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Module 21 - Test Methods and Practices |
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Module 21 − Test Methods and Practices
Pages i , 1−1, 1−11, 1−21, 2−1, 2−11, 2−21, 2−31, 2−41, 3−1, 3−11, 3−21, 3−31, 4−1, 4−11, 5−1, 5−11, 5−21, 5−31, AI−1 to AI−3, Index
Figure 5-31. - VSWR test for transmitting antennas.
Figure 5-32. - Typical spectrum analyzer VSWR display.
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Figure 5-33. - VSWR test for receiving antennas.
Q-12. What precautions must be taken when sweeping a transmitting antenna?
Summary
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 applications are 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.
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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
5-33 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.
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REFERENCES
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-1. Distortion.
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 signals).
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 transmission line.
A-12. You must ensure that power induced from any adjacent transmitting antennas does not damage your test equipment.
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