Module 16—Introduction to Test Equipment
Pages i - ix
1-1 to 1-10
, 1-11 to 1-20
1-21 to 1-33
, 2-1 to 2-10
2-11 to 2-20
, 2-21 to 2-27
3-1 to 3-10
, 3-11 to 3-20
3-21 to 3-30
, 3-31 to 3-34
4-1 to 4-10
, 4-11 to 4-20
4-21 to 4-28
, 5-1 to 5-10
5-11 to 5-20
, 5-21 to 5-30
5-31 to 5-40
, 6-1 to 6-10
6-11 to 6-20
, 6-21 to 6-30
6-31 to 6-40
, 6-41 to 6-46
Figure 2-7.—Electronic strobotac.
Table 2-1.—Strobotac Controls and Indicators
The normal speed range is from 110 to 25,000 rpm. At speeds below 600 rpm, "flicker" becomes a problem
because the human eye cannot retain successive images long enough to create the illusion of continuous motion. The
life of the strobotron lamp is approximately 250 hours if used at flashing speeds of less than 5,000 rpm, or 100
hours if used at higher speeds.
ELECTRICAL OUTPUT FREQUENCY
voltage sources are generated at a set frequency or range of frequencies. A FREQUENCY METER provides a means of
measuring this frequency. The electrical output frequency of ac power generators can be measured by a vibrating
reed, a tuned circuit, or by a crossed-coil, iron-vane type meter. The vibrating-reed device is the simplest type
of frequency meter. It has the advantage of being rugged enough to be mounted on generator control panels.
A simplified diagram of a vibrating-reed frequency meter is shown in figure 2-8, views A through D. In view A, you
can see that the current to be measured flows through the coil and exerts maximum attraction on the soft-iron
armature twice during each cycle. The armature is attached to the bar, which is mounted on a flexible support.
Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth, up to 130 hertz
are mounted on the bar (view B). The reed with a frequency of 110 hertz is marked 55 hertz; the one with a
frequency of 112 hertz is marked 56 hertz; the one with a frequency of 120 hertz is marked 60 hertz, and so forth.
Figure 2-8.—Vibrating-reed frequency meter.
When the coil is energized by a current with a frequency between 55 and 65 hertz, all the reeds are
vibrated slightly; but, the reed having a natural frequency closest to that of the energizing current vibrates
through a larger amplitude. The frequency is read from the scale value opposite the reed having the greatest
amplitude of vibration.
In some instruments, the reeds are the same length; but they are weighted by different amounts at the top so they
will have different natural rates of vibration. An end view of the reeds in the indicator is shown in view C. If
the energizing current has a frequency of 60 hertz, the reed marked 60 will vibrate the
greatest amount, as
shown. View D shows a hand-held vibrating-reed frequency meter mounted on the casing of a motor-generator.
TUNED CIRCUITS are used as filters for the passage or rejection of
specific frequencies. BANDPASS FILTERS and BAND-REJECT FILTERS are examples of this type. Tuned circuits have
certain characteristics that make them ideal for certain types of filters, especially where a high degree of
selectivity is desired. A series-tuned circuit offers a low impedance to currents of the particular frequency to
which the circuit is tuned and a relatively high impedance to currents of all other frequencies. A parallel-tuned
circuit, on the other hand, offers a very high impedance to currents of its natural, or resonant, frequency and a
relatively low impedance to others. If you feel you need to review the subject of tuned circuits at this time,
refer to NEETS, Module 9, Introduction to Wave-Generation and Wave- Shaping Circuits, for more information on
these circuits and their applications.
Frequency measurements in the AF range can be made by the comparison
method or the direct- reading frequency meter. Frequency comparisons can be made by the use of a calibrated af
generator in conjunction with either an oscilloscope or a modulator and a zero-beat indicating device.
Direct-reading frequency measurements can be made by instruments using series, frequency-selective electrical
networks, bridge test sets having null indicators, or counting-type frequency meters.
Heterodyne frequency meters are available in several varieties. They measure the frequency of the unknown signal
by matching the unknown signal with a locally generated signal of the same frequency obtained from a calibrated,
precision oscillator. This method is normally referred to as zero beating. When a perfect frequency match is
obtained, it is indicated by the absence of a beat note (zero beat). The technician generally uses a set of
headphones to detect a zero-beat condition in the equipment being tested.
The basic heterodyne meter
(figure 2-9) is a calibrated variable oscillator, which heterodynes against the frequency to be measured. Coupling
is accomplished between the frequency meter and the output of the equipment under test. (NOTE: This coupling
should be in accordance with the step-by-step procedures listed in the technical manual for the frequency meter.)
The calibrated oscillator is then tuned so that the difference between the oscillator frequency and the unknown
frequency is in the af range. This difference in frequency is known as the BEAT FREQUENCY. As the two frequencies
are brought closer to the same value, the tone in the headset will decrease in pitch until it is replaced by a
series of rapid clicks. As the process is continued, the clicks will decrease in rapidity until they stop
altogether. This is the point of zero beat; that is, the point at which the frequency generated in the oscillator
of the frequency meter is equal to the frequency of the unknown signal being measured.
Figure 2-9.—Basic heterodyne meter (block diagram).
Q-7. In a heterodyne-type frequency meter, what is the difference between the oscillator frequency and
the unknown frequency?
For all practical purposes, the point of zero beat can be assumed when the clicks
are heard at infrequent intervals. Figure 2-10 illustrates the zero-beat concept. Maintaining a condition of
absolute silence in the earphones is extremely difficult when you are making this measurement. When the incoming
signal is strong, the clicks are sharp and distinct. When the signal is weak, the zero-beat condition is evidenced
by a slowly changing "swishing" or "rushing" sound in the headset. After the zero beat is obtained, the dial
reading corresponds to the frequency measured.
Figure 2-10.—Graph of sound heard in earphone when zero beating.
The manufacturer’s calibration book is a very important part of the frequency meter package; in fact,
the book is so important that it bears the same serial number as the heterodyne-type frequency meter itself.
Contained in this book is a list of the dial settings and the corresponding frequencies produced by that meter at
those dial settings. Operating instructions for the meter are also included.
WAVEMETERS are calibrated resonant circuits used to measure frequency. The accuracy of wavemeters is not as high
as that of heterodyne-type frequency meters; however, they have the advantage of being comparatively simple and
can be easily carried.
Q-8. What equipment uses a calibrated resonant circuit to measure frequency?
Any type of resonant circuit can be used in wavemeter applications. The exact kind of circuit used depends on the
frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used for
VLF through VHF wavemeters.
The simplified illustration of an absorption wavemeter, shown in figure 2-11,
consists of a pickup coil, a fixed capacitor, a lamp, a variable capacitor, and a calibrated dial. When the
wavemeter’s components are at resonance, maximum current flows in the loop, illuminating the lamp to maximum
brilliance. The calibrated dial setting is converted to a frequency by means of a chart, or graph, in the
instruction manual. If the lamp glows very brightly, the wavemeter should be coupled more loosely to the circuit.
For greatest accuracy, the wavemeter should be coupled so that its indicator lamp provides only a faint glow when
tuned to the resonant frequency.
Figure 2-11.—Absorption wavemeter circuit.
FREQUENCIES ABOVE THE AUDIO RANGE
The signal frequencies of radio and radar
equipments that operate in the UHF and SHF ranges can be measured by resonant, cavity-type wavemeters or resonant,
coaxial-line-type wavemeters. When properly calibrated, resonant-cavity and resonant-coaxial line wavemeters are
more accurate and have better stability than wavemeters used for measurements in the LF to VHF ranges. These
frequency-measuring instruments are often furnished as part of the equipment. They are also available as
general-purpose test sets.
Although many wavemeters are used in performing various functions, the cavity-type wavemeter is the
type most commonly used. Only this type is discussed in some detail.
Figure 2-12 shows a typical CAVITY WAVEMETER. The wavemeter is of the type commonly used for the measurement of
microwave frequencies. The device uses a resonant cavity. The resonant frequency of the cavity is varied by means
of a plunger, which is mechanically connected to a micrometer mechanism. Movement of the plunger into the cavity
reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity
(made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment being
tested is fed into the wavemeter through one of two inputs, A or B. The crystal rectifier then detects (rectifies)
the signal. The rectified current is indicated on current meter M.
Figure 2-12.—Typical cavity wavemeter.
Electronic Frequency Counters
Another device used to measure frequencies above
the audio range is the ELECTRONIC FREQUENCY COUNTER. Since this instrument will be covered in detail in a later
chapter, only a brief description is provided at this time.
The electronic frequency counter is a
high-speed electronic counter with an accurate, crystal- controlled time base. This combination provides a
frequency counter that automatically counts and displays the number of events occurring in a precise time
interval. The frequency counter itself does not generate any signal; it merely counts the recurring pulses fed to
WAVEFORM ANALYSIS can be made by observing displays of voltage and current variations with respect to
time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and
testing pulse-generating, pulse-forming, and pulse-amplifying circuits. The waveform visual display is also useful
for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage.
Waveform analysis is used in various electrical and electronic equipment troubleshooting. This section will
briefly discuss the oscilloscope and spectrum analyzer to provide you with basic knowledge of this test equipment.
Q-9. Name two instruments used to analyze waveforms.
USE OF THE OSCILLOSCOPE
The CATHODE-RAY OSCILLOSCOPE (CRO or O-SCOPE) is commonly used for the analysis of waveforms generated by
electronic equipment. Several types of cathode-ray oscilloscopes are available for making waveform analysis. The
oscilloscope required for a particular test is determined by characteristics such as input-frequency response,
input impedance, sensitivity, sweep rate, and the methods of sweep control. The SYNCHROSCOPE is an adaptation of
the cathode-ray oscilloscope. It features a wide-band amplifier, triggered sweep, and retrace blanking circuits.
These circuits are desirable for the analysis of pulse waveforms.
Oscilloscopes are also part of some
harmonic analysis test equipments that display harmonic energy levels. To effectively analyze waveform displays,
you must know the correct wave shape. The maintenance instructions manual for each piece of equipment illustrates
what waveforms you should observe at the various test points throughout the equipment. Waveforms that will be
observed at any one selected test point will differ; each waveform will depend on whether the operation of the
equipment is normal or abnormal.
The display observed on a cathode-ray oscilloscope is ordinarily one
similar to those shown in figure 2-13. Views A and B show the instantaneous voltage of the wave plotted against
time. Elapsed time (view A) is indicated by horizontal distance, from left to right, across the etched grid
(graticule) placed over the face of the tube. The amplitude (view B) of the wave is measured vertically on the
Figure 2-13.—Typical waveform displays.
The oscilloscope is also used to picture changes in quantities other than simply the voltages in
electric circuits. For example, if you need to see the changes in waveform of an electric current, you must first
send the current through a small resistor. You can then use the oscilloscope to view the voltage wave across the
resistor. Other quantities, such as temperatures, pressures, speeds, and accelerations, can be translated into
voltages by means of suitable transducers and then viewed on the oscilloscope. A detailed discussion of the
oscilloscope is presented in chapter 6 of this module.
USE OF THE SPECTRUM ANALYZER
The SPECTRUM ANALYZER is a device that sweeps over a band of frequencies to determine (1) what frequencies are
being produced by a specific circuit under test and (2) the amplitude of each frequency component. To accomplish
this, the spectrum analyzer first presents a pattern on a display. Then the relative amplitudes of the various
frequencies of the spectrum of the pattern are plotted (see figure 2-14). On the vertical, or Y axis, the
amplitudes are plotted; on the horizontal, or X axis, the frequencies (time base) are plotted. The overall pattern
of this display indicates the proportion of power present at the various frequencies within the SPECTRUM
(fundamental frequency with sideband frequencies).
Figure 2-14.—Spectrum analyzer pattern.
Q-10. What device sweeps a band of frequencies to determine frequencies and amplitudes of each
The spectrum analyzer is used to examine the frequency spectrum of radar
transmissions, local oscillators, test sets, and other equipment operating within its frequency range. Proper
interpretation of the displayed frequency spectrum enables you to determine the degree of efficiency of the
equipment under test. With experience, you will be able to determine definite areas of malfunctioning components
within equipment. In any event, successful spectrum analysis depends on the proper operation of a spectrum
analyzer and your ability to correctly interpret the displayed frequencies. Later, in chapter 6, we will discuss
the various controls, indicators, and connectors contained on the spectrum analyzer.
TESTING SEMICONDUCTOR DEVICES
Because of the reliability of semiconductor devices, servicing techniques developed for transistorized
equipment differ from those normally used for electron-tube circuits. Electron tubes are usually considered to be
the circuit component most susceptible to failure and are normally the first components to be tested. Transistors,
however, are capable of operating in excess of 30,000 hours at maximum rating without failure. They are often
soldered in the circuit in much the same manner as resistors and capacitors. Therefore, they are NOT so quickly
removed for testing as tubes.
Substitution of a semiconductor diode or transistor known to be in good
condition is one method of determining the quality of a questionable semiconductor device. This method should be
used only after you have made voltage and resistance measurements. This ensures the circuit has no defect that
damage the substitute semiconductor device. If more than one defective semiconductor is present in the
equipment section where trouble has been localized, the semiconductor replacement method becomes cumbersome.
Several semiconductors may have to be replaced before the trouble is corrected. To determine which stage(s) failed
and which semiconductors are not defective, you must test all the removed semiconductors. You can do this by
observing whether the equipment operates correctly as you reinsert each of the removed semiconductor devices into
Semiconductor diodes, such as general-purpose germanium and silicon
diodes, power silicon diodes, and microwave silicon diodes, can be tested effectively under actual operating
conditions. However, crystal-rectifier testers are available to determine dc characteristics that provide an
indication of crystal- diode quality.
A common type of crystal-diode test set is a combination
ohmmeter-ammeter. Measurements of forward resistance, back resistance, and reverse current can be made with this
equipment. Using the results of these measurements, you can determine the relative condition of these components
by comparing their measured values with typical values obtained from test information furnished with the test set
or from the manufacturer’s data sheets. A check that provides a rough indication of the rectifying property of a
diode is the comparison of the back-and-forward resistance of the diode at a specified voltage. A typical
back-to-forward-resistance ratio is on the order of 10 to 1, and a forward-resistance value of 50 to 80 ohms is
Q-11. What is the typical back-to-forward resistance ratio of a good-quality diode?
Testing Diodes with an Ohmmeter
A convenient test for a semiconductor diode requires only an
ohmmeter. The back-and-forward resistance can be measured at a voltage determined by the battery potential of the
ohmmeter and the resistance range at which the meter is set. When the test leads of the ohmmeter are connected to
the diode, a resistance will be measured that is different from the resistance indicated if the leads are
reversed. The smaller value is called the FORWARD RESISTANCE, and the larger value is called the BACK RESISTANCE.
If the ratio of back-to-forward resistance is greater than 10 to 1, the diode should be capable of functioning as
a rectifier. However, keep in mind that this is a very limited test that does not take into account the action of
the diode at voltages of different magnitudes and frequencies. (NOTE: This test should never be used to test
crystal mixer diodes in radars. It will destroy their sensitivity.)
Testing Diodes with
An oscilloscope can be used to graphically display the back-and-forward resistance
characteristics of a crystal diode. A circuit used in conjunction with an oscilloscope to make this test is shown
in figure 2- 15. This circuit uses the oscilloscope line-test voltage as the test signal. A series circuit
(composed of resistor R1 and the internal resistance in the line-test circuit) decreases a 3-volt, open-circuit
test voltage to a value of approximately 2 volts peak to peak.
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics