
Model 8640B
TM 9-4935-601-14-7&P
PERFORMANCE TESTS
4-22. RESIDUAL FM TEST (Cont'd)
PROCEDURE:
1. Connect equipment as shown in Figure 4-9 after setting test Signal Generator's controls as follows:
Meter Function .......................................................................................... LEVEL
COUNTER MODE: EXPAND .......................................................................... Off
LOCK .............................................................................. Off
Source ........................................................................... INT
AM
.........................................................................................................OFF
FM
........................................................................................................... AC
PEAK DEVIATION ................................................................................. 320 kHz
PEAK DEVIATION Vernier ....................................................................... Full cw
RANGE ......................................................................................... 256 - 512 MHz
FREQUENCY TUNE ............................................................................. 500 MHz
OUTPUT LEVEL ..................................................................................... -7 dBm
RF ON/OFF ....................................................................................................ON
2. Install shorting board in discriminator and calibrate it for 1 Vdc (at the output jack) for a full-scale meter reading.
Remove shorting board, prepare a 20 kHz Butterworth low-pass filter (from the filter kit), and install the filter in
the discriminator.
3. Set reference signal generator for a 500.10 MHz, CW signal at +13 dBm.
4. Connect discriminator to mixer. Set discriminator's range to 100 kHz and sensitivity to 0.01 Vrms. Fine tune
either generator for a full-scale meter reading on the discriminator.
5. Connect amplifier to discriminator output. Connect the voltmeter through the 15 kHz LPF to amplifier's output.
The signal out of the amplifier is 0.5 mVrms per 1 Hz (rms) of residual FM deviation, and the average voltmeter
reading should be less than 7.5 mVrms (i.e., <15 Hz (rms) residual FM).
--------------------7.5 mVrms
NOTE
Test setup calibration can be checked by setting the test generator's FM to INT, PEAK
DEVIATION to 5 kHz (vernier full cw), and MODULATION FREQUENCY to 1000 Hz. The voltmeter
should read 1.77 Vrms.
6. Connect the capacitor between amplifier and filter. Replace 15 kHz LPF with 3 kHz LPF. The average voltmeter
reading should be less than 2.5 mVrms (i.e., <5 Hz (rms) residual FM).
------------------2.5 mVrms
7. Set test Signal Generator's PEAK DEVIATION switch to 2.56 MHz. The average voltmeter reading should be
less than 7.5 mVrms (i.e., <15 Hz (rms) residual FM).
------------------7.5 mVrms
4-23
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