Model 8640B TM 9-4935-601-14-7&P
ADJUSTMENTS
5-40. FM SENSITIVITY ADJUSTMENT (Cont'd)
3. Set Signal Generator's FM switch to AC. Adjust variable-phase generator for a frequency counter reading of
16.63 kHz at 707 mVrms as read on DVM.
4. Adjust MID FM SENS adjustment, A3A4R3, for at least 50 dB of carrier null.
NOTE
The carrier is the center spectrum line on the display. A 50 dB null is when it drops 50 dB below
its CW amplitude (set in step 2).
5. Set Signal Generator's FREQUENCY TUNE to 16 MHz. Adjust analyzer to center the carrier on the display.
Adjust LOW FM SENS adjustment, A3A4R2 for at least 50 dB of carrier null.
6. Set Signal Generator's FREQUENCY TUNE to 32 MHz. Adjust analyzer to center the carrier on the display.
Adjust HI FM SENS adjustment, A3A4R4, for at least 50 dB of carrier null.
7. Repeat steps 4 through 6 until carrier null is ≥ 50 dB at 16, 24, and 32 MHz.
8. Perform the FM distortion and FM sensitivity and accuracy tests in Section IV.
5-41. INTERNAL REFERENCE FREQUENCY ADJUSTMENT
REFERENCE:
Service Sheet 19.
DESCRIPTION:
An oscilloscope is used to display a Lissajous figure (2:1) to set the internal reference frequency. The Lissajous figure is
derived from the 10 MHz reference of a frequency counter and the Signal Generator's 5 MHz internal reference. This
procedure should be performed whenever the internal reference is found to be out of specification.
5-34
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