
Model 8640B
TM 9-4935-601-14-7&P
PERFORMANCE TESTS
4-38. FM 3 dB BANDWIDTH TEST (Cont'd)
AM ................................................................................................................OFF
FM ................................................................................................................OFF
PEAK DEVIATION ................................................................................... 80 kHz
PEAK DEVIATION Vernier........................................................................ Full cw
RANGE ...............................................................................................8 -16 MHz
FREQUENCY TUNE .................................................................................8 MHz
OUTPUT LEVEL ...................................................................................+13 dBm
RF ON/OFF ....................................................................................................ON
2. Prepare a 1 MHz Butterworth low-pass filter and install it in the discriminator. Set discriminator's range to 10 MHz and
input sensitivity to 1V.
3. Set Signal Generator's FM switch to DC. Set spectrum analyzer's resolution bandwidth to 3 kHz and its center
frequency controls to 1 kHz (with no sweep). Set analyzer's tracking generator output level for 80 kHz peak deviation
as read on generator's panel meter. Set the analyzer's frequency controls for a 0 to 250 kHz sweep. Set the
analyzer's display for 2 dB per division; adjust the display reference level controls to display the demodulated sweep.
4. Measure the sweep on the analyzer's display. Total amplitude variation from 20 Hz to 250 kHz should be <3 dB.
________3 dB
NOTE
If the FM discriminator's incidental AM rejection is insufficient, the generator could appear to be
out of specification. To check the discriminator, note analyzer's reading (in dBm), set
generator's AM switch to AC and connect analyzer's tracking generator output to AM INPUT. Set
MODULATION for 10% as read on panel meter. The analyzer should read >30 dB below the
reading noted above. If it does not, adjust discriminator sensitivity and trigger level (or
generator's OUTPUT LEVEL vernier) until it does. Then repeat steps 2 through 6.
4-39. FM DISTORTION TEST
SPECIFICATION:
FM Distortion: (at 400 and 1000 Hz rates)
<1% for deviations up to 1/8 maximum allowable.
<3% for maximum allowable deviation.
4-62
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