
Model 8640B
TM 9-4935-601-14-7&P
PERFORMANCE TESTS
4-39. FM DISTORTION TEST (Cont'd)
2. Using the filter kit, prepare a 1 MHz Butterworth low-pass filter and install it in the discriminator.
3. Set discriminator's range to 10 MHz and sensitivity to 1V.
4. Set spectrum analyzer's resolution bandwidth to 100 Hz and its center frequency controls for a 0 to 5 kHz span. Set
the display for 10 dB per division.
5. Use generator's PEAK DEVIATION vernier to set 80 kHz of peak deviation (as read on panel meter). Use analyzer's
display reference level controls to set the demodulated 1 kHz signal to the top (reference) graticule line on the display.
6. Note the level of the 1 kHz signal's harmonics (2 kHz, 3 kHz, etc.). For less than 3% distortion, they should be more
than 30.5 dB below the reference graticule line.
Maximum Deviation: 30.5 dB
7. Set generator's PEAK DEVIATION switch to 10 kHz. If necessary, use generator's PEAK DEVIATION vernier to set
10 kHz of peak deviation; use analyzer's display reference level controls to set the demodulated 1 kHz signal to the
reference graticule line.
8. For less than 1% distortion, the 1 kHz signal's harmonics should be more than 40 dB below the reference graticule
line.
1/8 Maximum Deviation: 40 dB
4-40. FM SENSITIVITY AND ACCURACY TEST
SPECIFICATION:
External FM Sensitivity: 1 volt peak yields maximum deviation indicated on PEAK DEVIATION switch with FM vernier at
full cw position.
External FM Sensitivity Accuracy: +6% from 15° to 35°C for FM excluding max peak deviation position. Maximum peak
deviation position, +9% typically.
Indicated FM Accuracy: (400 Hz and 1 kHz rates using internal meter) +10% of meter reading (for greater than 10% of
full scale).
DESCRIPTION:
The Signal Generator's FM sensitivity is checked using the carrier (Bessel) null technique. An externally applied 1 Vpk
signal is used to FM the generator. The modulation signal's frequency is adjusted for the first order null of the carrier and
the frequency is measured to find peak deviation. (For the first order null of the carrier, peak deviation equals 2.405
times the modulation rate.) The panel meter accuracy is found by comparing its reading to the given peak deviation. The
reference generator and mixer convert the signal into the range of the spectrum analyzer.
4-64
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