
Model 8640B
TM 9-4935-601-14-7&P
ADJUSTMENTS
5-44. FM LINEARITY ADJUSTMENT (Alternate) (Cont'd)
2. Using the filter kit, prepare a 25 kHz Butterworth low-pass filter and install it in the discriminator.
3. Set the discriminator's range to 1 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 5 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. Adjust POS SHAPE and NEG SHAPE adjustments, A7R12 and A7R41, for minimum distortion.
Observe both second and third harmonics.
7. Slowly tune from 0.5 to 1 MHz and observe distortion. If harmonics are less than 30 dB down (3% distortion) or if
it is desired to minimize distortion across the band, adjust A7R12 and A7R41 for best compromise. However,
harmonics must always be greater than 30 dB down.
8. Perform the FM sensitivity adjustment (5-40).
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