement produce the magnetic field? As shown in figure 3-16, the fixed coils are connected in series and positioned coaxially (in line) with a space between them. The two movable coils are also positioned coaxially and are connected in series. The two pairs of coils (fixed pair and movable pair) are also connected in series with each other. The movable coil is pivot-mounted between the fixed coils. The main shaft on which the movable coils are mounted is restrained by spiral springs that restore the pointer to zero when no current is flowing through the coil. These springs also act as conductors for delivering current to the movable coils. Since these conducting springs are very small, the meter cannot carry a high value of current.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-33 UNCLASSIFIED

Q-28. What is the limiting factor as to the amount of current an electrodynamometer meter movement can handle? 3.3.1 Meter Accuracy The meter is mechanically damped by means of aluminum vanes that move in enclosed air chambers. Although very accurate, electrodynamometer-type meters do not have the sensitivity of the D’Arsonval-type meter movement. For this reason, you will not find them used outside of the laboratory environment to a large extent.

Figure 3-16 Internal construction of an electrodynamometer

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-34 UNCLASSIFIED 3.3.2 Meter Movement The primary advantage of the electrodynamometer-type meter movement is that it can be used to measure alternating as well as direct current. If you apply alternating current to the standard galvanometer-type meter, it will not produce a usable reading. Instead, the meter will vibrate at or near the zero reading. On one-half cycle of the ac, the meter is deflected to the left and on the other half cycle to the right. Since the frequencies you will be measuring are 60 hertz or greater, the meter is incapable of mechanically responding at this speed. The result is simply a vibration near the zero point; in addition, no useful reading of voltage or current is obtained. This problem does not exist with the electrodynamometer-type movement. Current flow through the stationary (fixed) coils sets up a magnetic field. Current flow through the moving coils sets up an opposing magnetic field. With two magnetic fields opposing, the pointer deflects to the right. If the current reverses direction, the magnetic fields of both sets of coils will be reversed. With both fields reversed, the coils still oppose each other, and the pointer still deflects to the right. Therefore, no rectifying devices are required to enable the electrodynamometer meter movement to read both ac and dc. Rectifying devices are required for the D’Arsonval-type meter movement to enable it to be used for measuring ac voltages and currents.

Q-29. What is the primary advantage of the electrodynamometer-type meter over the D’Arsonval-type meter?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-35 UNCLASSIFIED 3.3.3 Voltmeter When an electrodynamometer is used as a voltmeter, no problems in construction are encountered because the current required is not more than 0.1 ampere. This amount of current can be handled easily by the spiral springs. When the electrodynamometer is used as a voltmeter, its internal connections and construction are as shown in view A of figure 3-17. Fixed coils a and b are wound of fine wire since the current flow through them will not exceed 0.1 ampere. They are connected directly in series with movable coil c and the series current-limiting resistor.

Figure 3-17 Circuit arrangement of electrodynamometer for use as a voltmeter and an ammeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-36 UNCLASSIFIED 3.3.4 Ammeter When the electrodynamometer is used as an ammeter, a special type of construction must be used. This is because the large currents that flow through the meter cannot be carried through the moving coils. In the ammeter in view B of figure 3-17, stationary coils a and b are wound of heavier wire to carry up to 5.0 amperes. An inductive shunt (XL) is wired in parallel with the moving coils and permits only a small part of the total current to flow through the moving coil. The current flowing through the moving coil is directly proportional to the total current flowing through the instrument. The shunt has the same ratio of reactance to resistance as the moving coil does. Therefore, the instrument will be reasonably correct at frequencies at which it is used if ac currents are to be measured.

3.3.5 Wattmeter Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero position in the absence of a signal.

Figure 3-18 Simplified electrodynamometer wattmeter circuit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-37 UNCLASSIFIED 3.3.5.1 Wattmeter Connection The current coil of the wattmeter is connected in series with the circuit (load), and the voltage coil is connected across the line. When line current flows through the current coil of a wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to the line current. The voltage coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for this connection is to make the voltage-coil circuit of the meter as purely resistive as possible. As a result, current in the voltage circuit is practically in phase with line voltage. Therefore, when voltage is impressed on the voltage circuit, current is proportional to and in phase with the line voltage. Figure 3-19 shows the proper way to connect a wattmeter into a circuit.

3.3.5.2 Wattmeter Errors Electrodynamic wattmeters are subject to errors arising from such factors as temperature and frequency. For example, heat through the coils eventually causes the small springs attached to the pointer to lengthen and lose tension, which produces deflection errors. Large currents through the wattmeter also produce a noticeable deflection error. These errors are caused by the heat (I 2R) loss through coils from the application of high currents. Because of this, the maximum current range of electrodynamic wattmeters is normally restricted to approximately 20 amperes. The voltage range of wattmeters is usually limited to several hundred volts because of heat dissipation within the voltage circuit. However, the voltage range can be extended by the use of voltage multipliers.

Figure 3-19 Wattmeter connection

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-38 UNCLASSIFIED Good-quality, portable wattmeters usually have an accuracy of 0.2 to 0.25 percent. You must remember, though, that electrodynamic wattmeter errors increase with frequency. For the higher frequency and power ranges, special types of wattmeters are made specifically for those ranges. We will discuss two such wattmeters in chapter 5 of this module

3.3.5.3 Wattmeter Overloads The wattmeter consists of two circuits, either of which will be damaged if too much current passes through them. You should be especially aware of this fact because the reading on the instrument will not tell you whether or not the coils are being overheated. If an ammeter or voltmeter is overloaded, the pointer will indicate beyond the upper limit of its scale. In the wattmeter, both the current and potential circuit may carry such an overload that their insulations burn; yet the pointer may be only part of the way up the scale. This is because the position of the pointer depends upon the power factor of the circuit as well as upon the voltage and current. Therefore, a low power-factor circuit will provide a very low reading on the wattmeter. The reading will be low, even when the current and voltage circuits are loaded to the maximum safe limit. The safe rating for each wattmeter is always distinctly rated, not in watts, but in volts and amperes.

3.4 TECHNIQUES FOR METER USE We have considered the more common meters; now let’s consider some of the techniques employed in their use. The techniques suggested here are not all-inclusive. You will find, as you develop your technical skills, other variations and techniques in use. Consider the techniques for measuring current in a circuit. You can accomplish this by placing an ammeter in series with the circuit or by measuring the voltage across a resistor of known value and using Ohm’s law to figure current. This last technique has the advantage of eliminating the necessity of opening the circuit to connect the ammeter.

3.4.1 Continuity Tests Open circuits are those in which the flow of current is interrupted by a broken wire, defective switch, or any means by which the current cannot flow. The test used to detect open circuits (or to see if the circuit is complete or continuous) is continuity testing.

An ohmmeter (which contains its own batteries) is excellent for use in a continuity test. Normally, continuity tests are performed in circuits where the resistance is very low, such as the resistance of a copper conductor. An open is indicated in these circuits by a very high or infinite resistance between two continuously connected points.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-39 UNCLASSIFIED Figure 3-20 shows a continuity test of a cable that connects two electronic units. Notice that both plugs are disconnected and the ohmmeter is in series with conductor D under test. The power should be off. When checking conductors A, B, and C (connection of ohmmeter to conductors not shown), the current from the ohmmeter flows through plug 2 (female) through conductor A, B, or C to plug 1 (female). From plug 1, current passes through the jumper to the chassis, which is "grounded" to the ship’s structure. The metal structure serves as the return path to the chassis of unit 2 and completes the circuit through the series-connected ohmmeter. The ohmmeter indicates a low resistance because no break exists in conductors A, B, or C. However, checking conductor D reveals an open. The ohmmeter is shown indicating maximum resistance because current cannot flow in an open circuit. With an open circuit, the ohmmeter needle is all the way to the left since it is a series-type ohmmeter (reads right to left).

Where conditions are such that the ship’s structure cannot be used as the return path, one of the other conductors (known to be good) may be used. For example, to check D, you can connect a jumper from pin D to pin A of plug 1 (female) and the ohmmeter leads to pins D and A of plug 2 (female). This technique will also reveal the open in the circuit.

Figure 3-20 Continuity test

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-40 UNCLASSIFIED 3.4.2 Testing For Grounds Grounded circuits are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship. Grounds can have many causes. The two most common are the fraying of insulation from a wire and moisture-soaked insulation. The fraying of insulation from a wire allows bare wire to come into contact with the metal ground. Moisture-soaked insulation causes reduced insulation resistance (also classified as a ground).

Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped circuit breakers, however, can also result from a short circuit other than a ground. A high-resistance ground can also occur when current is increased significantly but not enough to rupture the fuse or trip the circuit breaker.

CAUTION

Before testing any circuit, ensure the circuit under test has been de-energized and checked with a safety shorting probe.

In testing for grounds, you may use a megger or an ohmmeter. Measuring the resistance to ground from points in a circuit determines if the point is grounded. Referring again to figure 3-20, you can see one possible means of testing a cable for grounds. If the jumper is removed from pin D of plug 1 (female), a test for ground can be made for each conductor in the cable. You can do this by connecting one meter lead to ground and the other to each of the pins of either of the plugs. A low resistance indicates that some part of that conductor or one of the plug assemblies is grounded. Both plugs must be removed from their units; if only one plug is removed, a false indication is possible because a conductor may be grounded through the unit.

3.4.3 Testing For Shorts A short circuit, other than a grounded one, is one where two conductors touch each other directly or through another conducting element. Two conductors with frayed insulation may touch and cause a short. Too much solder on the pin of a connector may short to the adjacent pin. In a short circuit, enough current may or may not flow to blow a fuse or open a circuit breaker. A short may occur between two cables carrying signals but might not be indicated by a blown fuse.

Shorts occur in many components, such as transformers, motor windings, and capacitors. The major test method used to detect shorts in such components is to measure resistance. The indicated resistance is then compared with the resistance given on schematics or in the equipment technical manuals to determine whether the measured value is within specifications.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-41 UNCLASSIFIED An ohmmeter is the device used to check for shorts. You can use the ohmmeter to detect a short between two conductors by measuring the resistance between them (be sure electrical power has been disconnected). A low resistance reading indicates a short. You can test the circuit in figure 3-20 for a short by first removing the jumper and disconnecting both plugs; you then measure the resistance between the two suspended conductors.

WARNING

The following section discusses voltage measurements on live circuits. BE SURE YOU ALWAYS FOLLOW PRESCRIBED SAFETY RULES WHEN MEASURING VOLTAGES.

3.4.4 Voltage Tests Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must be followed to prevent injury to personnel and damage to the equipment. You will find in your maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is being applied to the circuit.

The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since defective parts in a circuit can cause higher than normal voltages to be present at the point of test, the highest voltmeter range available should be used first. Once you have obtained a reading, determine if a lower scale can be used that will cause no damage to the meter movement. If so, use the lower scale. This provides a more accurate reading.

Another consideration in the circuit voltage test is the resistance and current in the circuit. A low resistance in a high-current circuit could result in considerable voltage drop, whereas the same resistance in a low-current circuit may be minimal. Abnormal resistance in part of a circuit can be checked with either an ohmmeter or a voltmeter. Where practical, an ohmmeter should be used because the test is then carried out with a "dead" circuit.

The majority of the electronic circuits you will encounter in equipment will be low- current circuits, and most voltage readings will be direct current. Also, many of the schematics will indicate the voltages at various test points. Therefore, if you suspect that a certain stage is defective, you can check the voltage by connecting a voltmeter from the test point to ground. If the suspected stage is not defective, the voltmeter readings should match the voltages given on the schematic.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-42 UNCLASSIFIED Some technical manuals also contain voltage charts on which all the voltage measurements are tabulated. These charts usually indicate the sensitivity of the meter (for example, 20,000 ohms/volt) used to obtain the voltage readings for the chart. To obtain comparable results, you must use a voltmeter of the same sensitivity (or greater) as that specified. Make certain that the voltmeter is not "loading" the circuit while taking a measurement. If the meter resistance is not considerably higher than the circuit resistance, the reading will be markedly lower than the true circuit voltage because of the voltmeter’s loading effect. (To calculate meter resistance, multiply the rated ohms-per- volt sensitivity value of the meter by the scale in use. For example, a 1,000-ohms-per-volt meter set to the 300-volt scale will have a resistance of 300,000 ohms.)

3.4.5 Resistance Tests Before checking the resistance of a circuit or of a part, make certain that the power has been turned off. Also make sure capacitors in the associated circuit are fully discharged. To check continuity, always use the lowest ohmmeter range. If the highest range is used, the meter may indicate zero, even though appreciable resistance is present in the circuit. Conversely, to check a high resistance, use the highest scale since the lower range scale may indicate infinity, even though the resistance is less than a megohm. In making resistance tests, you must remember that even though the external ohmmeter leads are connected in parallel with the circuit to be measured, the internal meter circuitry is electrically connected in series.

In making resistance tests, take into account that other circuits containing resistances and capacitances may be in parallel with the circuit to be measured. Erroneous conclusions may be drawn from readings obtained in such cases. Remember, a capacitor blocks the dc flow from the ohmmeter. To obtain an accurate reading when other parts are connected across the suspected circuit, disconnect one end of the circuit to be measured from the equipment. For example, many of the resistors in major components and subassemblies are connected across transformer windings. To obtain a valid resistance measurement, you must isolate the resistors to be measured from the shunt resistances of the coils of the transformers.

Resistance tests are also used to check a component for grounds. In these tests, the component to be tested should be disconnected from the rest of the circuit so that no normal circuit ground will exist. Dismounting the component to be checked is not necessary. The ohmmeter is set for a high-resistance range. Then the ohmmeter is connected between ground and each electrically separate circuit of the component being tested. Any resistance reading less than infinity indicates at least a partial ground. You can also check capacitors suspected of being short-circuited by measuring the resistance. To check a capacitor suspected of being open, temporarily shunt a known good capacitor then recheck the performance of the circuit.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-43 UNCLASSIFIED CAUTION

To avoid possible damage to equipment during resistance tests, observe the following precautions:

• Always connect an ammeter in series—never in parallel. • Connect a voltmeter in parallel. • Never connect an ohmmeter to a live circuit. • Observe polarity when using a dc ammeter or a dc voltmeter. • View meters directly from the front. When viewed from an angle off to the side, an incorrect reading will result because of OPTICAL PARALLAX. (Parallax was covered in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.) • Always choose an instrument suitable for the measurement desired. • Select the highest range first and then switch to the proper range. • In using a meter, choose a scale that will result in an indication as near midscale as possible. • Do not mount or use instruments in the presence of a strong magnetic field. • Remember, a low internal resistance voltmeter (low sensitivity) may shunt the circuit being measured and result in incorrect readings.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-44 UNCLASSIFIED 3.5 SUMMARY The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your study of test equipment.

A permanent-magnet, moving-coil meter movement (D’ARSONVAL movement) uses the interaction of magnetic fields to produce movement.

DAMPING is used to smooth out the vibration and to help prevent overshooting of the meter pointer.

ELECTRODYNAMOMETER movements are usually used in wattmeters. They operate much like the D’Arsonval meter movement, except field coils are used instead of a permanent magnet. Electrodynamometer movements measure either ac or dc without the use of a rectifier.

A SHUNT is a physically large, low-resistance conductor connected in parallel with the meter terminals. It carries the majority of the load current so that only a small portion of the total current will flow through the meter coil.

An AMMETER measures current and is always connected in series with the circuit being measured. An ammeter should have a low resistance so that the effect of the ammeter on the circuit will be kept to a minimum.

VOLTMETERS are used to measure voltage and are always connected in parallel with the circuit being measured. A voltmeter should have a high resistance compared to the circuit being measured to minimize the loading effect. Voltmeter sensitivity is expressed in ohms per volt.

OHMMETERS are used to measure resistance and to check continuity. An ohmmeter is electrically connected in series with the resistance being measured. The ohmmeter range, which allows a midscale deflection, should be used.

A MEGOHMMETER (MEGGER) is used to measure very high resistance, such as the insulation of wiring.

A WATTMETER is usually an electrodynamometer and is used to measure power.

A CONTINUITY TEST is accomplished with an ohmmeter. This test is used to check for opens (or to see if the circuit is complete or continuous).

GROUNDED CIRCUITS are caused by some conducting part of the circuit making contact either directly or indirectly with the metallic structure of the ship or chassis. In testing for grounds, you may use either an ohmmeter or a megger.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-45 UNCLASSIFIED A SHORT CIRCUIT, other than a grounded one, is where two conductors touch each other directly or through another conducting element. An ohmmeter is used to test for shorts.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-46 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q29.

A-1. Self-excited.

A-2. Phosphor bronze ribbons.

A-3. The pointer arrangement and the light and mirror arrangement.

A-4. Coil balance.

A-5. Hairspring.

A-6. Hairspring.

A-7. Makes it possible to have a more linear scale than if the poles were flat.

A-8. Shunt.

A-9. Zero-temperature coefficient.

A-10. Midscale.

A-11. In series.

A-12. Negative, positive.

A-13. False.

A-14. Meter-loading.

A-15. A multimeter (high resistance) is placed in series with the coil of the meter.

A-16. The current required for full-scale deflection, and the range of the voltage to be measured.

A-17. In parallel.

A-18. High.

A-19. Ohms per volt.

A-20. Megohmmeter (megger).

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

3-47 UNCLASSIFIED A-21. 1. A source of dc potential. 2. One or more resistors (one of which is variable).

A-22. Zero.

A-23. 1/10.

A-24. Shunts leakage current, which prevents false readings.

A-25. 500.

A-26. Friction clutches.

A-27. Fixed coils.

A-28. Size of spiral conducting.

A-29. The electrodynamometer-type meter can be used to measure both ac and dc currents.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-1 UNCLASSIFIED 4 COMMON TEST EQUIPMENT LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

1. Describe the proper operating procedures for using the multimeter. 2. Describe the proper operating procedures for using the digital multimeter. 3. Describe the proper operating procedures for using the differential voltmeter. 4. Describe the proper operation of the transistor tester. 5. Describe the proper procedure for using the RCL bridge to measure resistance, capacitance, and inductance.

4.1 INTRODUCTION In the previous chapters, you have learned how to use some basic and miscellaneous measuring instruments to perform required maintenance and upkeep of electronic systems and components. You were also introduced to the construction and operation of basic meter movements in test equipment. This chapter will introduce you to some of the testing instruments commonly used in the Navy today.

4.2 MULTIMETERS During troubleshooting, you will often be required to measure voltage, current, and resistance. Rather than using three or more separate meters for these measurements, you can use the MULTIMETER. The multimeter contains circuitry that allows it to be used as a voltmeter, an ammeter, or an ohmmeter. A multimeter is often called a VOLT-OHM- MILLIAMMETER (VOM).

One of the greatest advantages of a VOM is that no external power source is required for its operation; therefore, no warm-up is necessary. Other advantages are its portability, versatility, and freedom from calibration errors caused by aging tubes, line voltage variations, and so forth.

Q-1. What is one of the greatest advantages of a VOM? Two disadvantages are that (1) the VOM tends to "load" the circuit under test, and (2) the meter movement is easily damaged as a result of improper testing procedures.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-2 UNCLASSIFIED CAUTION

Never press down on or place any object on the glass face of any multimeter. This can disable the meter movement from operating properly or cause damage.

4.2.1 Measuring Resistance, Voltage, and Current with a Vom In the discussion that follows, you will become familiar with the operation and use of the multimeter in measuring resistance, voltage, and current.

The meter selected for this discussion is the Simpson 260 multimeter, as shown in figure 4-1. The Simpson 260 is a typical VOM used in the Navy today.

Figure 4-1 Simpson 260 Series 6XLP Volt-Ohm-Milliammeter (VOM)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-3 UNCLASSIFIED The multimeter has two selector switches. The switch on the lower left is the function switch, and the one in the lower center is the range switch. The function switch selects the type of current you will be measuring (+dc, −dc, or ac). The range switch is a 12- position switch that selects the range of ohmmeter, voltmeter, or milliammeter measurements you will make.

The multimeter is equipped with a pair of test leads; red is the positive lead and black is the negative, or common, lead. Eight jacks are located on the lower part of the front panel. To prepare the meter for use, simply insert the test leads into the proper jacks to obtain the circuit and range desired for each application. In most applications, the black lead will be inserted into the jack marked at the lower left with a negative sign (−) or with the word COMMON.

4.2.1.1 Measuring Resistance Before proceeding, you should be aware of the following important safety precaution that must be observed when using the ohmmeter function of a VOM:

CAUTION

Never connect an ohmmeter to a "hot" (energized) circuit. Be sure that no power is applied and that all capacitors are discharged.

Q-2. Before you connect a VOM in a circuit for an ohmmeter reading, in what condition must the circuit be? The internal components of the multimeter use very little current and are protected from damage by an overload protection circuit (fuse or circuit breaker). However, damage may still occur if you neglect the safety precaution in the CAUTION instructions above.

Because no external power is applied to the component being tested in a resistance check, a logical question you may ask is, Where does the power for deflection of the ohmmeter come from? The multimeter contains its own two-battery power supply inside the case. The resistive components inside the multimeter are of such values that when the leads are connected together (no resistance), the meter indicates a full-scale deflection. Because there is no resistance between the shorted leads, full-scale deflection represents zero resistance.

Before making a measurement, you must zero the ohmmeter to ensure accurate readings. This is accomplished by shorting the leads together and adjusting the OHMS ADJ control so that the pointer is pointing directly at the zero mark on the OHMS scale. The ZERO OHMS control is continuously variable and is used to adjust the meter circuit sensitivity to compensate for battery aging in the ohmmeter circuits.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-4 UNCLASSIFIED An important point to remember when you are making an accurate resistance measurement is to "zero" the meter each time you select a new range. If this is not done, the readings you obtain will probably be incorrect.

When making a resistance measurement on a resistor, you must give the following considerations to the resistor being tested:

• The resistor must be electrically isolated. In some instances, a soldered connection will have to be disconnected to isolate the resistor. Generally, isolating one side of the resistor is satisfactory for you to make an accurate reading. • The meter leads must make good electrical contact with the resistor leads. Points of contact should be checked for dirt, grease, varnish, paint or any other material that may affect current flow. • Touch only the insulated portions of the test leads. Your body has a certain amount of resistance, which the ohmmeter will measure if you touch the uninsulated portions of the leads.

Figure 4-2 is a functional block diagram of the ohmmeter circuit in a VOM. The proper method of checking a resistor is to connect the red lead to one end of the resistor and the black lead to the other end of the resistor.

Because zero resistance causes full-scale deflection, you should realize that the deflection of the meter is inversely proportional to the resistance being tested; that is, for a small resistance value, the deflection will be nearly full scale; and for a large resistance value, the deflection will be considerably less. This means that the left portion of the OHMS scale represents high resistance; the right side of the scale represents low resistance. Zero resistance (a short circuit) is indicated on the extreme right side of the scale; infinite resistance (an open circuit) is located on the extreme left side of the scale.

Figure 4-2 Functional block diagram of an ohmmeter circuit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-5 UNCLASSIFIED Notice that you read the OHMS scale on the multimeter from RIGHT to LEFT. For example, the pointer of the multimeter in figure 4-3 indicates 8.0 ohms. To determine the actual value of a resistor, multiply the reading on the meter scale by the range switch setting (R × 1, R × 100, or R × 10,000).

Q-3. When taking resistance readings with a VOM, you will obtain the most accurate readings at or near what part of the scale? To explain the relationship between the meter readings and the range switch setting, let’s use an example. Suppose you have a 2,400-ohm resistor, which you have identified by the resistor color code. With the range switch in the R × 1 position, you connect the meter across the resistor. The meter point then deflects between 200 and the point labeled with the infinity symbol (∞) on the extreme left side of the scale. Because the R × 1 range is selected, you multiply the reading by 1. Obviously, the scale reading is not accurate enough. Therefore, you move the range selector switch to the next higher scale position (R × 100) to obtain a more easily read value.

Figure 4-3 Ohmmeter scale

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-6 UNCLASSIFIED In the R × 100 position, you again zero the meter. This time, the pointer moves to the 24 mark on the scale. Because the R × 100 scale is selected, the reading is multiplied by 100. This gives a more accurate reading of 2,400 ohms (24 times 100).

If you position the range switch to the R × 10,000 scale, accuracy decreases. The most accurate readings are obtained at or near midscale. Other VOM instruments have ranges with other settings, such as R × 10, R × 100, or R × 1,000, to make it easier to make such readings.

Another thing to remember when you are measuring resistance is the tolerance of the resistor. If the tolerance of the resistor in the preceding example is 10 percent, we would expect a reading between approximately 2,160 and 2,640 ohms. If the reading is not within these limits, the resistor has probably changed value and should be discarded.

An open resistor will indicate no deflection on the meter. A shorted resistor causes full- scale deflection to the right on the lowest range scale, such as if the leads were shorted together.

4.2.1.2 Measuring dc Voltages You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two positions: +DC or −DC. The meter leads, as in the case of the ohmmeter function, must be connected to the proper meter jacks. When you measure dc voltages, be sure the red lead is the positive lead and the black lead is the negative, or common, lead. View A of figure 4-4 is a functional block diagram of dc voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages.

When the meter is connected in a circuit, it becomes a circuit component. Because all meters have some resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter depends on the amount of voltage being measured and the position of the function switch.

Some multimeters use a 20,000 ohms-per-volt meter sensitivity for measuring dc voltage and a 5,000 ohms-per-volt sensitivity for measuring ac voltage. The higher the meter resistance, the less it will load the circuit. The idea is to keep circuit loading to an absolute minimum so that the circuit under test is unaffected by the meter. In this way, you can get a clearer picture of what the circuit malfunction is, not the effect of the meter on the circuit.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-7 UNCLASSIFIED

Again, refer to figure 4-4. With the function switch set to either +DC or −DC, let’s consider the effect of the range switch on the meter scale to be used. When measuring dc voltages, you have eight voltage ranges available: .25V, 2.5V, 10V, 50V, 250V, and 500V (1- and 1,000-volt special application plug-ins are also available). The setting of the range switch determines the maximum value represented on the meter. When measuring dc voltages, use the scale marked DC (figure 4-3). The last number at the extreme right side of the DC scale indicates the maximum value of the range being used. When the range switch is in the 2.5V position, the scale represents a maximum of 2.5 volts.

To simplify the relationship between the digits on the meter scale and the setting of the range switch, always use the multiple of the full-scale-deflection digits on the meter face that correspond to the numbers of the range switch. For example, use the 250 scale for the 250MV jack, 2.5V, and 250V ranges; the 50 on the scale for 50V and 500V ranges; and the 10 on the scale for the 10V and 1,000V ranges.

For explanation purposes, let’s assume you wish to measure 30 volts dc. In this case, select the next higher range position, 50V. When you place the range switch to the 50V position (as shown in view B of figure 4-4), the meter pointer should rise from a little more than midscale to 30, which represents 30 volts dc.

Figure 4-4 Functional block diagram of dc voltage circuits

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-8 UNCLASSIFIED When measuring a known dc voltage, position the range switch to a setting that will cause approximately midscale deflection. Readings taken near the center of the scale are the most accurate. When measuring an unknown dc voltage, always begin on the highest voltage range. Using the range switch, work down to an appropriate range. If the meter pointer moves to the left, you should reverse the polarity of the function switch.

CAUTION

Always check the polarity before connecting the meter.

Q-4. Besides setting up the meter for expected voltage ranges, what must be strictly observed when taking dc voltage readings? Now let’s discuss how you take a voltage measurement on a component within a circuit. As an example, let’s measure the voltage drop across the resistor shown in figure 4-5.

When measuring a dc voltage drop across a component in a circuit, you must connect the voltmeter in parallel with the component. As you can see in figure 4-5, the positive (red) lead is connected to the positive side of the resistor, and the negative (black) lead is connected to the negative side. A voltage reading is obtained on the meter when current flows through the resistor.

Some voltmeter readings will require the use of a ground as a reference point. Under these conditions, one voltmeter lead is connected to the equipment ground, and the other lead is connected to the test point where voltage is to be measured. Be sure to observe polarity.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-9 UNCLASSIFIED

Figure 4-5 Measuring the voltage drop of a resistor

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-10 UNCLASSIFIED 4.2.1.3 Measuring ac Voltages To measure ac voltages, you must set the function switch to the AC position. The same procedure used to measure dc voltages applies, except that in reading the voltage, you use the AC volts scale (the polarity of the test leads is not important). When measuring very low or high frequencies of ac voltages, you should be aware that the multimeter has a tendency to be inaccurate. View A of figure 4-6 is a functional block diagram of the ac and output voltage circuits in the multimeter. View B shows the jacks and switch positions used to measure ac voltages.

Figure 4-6 Functional block diagram of ac and output voltage circuits

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-11 UNCLASSIFIED 4.2.1.4 Measuring Output Voltages You will often measure the ac component of an output voltage where both ac and dc voltage levels exist. This occurs primarily in amplifier circuits.

The multimeter has a 0.1-microfarad, 400-volt blocking capacitor in series with the OUTPUT jack. The capacitor blocks the dc component of the current in the circuit under test, but allows the ac component to pass on to the indicating circuits.

CAUTION

When using OUTPUT, do not attempt to use the meter in a circuit in which the dc voltage component exceeds the 400-volt rating of the blocking capacitor.

To use the multimeter to measure output voltage, you must follow these steps:

1. Set the function switch to AC.

2. Plug the black test lead into the COMMON jack and the red test lead into the OUTPUT jack.

3. Set the range switch at the appropriate range position, marked as 2.5V, 10V, 50V, or 250V.

4. Connect the test leads to the component being measured with the black test lead to the negative side of the component.

5. Turn on the power in the test circuit. Read the output voltage on the appropriate ac voltage scale. For the 2.5V range, read the value directly on the scale marked 2.5. For the 10V, 50V, or 250V range, use the red scale marked AC and read the black figures immediately above the scale.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-12 UNCLASSIFIED 4.2.1.5 Measuring Current The multimeter can function as an ammeter to measure current flow.

CAUTION

When using the multimeter as a current-indicating instrument, NEVER connect the test leads directly across a voltage. ALWAYS connect the instrument in series with the load.

To use the multimeter as an ammeter, you must take the following steps:

1. Set the function switch at +DC (assuming the current to be positive).

2. Plug the black test lead in the COMMON jack and the red test lead into the + jack.

3. Set the range switch at one of the five ampere-range positions.

4. Ensure the equipment is OFF and then physically open the circuit in which the current is being measured.

5. Connect the VOM in series with the circuit, ensuring that proper polarity is observed when making this connection.

6. Turn the equipment ON and then read the current on the DC scale. (This is the same scale used to measure dc voltages.)

The setting of the range switch determines the maximum value represented by the DC scale. Always use the range scale that corresponds to the range switch setting.

CAUTION

Never attempt to measure currents greater than the setting of the range switch. Increase the range with a shunt, if necessary, but do not exceed the marked current.

When measuring unknown currents, follow the same procedures as when measuring voltages. Always start with the highest range available and work down. Use the range that gives approximately half-scale deflection. If this procedure isn’t followed, the meter could be burned out.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-13 UNCLASSIFIED Figure 4-7 is a functional block diagram of the dc current circuits in a multimeter.

4.2.1.6 Accessories A dc high-voltage probe is available for use with the multimeter. The probe extends the range of the multimeter in a safe and convenient manner. It is primarily used to measure high-voltage, low-power, dc-current sources, such as the anode supplies in television receivers and other cathode-ray tube circuitry.

CAUTION

Do not use this probe on electrical equipment that can deliver high power under short-circuit conditions, such as from a large dc motor-generator set.

Also available is an ac high-voltage probe. The 10,000-volt ac probe is similar to the high-voltage dc probe with the following exceptions:

• The ac high-voltage probe is designed primarily to extend the range of a 5,000- ohms-per-volt VOM.

• The probe is used with the VOM in the 10V AC position.

• You take readings on the 0-10V AC scale and multiply by 1,000.

Figure 4-7 Functional block diagram of dc current circuits

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-14 UNCLASSIFIED 4.2.2 Electronic Digital Multimeter As you studied in chapter 3 (externally excited meters), placing a meter into a circuit causes energy to be taken from the circuit. The amount of energy taken depends on the sensitivity of the meter. In some cases, this energy loss cannot be tolerated. For example, in extremely sensitive circuits, such as oscillator grid circuits and automatic volume control circuits, degradation of normal circuit operation will occur. This often results in failure to obtain a usable indication of the fault. The use of electronic multimeters is practical in these sensitive electronic circuits. The higher the input impedance of a meter, the less the loading effect and the more accurate the measurements taken. Electronic multimeters have considerably greater input impedances than do nonelectronic multimeters.

One example of a typical electronic multimeter in use within the Navy is the electronic Model 8000A Digital Multimeter. Most electronic digital multimeters overcome the disadvantage of requiring a continuous external power source by combining an external ac source with an internal rechargeable battery. Another advantage of this meter is that it can be read directly and does not use a scale. Figure 4-8 shows the model 8000A multimeter.

Figure 4-8 Digital voltmeter 8000A operating features

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-15 UNCLASSIFIED 4.2.2.1 Operating Features The locations of all controls, connectors, and indicators are shown in figure 4-8. The INPUT terminals, located on the left-hand side of the meter face, provide input connections for voltage or resistance (V-?) and milliampere current (MA) measurements with respect to the common terminal. The readout section, located across the upper half of the meter face, contains light-emitting diode (LED) indicators. They display the measured input and polarity signs for dc measurements. The POWER switch, located on the lower right-hand side of the meter face, is a push-button switch used to energize the instrument. The RANGE switches, located on the lower, middle, right-hand portion of the meter face, select the voltage (200 millivolts, 2, 20, 200, or 1,200 volts), current (200 microamperes, 2, 200, or 2,000 milliamperes), and resistance (200 ohms, 2, 20, 200, or 2,000 kilohms) ranges. The FUNCTION switches, located on the lower, middle, left-hand portion of the meter face, select the voltage, current, or resistance modes. The MA input terminal is also a fuse holder for the current protection fuse.

4.2.2.2 Internal Battery Models Power is supplied by internal rechargeable batteries that allow the instrument to operate for at least 8 hours. Recharging the batteries is accomplished by switching the POWER switch to OFF and connecting the instrument to an ac power line. You can use the instrument when recharging the batteries on ac power, but the recharging time will be extended.

Q-5. Power for the electronic digital multimeter is normally supplied by what internal power source? 4.2.2.3 Overload Protection An overload condition is indicated by the simultaneous flashing of the display readouts. The dc voltage function can withstand up to 1,200 volts dc or 1,200 volts root-mean- square (rms) on any range.

Q-6. How is an overload condition indicated by the electronic digital multimeter? The ac voltage function can sustain up to 1,200 volts rms on the 20-, 200-, and 1,200-volt ranges and 500 volts rms on the 200-millivolt and 2-volt ranges. The current input fuse is protected above 2 amperes rms. Protection for the resistance function is to 130 volts rms in the 200-ohm and 2-kilohm ranges, and 250 volts rms in the 20-kilohm through 20- megohm ranges.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-16 UNCLASSIFIED 4.2.2.4 Basic Digital Multimeter Measurement Table 4-1 lists the proper function push buttons, range push buttons, and input terminal connections for performing specific measurements with the model 8000A.

Table 4-1 Basic Measurement Instructions MEASUREMENT FUNCTION RANGE INPUT CONNECTION MAXIMUM OVERLOAD REMARKS DC Volts DCV 200MV, 2, 20, 200, or 1200V V – Ω and COMMON 1200V dc or 1200V rms (sinusoidal) Auto-polarity DC Milliamperes DC MA 200μA, 2, 20, or 2000MA MA and COMMON 2A (fuse protected)

AC Volts ACV 200MV, 2, 20, 200, or 1200V V – Ω and COMMON 1200V rms (sinusoidal), not to exceed 107 V-Hz on 20, 200, 1200V ranges. 500V rms (sinusoidal) on 200mV and 2V ranges

AC Milliamperes AC MA 200μA, 2, 20, or 2000MA MA and COMMON 2A (fuse protected)

Kilohms KΩ 200Ω, 2, 20, 200, or 2000KΩ V – Ω and COMMON 130V rms, 200Ω and 2KΩ ranges. 250V rms, 20kΩ thru 2000kΩ ranges.

Megohms 20MΩ Any V – Ω and COMMON 250V rms Ranges switches non- functional

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-17 UNCLASSIFIED 4.2.2.5 Block Diagram Analysis Figure 4-9 is a block diagram of an electronic digital multimeter. Note that the block diagram divides the instrument into three major sections: the SIGNAL CONDITIONING section, the ANALOG-TODIGITAL CONVERTER section, and the DISPLAY section.

The signal conditioning section provides a dc analog voltage, characteristic of the applied input, to the analog-to-digital converter section. This task is accomplished by the input voltage divider, current shunts, ac converter, active filter, and associated switching.

The analog-to-digital (a/d) converter section changes the dc output voltage from the signal conditioning section to digital information. The a/d converter uses a voltage-to- frequency conversion technique. A dc voltage at the input of the a/d converter is changed to a frequency by the analog integrated circuit (ic). This frequency is characteristic of the magnitude and polarity of the dc input voltage. Counting of the output frequency from the analog ic is accomplished by the digital ic. The resulting count is transferred in binary format to the display section. (Binary number systems are covered in NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits.)

The display section takes the digital (binary) information from the a/d converter section, decodes it, and visually displays it. The decoded digital information is displayed on numerical LED readouts.

Figure 4-9 Model 8000A block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-18 UNCLASSIFIED Q-7. In an electronic digital multimeter, the digital information is displayed by what type of numerical readouts? 4.2.2.6 Accessories Several accessories are available for use with the electronic digital multimeter. One accessory is the test lead kit, shown in figure 4-10. The kit contains two color-coded test leads with threaded adapters. These adapters attach to banana plugs, pin tips, test prod tips, alligator clips, and binding post lugs.

Figure 4-10 Test lead kit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-19 UNCLASSIFIED Figure 4-11 shows a high-current probe. This probe extends the ac current measurement capability from 2 to 600 amperes at frequencies up to 400 hertz.

Figure 4-12 shows a high-voltage probe. The probe extends the dc voltage range to 30 kilovolts.

Figure 4-11 Ac high-current probe Figure 4-12 High-voltage probe

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-20 UNCLASSIFIED Figure 4-13 shows a high-frequency probe, which allows measurements over a frequency range of 10 kilohertz to 500 megahertz.

Figure 4-13 High-frequency probe

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-21 UNCLASSIFIED 4.3 AC/DC DIFFERENTIAL VOLTMETER The DIFFERENTIAL VOLTMETER provides extremely accurate voltage measurements and is a highly reliable piece of precision test equipment. Its general function is to compare an unknown voltage with a known internal reference voltage and to indicate the difference in their values. The differential voltmeter in common use in the Navy today is the model 893A (figure 4-14).

Q-8. What is the general function of the differential voltmeter? The differential voltmeter can be used as a conventional TRANSISTORIZED ELECTRONIC VOLTMETER (TVM) and a DIFFERENTIAL NULL VOLTMETER. It can also be used to measure variations of a voltage near some known value (NULL DETECTOR), high resistance values (MEGGOMETER), and for dBm measurements.

Figure 4-14 Ac/dc differential voltmeter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-22 UNCLASSIFIED 4.3.1 Meter Design Characteristics The differential voltmeter is a solid-state instrument that provides the capability of making dc voltage measurements from +/− 10 microvolts to +/− 1,100 volts. Ac voltages from 0.001 to 1,100 volts can be measured over a frequency range from 5 hertz to 100 kilohertz. Both of these measurements can be made without concern for loading the circuit. The differential voltmeter has four voltage readout dials that vary the resistance of the divider assembly as described above.

The differential voltmeter uses a built-in NULL DETECTOR to measure an unknown voltage. The meter circuitry compares the unknown voltage to a known, adjustable reference voltage supplied by the meter. The reference voltage is provided by a high- voltage dc power supply and decade resistor divider assembly strings that are set by voltage readout dials. In this way, the output from the high-voltage power supply can be precisely divided into increments as small as 10 microvolts. The readout dials are used to adjust the meter pointer to 0 and the unknown voltage is then read from the voltage dials.

A primary feature of the differential voltmeter is that it does not draw current from the unknown source for dc measurements when the measurement is obtained. Therefore, the determination of the unknown dc potential is independent of its source.

4.3.2 Front Panel Controls The front panel of a typical differential voltmeter is shown in figure 4-15. With a few differences, the controls and terminals are similar to those used on other differential voltmeters. The NULL SENSE switch selects the conventional TVM mode of operation and the various full-scale null detector sensitivity ranges when the instrument is operated in the differential mode of operation. The RANGE switch allows selection of the desired input voltage range, positions the readout dial decimal point, and selects the various ranges of the NULL SENSE switch. The readout dials provide a digital readout of the measured voltage when the instrument is in the differential mode.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-23 UNCLASSIFIED

4.3.3 Modes of Operation There are two primary modes of operation: the conventional transistorized voltmeter mode and the differential null mode. These modes are described in the next paragraphs.

4.3.3.1 Conventional Transistorized Voltmeter (TVM) Mode When the instrument is used as a conventional transistorized dc voltmeter, the circuitry is connected as shown in figure 4-16. The null detector drives the front panel meter and provides a full-scale meter deflection for any full-scale input. Positive or negative voltage measurements are made by reversing the meter terminals through the contacts of the MODE switch.

Figure 4-15 Controls, terminals, and indicators

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-24 UNCLASSIFIED 4.3.3.2 Differential Null Mode When the instrument is used as a dc differential voltmeter, the MODE and NULL SENS switches in figure 4-16 are placed to their respective +/− dc and desired full-scale meter sensitivity positions. In this mode of operation, the NULL SENS switch selects a suitable resistance value to determine the full-scale sensitivity of the meter. The dc input voltage applied to the instrument is then compared with the null detector, and any resulting difference is used to drive the meter. The meter terminals can be reversed through the contacts of the MODE switch for +/− dc voltage measurements.

Figure 4-16 Ac/dc differential voltmeter block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-25 UNCLASSIFIED 4.3.4 Functional Block Diagram Figure 4-16 is a functional block diagram of a differential voltmeter. The circuitry is made up of a reference supply, a resistive divider, a dc input divider, an ac converter, a null detector, and a meter. The circuitry is interconnected by various switching arrangements when you perform the desired ac or dc conventional or differential voltage measurements.

Placing the MODE switch in figure 4-16 to the AC position connects the instrument circuitry as a conventional transistorized ac voltmeter. A full-scale input voltage at the input terminals of the instrument results in a voltage being applied to the input of the null detector. The null detector drives the front panel meter that indicates the value of the measured ac voltage.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-26 UNCLASSIFIED 4.4 TRANSISTOR TESTERS Laboratory transistor test sets are used in experimental work to test characteristics of transistors. For maintenance and repair, however, checking all transistor parameters is not necessary. A check of two or three performance characteristics is usually sufficient to determine whether a transistor needs to be replaced.

Two of the most important parameters used for transistor testing are the transistor CURRENT GAIN (BETA) and the COLLECTOR LEAKAGE or REVERSE CURRENT (ICO). Two other tests that can be accomplished include the electrode resistance and diode measurements. You may want to review NEETS, Module 7, Introduction to Solid- State Devices and Power Supplies, for a review of transistors before continuing this section.

The Semiconductor Test Set AN/USM-206A (figure 4-17) is a rugged, field-type tester designed to test transistors and semiconductor diodes. The set will measure the beta of a transistor, the resistance appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It will also measure a shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition of its own batteries.

To assure that accurate and useful information is gained from the transistor tester, you should make the following preliminary checks of the tester before testing any transistors:

1. With the POLARITY switch in the OFF position, the meter pointer should indicate exactly zero. (When required, rotate the meter ZERO ADJUST KNOB on the front of the meter to fulfill this requirement.) To prevent battery drain, be sure to leave the POLARITY switch in the OFF position when measurements are not actually being made.

2. Always check the condition of the test set batteries. To make this check, disconnect the test set power cord, place the polarity switch in the PNP position, and place the function switch first to BAT. 1 and then to BAT. 2. In both BAT positions, the meter pointer should move so as to indicate within the red BAT box.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-27 UNCLASSIFIED

4.4.1 Beta Measurements If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side below the meter. If the transistor is to be tested in the circuit, at least 300 ohms must exist between EB (emitter to base), C-B (collector to base), and C-E (collector to emitter) for accurate measurement.

Figure 4-17 Semiconductor test set

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-28 UNCLASSIFIED Initial setting of the test set controls is performed as follows:

1. Set the function switch to BETA.

2. Set the POLARITY switch to PNP or NPN (depending on the type of transistor under test).

3. Set the RANGE switch to X10.

4. Adjust METER ZERO for zero meter indication (transistor disconnected).

5. The POLARITY switch should remain OFF while the transistor is connected to or disconnected from the test set; it should then be set to PNP or NPN, as in step 2 above.

If the beta reading is less than 10, perform the following steps:

1. Reset the RANGE switch to X1 and reset the meter to zero.

2. After connecting the yellow test lead to the emitter, the green test lead to the base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower right-hand corner of the test set.

3. When testing grounded equipment, unplug the 115-volt line cord and use battery operation. A beta reading is attained by multiplying the meter reading times the RANGE switch setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is normal for the type of transistor under test.

4.4.2 ICO Measurements Adjust the METER ZERO control for a zero meter indication. Plug the transistor to be tested into the jack, or connect the test leads to the device. Set the PNP/NPN switch to correspond with the type of transistor under test. Set the function switch to ICO and the RANGE switch to X0.1, X1.0, or X10, as specified by the transistor data book for allowable leakage. Read leakage on the bottom scale and multiply by the range setting figure as required.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-29 UNCLASSIFIED 4.4.3 Electrode Resistance Measurements Connect the in-circuit probe test leads to the transistor with the yellow lead to the emitter, the green lead to the base, and the blue lead to the collector. Set the function switch to the OHMS E-B position and read the resistance between the emitter and base electrode on the center scale of the meter marked OHMS.

To read the resistance between the collector and base and the collector and emitter, set the function switch to OHMS C-B and OHMS C-E, respectively. These in-circuit electrode resistance measurements are used to correctly interpret the in-circuit beta measurements. The accuracy of beta times 1 and 10 range is ±15 percent only when the emitter-to-base load is equal to or greater than 300 ohms.

4.4.4 Diode Measurements Diode in-circuit quality measurements are made by connecting the green test lead to the cathode and the yellow test lead to the anode. Set the function switch to DIODE IN/CKT and the RANGE switch to times 1 position. Ensure that the meter has been properly zeroed on this scale. If the meter reads downscale, reverse the polarity switch. If the meter reads less than midscale, the diode under test is either open or shorted. The related circuit impedance of this test is less than 25 ohms.

4.5 RESISTANCE-CAPACITANCE-INDUCTANCE (RCL) BRIDGES Resistance, capacitance, and inductance can be measured with precise accuracy by alternating-current bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of combinations. These bridges operate on the principle of the Wheatstone bridge; that is, an unknown resistance is balanced against known resistances and, after the bridge has been balanced, the unknown resistance is calculated in terms of the known resistance.

The universal Impedance Bridge, Model 250DE (shown in figure 4-18) is used to measure resistance, capacitance, and inductance (RCL) values. It is also used to make other special tests, such as determining the turns ratio of transformers and capacitor quality tests. This instrument is self-contained, except for a source of line power, and has an approximate 500-hour battery life expectancy. It has its own source of 1,000-hertz bridge current with a sensitive bridge balance indicator and an adjustable source of direct current for electrolytic capacitor and resistance testing. The bridge also contains a meter with suitable ranges to test for current leakage on electrolytic capacitors.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-30 UNCLASSIFIED

4.5.1 Controls Figure 4-18 is a panel view of the model 250DE bridge switches, dials, controls, and connections. Refer to the figure as we briefly discuss some of the switches and dials below.

• The FUNCTION switch selects the type of bridge circuit that will measure resistance, capacitance, or inductance. • The RANGE switch selects the multiplier for each function. • L-R-C decade dials are a DEKASTAT decade resistor that is the main balancing element of the bridge. The setting of the dials after the bridge is balanced indicates the value of inductance, resistance, or capacitance. • The D-Q dial is used to balance the phase of the capacitance or inductance of the bridge. The setting of the dial after the bridge is balanced indicates the value of dissipation factor (D) or storage factor (Q).

Figure 4-18 Resistance-capacitance-inductance bridge

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-31 UNCLASSIFIED • The GEN-DET switch selects bridge generator and detector connections, ac or dc, internal or external generator. The switch also connects the internal batteries to the battery test circuit. • The DET GAIN control adjusts the sensitivity of the ac-dc detector and turns on power to the generator.

4.5.2 Connections L, R, and C terminals 1, 2, and 3 are used to connect unknown resistors, inductors, and capacitors to the bridge. Resistors and inductors are connected between terminals 1 and 2, and capacitors are connected between terminals 2 and 3. EXT BIAS terminals are normally connected with a shorting lug. They allow insertion of a dc voltage or current to bias capacitors or inductors. EXT DET connector is a BNC coaxial socket that allows an external detector to be used with the instrument. It is connected to the bridge at ALL TIMES.

EXT D-Q terminals are normally connected with a shorting lug. They allow an external rheostat to extend the range of the D-Q dial. EXT GEN terminals provide a connection to the bridge for an external generator. When the GEN-DET switch is in the AC EXT GEN position, the terminals connect an isolation transformer so that a grounded external generator can be used. When the GEN-DET switch is in the DC EXT GEN position, the terminals are connected directly to the bridge.

4.5.3 Battery The model 250DE bridge has a battery supply consisting of four 1.5 V dc batteries with an expected life of 500 hours. The battery power supply should be checked before each day’s operation. Turn DET GAIN control to 1 and set GEN-DET switch to BATT. TEST (battery test). If the meter deflects beyond the BAT OK mark, the battery is good.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-32 UNCLASSIFIED 4.5.4 Resistance Measurements Resistance is usually measured with direct current for maximum accuracy. The model 250DE bridge can be used to measure resistance with alternating current, but external reactance compensation is usually required. On high-resistance ranges, care should be taken to avoid leakage across a resistor under test. Insulation with a resistance of 109 ohms, which is adequate for most purposes, will cause a measurement error of 1 percent if it shunts a 10-megohm resistor. Using the following steps, you will be able to measure dc resistance ONLY:

1. Turn the DET GAIN control to 2.

2. Set the FUNCTION switch to R × 1 or R × 10.

3. Set L-R-C decade dials to 3.000.

4. Connect the unknown resistor to R-L terminals 1 and 2.

5. Set the GEN-DET switch to INT DC.

6. Adjust the RANGE switch for minimum detector deflection.

7. Adjust L-R-C decade dials for null, turning the DET GAIN control clockwise to increase sensitivity as necessary.

8. The measured resistance is the product of the L-R-C decade dial setting times the RANGE and FUNCTION switch settings.

4.5.5 Capacitance Measurements Capacitance is measured in terms of a two-element equivalent circuit consisting of a capacitor in series with a resistor. The internal ac generator and detector of the model 250DE bridge are tuned to 1 kilohertz. Other frequencies can be used, but an external generator and detector are required. The D and Q ranges of the bridge can be extended by use of an external rheostat connected to the terminals provided. The measured capacitance is the product of the L-R-C dial setting times the setting of the RANGE switch. Using the following steps, you can make a standard capacitance measurement:

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-33 UNCLASSIFIED 1. Turn the DET GAIN control to 1.

2. Set the FUNCTION switch to C, D × 0.1 or D × 0.01 SERIES.

3. Set L-R-C decade dials to 3.000 and D-Q dial to 0.

4. Connect the unknown capacitor to C terminals 2 and 3.

5. Set the GEN DET switch to INT 1 kHz.

6. Adjust the RANGE SWITCH for minimum detector deflection.

7. Adjust L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning the DET GAIN control clockwise to increase sensitivity as necessary.

8. The measured capacitance is the product of the L-R-C decade dial settings.

9. The measured dissipation factor (D) is the product of the D-Q setting times the FUNCTION switch setting.

4.5.6 Inductance Measurements Inductance is measured in terms of a two-element equivalent circuit consisting of an inductance either in series or in parallel with a resistance. The internal ac generator and detector of the model 250DE bridge are tuned to 1 kHz. Other frequencies can be used, but like capacitance measurements, an external generator and detector are required. When inductance is being measured in ac or dc, it should be realized that iron-core inductors are sensitive to current variations. Quantitative measurements of dc effects can be made by supplying current to the unknown inductor through the EXT BIAS terminal. Use the following steps to make inductance measurements:

1. Turn the DET GAIN control to 1.

2. Set the FUNCTION switch to L PARALLEL if Q is greater than 10 to L SERIES if Q is less than 10.

3. Set L-R-C decade dials to 3.000 and D-Q dial to maximum.

4. Connect the unknown inductor to the R-L terminals 1 and 2

5. Set the GEN DET switch to INT 1 kHz.

6. Adjust the RANGE switch for minimum detector deflection.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-34 UNCLASSIFIED 7. Adjust the L-R-C decade dials and D-Q dial alternately for a minimum meter deflection, turning the DET GAIN control clockwise to increase sensitivity as necessary.

8. The measured inductance is the product of the L-R-C decade dial setting times the RANGE switch setting.

9. The measured storage factor (Q) is read directly from the D-Q dial, inner scale for parallel and outer scale for series inductance.

4.6 SUMMARY The important points of this chapter are summarized in the following paragraphs. You should be familiar with these points before continuing with your studies of test equipment.

A MULTIMETER is a single meter that combines the functions of a dc ammeter, a dc voltmeter, an ac ammeter, an ac voltmeter, and an ohmmeter. Observe the following safety precautions when using a multimeter:

• De-energize and discharge the circuit completely before connecting a multimeter.

• Never apply power to the circuit while you are measuring resistance with an ohmmeter.

• Connect the ammeter in series for current measurements and in parallel for voltage measurements.

• Be certain the multimeter is switched to ac before attempting to measure ac circuits.

• Observe proper dc polarity when measuring dc circuits.

• Always start with the highest voltage or current range.

• Select a final range that allows a reading near the middle of the scale.

• Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance measurement.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-35 UNCLASSIFIED An ELECTRONIC DIGITAL MULTIMETER is used in sensitive electronic circuits where only extremely small amounts of energy can be extracted without disturbing the circuits under test, or causing them to be inoperative.

The DIFFERENTIAL VOLTMETER is a precision piece of test equipment used to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values.

A SEMICONDUCTOR TEST SET is used to measure the beta of a transistor, the resistance appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It also measures a shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition of its own batteries.

Resistance, capacitance, and inductance are measured for precise accuracy by RCL BRIDGES. They are composed of capacitors, inductors, and resistors and operate on the principle of the Wheatstone bridge.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

4-36 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q8.

A-1. No external power source is required.

A-2. De-energized.

A-3. Midscale.

A-4. Polarity.

A-5. Rechargeable batteries.

A-6. Simultaneous flashing of display readouts.

A-7. Light-emitting diodes.

A-8. To compare an unknown voltage with a known reference voltage and indicate the difference in their values.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-1 UNCLASSIFIED 5 SPECIAL-APPLICATION TEST EQUIPMENT LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

1. Explain the theory of operation of two types of power meters. 2. Describe the purpose of the controls and indicators found on power meters. 3. Describe the proper procedure for taking power measurements for incident and reflected energy. 4. Describe the uses and purposes of the controls and indicators found on the signal generator. 5. Explain the theory of operation of a typical frequency counter. 6. Describe the uses and purposes of the controls and indicators found on the frequency counter. 7. Explain the uses and purposes of the controls and indicators found on the Huntron Tracker 2000. 8. Describe the proper procedures for troubleshooting with a logic probe. 9. Describe the proper procedures for troubleshooting using the Huntron Tracker 2000.

5.1 INTRODUCTION In chapters 3 and 4, you studied the more common pieces of test equipment. As a technician, you will routinely use this test equipment to troubleshoot and perform maintenance on electronic equipment. However, the equipments you will study in this chapter may or may not be found in your shop. This is because these equipments have specific or specialized uses. Unless your rating is involved with the equipment with which they are used, you may not have reason to use them. They are presented here so that you will be familiar with their overall function should the need arise. The equipments you will study in this chapter are power meters, signal generators, frequency counters, and integrated circuit-testing devices.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-2 UNCLASSIFIED 5.2 POWER METERS As a technician, you will use a POWER METER to measure power. There are various types of power meters, some of which are called WATTMETERS. Figure 5-1 shows the AN/URM-120 wattmeter, which is one type of power meter commonly used in the Navy. This particular power meter measures power directly; that is, you connect it directly between the transmitter output (rf source) and the load, most likely an antenna.

Other types of power meters measure power indirectly; that is, they sample power in other ways - but not by being placed directly between the output of the transmitter and the load. Let's discuss the direct-measuring power meter first; then we'll talk about an indirect-measuring power meter.

Figure 5-1 Wattmeter (AN/URM-120)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-3 UNCLASSIFIED 5.2.1 Direct-Measuring Power Meters The direct-measuring power meter is designed to measure incident (forward) and reflected (reverse) rf power from 50 to 1,000 watts at 2 MHz to 30 MHz and 10 to 500 watts at 30 MHz to 1,000 MHz. Three separate COUPLER-DETECTORS (sometimes called ATTENUATORS), each rated to cover a portion of the frequency and power ranges, are provided with the wattmeter. These devices couple the rf signal into the wattmeter and detect the signal. The coupler-detector knob projects through the top of the wattmeter case, as shown on the AN/URM-120 wattmeter in figure 5-1.

A nameplate on the top of the POWER RANGE knob indicates the power range. The POWER RANGE knob can be rotated 360º to the desired power range. The coupler- detector rotates 180º inside the metal case for either forward or reverse power measurements. Also located inside the metal case are the indicating meter and cable for interconnecting the meter to the coupler-detector. The LOCKING knob locks the coupler- detector and POWER RANGE knobs in place.

Two N-TYPE connectors (one male and one female) are located on either side of the wattmeter case to connect the instrument between the power source and the load. The upper and lower parts of the wattmeter are held together with quick-action fasteners, which permit easy access to the inside of the wattmeter.

Power measurements are made by inserting the proper coupler-detector and connecting the wattmeter in the transmission line between the load and the rf power source. To measure incident power with the wattmeter, rotate the arrow on the COUPLER- DETECTOR knob toward the load, and position the POWER RANGE knob for peak meter reading. To measure reflected power, position the arrow toward the rf power source.

In effect, rotating the coupler-detector causes the coupler to respond only to a wave traveling in a particular direction, either to (incident) or from (reflected) the load. It will be unaffected by a wave traveling in the opposite direction. A diode rectifier in the coupler rectifies the energy detected by the coupler. This detected rf energy is measured across a known impedance to obtain either incident or reflected power.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-4 UNCLASSIFIED 5.2.1.1 Operating the Wattmeter Always de-energize and tag the rf power source before measuring incident power. Insert the proper coupler-detector for the rf power being measured into the wattmeter case. Remove the wire shunt (not shown in figure 5-1) from the meter terminals, then connect the wattmeter into the transmission line, either at the load or the rf source. Ensure that all connections are tight.

Position the POWER RANGE knob to a value higher than the rated power of the rf source.

CAUTION

If the rated power to be measured is not known, place the POWER RANGE knob in the highest power position before turning on the power source.

Q-1. To measure incident power, you must rotate the coupler-detector of the wattmeter so that the arrow indicating power flow points toward which end of the transmission line? Reflected power is measured in the same manner as described for incident power, except that the coupler-detector is rotated so that the arrow points toward the rf source.

After completing power measurements, de-energize the rf source, disconnect the wattmeter from the transmission line, and place the wire shunt on the meter terminals.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-5 UNCLASSIFIED 5.2.1.2 Interpreting Power Measurements Made by the Wattmeter The rf power measurements made by the wattmeter are used to determine the voltage standing wave ratio (VSWR) of the load and the power absorbed by the load. (VSWR is covered in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas.) The VSWR can be determined from a chart provided in the wattmeter technical manual, or it can be calculated (as shown in the following example for a UHF transmitter) by the formula below (Pi is the incident power, and Pr is the reflected power as measured by the wattmeter):

Where:

Pi = 30 watts

Pr = 0.5 watts

VSWR = �Pi + �Pr �Pi − �Pr

VSWR = √30 + √0.5 √30 – √0.5

= 5.47 + 0.71 5.47 − 0.71

= 6.18 4.76

= 1.30

The example above results in a standing wave ratio expressed as 1.3 to 1. In a perfectly matched transmission line where there is no reflected power (P r = 0), the standing wave ratio would be 1 to 1. A standing wave ratio of 1.5 to 1 indicates a 5-percent reflection of energy (loss) and is considered to be the maximum allowable loss. So, our example is within allowable limits.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-6 UNCLASSIFIED If the standing wave ratio is greater than 1.5 to 1, then the transmission line efficiency has decreased and troubleshooting is necessary. An excellent discussion of the reasons for standing wave ratio increases is presented in EIMB, Test Methods and Practices, NAVSEA 0967-LP-000-0130.

You can determine the rf power absorbed by the load simply by subtracting the reflected power reading from the incident power reading made by the wattmeter (30 watts - 0.5 watts = 29.5 watts).

The power meter just discussed is often described as an IN-LINE POWER METER because readings are taken while the power meter is connected in series with the transmission line. Another type of power meter used by the Navy measures power indirectly. An example of an indirect-measuring power meter is described in the next section.

5.2.2 Indirect-Measuring Power Meters An example of an indirect-measuring power meter is the HP-431C, shown in figure 5-2. The controls, connectors, and indicators for the power meter are illustrated in figure 5-3. This power meter can be operated from either an ac or dc primary power source. The ac source can be either 115 or 230 volts at 50 to 400 hertz. The dc source is a 24-volt rechargeable battery. Overall circuit operation of the power meter is shown in the block diagram in figure 5-4.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-7 UNCLASSIFIED

Figure 5-2 Power meter (HP 431C)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-8 UNCLASSIFIED

Figure 5-3 Power meter controls, indicators, and connectors

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-9 UNCLASSIFIED

The HP-431C power meter indirectly measures microwave frequency power by using two bridge circuits - the detection bridge and the compensation and metering bridge. The detection bridge incorporates a 10-kilohertz (kHz) oscillator in which the amplitude is determined by the amount of heating of the thermistors in that bridge caused by microwave power. (Thermistors were covered in chapter 2 of this module.) The compensation and metering bridge contains thermistors that are affected by the same microwave power heating as those of the detection bridge.

An unbalance in the metering bridge produces a 10-kHz error signal. This error signal, plus the 10-kHz bias that is taken directly from the 10-kHz OSCILLATOR-AMPLIFIER, is mixed in the SYNCHRONOUS DETECTOR. The synchronous detector produces a dc current (I dc) that is proportional to the 10-kHz error signal. The Idc error signal is fed back to the compensation and metering bridge, where it substitutes for the 10-kHz power in heating the thermistor and drives the bridge toward a state of balance. The dc output of the synchronous detector also operates the meter circuit for a visual indication of power.

Figure 5-4 Power meter block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-10 UNCLASSIFIED Q-2. What condition produces the 10-kHz error signal generated by the metering bridge in the HP-431C power meter? The HP-431C power meter measures rf power from 10 microwatts (−20 dBm) to 10 milliwatts (+10 dBm) full scale in the 10 MHz to 18 GHz for a 50-ohm coaxial system and 2.6 GHz to 40 GHz for a waveguide system.

5.3 SIGNAL GENERATORS Standard sources of ac energy, both audio frequency (af) and radio frequency (rf), are often used in the maintenance of electronic equipment. These sources, called SIGNAL GENERATORS, are used to test and align all types of transmitters and receivers. They are also used to troubleshoot various electronic devices and to measure frequency.

The function of a signal generator is to produce alternating current (ac) of the desired frequencies and amplitudes with the necessary modulation for testing or measuring circuits. (Modulation was discussed in NEETS Module 12, Modulation Principles.) It is important that the amplitude of the signal generated by the signal generator be correct. In many signal generators, output meters are included in the equipment to adjust and maintain the output at standard levels over wide ranges of frequencies.

When using the signal generator, you connect the output test signal into the circuit being tested. You can then trace the progress of the test signal through the equipment by using electronic voltmeters or oscilloscopes. In many signal generators, calibrated networks of resistors, called ATTENUATORS, are provided. You use attenuators in signal generators to regulate the voltage of the output signal. Only accurately calibrated attenuators can be used because the signal strength of the generators must be regulated to avoid overloading the circuit receiving the signal.

Q-3. In signal generators, what device is used to regulate the voltage of the output signal? There are many types of signal generators. They are classified by use and the frequency range covered as AUDIO-FREQUENCY (AF) GENERATORS, VIDEO SIGNAL GENERATORS, RADIO-FREQUENCY (RF) GENERATORS, FREQUENCY- MODULATED RF GENERATORS, and other special types, which combine frequency ranges.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-11 UNCLASSIFIED 5.3.1 Audio and Video Signal Generators AUDIO SIGNAL GENERATORS produce stable af signals used for testing audio equipment. VIDEO SIGNAL GENERATORS produce signals that include the audio range and extend into the rf range. These signal generators are used to test video amplifiers and other wideband circuits. In both audio and video signal generators (figure 5-5), major components include a POWER SUPPLY, an OSCILLATOR (or oscillators), one or more AMPLIFIERS, and an OUTPUT CONTROL.

In the audio and video generators that produce a beat-frequency, the output frequency is produced by mixing the signals of two separate rf oscillators. One is fixed in frequency, and the other is variable. The difference between the frequencies of the two oscillators is equal to the desired audio or video frequency.

Audio signal generators often include resistance-capacitance (rc) oscillators in which the af is directly produced. In these signal generators, a resistance-capacitance circuit is the frequency-determining part of the oscillator. The frequency varies when either the resistance or the capacitance is changed in value.

Figure 5-5 Af and video signal generator block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-12 UNCLASSIFIED In other signal generators, however, the capacitance alone is often chosen as the only variable element. The change in frequency that can be produced by this method is limited, and it is usually necessary to cover the entire range of the generator in frequency steps. This is usually accomplished by providing several rc circuits, each corresponding to a specific portion of the entire range of frequency values. The circuits in the oscillator are switched one at a time to provide the desired portion of the af range.

The amplifier section of the block diagram (figure 5-5) usually consists of a voltage amplifier and one or two power amplifiers, which are coupled by means of rc networks. The output of the final power amplifier is often coupled to the output control (attenuator) by means of an output transformer.

The output control section regulates the amplitude of the signal. A commonly used af signal generator is the model SPN audio oscillator shown in figure 5-6. The model SPN is a programmable synthesized signal generator designed to provide a stable, low-distortion, wide-amplitude range signal over a 1-Hz to 1.3-MHz frequency range. Additionally, the equipment provides a square wave output and means for swept operation with an external signal and can be remotely controlled via an IEEE-488 bus. For the proper operation of any test equipment, you should always refer to the specific technical manual that describes its use.

Figure 5-6 Model SPN audio oscillator

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-13 UNCLASSIFIED 5.3.2 Radio-Frequency (Rf) Signal Generators In addition to the necessary power supply, a typical rf signal generator contains three other main sections: an OSCILLATOR CIRCUIT, a MODULATOR, and an OUTPUT CONTROL CIRCUIT. The modulator modulates the rf signal of the oscillator. In addition, most rf generators are provided with connections through which an external source of modulation of any desired waveform can be applied to the generated signal. Metal shielding surrounds the unit to prevent signals from the oscillator from affecting the circuit under test.

Q-4. Name the three main sections of a typical rf signal generator. A block diagram of a representative rf signal generator is shown in figure 5-7. The function of the oscillator stage is to produce a signal that can be accurately set in frequency at any point within the range of the generator. The type of oscillator circuit used depends on the range of frequencies for which the generator is designed. In lower frequency rf signal generators, the oscillating circuit consists of one of a group of coils combined with a variable capacitor. One of the coils is selected by the position of a range selector switch that connects the coil to a capacitor to provide an inductance-capacitance circuit. The inductive-capacitance circuit then has the correct range of resonant frequencies.

Figure 5-7 Rf signal generator block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-14 UNCLASSIFIED The function of the modulator is to produce an audio (or video) modulating signal that can be superimposed on the rf signal produced by the oscillator. The modulating signal may be provided by an audio oscillator within the generator. This is termed INTERNAL MODULATION. It may also be derived from an external source. This is termed EXTERNAL MODULATION. In some signal generators, either of these two methods of modulation can be employed. In addition, a means of disabling the modulator section is available so that the pure, unmodulated signal from the oscillator can be used when desired.

Q-5. What is the function of the modulating circuit? The type of modulation selected depends on the application of the particular signal generator. The modulating signal may be a sine wave, a square wave, or pulses of varying duration. In some special generators, provision is made for pulses over a wide range of repetition rates and widths.

The output circuit of the rf signal generator usually contains a calibrated attenuator and an output level meter. The output level meter provides an indication and permits control of the output voltage of the generator. The attenuator allows you to select the amount of this output. The attenuator is made up of a group of resistors that form a voltage-dropping circuit.

It is controlled by a control calibrated in microvolts. When the control is adjusted so that the output meter reads unity (1.0), the reading on the attenuator control gives the exact value (no multiplication factor) of the output in microvolts. If an output voltage at a lower value is desired, the attenuator control is varied until the meter indicates some decimal value less than 1. This decimal is multiplied by the attenuator reading to give the actual output in microvolts.

An rf signal generator currently in wide use by the Navy is the HP 8640B (figure 5-8). The HP 8640B signal generator covers the frequency range of 500 kilohertz to 512 megahertz, and can be extended to 1,024 MHz by using adapters.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-15 UNCLASSIFIED

This completes our discussion of signal generators. The following section deals with an instrument that measures frequency - the FREQUENCY COUNTER.

Figure 5-8 Rf signal generator (HP-model 8640B)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-16 UNCLASSIFIED 5.4 FREQUENCY COUNTERS The signal generators you studied in the previous section provide signals for use in testing, aligning, and troubleshooting electronic equipment. Now, we will study the FREQUENCY COUNTER, an instrument that measures frequencies. Frequency counters are used to measure frequencies already in existence. An example of a typical frequency counter, the model 5328A, is shown in figure 5-9.

5.4.1 General Description of the Frequency Counter The following description is for the model 5328A counter only. Other counters use different techniques to derive the displayed frequency. The outputs available vary from counter to counter. For further information on other types of counters and their uses, refer to EIMB, Test Equipment, NAVSEA 0967-LP-000-0040.

The model 5328A is a portable, solid-state electronic frequency counter. It is used to precisely measure and display, using a nine-digit LED readout, frequency, period, period average, time interval, time interval average, and ratio of electronic frequency signals. This frequency counter can also provide a 1-MHz and 10-MHz output signal through the back-panel BNC.

Q-6. What frequencies are provided through the back-panel BNC?

Figure 5-9 Model 5328A 500 MHz universal frequency counter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-17 UNCLASSIFIED The model 5328A frequency counter can be divided into four major internal subsections: the main counter section, input section, power supply, and interface bus section. Additionally, two separate (front panel) input channels provide for time interval measurements. Each channel has an attenuator, trigger slope selector, level control, ac or dc coupling, and an oscilloscope marker output. A third input channel is provided to allow the measurement of 30 MHz to 500 MHz with a maximum input of 5 volts rms with a fused-protected connection.

Front-panel controls are provided for you to do the function selection, frequency resolution, sample rate, and reset display. Also, a push-button control on the front panel allows the unit to be used in the operational or standby mode (power applied to the crystal oven to eliminate warm-up). Rear-panel connectors provide the gated output frequencies and an input for an external frequency standard. A detachable front cover is used to store accessory cables and connectors.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-18 UNCLASSIFIED 5.4.1.1 Controls and Indicators Figures 5-10 and 5-11 show all the front- and rear-panel operating controls and indicators. Refer to tables 5-1 and 5-2 for a description of each of the numbered controls and connectors shown in figures 5-10 and 5-11, respectively.

Figure 5-10 Front-panel controls and indicators Figure 5-11 Rear-panel controls and connectors

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-19 UNCLASSIFIED Table 5-1 Front Panel Controls and Connectors NO. NAME DESCRIPTION 1 LINE switch This is a two-position switch that can be selected for STBY or OPER. When in STBY, the power supply provides power to the oven of the high stability time base to maintain a constant temperature for the crystal. When in OPER, the power supplies normal power to the unit for operation. 2 RESET This button, when pressed, resets the display and internal count to zero. When pressed continuously, this button lights all segments of the LED display and all annunciator LEDs for a LED test. 3 FUNCTION This is a 10-position selector used to select the mode of operation. FREQ A This position, when selected, allows the counter to measure frequency at channel A. FREQ C This position, when selected, allows the counter to measure the frequency at channel C. PER A This position, when selected, allows the counter to measure the period at channel A. PER AVG A This position, when selected, allows the counter to make a period average measurement of the signal at channel A. The number of periods over which the average is made is determined by the RESOLUTION switch selection. T. I. AVG A!B This position, when selected, allows the counter to make a time interval measurement of a signal applied to channel A. The number of time intervals over which the average will be made is determined by the RESOLUTION switch selection made. T. I. A!B This position, when selected, allows the counter to make a time interval measurement. The start signal would be applied to channel A and the stop signal applied to channel B. CHECK This position, when selected, applies a 10-MHz signal to the decade counting assemblies, verifying operation of the SAMPLE RATE control, RESOLUTION switch, and RESET. 3 Continued RATIO C/A This position, when selected, allows the counter to measure the ratio of the frequency at channel C to the frequency at channel A. RATIO B/A This position, when selected, allows the counter to measure the ratio of the frequency at channel B to the frequency at channel A. Top This position is blank and has no function. 4 FREQ RESOLUTION, N This is an eight-position switch that is used to select the resolution in frequency measurements and N for totalizing and averaging measurements. This also determines how long the main gate is open for frequency measurements.

N GATE TIME (seconds) RESOLUTION (Hz) 1 1 X 10-6 1M 10 10 X 10-6 100k 102 100 X 10-6 10k 103 1 X 10-3 1k 104 .01 100 105 .1 10 106 1 1 107 10 .1

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-20 UNCLASSIFIED NO. NAME DESCRIPTION 5 SAMPLE RATE control This is used to vary the time between measurements continuously from approximately 2 milliseconds to an indefinite hold of the display. 6 500 MHz, 50 Ω This is the channel C input connector. 7, 14 OUTPUT MARKER These are the channel A and B Schmidt trigger outputs used to indicate when a channel is triggered by 0 to 300 mV levels with less than 20 nanoseconds delay. 8, 13 AC/DC These control the selection of ac or dc coupling for the input signal. When the input amplifier control switch (No. 18) is in COM A, channel B coupling is determined by setting of channel A coupling switch. 9, 12 ATTEN switches These select the attenuation of the input signal. The signal amplitude is reduced by 10 in ×10 and by 100 in ×100. When the input amplifier control switch (No. 18) is in COM A, the channel B attenuation is determined by the channel A attenuation switch. 10, 11 INPUT A and B These are the input BNC connectors for channels A and B. 15, 20 LEVEL A/B These controls are used in conjunction with the ATTEN (Nos. 9, 12) to select the voltage at which triggering will occur. With X1 attenuator selected, the level is variable at ±2.5 volts, and ×100 at ±250 volts. 16, 21 Channel A and B triggering lights The light will blink for the associated channel when triggering is occurring. When the light is off, the input signal is below triggering level and on when the signal is above triggering level. 17, 19 Channel A and B SLOPE switch These switches control the selection of triggering on either the positive or negative slope of the input signal. 18 COM A/SEP This is the input amplifier control switch that selects independent operation of channels A and B in SEP (separate) position. When in COM A (common A) position, the signal at A is also applied to channel B; this disconnects the channel B input circuitry. Channel B coupling and attenuation are then determined by the channel A setting. 22 OVERLOAD This indicator will flash on and off if more than 5 volts is applied to channel C input. 23 K, S, M, n, and Hz These will light up to show the appropriate units multiplier of the measurement being taken. 24 LED display This is a nine-digit LED display that shows the numerical measurement taken. 25 OVFL This indicates an overflow of one or more of the most significant digits (leftmost from the decimal point) are not displayed. 26 RMT Lights when the unit is in remote operation. 27 GATE Lights when the counter's main gate is open and a measurement is in progress.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-21 UNCLASSIFIED Table 5-2 Rear-Panel Controls and Connectors NO. NAME FUNCTION 1 VOLTAGE SELECTOR Used to select 115 or 230 volt operation. 2 LINE FUSE This requires the insertion of a 2.0 amp fuse for 115 volt or 1.0 amp fuse for 230 volt operation. 3 Input ac connector Used to connect the input ac to the unit. 4 ARM When this switch is in the OFF position, the counter is armed by the signal that is selected for measurement. In the ON position, the measurement is armed by an input other than the input being measured. 5 EXT OSC This input connector allows a separate outside signal to be used for the time base. 6 1 MHz OUT and 10 MHz OUT These connectors allow an internal oscillator signal to be used externally when connected. 7 GATE/MARKER OUT Applies a high to the output when the main gate is open. 8 HP-IB This is an interface bus connector that allows the unit to receive programming instructions.

5.4.1.2 Frequency Measurement As discussed previously, the model 5328A frequency counter is capable of measuring frequency, time period (inverse of frequency), ratio, and time interval. We will start with frequency. When the FUNCTION selector is in the FREQ A or FREQ C position, the counter measures the frequency, f, by accumulating the number of cycles, n, of the input signal that occurs over the time period, t. This is expressed by:

f = n t

The basic counter elements necessary to perform this measurement are shown in figure 5- 12. The INPUT AMPLIFIER/TRIGGER essentially conditions the input signal to a format that is compatible with the internal circuitry of the counter. As figure 5-12 indicates, the output of the amplifier/trigger corresponds directly to the input signal.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-22 UNCLASSIFIED

The TIME BASE OSCILLATOR is a 10-MHz temperature-controlled (oven-regulated) precision, crystal oscillator used for the time base element from which time, t, is derived. DECADE DIVIDERS take the time base oscillator signal as the input and provide a pulse train, whose frequency is variable in decade steps. This frequency can be controlled by the FREQ RESOLUTION, N switch. The time, t, is determined by the period of this pulse train.

The heart of the counter is the MAIN GATE. When the gate is opened, pulses from the amplifier/trigger are allowed to pass through. The opening and closing of the main gate is controlled by the decade divider output to the main gate flip-flop. The output of the MAIN GATE is then sent to the DECADE COUNTING ASSEMBLIES (DCAs), where the pulses are combined and displayed after the gate is closed.

If the FREQUENCY RESOLUTION, N selection switch, is set for 10 6, the main gate is open for 1 second, and the decade counting assemblies display the frequency of the input signal in hertz (refer to figure 5-10, FREQUENCY RESOLUTION, N selection switch).

Figure 5-12 Basic elements of the frequency counter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-23 UNCLASSIFIED 5.4.1.3 Period Measurement Period, the inverse of frequency, can be measured with the counter by reversing the inputs to the main gate. With the FUNCTION selector switch in the PER A position, the input signal controls the duration over which the main gate is open and the decade divider output is counted by the DCAs (see figure 5-13). The duration of the count is one cycle or period of the input signal. When the FUNCTION selector is in the PER AVG A position, the unused decades in the decade divider chain are used to divide the amplifier/trigger output so that the gate remains open for decade steps of each input period rather than a single period. This is the basis for multiple period averaging. Period and period averaging techniques are used to increase measurement accuracy on low-frequency measurements.

5.4.1.4 Ratio Measurement Placing the FUNCTION selector switch to RATIO C/A OR B/A sets the counter to measure the ratio of the signal frequency at channel C or B to the signal frequency at channel A. Using the same configuration as in figure 5-13 and replacing the time base with a second input frequency, f 2, you can measure the ratio of f 2/f. The signal at frequency f can be divided into decade steps in the same manner as multiple period averaging for higher resolution.

Figure 5-13 Measuring period

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-24 UNCLASSIFIED 5.4.1.5 Time Interval Measurement Figure 5-14 illustrates the configuration for the measurement of time between two events or time interval. This is done by placing the FUNCTION selector in the T.I.AVG AB position. The START input opens the main gate, and the STOP input closes it. The START input is applied to channel A, and the STOP input is applied to channel B. The decade divider output (clock pulses) is counted, and the display shows the elapsed time between START and STOP signals, as shown in figure 5-15.

Figure 5-14 Basic elements of a time interval counter

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-25 UNCLASSIFIED

5.4.1.6 Resolution The resolution of the measurement is determined by the frequency of the counted clock (for example, a 10-MHz clock provides 100 ns resolution [see figure 5-10, FREQUENCY RESOLUTION, N selection switch]). The input amplifier, main gate, and DCAs (elements of the time interval counter) must operate at speeds consistent with the clock frequency; otherwise the instrument's resolution would be meaningless. Clock frequencies of 1, 10, and 100 MHz, and other 10n frequencies, are preferred, since the accumulated count, with the appropriate placement of the decimal point, gives a direct readout of the time interval. This explains why the conventional time interval counter is presently limited to 10 nanoseconds, a clock frequency of 100 MHz. One GHz is beyond reach, and a clock frequency of 200 MHz would require some arithmetic processing of the accumulated count in the DCAs to enable time to be displayed directly.

Figure 5-15 Clock pulses

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-26 UNCLASSIFIED 5.4.1.7 Time Interval Averaging The time interval averaging technique is based on the fact that if the ±1 count error is truly random, it can be reduced by averaging a number of measurements. The words "truly random" are significant. For time interval averaging to work, the time interval must (1) be repetitive and (2) have a repetition frequency that is asynchronous to the instrument's clock. Under these conditions, the resolution of the measurement is:

resolution = ±1 count √N

where N = number of time intervals averaged

With averaging, resolution of a time interval measurement is limited only by the noise inherent in the instrument. The 5328A can obtain 10-picoseconds resolution. Most time interval averaging has one severe limitation: The clock period limits the minimum measurable time interval. With the FUNCTION selector switch in the T.I. AVG AB position, synchronizers are used to remove this limitation. These synchronizers enable the 5328A to measure intervals as short as 100 picoseconds.

Referring to figure 5-16, note that the input waveshape shows a repetitive time interval, which is asynchronous to the square wave clock. When these signals are applied to the main gate, with no synchronizers, an output similar to the third waveform results. Since the DCAs are designed to count at the clock frequency and are unable to accept pulses of shorter duration than the clock, the resulting counts accumulated in the DCAs will be in error, as shown in the fourth waveform. This problem is alleviated by the synchronizers, which are designed to detect leading edges of the clock pulses that occur while the gate is open. They detect and reconstruct the leading edges, making the pulses applied to the DCAs the same duration as the clock, as shown in the fifth waveform. Occasionally, when the input time interval repetition is synchronous with the internal clock, time interval averaging cannot be performed.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-27 UNCLASSIFIED

This ends our discussion on electronic frequency counters. Now, we'll study an area of electronics test equipment that is becoming more widespread and important each day - the testing of electronic logic components. A test instrument of value for any technician who works on digital equipment is the LOGIC PROBE, which is an integrated circuit- testing device.

Figure 5-16 Synchronizer operation with time interval averaging

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-28 UNCLASSIFIED 5.5 INTEGRATED CIRCUIT-TESTING DEVICES Digital integrated circuits are relatively easy to troubleshoot and test because of the limited numbers of input and output combinations involved in circuits. The two-state conditions in logic circuits are often referred to as (1) low or high, (2) on or off, or (3) one or zero (1 or 0).

Other terminology may also be used. Any particular integrated circuit (IC) can be tested by simply comparing it to a known good one. The LOGIC PROBE is a device that can be of great value in troubleshooting digital integrated logic circuits.

The ideal logic probe has the following characteristics:

• It will detect a steady logic level. • It will detect a train of logic levels. • It will detect an open circuit. • It will detect a high-speed transient pulse. • It will have over-voltage protection. • It will be small, light, and easy to handle.

The use of a suitable logic probe can greatly simplify your troubleshooting of logic levels through digital integrated logic circuitry. It can immediately show you whether a specific point in the circuit is low, high, open, or pulsing. Some probes have a feature that detects and displays high-speed transient pulses as small as 10 nanoseconds wide. These probes are usually connected directly to the power supply of the device being tested, although a few have internal batteries.

Most IC failures show up in a circuit as a constant high or low level. Because of this, logic probes provide a quick, inexpensive way of locating the fault. They can also display the single, short-duration pulse that is hard to detect on an oscilloscope. Figure 5-17 shows a basic logic probe.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-29 UNCLASSIFIED

The logic probe can be powered from the supply of the circuit under test or from a regulated dc power supply. If a separate power supply is used, the ground points of the power supply and circuit under test should be connected together.

The display LED (light-emitting diode) near the probe tip provides an immediate indication of the logic state existing in the circuit under test. The LED will provide any of four indications: (1) off, (2) dim (about one-half brilliance), (3) bright (full brilliance), and (4) flashing on and off. The LED is normally in the dim state and must be driven to one of the other three states by voltage levels at the probe tip. The LED is usually bright for inputs above the logic "1" threshold and off for inputs below the logic "0" threshold. The LED is usually dim for voltages between the logic "1" and logic "0" thresholds and for open circuits.

Figure 5-17 Basic logic probe

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-30 UNCLASSIFIED Q-7. The LED lamps of a typical logic probe are normally in what state? Another logic circuit analysis technique is useful with the logic probe. This technique is to run the circuit under test at its normal clock (timing) rate while monitoring for various control signals, such as RESET, START, STOP, SHIFT, TRANSFER, or CLOCK. Questions such as "Is the counter operating?" are quickly resolved by noting if the probe indicator is flashing on and off, indicating that pulse train activity is present.

This ends our discussion on logic probes. Now, we'll study another piece of electronics test equipment that is used in evaluating integrated circuits, the HUNTRON TRACKER 2000.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-31 UNCLASSIFIED 5.6 HUNTRON TRACKER 2000 The logic probe we just discussed is but one specialized tool used to isolate problems to the component level. Another device you can use is the Huntron Tracker 2000. It is a very versatile electronic troubleshooting tool that is used to evaluate suspect components and/or locate defective components on de-energized circuit cards quickly and safely without requiring the removal of component leads. The unit provides a built-in display that allows you to visually analyze the component under test conditions.

CAUTION Before connecting the Huntron Tracker 2000, you must first secure all power, then discharge all high-voltage capacitors.

5.6.1 Physical Features Because the Tracker 2000 has so many controls and indicators, it would impractical to cover each within this chapter. We will therefore concentrate our discussion only to the externally accessible features. To find information on internal controls and indicators, you should review the most current technical manual with up-to-date changes entered for the unit being used.

5.6.1.1 Front Panel The front-panel (figure 5-18) design allows you to easily select the desired function. All the push buttons are the momentary action type and have light-emitting diode (LED) indicators embedded in them to show the functions that are active, by lighting up when active. A detailed description of each item on the front panel is provided in table 5-3.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-32 UNCLASSIFIED

Table 5-3 Front-Panel Controls and Connectors NO. NAME DESCRIPTION 1 HORIZ control When adjusted this controls the horizontal position of the CRT display. 2 VERT control When adjusted, this controls the vertical position of the CRT display. 3 POWER on/off and INTENSITY control switch When this is rotated clockwise, the power is turned on. Further adjustment of the switch controls the intensity of the CRT display. When it is rotated fully counterclockwise, the power is turned off. 4 TRACE ROTATE control When adjusted, this controls the trace rotation of the CRT. 5 CRT display This displays the signatures produced by the unit. 6 RANGE selectors These are push buttons that are used to select one of four impedance ranges: LOW, MEDIUM 1, MEDIUM 2, and HIGH. 7 AUTO selector This push button, when selected, initiates automatic scanning of the four ranges from low to high. The speed of the scanning is determined by the RATE control (item 14) 8 CHANNEL A selector This push button, when selected, causes channel A to be displayed on the CRT. 9 CHANNEL A test plug This is a fused test lead connector that is active when channel A is selected. All test lead connectors accept standard banana plugs. 10 ALT selector When selected, this push button causes the unit to alternate between channel A and channel B. The speed of this is determined by the RATE control (item 14). Figure 5-18 Front Panel

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-33 UNCLASSIFIED NO. NAME DESCRIPTION 11 COM test plug This test lead connector is the instrument common to and the common reference point for channel A and channel B. 12 CHANNEL B selector This push button, when selected, causes channel B to be displayed on the CRT. 13 CHANNEL B test plug This is a fused test lead connector that is active when channel B is selected. 14 RATE control This controls the channel alteration and/or the range scanning. 15 G1 and G2 plugs These are used for the pulse generator output test leads. 16 WIDTH control This controls the duty cycle of the pulse generator. 17 LEVEL control This controls the amplitude of the internal pulse generator. 18 PULSE GENERATOR selectors These push buttons are used to select the output modes of the pulse generator: positive (+), negative (-) or PULSE/DC. 19 FREQUENCY selectors These push buttons are used to select one of the three test signal frequencies: 50/60 Hz, 400 Hz, 2000 Hz.

5.6.1.2 Cathode-Ray Tube (CRT) Display The CRT display (figure 5-19) is used to view the signature of the component under test. The display has a graticule consisting of horizontal and vertical axes. The horizontal axis is used to represent voltage with the vertical axis being used to represent current. The axes divide the display into four quadrants. Each quadrant displays a different portion of the signature for the component under test.

Figure 5-19 CRT display

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-34 UNCLASSIFIED • Quadrant 1 displays positive voltage and positive current. • Quadrant 2 displays negative voltage and positive current. • Quadrant 3 displays negative voltage and negative current. • Quadrant 4 displays positive voltage and negative current.

Q-8. On the CRT display, what information is displayed in Quadrant 4? The horizontal axis (see figure 5-19) is divided into eight equal divisions, allowing the technician to estimate the voltage at which changes occur in the signature for the component being tested. The associated approximate horizontal sensitivities for each range are:

• High = 15.0 Volts/Div. • Medium 2 = 5.0 Volts/Div. • Medium 1 = 3.75 Volts/Div. • Low = 2.5 Volts/Div.

5.6.1.3 Back Panel The back panel (figure 5-20) provides three additional controls and connectors. One is the accessory output connector (ACC), which provides a clock signal and power for the Huntron Switcher Model HSR410. Next, is the power cord connector used to provide the required ac line voltage used to power the unit. The last item found on the back panel is the FOCUS control; this controls the focus for the front-panel CRT display.

Figure 5-20 Back panel

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-35 UNCLASSIFIED 5.6.2 Operation The following sections explain how to use most of the front- and back-panel controls and connectors. Each control and connector has already been briefly described. To review this information, refer to figure 5-18 and table 5-3.

5.6.2.1 Initial Setup First, turn the POWER/INTENSITY knob located on the front panel to the clockwise, on, position. Under normal conditions, the unit will come on with the following LEDs illuminated: power, channel A, 50/60 Hz, low range, and pulse/DC.

Focusing the CRT is very critical for the technician to be able to properly analyze the signature being displayed. This is done by first turning the INTENSITY control to a level comfortable for the eye, and then adjusting the FOCUS control (back panel) for the narrowest possible trace.

Aligning the trace will help determine, during troubleshooting, which quadrant the portion of the signature is in during a change. With a short circuit applied to channel A (connect a cable between jack A and COM), adjust the TRACE ROTATE control until the trace is as close to parallel as possible to the vertical axis. Then, adjust the HORIZ (horizontal) control until the vertical trace is as close to even with the vertical axis as possible.

Q-9. When aligning the trace with a short applied to channel A, which control should be adjusted to bring the trace parallel to the vertical axis? With an Open applied to channel A (nothing connected to A and COM test plug), adjust the VERT (vertical) control until the horizontal trace is as close to even with the horizontal axis as possible. Once set, these adjustments should not need readjusting during the unit's operation. However, remember that each time the unit is used, this process will need to be repeated.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-36 UNCLASSIFIED 5.6.2.2 Range Selection Four impedance ranges (LOW, MED 1, MED 2, and HIGH) can be selected on the Tracker 2000. These ranges will become active when the appropriate front-panel button is pressed. To obtain the most useful signature display when troubleshooting a component, you should always start by selecting one of the two medium ranges (medium 1 or medium 2). If the display appears to be an open (horizontal trace), then the next higher range should be selected to get a more accurate signature for analysis. If the display appears to be a short (vertical trace), then the next lower range should be selected.

The AUTO feature, when selected, will allow the unit to automatically scan through the four ranges at a speed set by the technician using the RATE control knob. This allows the user to keep his or her hands free to hold test leads while still being able to observe the component under test signature for analysis. The HIGH LOCKOUT, when selected by the technician, prevents the unit from functioning in the HIGH range in either the manual or AUTO mode.

5.6.2.3 Channel Selection You can select two channels by pressing the channel A (test probes connected to A jack and COM) or channel B (test probes connected to B and COM jacks) push button on the front panel. When using a single channel, you should plug the red probe into the corresponding channel test jack, and plug the black test lead into the common test jack. When testing a component, you should connect the red probe to the positive terminal and the black probe to the negative terminal of the component under test. Following this procedure every time will ensure that the signature for the component under test will be displayed in the correct quadrants of the CRT display.

The ALT (alternate) mode provides automatic switching back and forth between channel A and channel B. This allows you to easily compare two components or the same test points on two circuit boards. You select the ALT mode by pressing the ALT push button on the front panel. The rate of switching between channels A and B can be varied by adjusting the RATE control knob on the front panel. You will find that the ALT mode feature is very useful for comparing a known good component with the same type of component that is of unknown quality.

Figure 5-21 shows a typical way of connecting the unit to a known good circuit board and a board under test. This test mode uses the supplied common test leads to connect two equivalent points on the boards to the common test jack. Note that the black probe is now being used in the channel B jack rather than the COM jack. When the technician uses the ALT and AUTO features together, each channel is displayed before the range selection will change. Figure 5-22 shows the sequence of these changes.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-37 UNCLASSIFIED

Figure 5-21 ALT (alternate) mode setup Figure 5-22 AUTO/ALT sequence

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-38 UNCLASSIFIED 5.6.2.4 Frequency Selection There are three test signal frequencies (50/60 Hz, 400 Hz, and 2000 Hz) that can be selected and then provided by pressing the appropriate front-panel push button. During most troubleshooting evolutions, the 50/60 Hz test signal is the best to start with. The 400 Hz and 2000 hz frequencies are generally used to view small amounts of capacitance or large amounts of inductance.

5.6.2.5 Pulse Generator A built-in pulse generator is also provided with the Tracker 2000. It allows the technician to do dynamic, in-circuit testing of certain devices in their active mode. In addition to using the red and black probes, you can connect the output of the pulse generator to the control input of the device to be tested with one of the blue micro clips provided with the unit. The pulse generator has two outputs (G1 and G2 jacks) so that three devices can also be tested in the ALT (alternate) mode. Figure 5-23 shows a way of connecting the unit in the ALT mode using the pulse generator.

There are a variety of output waveforms available using the pulse generator selection buttons, as shown in figure 5-24. First, the technician must select the PULSE mode or DC mode using the PULSE/DC button located on the front panel. In the PULSE mode, the PULSE/DC LED flashes at a slow rate. While in the DC mode, this LED will be continuously on.

Figure 5-23 Pulse generator comparison mode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-39 UNCLASSIFIED

Next, the technician needs to select the polarity of the output desired by using the positive (+) and/or negative (−) push buttons. All three buttons only function in a push-on/push- off mode and will only interact with each other to avoid the NOT ALLOWED state found in figure 5-24.

Once the specific output type has been selected, the output desired by the technician is set using the LEVEL and WIDTH controls. The LEVEL control is used to vary the magnitude of the output amplitude from zero to 5 volts (peak or DC). During the PULSE mode, the WIDTH control will adjust the cycle of the pulse output from low to 50 percent maximum (square wave).

The start of a pulse will be triggered by the appropriate zero crossing of the test signal, which results in the pulse frequency being equal to the selected test signal frequency.

The end of the pulse is determined by the WIDTH control setting, which determines the cycle length. The WIDTH control, however, has no effect when the DC mode is selected.

Figure 5-24 Pulse generator selector chart

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-40 UNCLASSIFIED 5.6.3 Functional Overview There are six major sections in the Tracker 2000, as shown in the block diagram in figure 5-25. The control logic section controls the selection of the channel, frequency, impedance range, and pulse generator mode according to the front-panel buttons pushed by the technician. The oscillator provides the test signal that is used by the signal and pulse generator sections. In the signal section, the test jacks are driven by the test signal, while signal conditioners monitor the jacks and produce the horizontal and vertical signals used by the CRT section to produce a component signature on the display. The pulse generator provides an added source for testing three additional terminal devices. The power supply produces all the required voltages needed to operate the Tracker 2000.

Figure 5-25 Tracker 2000 block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-41 UNCLASSIFIED 5.6.3.1 Control Logic The control logic senses which button is pushed on the front panel. Since the buttons are the momentary action type, the logic must remember what button was pushed, turn on the LED indicator within the button, and activate the appropriate configuration of the oscillator, signal, and pulse generator sections.

After a button is pushed, the unit will remain in that configuration until another selection is made or the power is secured. The HIGH LOCKOUT and PULSE GENERATOR push buttons are the only ones that require repetitive pressing to be turned on or off.

The channel relay is controlled by CHANNEL buttons A, ALT, and B. The relay is a single-pole, double-throw type and is de-energized for channel A and energized for channel B. If CHANNEL A is already selected and the CHANNEL B button is pressed, CHANNEL A will be canceled and CHANNEL B selected. When the ALT button is pressed, another control line is set, which enables an internal clock to toggle the channel relay on and off, causing the unit to alternate between channels A and B.

The internal clock is controlled by the RATE control knob on the front panel. When the ALT mode is active, the LEDs within the A and B buttons flash alternately, and the ALT LED is on continuously. Pressing either of the channel buttons will cancel the ALT mode, and the selected channel will then become active.

The FREQUENCY buttons (50/60, 400, and 2000 Hz) directly control the operation of the oscillator and the pulse generator. The RANGE buttons (LOW, MED 1, MED 2, and HIGH) control four relays in the signal section that select the appropriate terminal characteristics for each impedance range (table 5-4).

Table 5-4 Terminal Characteristics for Impedance ranges RANGE OPEN CIRCUIT VOLTAGE (Vp) SHORT CIRCUIT CURRENT (mAr) High 60 0.57 Medium 2 20 0.53 Medium 1 15 8.5 Low 10 132

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-42 UNCLASSIFIED You can select the four ranges manually by pressing the RANGE button, or you can scan them automatically by using the AUTO function. When AUTO is activated, the control logic will follow the sequence LOW, MED 1, MED 2, and HIGH over and over if HIGH LOCKOUT is off. The current active range is always indicated by the LED for the range selected. The AUTO mode will stay active until you select a particular range by pressing its associated button. While AUTO is active, the AUTO LED is continuously on.

The speed at which the ranges are scanned is controlled by the front-panel RATE control knob. This allows you to adjust the time each range is displayed for signature analysis on the CRT display. If AUTO and ALT (alternate) are active at the same time, the RATE control affects the speed of both functions with ALT having priority. This is done so that the two channels can be compared to each other within one range before the next range is selected (figure 5-26).

The HIGH LOCKOUT function disables the HIGH range and limits the maximum test signal to 20 volts peak vice 60 volts peak. When you select the manual mode (AUTO off), activating the HIGH LOCKOUT prevents the HIGH range from being selected. If the HIGH range is active when the HIGH LOCKOUT is pressed, the HIGH range is canceled and the next lower range (MED 2) will be selected and become active. When you select the AUTO mode, the RANGE sequence with the HIGH LOCKOUT active will start with LOW, and sequence through MED 1, MED 2, back to LOW, and continue until the AUTO mode is stopped.

The PULSE GENERATOR buttons (positive [+], negative [−], and PULSE/DC) toggle control lines that change the polarity and output type of the pulse generator.

Figure 5-26 Range scanning sequence with AUTO and ALT active

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-43 UNCLASSIFIED 5.6.3.2 Oscillator The oscillator produces a constant amplitude, low distortion sine wave test signal. The frequency of the test signal is programmable between one variable frequency (50/60 Hz), and two fixed frequencies (400 Hz and 2000 Hz). The variable frequency depends on the input power line used for the Tracker 2000; a 50-Hz line produces a 50-Hz test signal, and a 60-Hz line produces a 60-Hz test signal. If a 400-Hz power line is used, an 80-Hz test signal is provided. This versatility is built in to ensure you will always have low, medium, and high frequencies to work with.

5.6.3.3 Signal Section The signal section is considered to be the heart of the Tracker 2000. In this section, the signal from the oscillator is applied across two terminals of a device being tested via the front-panel jacks.

The test signal causes a current to flow through the device and a voltage drop to occur across its terminals. The current flow causes a vertical deflection of the trace on the CRT display, while the voltage drop across the device causes a horizontal deflection of the trace on the CRT display. By combining these, the current-voltage signature of the device being tested is displayed on the CRT.

The test signal appears at the front-panel test jacks as though it is being originated by a voltage source (V s) with a series output impedance (Zs). An equivalent circuit of the signal section is shown in figure 5-27. As you can see, the figure also shows how the terminal voltage affects the horizontal deflection plates of the CRT, and how the current through the terminals affects the vertical deflection plates through current sensing point I.

Figure 5-27 Signal section equivalent circuit

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-44 UNCLASSIFIED An open circuit has zero current flowing through the terminals and maximum voltage drop across the terminals. In all ranges, this is represented by a straight horizontal trace from left to right on the CRT display, as seen in figure 5-28 view A. When a short occurs, maximum current flows through the terminals, and the voltage drop is considered to be zero. This occurs in all ranges and is represented by a straight vertical trace from top to bottom of the CRT display, as seen in figure 5-28 view B.

5.6.3.4 Pulse Generator The pulse generator provides dynamic testing for certain types of devices by driving the control input of the device under test. The normal two-terminal mode of testing can be considered a static test, since devices with three or more terminals are not tested in their active mode. However, with the pulse generator, an in-circuit active test is possible.

In the PULSE mode, this circuit uses the zero crossing of the test signal to trigger the start of the pulse. When positive (+) is enabled, a positive-going zero crossing triggers a positive pulse. When negative (−) is enabled, a negative-going zero crossing triggers a negative pulse. If both are enabled, then both positive and negative pulses are produced on alternate crossings (composite pulses). Once a pulse is triggered, its duration is set by the WIDTH control knob. Figure 5-29 shows the waveforms for three pulse polarity types at various settings of the WIDTH control.

Figure 5-28 Open and short circuit display

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-45 UNCLASSIFIED

The LEVEL control adjusts the peak of each pulse from zero to 5 volts with the polarity dependent on the pulse polarity selected. When an open circuit is present, a maximum output of 5 volts peak-to-peak is present with either positive or negative selected, and 10 volts peak-to-peak when the composite pulse is active.

Q-10. What minimum/maximum voltage level can be attained in the pulse generator section by adjusting the LEVEL control? In the DC mode, a zero-to-5-volts DC level is produced at G1 and G2 on the front panel. The polarity of the level is controlled by the positive and negative buttons. By pressing the positive button, you enable a positive DC output and disable the negative button. By pressing the negative button, you enable a negative DC voltage only if the positive button is in the off position. The WIDTH control knob has no effect in the DC mode of operation.

Figure 5-29 Pulse generator waveforms

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-46 UNCLASSIFIED 5.6.3.5 CRT Display The CRT deflection drivers boost the low-level outputs from the signal section to the higher voltage levels needed by the deflection plates in the CRT. The HORIZ (horizontal) and VERT (vertical) controls on the front panel adjust the position of the CRT trace. The TRACE ROTATE control on the front panel is used to adjust the short circuit vertical trace to be parallel with the vertical axis on the CRT graticule.

Three other controls (INTENSITY, FOCUS, and astigmatism) are used to adjust the proper brightness and clarity of the trace. The front-panel INTENSITY control is the primary way to adjust the visual quality of the trace. FOCUS is located on the back panel and is used as your trimming adjustment. Astigmatism is an internal adjustment and is set at the factory.

5.6.3.6 Power Supply The power supply is an ac-line-operated power supply that is turned on and off by the POWER/INTENSITY knob located on the front panel. Once power is turned on, the power supply provides 12 V dc (nominal) and ±5 V dc (regulated) for normal circuit operational use in the oscillator, pulse generator, signal, and control logic sections of the Tracker 2000.

The other outputs from the power supply are provided to the CRT display section. The CRT is provided with a filament voltage of 6.3 V rms, +180 V dc for the deflection driver circuits, and a regulated −1320 V dc for the CRT acceleration voltage.

5.6.4 Component Testing Now that you have a basic understanding of how the Tracker 2000 works, we will show you a few examples of different components with values and their associated displays. Because of the large number of different values that can be given to any component, this section will present only a few.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-47 UNCLASSIFIED 5.6.4.1 Testing Resistors A resistance across the test probes will cause the trace of the Tracker 2000 to rotate in a counter-clockwise direction around its center axis from an open circuit position. The degree of rotation is directly related to the resistance value. The higher the value, the less rotation will be observed.

LOW RANGE - The low range is designed to test for resistance values between 1Ω and 1K. Figure 5-30 shows the effect of resistance on the angle of rotation in low range. A 1Ω resistor causes almost 90º of rotation, and a 50Ω resistor produces a 45º rotation. A 400Ω resistor causes a very small rotation angle. Resistors lower than 1Ω will appear as a short circuit (vertical trace), and resistance values above 400Ω will look like an open circuit (horizontal trace).

Figure 5-30 Effects of resistance on the rotation angle in low range

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-48 UNCLASSIFIED MEDIUM 1 RANGE - The medium 1 range is designed to test for resistance values between 50Ω and 10KΩ. Figure 5-31 shows the signatures for a 50Ω resistor, a 1KΩ resistor, and a 10KΩ resistor using the medium 1 range. Resistors that are smaller than 50Ω display a signature that is almost a vertical line. A 1KΩ resistor causes a change in the angle of rotation of 45º, whereas the display of a 10KΩ resistor shows only a slight rotation. Resistance values under test higher than 10KΩ produce a signature with such a small rotation angle that it almost appears to be a horizontal line.

Q-11. Medium 1 range is designed to check what resistance values?

Figure 5-31 Effects of resistance on the rotation angle in medium 1 range

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-49 UNCLASSIFIED MEDIUM 2 RANGE - The medium 2 range is designed to test for resistance values between 1KΩ and 200KΩ Figure 5-32 shows the signatures for a 1KΩ resistor, a 15KΩ resistor, and a 200KΩ resistor in the medium 2 range. Resistance values that are smaller than 1KΩ will appear to be almost a vertical line. A 15KΩ resistor causes a change in the angle of rotation of 45º, whereas the display for a 200KΩ resistor shows only a slight rotation. When resistance values being tested are higher than 200KΩ, the displayed signature that they produce will have such a small rotation that it appears to be almost a horizontal line.

HIGH RANGE - The high range is designed to test resistance values between 3KΩ and 1MΩ. Figure 5-33 shows the signatures that would be displayed for a 3KΩ resistor, a 50KΩ resistor, and a 1MΩ resistor using the high range. Resistors that are smaller than 3K_ will appear to be almost a vertical line. A 50KΩ resistor will cause a change in the angle of rotation of 45º, whereas the display for a 1MΩ resistor shows only a slight rotation that is very close to a horizontal line. Resistance values higher than 1MΩ will produce such a small rotation that it appears to be a horizontal line.

Figure 5-32 Effects of resistance on the rotation angle in medium 2 range Figure 5-33 Effects of resistance on the rotation angle in high range

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-50 UNCLASSIFIED 5.6.4.2 Testing Capacitors When you test capacitors, the signature will be displayed as an ellipse. The size and shape of the ellipse depend on the capacitor value, test signal frequency, and the selected impedance range. Figure 5-34 shows the signature of a 0.22F capacitor in each of the 12 combinations of range and frequency. As you review this figure, you will notice that the signature appears to be an open circuit in the low range at 60 Hz; while in the high range at 2000 Hz, the signature appears to be a short. Between these, the signatures displayed are a variety of ellipsoids, which demonstrates that certain range and frequency combinations are better than others for examining a capacitor.

Figure 5-34 Signature of a 0.22μF capacitor

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-51 UNCLASSIFIED Table 5-5 lists the range of capacitance covered by each of the 12 range and frequency combinations for the Tracker 2000. The lowest value of capacitance in each combination will produce a signature of a very narrow horizontal ellipsoid. Capacitors with less of a value than these will appear to be an open. The uppermost value of capacitance in each combination will produce a very narrow vertical ellipsoid signature. Capacitors of greater value than these will appear as a vertical line signature of a short circuit.

Table 5-5 Min/Max Capacitance Values RANGE TEST FREQUENCY 50/60 Hz 400 Hz 2000 Hz HIGH .001µF-1µF 500pF-.1µF 100pF-.02µF MEDIUM 1 .01µF-2µF .001µ-.5µF 200pF-.05µF MEDIUM 2 .2µF-50µF .02µF-5µF .005µ-1µF LOW 5µF-2000µF .5mF-100µF .2mF-25µF

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-52 UNCLASSIFIED 5.6.4.3 Testing Inductors Inductors, like capacitors, produce an elliptical signature on the Tracker 2000. Figure 5- 35 shows you the signatures produced in each of the 12 range and frequency combinations by a 250mH inductor.

Figure 5-35 Signatures of a 250mH inductor

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-53 UNCLASSIFIED Ferrite inductors can also be checked using this unit; however, the signature produced will be different. Ferrite inductors operate well at high frequencies, but saturate at low frequencies. Figure 5-36 shows the signature of a 490mH ferrite inductor tested at 60 Hz. In low and medium 1, you can see that the signature shows distortion. However, in medium 2 and high ranges, the impedance of the inductor is low compared with the internal impedance of the Tracker 2000, so the signatures are a split vertical trace. Figures 5-37 and 5-38 show the same 490mH inductor being tested at 400 Hz and 2000 Hz.

Figure 5-36 Signatures of a 490mH ferrite inductor tested at 60 Hz.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-54 UNCLASSIFIED

Figure 5-37 Signature of a 490mH ferrite inductor at 400 Hz. Figure 5-38 Signature of a 490mH ferrite inductor at 2000 Hz.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-55 UNCLASSIFIED 5.7 SUMMARY Now that we have completed this chapter, let's review the more important points.

The type of meter used to measure power is the POWER METER.

One type of wattmeter is the AN/URM-120. It is an IN-LINE TYPE WATTMETER.

The principal function of a SIGNAL GENERATOR is to produce an alternating voltage of the desired frequency and amplitude, which has the necessary modulation for the test or measurement concerned.

There are basically two types of signal generators: AF and RF FREQUENCY GENERATORS.

The instrument used to determine the frequency of a signal is the FREQUENCY COUNTER.

An instrument of great value to a technician in troubleshooting digital integrated logic circuits is the LOGIC PROBE.

The HUNTRON TRACKER 2000 is a versatile piece of test equipment that is used to compare known good devices against those of unknown quality or troubleshoot to the component level after power is disconnected to the device under test.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

5-56 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q11.

A-1. Load.

A-2. An unbalance in the metering bridge.

A-3. Attenuator.

A-4. Oscillator circuit, modulator, and output control circuit.

A-5. To produce an af (or video) signal that can be superimposed on the rf signal produced by the oscillator.

A-6. 1 MHz and 10 MHz.

A-7. DIM.

A-8. Positive voltage (+V) and negative current (-I).

A-9. TRACE ROTATE control.

A-10. 0 to 5 volts.

A-11. 50 Ω to 10KΩ.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-1 UNCLASSIFIED 6 THE OSCILLOSCOPE AND SPECTRUM ANALYZER LEARNING OBJECTIVES

Upon completing this chapter, you should be able to:

1. Describe the purpose of the CRT used in the oscilloscope. 2. Explain the operation of an oscilloscope. 3. Describe the purpose of the controls and indicators found on an oscilloscope. 4. Describe the proper procedure for using a dual-trace oscilloscope. 5. Describe the accessory probes available for use with a dual-trace oscilloscope. 6. Explain the operation of the spectrum analyzer. 7. Describe the purpose of the controls and indicators found on the spectrum analyzer.

6.1 INTRODUCTION One of the most widely used pieces of electronic test equipment is the OSCILLOSCOPE. An oscilloscope is used to show the shape of a video pulse appearing at a selected equipment test point. Although some oscilloscopes are better than others in accurately showing video pulses, all function in fundamentally the same way. If you learn how one oscilloscope operates, you will be able to learn others.

As you will learn in this chapter, there are many different types of oscilloscopes - varying in complexity from the simple to the complex. Before we get into our discussion of the dual-trace oscilloscope, we will first present a general overview of basic single-trace oscilloscope operation. Shortly, we will see how oscilloscopes use a CATHODE-RAY TUBE (CRT) in which controlled electron beams are used to present a visible pattern of graphical data on a fluorescent screen.

Another piece of test equipment used is the SPECTRUM ANALYZER. This test equipment is used to sweep over a band of frequencies to determine what frequencies are being produced by a specific circuit under test, and then the amplitude of each frequency component. An accurate interpretation of the display will allow you to determine the efficiency of the equipment being tested.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-2 UNCLASSIFIED 6.2 CATHODE-RAY TUBES A detailed discussion of CATHODE-RAY TUBES (CRTs) is presented in NEETS, Module 6, Electronic Emission, Tubes, and Power Supplies. Before continuing with your study of CRTs in this section, you may want to review chapter 2 of that module.

Cathode-ray tubes used in oscilloscopes consist of an ELECTRON GUN, a DEFLECTION SYSTEM, and a FLUORESCENT SCREEN. All of these elements are enclosed in the evacuated space inside the glass CRT. The electron gun generates electrons and focuses them into a narrow beam. The deflection system moves the beam horizontally and vertically across the screen. The screen is coated with a phosphorous material that glows when struck by the electrons. Figure 6-1 shows the construction of a CRT.

6.2.1 Electron Gun The ELECTRON GUN consists of a HEATER and a CATHODE to generate electrons, a CONTROL GRID to control brightness by controlling electron flow, and two ANODES (FIRST and SECOND). The main purpose of the first (FOCUSING) anode is to focus the electrons into a narrow beam on the screen. The second (ACCELERATING) anode accelerates the electrons as they pass. The control grid is cylindrical and has a small opening in a baffle at one end. The anodes consist of two cylinders that contain baffles (or plates) with small holes in their centers.

Figure 6-1 Construction of a CRT

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-3 UNCLASSIFIED Q-1. What element controls the number of electrons striking the screen? Q-2. What element is controlled to focus the beam? 6.2.1.1 Cathode and Control Grid As in most conventional electron tubes, the cathode is indirectly heated and emits a cloud of electrons. The control grid is a hollow metal tube placed over the cathode. A small opening is located in the center of a baffle at the end opposite the cathode. The control grid is maintained at a negative potential with respect to the cathode to keep the electrons bunched together.

A high positive potential on the anodes pulls electrons through the hole in the grid. Because the grid is near the cathode, it can control the number of electrons that are emitted. As in an ordinary electron tube, the negative voltage of the grid can be varied either to control electron flow or stop it completely. The brightness (intensity) of the image on the fluorescent screen is determined by the number of electrons striking the screen. This is controlled by the voltage on the control grid.

6.2.1.2 Electrostatic Lenses and Focusing The electron beam is focused by two ELECTROSTATIC FIELDS that exist between the control grid and first anode and between the first and second anodes.

Figure 6-2 shows you how electrons move through the electron gun. The electrostatic field areas are often referred to as LENSES because the fields bend electron streams in the same manner that optical lenses bend light rays. The first electrostatic lens cause the electrons to cross at the first focal point within the field. The second lens bend the spreading streams and return them to a new, second focal point at the CRT.

Q-3. Why are the electrostatic fields between the electron gun elements called lenses?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-4 UNCLASSIFIED

Figure 6-2 also shows the relative voltage relationships on the electron-gun elements. The cathode (K) is at a fixed positive voltage with respect to ground. The grid is at a variable negative voltage with respect to the cathode. A fixed positive voltage of several thousand volts is connected to the second (accelerating) anode. The potential of the first (focusing) anode is less positive than the potential of the second anode. The first anode can be varied to place the focal point of the electron beam on the screen of the tube. Control-grid potential is established at the proper level to allow the correct number of electrons through the gun for the desired image intensity.

Q-4. What is the function of the second anode?

Figure 6-2 Formation of an electron beam

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-5 UNCLASSIFIED 6.2.2 Electron Beam-Deflection System The electron beam is developed, focused, and accelerated by the electron gun. The beam appears on the screen of the CRT as a small, bright dot. If the beam is left in one position, the electrons will soon burn away the illuminating coating in that one area. To be of any use, the beam must be able to move. As you have studied, an electrostatic field can bend the path of a moving electron.

As you have seen in the previous illustrations, the beam of electrons passes through an electrostatic field between two plates. You should remember that electrons are negatively charged and that they will be deflected in the direction of the electric force (from negative to positive). This deflection causes the electrons to follow a curved path while in the electrostatic field.

When the electrons leave the electrostatic field, they will take a straight path to the screen at the angle at which they left the field. Because they were all deflected equally, the electrons will be traveling toward the same spot. Of course, the proper voltages must exist on the anodes to produce the electrostatic field. Changing these voltages changes the focal point of the beam and causes the electron beam to strike the CRT at a different point.

6.2.2.1 Factors Influencing Deflection The ANGLE OF DEFLECTION (the angle the outgoing electron beam makes with the CRT center line axis between the plates) depends on the following factors:

• Length of the deflection field; • Spacing between the deflection plates; • The difference of potential between the plates; and • The accelerating voltage on the second anode.

LENGTH OF DEFLECTION FIELD - As shown in figure 6-3, a long field (long deflection plates) has more time to exert its deflecting forces on an electron beam than does a shorter field (short deflection plates). Therefore, the longer deflection plates can bend the beam to a greater deflection angle.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-6 UNCLASSIFIED

Q-5. What effect do longer deflection plates have on the electron beam? SPACING BETWEEN PLATES - As shown in figure 6-4, the closer together the plates, the more effect the electric force has on the deflection angle of the electron beam.

Figure 6-3 Factors influencing length of field Figure 6-4 Spacing between plates

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-7 UNCLASSIFIED DIFFERENCE OF POTENTIAL - The potential on the plates (figure 6-5) can be varied to cause a wider or narrower deflection angle. The greater the potential, the wider the deflection angle.

Q-7. Is the deflection angle greater with higher or lower potential on the plates? BEAM ACCELERATION - The faster the electrons are moving, the smaller their deflection angle will be, as shown in figure 6-6.

Q-8. Is the deflection angle greater when the beam is moving faster or slower?

Figure 6-5 Differences of potential Figure 6-6 Beam acceleration

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-8 UNCLASSIFIED 6.2.2.2 Vertical and Horizontal Plates If two sets of deflection plates are placed at right angles to each other inside a CRT (figure 6-7), the electron beam can be controlled in any direction. By varying the potential of the vertical-deflection plates, you can make the spot (beam) on the face of the tube move vertically. The distance the beam moves will be proportional to the change in potential difference between the plates. Changing the potential difference between the horizontal-deflection plates will cause the beam to move a given distance from one side to the other. Directions other than up-down and left-right are achieved by a combination of horizontal and vertical movement.

As shown in figure 6-8, position X of the beam is in the center. It can be moved to position Y by going up 2 units and then right 2 units. Movement of the beam is the result of the simultaneous action of both sets of deflection plates. The electrostatic field between the vertical plates moves the electrons up an amount proportional to 2 units on the screen. As the beam passes between the horizontal plates, it moves to the right an amount proportional to 2 units on the screen.

Figure 6-7 Deflection plate arrangement

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-9 UNCLASSIFIED

If the amount of deflection from the left and down occurred so that each set of plates acted at the same time, the picture would be like the one in view A of figure 6-9. For example, if the vertical plates moved the beam downward (starting from point X) at the rate of 3 units per second and the horizontal plates moved it to the left at the rate of 1 unit per second, both movements would have been completed in 1 second at point Y. The result would be a straight line.

Figure 6-8 Beam movement on the CRT Figure 6-9 Deflection of the beam

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-10 UNCLASSIFIED In view B, the potentials on the vertical and horizontal plates change at the same rate. In the same time period, say 1 second, both plates move the beam 1 unit. The horizontal plates have completed their task at the end of 1 second, but the vertical plates have moved the beam only one-third of the required distance. In this case, the picture in view B would appear on the screen.

6.2.2.3 Beam-deflection Plate Action Recall from your study of chapter 2 of this module that waveforms are described in terms of amplitude versus time. You have just seen how the movement of the CRT beam depends on both potential (amplitude) and time.

Q-9. Waveforms are described in terms of what two functions? VERTICAL-DEFLECTION PLATES - We will use figure 6-10 to explain the action of the vertical-deflection plates in signal amplitude measurements. As this discussion begins, remember that vertical-deflection plates are used to show amplitude of a signal, and horizontal-deflection plates are used to show time and/or frequency relationships.

Figure 6-10 Amplitude versus time

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-11 UNCLASSIFIED 1. From T0 to T1, the vertical plates maintain their static difference in potential and the beam stays at 0 units; the T0 to T1 change causes an increasing potential difference in the horizontal plates, and the beam moves 1 unit to the right.

2. At T1, a positive potential difference change in the vertical plates occurs, which causes the beam to move up (instantaneously) 2 units. This vertical (amplitude) beam location is maintained from T1 to T4; horizontal beam movement continues moving to the right as 3 units of time pass.

3. At T4, an instantaneous negative change in potential of 4 units in amplitude occurs, and the beam moves from +2 to -2 units.

4. From T4 to T7, the beam remains at -2 units. During this time period, the beam continues moving horizontally to the right, indicating the passage of time.

5. At T7, a positive increase of amplitude occurs, and the beam moves vertically from -2 to 0 units. From T7 to T8, no change occurs in vertical beam movement; however, horizontal movement continues with time.

The vertical-plate potential difference follows the voltage of the waveform. The horizontal-plate potential follows the passage of time. Together, they produce the image (trace) produced on the screen by the moving beam.

Q-10. The vertical-deflection plates are used to reproduce what function? Q-11. The horizontal-deflection plates are used to produce what function? HORIZONTAL-DEFLECTION PLATES - Now let's look at horizontal-deflection action. Assume that the resistance of the potentiometer shown in figure 6-11 is spread evenly along its length. When the arm of the potentiometer is at the middle position, the same potential exists on each plate. Since there is zero potential difference between the plates, an electrostatic field is not moved downward at a uniform rate; the right plate will become more positive than the left (you are looking down through the top of the CRT). The electron beam will move to the right from screen point 0 through points 1, 2, 3, and 4 in equal time intervals.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-12 UNCLASSIFIED

If the potentiometer arm is moved at the same rate in the opposite direction, the right plate will decrease in positive potential until the beam returns to the 0 position. At that point, the potential difference between the plates is again zero. Moving the arm toward the other end of the resistance causes the left plate to become more positive than the right, and the beam moves from screen points 0 through 4. If the movement of the potentiometer arm is at a uniform (linear) rate, the beam moves at a uniform rate.

Notice that the ends of the deflection plates are bent outward to permit wide-angle deflection of the beam. The vertical plates are bent up and down in the same manner.

Q-12. Why are the ends of the deflection plates bent outward? For ease of explanation, the manual movement of the potentiometer arm is satisfactory to introduce you to horizontal beam movement. However, in the oscilloscope this is not how horizontal deflection is accomplished. Beam movement voltages are produced much faster by sawtooth circuitry. You may want to review the sawtooth generation section in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuitry before continuing. Nearly all oscilloscopes with electrostatic deflection apply a sawtooth voltage to the horizontal plates to produce horizontal deflection of the beam, as shown in figure 6-12.

Figure 6-11 Horizontal plates (top view)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-13 UNCLASSIFIED

In the figure, the sawtooth generator replaces the potentiometer and is connected to both horizontal plates of the CRT. At the reference line, the potential on both plates is equal. Below the line, the left plate is more positive and the right plate is less positive. This causes the beam to move left. Above the line, the right plate is made more positive than the left and the beam moves to the right. The waveform amplitude causes a uniform movement of the beam across the screen (called TRACE). RETRACE time, shown at the trailing edge of the waveform, quickly deflects the beam back to the starting point.

6.2.3 CRT Graticule A GRATICULE was used in our previous discussion in figure 6-10. It is simply a calibrated scale (made of clear plastic) of amplitude versus time that is placed on the face of the CRT.

The graticule can be used to determine the voltage of waveforms because the DEFLECTION SENSITIVITY of a CRT is uniform throughout the vertical plane of the screen. Deflection sensitivity states the number of inches, centimeters, or millimeters a beam will be deflected for each volt of potential difference applied to the deflection plates. It is directly proportional to the physical length of the deflection plates and their distance from the screen and inversely proportional to the distance between the plates and to the second-anode voltage. Deflection sensitivity is a constant that is dependent on the construction of the tube.

Deflection sensitivity for a given CRT might typically be 0.2 millimeters per volt. This means the spot on the screen will be deflected 0.2 millimeters (about 0.008 inch) when a difference of 1 volt exists between the plates. Sometimes the reciprocal of deflection sensitivity (called DEFLECTION FACTOR) is given. The deflection factor for the example given would be 125 volts per inch (1/0.008).

Figure 6-12 Sawtooth generator

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-14 UNCLASSIFIED Q-13. What term is used to describe the reciprocal of deflection sensitivity of a scope? In the above example, 125 volts applied between one set of plates would deflect the beam 1 inch on the screen. This means that the deflection caused by small signals would likely not be observed. For this reason, the deflection plates are connected to amplifiers that magnify the signals applied to the vertical input of the scope.

Assume, for example, that a peak-to-peak value of a known voltage applied to the oscilloscope indicates that each inch marking on the graticule is equal to 60 volts. Each of the 10 subdivisions will, therefore, equal a value of 6 volts. Most oscilloscopes have ATTENUATOR controls to decrease or GAIN controls to increase the strength of a signal before it is placed on the deflection plates. Attenuator and gain settings must not be disturbed after the calibration has been made. For maximum accuracy, you should recalibrate the graticule each time a voltage is to be measured.

6.2.4 CRT Designations Cathode-ray tubes are identified by a tube number, such as 2AP1, 2BP4, or 5AP1A. The first number identifies the diameter of the tube face. Typical diameters are 2 inches, 5 inches, and 7 inches. The first letter designates the order in which a tube of a given diameter was registered. The letter-digit combination indicates the type of phosphor (glowing material) used on the inside of the screen. Phosphor P1, which is used in most oscilloscopes, produces a green light at medium PERSISTENCE. Persistence refers to the length of time the phosphor glows after the electron beam is removed. P4 provides a white light and has a short persistence. If a letter appears at the end, it signifies the number of the modification after the original design.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-15 UNCLASSIFIED 6.3 OSCILLOSCOPE CONTROL COMPONENTS Although the CRT is a highly versatile device, it cannot operate without control circuits. The type of control circuits required depends on the purpose of the equipment in which the CRT is used.

There are many different types of oscilloscopes. They vary from relatively simple test instruments to highly accurate laboratory models. Although oscilloscopes have different types of circuits, most can be divided into the basic sections shown in figure 6-13: (1) a CRT, (2) a group of control circuits that control the waveform fed to the CRT, (3) a power supply, (4) sweep circuitry, and (5) deflection circuitry.

Q-14. List the circuits that all oscilloscopes have in common.

Figure 6-13 Block diagram of an oscilloscope

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-16 UNCLASSIFIED Figure 6-14 is a drawing of the front panel of a dual-trace, general-purpose oscilloscope. Oscilloscopes vary greatly in the number of controls and connectors. Usually, the more controls and connectors, the more versatile the instrument. Regardless of the number, all oscilloscopes have similar controls and connectors. Once you learn the fundamental operation of these common controls, you can move with relative ease from one model of oscilloscope to another. Occasionally, controls that serve similar functions will be labeled differently from one model to another. However, you will find that most controls are logically grouped and that their names usually indicate their function.

The oscilloscope in figure 6-14 is called DUAL-TRACE because it can accept and display two vertical signal inputs at the same time - usually for comparison of the two signals or one signal and a reference signal. This scope can also accept just one input. In this case, it is used as a SINGLE-TRACE OSCILLOSCOPE. For the following discussion, we will consider this to be a single-trace oscilloscope. The oscilloscope in the figure is commonly used in the fleet. You are likely to use this one (model AN/USM- 425) or one very similar to it. Let's now look at the front panel controls.

Figure 6-14 Dual-trace oscilloscope

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-17 UNCLASSIFIED 6.3.1 Components Used to Display the Waveform The CRT DISPLAY SCREEN is used to display the signal (figure 6-15). It allows you to make accurate measurements using the vertical and horizontal graticules, as discussed earlier.

Figure 6-15 CRT display and graticule

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-18 UNCLASSIFIED 6.3.2 Components Used to Adjust CRT Display Quality The controls in figure 6-16 allow you to adjust for a clear signal display. They also allow you to adjust the display position and magnify the horizontal trace by a factor of 10 (X10). Keep in mind that the controls may be labeled differently from one model to another, depending on the manufacturer. Refer to figure 6-16 as you study the control descriptions in the next paragraphs.

6.3.2.1 INTEN (Intensity) Control The INTEN (intensity) control (sometimes called BRIGHTNESS) adjusts the brightness of the beam on the CRT. The control is rotated in a clockwise direction to increase the intensity of the beam and should be adjusted to a minimum brightness level that is comfortable for viewing.

Figure 6-16 Quality adjustment for CRT display

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-19 UNCLASSIFIED 6.3.2.2 FOCUS and ASTIG (Astigmatism) Controls The FOCUS control adjusts the beam size. The ASTIG (astigmatism) control adjusts the beam shape. The FOCUS and ASTIG controls are adjusted together to produce a small, clearly defined circular dot. When displaying a line trace, you will use these same controls to produce a well-defined line. Figure 6-17, view A, shows an out-of-focus beam dot. View B shows the beam in focus. Views C and D show out-of-focus and in-focus traces, respectively.

6.3.2.3 TRACE ROTATION Control The TRACE ROTATION control (figure 6-16) allows for minor adjustments of the horizontal portion of the trace so that you can align it with the horizontal lines on the graticule.

Figure 6-17 Effects of FOCUS and ASTIG (astigmatism) controls

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-20 UNCLASSIFIED 6.3.2.4 BEAM FINDER Control Occasionally, the trace will actually be located off the CRT (up or down or to the left or right) because of the orientation of the deflection plates. When pushed, the BEAM FINDER (figure 6-16) pulls the beam onto the screen so that you can use the horizontal and vertical POSITION controls to center the spot.

6.3.2.5 Horizontal and Vertical POSITION Controls The horizontal and vertical POSITION controls (figure 6-16) are used to position the trace. Because the graticule is often drawn to represent a graph, some oscilloscopes have the positioning controls labeled to correspond to the X and Y axes of the graph. The X axis represents horizontal movement; the Y axis represents the vertical movement. Figure 6-18 shows the effects of positioning controls on the trace.

In view A, the horizontal control has been adjusted to move the trace too far to the right; in view B, the trace has been moved too far to the left. In view C, the vertical POSITION control (discussed later) has been adjusted to move the trace too close to the top; in view D, the trace has been moved too close to the bottom. View E (figure 6-18) shows the trace properly positioned.

Figure 6-18 Effects of horizontal and vertical controls

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-21 UNCLASSIFIED 6.3.2.6 10X MAG (Magnifier) Switch The 10X MAG (magnifier) switch (figure 6-16) allows you to magnify the displayed signal by a factor of 10 in the horizontal direction. This ability is important when you need to expand the signal to evaluate it carefully.

6.3.3 Components Used to Determine the Amplitude of a Signal We will now discuss the dual-trace components of the scope. You will use these components to determine the amplitude of a signal. Notice in figure 6-19 that the highlighted section at the upper left of the scope looks just the same as the section at the lower left of the scope. This reveals the dual-trace capability section of the scope. The upper left section is the CH (channel) 1 input and is the same as the CH 2 input at the lower left. An input to both inputs at the same time will produce two independent traces on the CRT and use the dual-trace capability of the scope.

Figure 6-19 Components that determine amplitude

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-22 UNCLASSIFIED For purposes of this introductory discussion, we will present only CH (channel) 1. You should realize that the information presented also applies to CH 2.

6.3.3.1 Vertical POSITION Control The vertical POSITION control allows you to move the beam position up or down, as discussed earlier.

6.3.3.2 Input Connector The vertical input (or signal input) jack connects the signal to be examined to the vertical-deflection amplifier. Some oscilloscopes may have two input jacks, one labeled AC and the other labeled DC. Other models may have a single input jack with an associated switch, such as the AC GRD DC switch in figure 6-19. This switch is used to select the ac or dc connection. In the DC position, the signal is connected directly to the vertical-deflection amplifier; in the AC position, the signal is first fed through a capacitor. Figure 6-20 shows the schematic of one arrangement.

Figure 6-20 Vertical input arrangement

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-23 UNCLASSIFIED The VERTICAL-DEFLECTION AMPLIFIER increases the amplitude of the input signal level required for the deflection of the CRT beam. The deflection amplifier must not have any other effect on the signal, such as changing the shape (called DISTORTION). Figure 6-21 shows the results of distortion occurring in a deflection amplifier.

6.3.3.3 Attenuator Control An amplifier can handle only a limited range of input amplitudes before it begins to distort the signal. Signal distortion is prevented in oscilloscopes by the incorporation of circuitry that permits adjustment of the input signal amplitude to a level that prevents distortion from occurring. This adjustment is called the ATTENUATOR control in some scopes (VOLTS/DIV and VAR in figure 6-19). This control extends the usefulness of the oscilloscope by enabling it to handle a wide range of signal amplitudes.

The attenuator usually consists of two controls. One is a multi-position (VOLTS/DIV) control, and the other is a variable (VAR) potentiometer. Each position of the control may be marked either as to the amount of voltage required to deflect the beam a unit distance, such as VOLTS/DIV, or as to the amount of attenuation (called the DEFLECTION FACTOR) given to the signal, such as 100, 10, or 1.

Figure 6-21 Deflection amplifier distortion

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-24 UNCLASSIFIED Suppose the .5 VOLTS/DIV position were selected. In this position, the beam would deflect vertically 1 division for every 0.5 volts of applied signal. If a sine wave occupied 4 divisions peak-to-peak, its amplitude would be 2 volts peak-to-peak (4 × 0.5), as shown in figure 6-22.

The vertical attenuator control (VOLTS/DIV in figure 6-19) provides a means of adjusting the input signal level to the amplifiers by steps. These steps are sequenced from low to high deflection factors. The potentiometer control (VAR in figure 6-19) provides a means of fine, or variable, control between steps. This control may be mounted separately, or it may be mounted on the attenuator control. When the control is mounted separately, it is often marked as FINE GAIN or simply GAIN. When mounted on the attenuator control, it is usually marked VARIABLE or VAR.

The variable control adds attenuation to the step that is selected. Since accurately calibrating a potentiometer is difficult, the variable control is either left unmarked or the front panel is marked off in some convenient units, such as 1-10 and 1-100. The attenuator control, however, can be accurately calibrated. To do this, you turn off the variable control to remove it from the attenuator circuit. This position is usually marked CAL (calibrate) on the panel, or an associated light indicates if the VAR control is on or off. In figure 6-19, the light called UNCAL indicates the VAR control is in the uncalibrated position.

Figure 6-22 Sine wave attenuation

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-25 UNCLASSIFIED 6.3.4 Components Used to Select the Vertical Operating Mode As we discussed earlier, channel 1 is being used to discuss basic operating procedures for the oscilloscope. Figure 6-23 shows how the vertical mode of operation is selected. The VERT MODE section contains push-button switches that enable you to select channel 1, channel 2, and several other vertical modes of operation. For the present discussion, note only that CH 1 is selected by these switches.

Figure 6-23 Vertical-deflection controls

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-26 UNCLASSIFIED 6.3.5 Components Used to Determine Period Time of the Display The TIME/DIV (figure 6-24) controls on the scope determine the period time of the displayed waveform. As we discussed earlier, the sweep generator develops the sawtooth waveform that is applied to the horizontal-deflection plates of the CRT. This sawtooth voltage causes the beam to move across the screen. This trace (sometimes called SWEEP) sets the frequency of the TIME BASE of the oscilloscope. The frequency of the time base is variable, which enables the oscilloscope to accept a wide range of input frequencies. Again, two controls are used (figure 6-24). One is a multi-position switch (TIME/DIV) that changes the frequency of the sweep generator in steps. The second control is a potentiometer (VAR) that varies the frequency between steps. Each step on the TIME/DIV control is calibrated. The front panel has markings that group the numbers into microseconds and milliseconds.

Figure 6-24 Period time of the waveform (TIME/DIV)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-27 UNCLASSIFIED The potentiometer is labeled VAR, and the panel has an UNCAL indicator that lights when the VAR control is in the variable position. When you desire to accurately measure the time of one cycle of an input signal, turn the VAR control to the CAL position and turn the TIME/DIV switch to select an appropriate time base. Suppose you choose the 10-microsecond position to display two cycles of an input signal, as shown in figure 6-25. One cycle occupies 3 centimeters (small divisions) along the horizontal axis. Each cm has a value of 10 microseconds. Therefore, the time for one cycle equals 30 microseconds (3 × 10). Recall that the frequency for a signal may be found by using the following procedure:

f = 1 time (t)

= 1 30 X 10−6

= 33.33 kHz

In selecting a time base, you should select one that is lower in frequency than the input signal. If the input signal requires 5 milliseconds to complete one cycle and the sawtooth is set for 0.5 milliseconds per centimeter with a 10-centimeter-wide graticule, then approximately one cycle will be displayed. If the time base is set for 1 millisecond per centimeter, approximately two cycles will be displayed. If the time base is set at a frequency higher than the input frequency, only a portion of the input signal will be displayed.

Figure 6-25 Time measurement of a waveform (TIME/DIV)

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-28 UNCLASSIFIED In the basic oscilloscope, the sweep generator runs continuously (FREE-RUNNING); in more elaborate oscilloscopes, it is normally turned off. In the oscilloscope we’re using as an example, the sweep generator can be triggered by the input signal or by a signal from some other source. (Triggering will be discussed later in this chapter.) This type of oscilloscope is called a triggered oscilloscope. The triggered oscilloscope permits more accurate time measurements to be made and provides a more stable presentation than the non-triggered-type oscilloscope.

On some oscilloscopes, you will find a 10 times (10X) magnification control. As previously mentioned, this allows the displayed sweep to be magnified by a factor of 10.

Q-15. When you select the time base to display a signal, should the time base be the same, higher, or lower than the input signal? 6.3.6 Components Used to Provide a Stable Display The triggering and level controls are used to synchronize the sweep generator with the input signal. This provides a stationary waveform display. If the input signal and horizontal sweep generator are unsynchronized, the pattern tends to jitter, making observations difficult.

The A TRIGGER controls at the lower right of the scope (figure 6-26) are used to control the stability of the oscilloscope CRT display. They are provided to permit you to select the source, polarity, and amplitude of the trigger signal. These controls, labeled A TRIGGER, LEVEL, SOURCE, and SLOPE, are described in the following paragraphs.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-29 UNCLASSIFIED

6.3.6.1 SOURCE Control The SOURCE control allows you to select the appropriate source of triggering. You can select input signals from channel 1 or 2, the line (60 hertz), or an external input.

Figure 6-26 Components that control stability

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-30 UNCLASSIFIED 6.3.6.2 TRIGGER LEVEL/SLOPE Controls The LEVEL control allows you to select the amplitude point of the trigger signal at which the sweep is triggered. The SLOPE lets you select the negative or positive slope of the trigger signal at which the sweep is triggered. The TRIGGER LEVEL (mounted with the TRIGGER SLOPE on some scopes) determines the voltage level required to trigger the sweep. For example, in the TRIGGER modes, the trigger is obtained from the signal to be displayed. The setting of the LEVEL control determines the amplitude point of the input waveform that will be displayed at the start of the sweep.

Figure 6-27 shows some of the displays for a channel that can be obtained for different TRIGGER LEVEL and TRIGGER SLOPE settings. The level is zero and the slope is positive in view A; view B also shows a zero level but a negative slope selection. View C shows the effects of a positive trigger level setting and positive trigger slope setting; view D displays a negative trigger level setting with a positive trigger slope setting. Views E and F have negative slope settings. The difference is that view E has a positive trigger level setting, whereas F has a negative trigger level setting.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-31 UNCLASSIFIED

In most scopes, an automatic function of the trigger circuitry allows a free-running trace without a trigger signal. However, when a trigger signal is applied, the circuit reverts to the triggered mode of operation and the sweep is no longer free running. This action provides a trace when no signal is applied.

Synchronization is also used to cause a free-running condition without a trigger signal. Synchronization is not the same as triggering. TRIGGERING refers to a specific action or event that initiates an operation. Without this event, the operation would not occur. In the case of the triggered sweep, the sweep will not be started until a trigger is applied. Each succeeding sweep must have a trigger before a sweep commences. SYNCHRONIZATION, however, means that an operation or event is brought into step with a second operation. Figure 6-27 Effects of SLOPE and TRIGGER LEVEL controls

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-32 UNCLASSIFIED A sweep circuit that uses synchronization instead of triggering will cause a previously free-running sweep to be locked in step with the synchronizing signal. The TRIGGER LEVEL control setting can be increased until synchronization occurs; but, until that time, an unstable pattern will appear on the CRT face.

6.3.6.3 COUPLING Section The COUPLING section allows you to select from four positions: AC, LF REJ, HF REJ, and DC. The AC position incorporates a coupling capacitor to block any dc component. The LF and HF REJ positions reject low- and high-frequency components, respectively. The DC position provides direct coupling to the trigger circuits. This is useful when you wish to view only the LF or HF component of a signal.

6.3.7 Components Used to Select Scope Triggering The TRIG MODE section in figure 6-28 allows for automatic triggering or normal triggering. In AUTO (automatic), the triggering will be free-running in the absence of a proper trigger input or will trigger on the input signal at frequencies above 20 hertz. In NORM (normal), the vertical channel input will trigger the sweep.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-33 UNCLASSIFIED

Figure 6-28 Components to select triggering

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-34 UNCLASSIFIED 6.3.8 Components Used to Select Horizontal-Deflection Mode For the present, notice only that the HORIZ DISPLAY (horizontal display) in figure 6-29 can be controlled by the TIME/DIV switch. Other switches in this section will be explained later in this chapter.

Figure 6-29 Components to select mode of horizontal deflection

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-35 UNCLASSIFIED 6.3.9 Components Used to Calibrate the Probe of the Scope In figure 6-30, you can see the components used to calibrate the test probe on the scope. A 1-volt, 2-kilohertz square wave signal is provided for you to adjust the probe for an accurate square wave and to check the vertical gain of the scope. You adjust the probe with a screwdriver, as shown in the figure.

6.3.10 Similarities Among Oscilloscopes The oscilloscope you use may differ in some respects from the one just covered. Controls and circuits may be identified by different names. Many of the circuits will be designed differently. However, all the functions will be fundamentally the same. Before using an oscilloscope, you should carefully study the operator’s manual that comes with it.

Figure 6-30 Components to calibrate probe

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-36 UNCLASSIFIED 6.4 USING THE OSCILLOSCOPE An oscilloscope can be used for several different types of measurements, such as time, phase, frequency, and amplitude of observed waveforms. Earlier in this chapter, you learned that the oscilloscope is most often used to study the shapes of waveforms when the performance of equipment is being checked. The patterns on the scope are compared with the signals that should appear at test points (according to the technical manual for the equipment under test). You can then determine if the equipment is operating according to peak performance standards.

Q-16. Oscilloscopes are used to measure what quantities? 6.4.1 Turning on the Scope Before turning on the scope, make sure it is plugged into the proper power source. This may seem obvious, but many technicians have turned all knobs on the front panel out of adjustment before they noticed that the power cord was not plugged in. On some scopes, the POWER switch is part of the INTEN (intensity) control. Turn or pull the knob until you hear a click or a panel light comes on (figure 6-31). Let the scope warm up for a few minutes so that voltages in all of the circuits become stabilized.

Figure 6-31 Components to energize scope

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-37 UNCLASSIFIED 6.4.2 Obtaining a Pattern on the Screen When adjusting a pattern onto the screen, adjust the INTEN (intensity) and FOCUS controls for a bright, sharp line. If other control settings are such that a dot instead of a line appears, turn down the intensity to prevent burning a hole in the screen coating. Because of the different speeds at which the beam travels across the screen, brightness and sharpness will vary at various frequency settings. For this reason, you may have to adjust the INTEN and FOCUS controls occasionally while taking readings.

6.4.3 Number of Cycles on the Screen Because distortion may exist at the beginning and end of a sweep, it is better to place two or three cycles of the waveform on the screen instead of just one, as shown in figure 6-32.

Figure 6-32 Proper signal presentation

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-38 UNCLASSIFIED The center cycle of three cycles provides you with an undistorted waveform in its correct phase. The center of a two-cycle presentation will appear inverted, but will be undistorted. To place waveforms on the CRT in this manner, you must understand the relationship between horizontal and vertical frequencies. The relationship between the frequencies of the waveform on the vertical plates and the sawtooth on the horizontal plates determines the number of cycles on the screen, as shown in figure 6-33.

The horizontal sweep frequency of the scope should always be kept lower than, or equal to, the waveform frequency; it should never be higher. If the sweep frequency were higher, only a portion of the waveform would be presented on the screen.

If, for example, three cycles of the waveform were to be displayed on the screen, the sweep frequency would be set to one-third the frequency of the input signal. If the input frequency were 12,000 hertz, the sweep frequency would be set at 4,000 hertz for a three- cycle scope presentation. For two cycles, the sweep frequency would be set at 6,000 hertz. If a single cycle were desired, the setting would be the same as the input frequency, 12,000 hertz.

Figure 6-33 Vertical versus horizontal relationship

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-39 UNCLASSIFIED 6.4.4 Dual-Trace Operation The information presented in the previous sections served as a general overview of basic single-trace oscilloscope operation using one channel and operating controls. Now, you will be introduced to DUALTRACE operation.

Dual-trace operation allows you to view two independent signal sources as a dual display on a single CRT. This operation allows an accurate means of making amplitude, phase, time displacement, or frequency comparisons and measurements between two signals.

A dual-trace oscilloscope should not be confused with a dual-beam oscilloscope. Dual- beam oscilloscopes produce two separate electron beams on a single scope, which can be individually or jointly controlled. Dual-trace refers to a single beam in a CRT that is shared by two channels.

Q-17. Scopes that produce two channels on a single CRT with a single beam are referred to as what types of scopes?

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-40 UNCLASSIFIED 6.4.4.1 Components Used to Select Vertical-Deflection Operating Mode The VERT MODE controls (figure 6-34) allow you to select the operating mode of the scope for vertical deflection.

CH 1 AND CH 2 - These controls allow you to display signals applied to either channel 1 or channel 2, as discussed earlier.

TRIGGER VIEW - The TRIG VIEW allows you to display the signal that is actually used to trigger the display. (Triggering was discussed earlier.)

Figure 6-34 Components to select vertical operating mode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-41 UNCLASSIFIED ALT - The ALT (alternate) mode (figure 6-35) of obtaining a dual trace uses the techniques of GATING between sweeps. This control allows the signal applied to channel 1 to be displayed in its entirety; then, channel 2 is displayed in its entirety. This method of display is continued alternately between the two channels. At slow speeds, one trace begins to fade while the other channel is being gated. Consequently, the ALT mode is not used for slow sweep speeds. The CHOP mode, shown in figure 6-36 (explained next), will not produce a satisfactory dual sweep at high speeds. The ALT mode is deficient at low speeds. Therefore, both are used on dual-trace oscilloscopes to complement each other and give the scope a more dynamic range of operation.

Figure 6-35 ALT (alternate) mode Figure 6-36 CHOP mode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-42 UNCLASSIFIED The output dc voltage references on each of the amplifiers are independently adjustable. Therefore, the beam will be deflected by different amounts on each channel if the voltage reference is different at each amplifier output. The output voltage from each amplifier is applied to the deflection plates through the gate. The gate is actually an electronic switch. In this application, it is commonly referred to as a BEAM SWITCH.

Switching is controlled by a high-frequency multivibrator in the CHOP mode. That is, the gate selects the output of one channel and then the other at a high-frequency rate (1200 kilohertz in most oscilloscopes). Because the switching time is very short in a good- quality oscilloscope, the resultant display is two sets of horizontally dashed lines, as shown in figure 6-37, view A.

Figure 6-37 Displaying CHOP mode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-43 UNCLASSIFIED Dashed line CH 1 is the output of one channel, while line CH 2 is the output of the other. The trace moves from left to right because of the sawtooth waveform applied to the horizontal plates. A more detailed analysis shows that the beam moves from CH 1 to CH 2 while the gate is connected to the output from one channel. Then, when the gate samples the output of the CH 2 during time 3 to 4, the beam is at a different vertical LOCATION. (This is assuming that CH 2 is at a different voltage reference.) The beam continues in the sequence 5 to 6, 7 to 8, 9 to 10, and 11 to 12 through the rest of one horizontal sweep.

When the chopping frequency is much higher than the horizontal sweep frequency, the number of dashes will be very large. For example, if the chopping occurs at 100 kilohertz and the sweep frequency is 1 kilohertz, each horizontal line would then appear as a series of closely spaced dots, as shown in figure 6-37 view B. As the sweep frequency becomes lower compared to the chopping frequency, the display will show apparently continuous traces; therefore, the CHOP mode is used at low sweep rates.

When signals are applied to the channel amplifiers (view A of figure 6-38), the outputs are changed according to the triggering signal (view B). The resultant pattern (view C) on the screen provides a time-base presentation of the signals of each channel.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-44 UNCLASSIFIED

ADD - The ADD switch (shown earlier in figure 6-34) algebraically adds the two signals of channels 1 and 2 together for display.

Figure 6-38 Dual-channel display in CHOP mode

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-45 UNCLASSIFIED 6.4.4.2 Other Dual-Trace Oscilloscope Controls Most dual-trace oscilloscopes have both an A and B time base for horizontal sweep control. Notice in the upper right corner on our example scope (figure 6-34) the COUPLING, SOURCE LEVEL, and SLOPE controls. These serve the same function as did those same controls in the A time-base section of the scope. The B time base is selected using the same A and B TIME/DIV control (pull out outer knob).

The use of the B time base is controlled by the HORIZ DISPLAY section discussed earlier in the A time-base section. However, inexperienced technicians generally do not use A and B time bases together in the MIXED, A INTEN (intensified), and B D'LYD (delayed) settings. These controls are fully explained in the applicable technical manuals; therefore, we will not discuss the controls in this chapter. Figure 6-39 is a block diagram of a basic dual-trace oscilloscope without the power supplies.

Figure 6-39 Basic dual-trace oscilloscope block diagram

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-46 UNCLASSIFIED 6.4.5 Accessories The basic dual-trace oscilloscope has one gun assembly and two vertical channels. However, there are many variations. The horizontal sweep channels vary somewhat from equipment to equipment. Some have one time-base circuit and others have two. These two are interdependent in some oscilloscopes and in others they are independently controlled. Also, most modern general-purpose oscilloscopes are constructed of modules. That is, most of the vertical circuitry is contained in a removable plug-in unit, and most of the horizontal circuitry is contained in another plug-in unit.

The main frame of the oscilloscope is often adapted for many other special applications by the design of a variety of plug-in assemblies. This modular feature provides much greater versatility than in a single-trace oscilloscope. For instance, to analyze the characteristics of a transistor, you can replace the dual-trace, plug-in module with a semiconductor curve-tracer plug-in module.

Other plug-in modules available with some oscilloscopes are high-gain, wide-bandwidth amplifiers; differential amplifiers; spectrum analyzers; physiological monitors; and other specialized units. Therefore, the dual-trace capability is a function of the type of plug-in unit that is used with some oscilloscopes.

To get maximum usefulness from an oscilloscope, you must have a means of connecting the desired signal to the oscilloscope input. Aside from cable connections between any equipment output and the oscilloscope input, a variety of probes are available to assist in monitoring signals at almost any point in a circuit. The more common types include 1- TO-1 PROBES, ATTENUATION PROBES, and CURRENT PROBES. Each of these probes may be supplied with several different tips to allow measurement of signals on any type of test point. Figure 6-40 shows some of the more common probe tips.

In choosing the probe to use for a particular measurement, you must consider such factors as circuit loading, signal amplitude, and scope sensitivity.

The 1-to-1 probe offers little or no attenuation of the signal under test and is, therefore, useful for measuring low-level signals. However, circuit loading with the 1-to-1 probe may be a problem. The impedance at the probe tip is the same as the input impedance of the oscilloscope.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-47 UNCLASSIFIED

An attenuator probe has an internal high-value resistor in series with the probe tip. This gives the probe a higher input impedance than that of the oscilloscope. Because of the higher input impedance, the probe can measure high-amplitude signals that would overdrive the vertical amplifier if connected directly to the oscilloscope. Figure 6-41 shows a schematic representation of a basic attenuation probe. The 9-megohm resistor in the probe and the 1-megohm input resistor of the oscilloscope form a 10-to-1 voltage divider.

Figure 6-40 Common probe tips Figure 6-41 Basic attenuation probe

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-48 UNCLASSIFIED Since the probe resistor is in series, the oscilloscope input resistance is 10 megohms when the probe is used. Thus, using the attenuator probe with the oscilloscope causes less circuit loading than using a 1-to-1 probe.

Before using an attenuator probe for measurement of high-frequency signals or for fast- rising waveforms, you must adjust the probe compensating capacitor (C1) according to instructions in the applicable technical manual. Some probes will have an IMPEDANCE EQUALIZER in the end of the cable that attaches to the oscilloscope. The impedance equalizer, when adjusted according to manufacturer’s instructions, assures proper impedance matching between the probe and oscilloscope. An improperly adjusted impedance equalizer will result in erroneous measurements, especially when you are measuring high frequencies or fast-rising signals.

More information on oscilloscope hook-ups can be found in Electronics Information Maintenance Books (EIMB), Test Methods and Practices.

Special current probes have been designed to use the electromagnetic fields produced by a current as it travels through a conductor. This type of probe is clamped around a conductor without disconnecting it from the circuit. The current probe is electrically insulated from the conductor, but the magnetic fields about the conductor induce a potential in the current probe that is proportional to the current through the conductor. Thus, the vertical deflection of the oscilloscope display will be directly proportional to the current through the conductor.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-49 UNCLASSIFIED 6.5 SPECTRUM ANALYZER The spectrum analyzer is used to examine the frequency spectrum of radar transmissions, local oscillators, test sets, and any other equipment operating within its testable frequency range. With experience, you will be able to determine definite areas of malfunctioning components within equipment. Successful spectrum analysis depends on the proper operation of a spectrum analyzer and your ability to correctly interpret the displayed frequencies. Although there are many types of spectrum analyzers, we will use the Tektronix, Model 492 for our discussion.

The spectrum analyzer accepts an electrical input signal and displays the frequency and amplitude of the signal on a CRT. On the vertical, or Y, axis, the amplitude is plotted. The frequency would then be found on the horizontal, or X, axis. The overall pattern of this display (figure 6-42) indicates the proportion of power present at the various frequencies within the spectrum (fundamental frequency with sideband frequencies).

Figure 6-42 Spectrum analyzer pattern

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-50 UNCLASSIFIED 6.5.1 Basic Functional Description The model 492 analyzer can be divided into six basic sections, as follows:

• Converter section; • Intermediate frequency (IF) section; • Display section; • Frequency control section; • Digital control section; and • Power and cooling section.

6.5.1.1 Converter Section The converter section actually consists of three frequency converters, made up of a mixer, local oscillator (LO), and required filters. Only one frequency can be converted at a time and pass through the filters to reach the next converter. The analysis frequency can, however, be changed by altering the frequency of the LO and adjusting the FREQUENCY control knob.

FIRST CONVERTER - The first (front end) converter changes the input signal to a usable IF signal that will either be 829 MHz or 2072 MHz. The IF signal to be produced is dependent on which measurement band selection is currently being used. The 829 MHz IF signal will be selected for bands 2 through 4, while the 2072 MHz IF signal is selected for bands 1 and 5 through 11.

Q-18. The first converter is also known by what other name? SECOND CONVERTER - The second converter actually contains two converters. Only one of these two converters in this section is ever operational, and selected as a result of the measurement band currently being used. The selected converter will convert the frequency received from the first converter to a usable (110 MHz) IF signal, which is then provided to the third converter.

THIRD CONVERTER - This converter takes the 110 MHz IF signal, amplifies it, and then converts it to the final IF of 10 MHz. This signal, in turn, is then passed on to the IF section.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-51 UNCLASSIFIED 6.5.1.2 IF Section The IF section receives the final IF signal and uses it to establish the system resolution by using selective filtering. System resolution is selected under microcomputer control among five bandwidths (1 MHz, 100 kHz, 10 kHz, 1 kHz, and 100 Hz). The gain for all bands are then leveled and logarithmically amplified. This is done so that each division of signal change on the CRT display remains equal in change to every other division on the CRT. For example, in the 10-dB-per-division mode, each division of change is equal to a 10 dB difference, regardless of whether the signal appears at the top or bottom of the CRT. The signal needed to produce the video output to the display section is then detected and provided.

6.5.1.3 Display Section The display section provides a representative display of the input signal on the CRT. It accomplishes this by performing the following functions:

• Receives the video signals from the IF section and processes these signals to adjust the vertical drive of the CRT;

• Receives the sweep voltages and processes these signals to produce the horizontal CRT drive plate voltage;

• Receives character data information and generates CRT plate drive signals to display alpha and numeric characters on the CRT;

• Receives control levels from the front panel beam controls and generates unblanking signals to control display presence, brightness, and focus.

The vertical deflection of the beam is increased as the output of the amplitude detector increases. The horizontal position is controlled by the frequency control section and is the frequency analyzed at that instant. The beam sweeps from left to right, low to high frequencies during its analysis. During this analysis, any time a signal is discovered, a vertical deflection will show the strength of the signal at the horizontal position that is the frequency. This results in a display of amplitude as a function of frequency.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-52 UNCLASSIFIED 6.5.1.4 Frequency Control Section The frequency control section accomplishes the tuning of the first and second LOs within the converter section. The frequency immediately being analyzed is controlled by the current frequencies of the LOs. To analyze another frequency, you must change an LO frequency to allow the new frequency to be converted to a 10 MHz signal by the converter section. Periodically, the unit sweeps and analyzes a frequency range centered on the frequency set by the FREQUENCY knob. Adjusting the FREQUENCY knob will cause the LOs to be tuned to the new frequency. Only the LOs of the first two converters can be changed to vary the frequency being analyzed.

6.5.1.5 Digital Control Section All the internal functions are controlled from the front panel through the use of a built-in microcomputer. The microcomputer uses an internal bus to receive or produce all communication or control to any section of the analyzer.

6.5.1.6 Power and Cooling Section The main power supply provides almost all the regulated voltages required to operate the unit. The display section provides the high voltage necessary for CRT operation.

The cooling system allows fresh cool air to be routed to all sections of the unit in proportion to the heat that is generated by each section.

6.5.2 Spectrum Analyzer Front Panel Controls, Indicators, and Connectors This section will describe the function of the front panel controls, indicators, and connectors. For a complete description of each function, refer to table 6-1 while reviewing the front panel in figure 6-43. The numbers located in column 1 of table 6-1 equate to the same numbers found on the front panel of figure 6-43. Because most operational functions of this spectrum analyzer are microprocessor-controlled, they are switch-selected rather than adjusted.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-53 UNCLASSIFIED

Figure 6-43 Spectrum analyzer front panel controls, indicators, and connectors

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-54 UNCLASSIFIED Table 6-1 Description of Front Panel Controls, Indicators, and Connectors ITEM FUNCTION DESCRIPTION 1 INTENSITY This knob controls the brightness of the CRT trace and the CRT readout display. The focus is electronically adjusted. 2 READOUT This push button switches the readout display on and off. All spectrum analyzer parameters are displayed except TIME/DIV. The brightness for this display is proportional to the trace brightness and can be readjusted on internal controls only by a qualified technician. 3 GRATILLUM This push button switches the graticule light on and off. 4 BASELINE CLIP This push button, when activated, clips (subdues) the intensity at the baseline. 5 TRIGGERING This area allows one of four triggering modes to be selected by push buttons that illuminate when active. When any of these four are selected, the others are canceled. 5a FREE RUN When activated, the sweep is free-running without regard to trigger signals. 5b INT When activated, the sweep is triggered by any signal at the left edge of the display with an amplitude of 1.0 divisions of the graticule or more. 5c LINE When activated, a sample of the ac power line voltage is used to trigger the sweep. 5d EXT When selected, the sweep is triggered by an external signal (applied through the back panel IN HORZ/TRIG connector) between a minimum and maximum of 0.5 and 50 volt peak. 6 SINGLE SWEEP This push button, plus a ready indicator (No. 7), provides the single sweep operation. When this operation is selected, one sweep is initiated after the sweep circuit has been triggered. Pushing this button does not cancel the other trigger modes. When single sweep is first selected, the present sweep is aborted, but the sweep circuit is not yet armed. An additional push is required to initially arm the sweep. The button must be pushed again to rearm the sweep circuit each time the sweep has run. To cancel single sweep, you must select one of the four trigger mode selections. 7 READY When single sweep is selected, this indicator lights while the sweep circuit is armed and ready for a trigger signal. The indicator stays lit until the sweep is complete. 8 MANUAL SCAN When the TIME/DIV (No. 9c) selector is in the MNL position, this control will manually vary the CRT beam across the full horizontal axis of the display. 9 TIME/DIV Is used to select sweep rates from 5μsec/div to 20μsec/div. This switch also selects AUTP, EXT, and MNL modes. 9a AUTO In this position, the sweep rate is selected by the microcomputer to maintain a calibrated display for any FREQ SPAN/DIV, RESOLUTION, and VIDEO FILTER combination. 9b EXT When selected, this control allows an external input source to be used with the sweep rates. 9c MNL When selected, this control is used in conjunction with No. 8 (see MANUAL SCAN, No. 8). 10 FREQUENCY This control is manually turned to allow you to tune to the center frequency. 11 FREQUENCY RANGE (band) These two push buttons are used to shift the center frequency up or down. Frequency range on the band is displayed on the CRT readout.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-55 UNCLASSIFIED ITEM FUNCTION DESCRIPTION 12 F This control is used for measuring the frequency difference between signals. When selected, the frequency readout goes to zero. It will then read out the deviation from this reference to the next frequency desired as the FREQUENCY knob is adjusted. 13 CAL When this is activated, the frequency readout can be calibrated to center the center frequency by adjusting the FREQUENCY control for the correct reading. When accomplished, you should deactivate the CAL mode. 14 DEGAUSS When this button is pressed, current through the local oscillator system is reduced to zero in order to minimize magnetism build-up around the LOs. This is done to enhance the center frequency display and amplitude accuracy. You should do this after every significant frequency change and before calibrating the center frequency. 15 IDENTIFY 500 kHz ONLY The signal identify feature can become functional only when the FREQ SPAN/DIV is set to 500 kHz. When activated (button lit), true signals will change in amplitude on every sweep. Images and spurious response signals will shift horizontally or go completely off the CRT display. To ensure that the signal is changing amplitude every sweep, you should decrease the sweep rate so that each sweep can be analyzed. 16 PHASE LOCK When this control is activated (button lit), it will reduce residual FM when narrow spans are selected. In narrow spans, the phase lock can be turned off or back on by pressing the button. Switching the PHASE LOCK off may cause the signal to shift position. In narrow spans, the signal could shift off the display; however, it will usually return to its phase locked position after a few moments. The microcomputer automatically selects PHASE LOCK for a span/division of 50 kHz or below in bands 1 through 3, 100 kHz or below for band 3, and 200 kHz for bands 5 and above. 17 AUTO RESOLUTION This push button, when activated, will automatically select the bandwidth for FREQ SPAN/DIV, TIME/DIV, and VIDEO FILTER. The internal microcomputer selects the bandwidth to maintain a calibrated display. This can be checked by changing the FREQ SPAN/DIV and observing the bandwidth change on the display. 18 FREQ SPAN/DIV This is a continuous detent control that selects the frequency span/div. The span/div currently selected is displayed on the CRT. The range of the span/div selection is dependent on the frequency band selected:

BAND NARROW SPAN WIDE SPAN 1-3 (0-7.1GHz) 10kHz/Div 200MHz/Div 4-5 (5.4-21GHz) 50 kHz/Div 500 MHz/Div 6 (18-26GHz) 50 kHz/Div 1 GHz/Div 7-8 (26-60GHz) 100 kHz/Div 2 GHz/Div 9 (60-90GHz) 200 kHz/Div 2 GHz/Div 10 (90-140GHz) 500 kHz/Div 5 GHz/Div 11 (140-220GHz) 500 kHz/Div 10 GHz/Div Two additional bands are provided: full band (max span) and 0 Hz span. When max span is selected, the span displayed is the full band. When zero span is selected, time/div is read out instead of span/div.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-56 UNCLASSIFIED ITEM FUNCTION DESCRIPTION 19 RESOLUTION BANDWIDTH This is also a continuous detent control that selects the resolution bandwidth. The bandwidth is shown on the CRT display. The range of adjustment is from 1 kHz to 1 MHz in decade steps. When you change the resolution bandwidth with this control, it will deactivate the AUTO RESOLUTION. 20 VERTICAL DISPLAY These four push buttons select the display mode. The scale factor can be seen on the CRT display. 20a 10dB/DIV When this is activated, the dynamic range of the display is calibrated to 80 dB, with each major graticule representing 10 dB. 20b 2dB/DIV When activated, this will increase the resolution so that each major graticule division represents 2 dB. 20c LIN When activated, this selects a linear display between zero volts (bottom graticule line) and the reference level (top graticule line) scaled in volts/division (see REFERENCE LEVEL, No. 23a). 20d PULSE STRETCHER When selected, this increases the fall time of the pulse signals so that very narrow pulses in a line spectrum display can be observed. 21 VIDEO FILTER One of two (NARROW OR WIDE) filters can be activated to reduce video bandwidth and high-frequency components for display noise averaging. The narrow filter is approximately 1/300th of the selected resolution bandwidth with the wide filter being 1/30th the bandwidth. Activating either one will cancel the other. To disable, completely switch filter off. 22 DIGITAL STORAGE Five push buttons and ON control operate the digital storage functions. With none of the push buttons activated, the display will not be stored. 22a VIEW A, VIEW B When either or both of these push buttons are selected, the push button illuminates, and the contents of memory A and/or memory B are displayed. With Save A mode off, data in a memory is interlaced with data from B memory. 22b B-SAVE A When activated, the differential (arithmetic difference) of data in B memory and the saved data in memory A are displayed. SAVE A mode is activated and SAVE A button will be lit. 22c MAX HOLD When activated, the digital storage memory retains the maximum signal amplitude at each memory location. This permits visual monitoring of signal frequency and amplitude at each memory location over an indefinite period of time. This feature is used to measure drift, stability, and record peak amplitude. 22d PEAK/AVERAGE This control selects the amplitude at which the vertical display is either peak detected or averaged. Video signals above the level set by the control (shown by a horizontal line or cursor) are peak detected and stored while video signals below the cursor are digitally averaged and stored. 23 MIN RF ATTEN This control is used to set the minimum amount of RF attenuation. Changing RF LEVEL will not decrease RF attenuation below that set by the MIN RF ATTEN selector. 23a REFERENCE LEVEL This is a continuous control that requests the microcomputer to change the reference level one step for each detent. In the 10 dB/DIV vertical-display mode, the steps are 1 dB or 0.25 dB if the FINE mode (No. 26) is selected.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-57 UNCLASSIFIED ITEM FUNCTION DESCRIPTION 23b MIN RF ATTEN DB This selects the lowest value of attenuation allowed: Actual RF attenuation is set by the microcomputer according to the logarithm selected by the MIN NOISE/MIN DISTORTION (No. 27) button. If RF attenuation is increased by changing MIN RF ATTEN, the microcomputer automatically changes IF gain to maintain the current reference level. 24 UNCAL This indicator lights when the display amplitude is no longer calibrated (selecting a sweep rate that is not compatible with the frequency span/div and resolution bandwidth). 25 LOG and AMPL CAL These adjustments calibrate the dynamic range of the display. The LOG calibrates any logarithm gain dB/Div, and the AMPL calibrates the reference level of the top graticule line at the top of the display. 26 FINE When activated, the REFERENCE LEVEL (No. 23a) switches in 1 dB increments for 10 dB/Div display mode, 0.25 dB for 2 dB/Div, and volts 1 dB for LIN display mode. 27 MIN NOISE/MIN DISTORTION This selects one of two logarithms used to control attenuator and IF gain. MIN NOISE (button illuminated) reduces the noise level by reducing attenuation and IF gain 10 dB. MIN DISTORTION (button not illuminated) reduces distortion to its minimum. To observe any changes, the RF attenuation displayed on the CRT readout must be 10 dB higher than that set by the MIN RF ATTEN selector. 28 POWER This is a pull switch that turns power on when extended. 29 RF INPUT This is a 50 ohm coaxial input jack used to input signals of 21GHz or below. The maximum nondestructive input signal level that can be applied to this input is +13 dBm or 30 mW. Signals above 10 dB may cause signal compression. 30 POSITION These controls are used to position the display on the horizontal and vertical axes. 31 CAL OUT This is an output jack that has a calibrated 20 dBm 100 MHz signal, with frequency markers spaced 100 MHz apart. The calibrated 100 MHz marker is used as a reference for calibrating the reference level and log scale. The combination of 100 MHz markers is used to check span and frequency readout accuracy. 32 OUTPUT 1ST AND 2ND LO These jacks provide access to the output of the respective LOs. The jacks must have 50 ohm terminators installed when not connected to an external device. 33 EXTERNAL MIXER When the EXTERNAL MIXER button is activated, bias is provided out the EXTERNAL MIXER port for external waveguide mixers. The IF output from the EXTERNAL MIXER is then applied through the EXTERNAL MIXER port to the second converter for use. 34 PEAKING This control varies the mixer bias for external mixers in the EXTERNAL MIXER mode. This control should be adjusted for maximum signal amplitude.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-58 UNCLASSIFIED 6.5.3 Normal Indications Upon Power on With power applied (power knob pulled out), the spectrum analyzer will automatically (upon microcomputer control) go into the following conditions. If you do not find these indications, there is a probably a problem with the unit.

• Vertical display: 10 dB/div;

• Frequency: 0.00 MHz;

• REF level: +30 dB;

• RF attenuation: 60 dB;

• Frequency range: 0.0 to 1.8 GHz;

• Auto resolution: 1 MHz;

• Resolution bandwidth: 1 MHz;

• Freq Span/Div: Max;

• Triggering: Free run;

• Readout: On;

• Digital storage: View A/View B On;

• All other indicators off or inactive.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-59 UNCLASSIFIED 6.6 SUMMARY Now that we have completed this chapter, we will briefly review the more important points covered.

A CATHODE-RAY TUBE (CRT) is used in an oscilloscope to display the waveforms.

The CRT used in oscilloscopes consists of an ELECTRON GUN, a DEFLECTION SYSTEM, and a FLUORESCENT SCREEN.

The ELECTRON BEAM in an oscilloscope is allowed to be controlled in any direction by means of HORIZONTAL- and VERTICAL-DEFLECTION PLATES.

VERTICAL-DEFLECTION PLATES are used to show AMPLITUDE of a signal.

HORIZONTAL-DEFLECTION PLATES are used to show TIME and/or FREQUENCY relationship.

A GRATICULE is a calibrated scale of AMPLITUDE VERSUS TIME that is placed on the face of the CRT.

A DUAL-TRACE OSCILLOSCOPE is designed to accept two vertical inputs at the same time. It uses a single beam of electrons shared by two channels.

The SPECTRUM ANALYZER accepts an electrical input signal and displays the signal’s frequency and amplitude on a CRT display.

NEETS Module 16, NAVEDTRA 14188A UNCLASSIFIED

6-60 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q18.

A-1. Control grid.

A-2. The first anode.

A-3. Because they bend electron streams in much the same manner that optical lenses bend light rays.

A-4. It accelerates the electrons emerging from the first anode.

A-5. A greater deflection angle.

A-6. A greater deflection angle.

A-7. Higher potential.

A-8. Slower beam.

A-9. Amplitude and time.

A-10. Amplitude.

A-11. Time and/or frequency relationships.

A-12. To permit wide-angle deflection of the beam.

A-13. Deflection factor.

A-14. A CRT, a group of control circuits, power supply, sweep circuitry, and deflection circuitry.

A-15. Lower.

A-16. Amplitude, phase, time, and frequency.

A-17. Dual-trace oscilloscopes.

A-18. Front end.