NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES MODULE 16- TEST EQUIPMENT

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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.

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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)

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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.

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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

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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

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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.

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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

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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.

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Figure 4-5 Measuring the voltage drop of a resistor

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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

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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.

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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.

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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.

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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.

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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

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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.

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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.

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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

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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.

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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:

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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.

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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.

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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.

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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.

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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.

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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

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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?

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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

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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.

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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

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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

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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

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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

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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

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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.

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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)

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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)

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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)

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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.

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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

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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.

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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

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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.

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6-33 UNCLASSIFIED

Figure 6-28 Components to select triggering

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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

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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

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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

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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

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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

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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?

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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

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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

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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

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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.

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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

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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

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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.

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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

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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.

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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

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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.

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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.

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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.

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6-53 UNCLASSIFIED

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.