Module 21—Test Methods and Practices
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to AI-3, Index
Figure 5-31.—VSWR test for transmitting antennas.
Figure 5-32.—Typical spectrum analyzer VSWR display.
Figure 5-33.—VSWR test for receiving antennas.
Q-12. What precautions must be taken when sweeping a transmitting antenna?
This chapter has presented information on waveform interpretation. The information that follows
summarizes the important points of this chapter.
Interpretation of a waveform is best accomplished with
test equipment that gives you a visual indication of the waveform. The most common devices used in systems
OSCILLOSCOPES and SPECTRUM ANALYZERS.
An amplitude-modulated signal can
be tested with either an oscilloscope or a spectrum analyzer to determine its percentage of modulation, sideband
characteristics, and carrier frequency. Frequency- modulated signals are normally tested with a spectrum analyzer
or a modulation analyzer.
Oscilloscopes are designed to view a time-domain waveform (amplitude versus time). Spectrum analyzers
are designed to view a frequency-domain waveform (amplitude versus frequency). One advantage of using a spectrum
analyzer is its ability to graphically display the composition of COMPLEX WAVEFORMS.
TIME-DOMAIN REFLECTOMETERS work on a principle similar to that used in radar. A precise
signal is produced by the time-domain reflectometer and injected into the device under test (usually a
transmission line); and the resulting reflections are displayed to discover such things as
impedance mismatches, opens, and shorts. The display sections of time-domain reflectometers are
calibrated to give you a graphical display of amplitude versus distance.
SWEPT-FREQUENCY TESTING is performed by using a TRACKING GENERATOR
to inject a signal into a device and then monitoring the output of the device with a spectrum analyzer. The
tracking generator is designed to sweep or scan through the entire frequency range of the device being tested. Its
sweep rate must be matched with the sweep rate of the spectrum analyzer.
Communications Systems, NAVTELCOMINST 2313.1, Naval Telecommunications Command, Washington, D.C., 1984.
EIMB, Test Methods and Practices, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980.
Modulation Principles, NAVEDTRA 172-12-00-83. Naval Education and Training Professional Development and Technology
Center, Pensacola, Fla., 1983.
ANSWERS TO QUESTIONS Q1. THROUGH Q12.
A-2. 60% to 95%.
A-3. The difference between the carrier frequency of an fm signal and its maximum
frequency excursion when modulated.
A-4. Amplitude versus frequency (the frequency domain of the
A-5. The same amount of intelligence can be transmitted with one-sixth of the output power
with less than one-half the bandwidth.
A-6. Both the bandwidth and the number of significant sidebands increase.
A-7. The ability of the
analyzer to discriminate between display signals of slightly different frequencies.
A-8. Both transmit a pulse and analyze the signal reflection.
A-9. A TDR will indicate the nature
of and the distance to or location of any faults.
A-10. To determine various characteristics of a
component, piece of equipment, or system over its operational frequency range.
A-11. SWR on the
A-12. You must ensure that power induced from any adjacent transmitting antennas
does not damage your test equipment.
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