
Model 8640B
TM 9-4935-601-14-7&P
ADJUSTMENTS
5-38. PRELIMINARY FM ADJUSTMENTS (Cont'd)
10. Reset PEAK DEVIATION switch to 2.56 MHz. Turn PEAK DEVIATION vernier and FREQUENCY TUNE control
through their ranges. The voltage at A7TP3 should remain less than 1.5 mVdc.
_____________1.5 mVdc
11. Set FM switch to OFF and note frequency displayed on generator's counter. Set FM to DC; the frequency should
change less than 800 Hz.
_____________800 Hz
12. To set VAR pot (VARACTOR BIAS), A7R19, connect DVM to VARACTOR ANODE test point, A7TP2, and check
that voltage is -14.70 ± 0.01 Vdc. If it is not, adjust A7R19 until it is.
13. Perform the FM Linearity Adjustment, paragraph 5-39.
5-39. FM LINEARITY ADJUSTMENT
REFERENCE:
Service Sheet 7.
DESCRIPTION:
The positive and negative shaping circuits are adjusted to match the characteristics of the varactors in the RF oscillator.
The reference output of a variable-phase generator is used to drive the Signal Generator's FM circuits; its variable phase
output is used to drive an oscilloscope's horizontal circuits and the FM linearity circuit. A discriminator is used to
demodulate the FM and the demodulated signal is subtracted (i.e., summed 180° out of phase) from the modulation
signal in the FM linearity circuit and fed to the oscilloscope's vertical circuits. The shaping circuits are then adjusted for
the flatest trace possible on the oscilloscope's display. A reference signal generator and a mixer are used to down-
convert the test generator's output to within the range of the discriminator.
NOTE
The Preliminary FM Adjustment (5-38) should be made before performing this adjustment.
A simpler method for adjusting FM linearity, using less test equipment, is presented in paragraph
5-44. This alternate method however, is not as effective for locating the source of FM distortion
when used in troubleshooting.
5-29
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