35. Check Incidental Frequency Modulation
NOTE
Signal source must supply a very stable 200 MHz
signal to accurately measure incidental FM and
the Type 491 must be on a vibration-free platform.
a. Equipment setup is given in step 34.
b. Set the DISPERSION RANGE switch to kHz/DIV and
the DISPERSION to 500 kHz/div. Set the TIME/DIV to .1 s.
c. Apply a 200 MHz signal from the Time-Mark Generator
through a 20 dB attenuator to band A RF INPUT connector
and center the IF feedthrough signal on screen.
d. Change the DISPERSION-COUPLED RESOLUTION to
1 kHz/div, adjusting the IF CENTER FREQ control to keep
the signal centered on screen.
e. Adjust the GAIN control for an 8 division signal
amplitude.
f. Check the display frequency modulation (see Fig. 6-37).
Must not exceed 1 minor division
(≤200 Hz).
g. Change the DISPERSION to 100 kHz/div and move the
IF feedthrough signal off screen with the IF CENTER FRE-
QUENCY control. Center the tunable 200 MHz signal on
screen with the RF CENTER FREQUENCY controls.
h. Turn the INT REF FREQ on and phase lock the display.
See Operating Instructions.
i. Decrease the DISPERSION to 1 kHz/div, keeping the
phase locked signal on screen with the IF CENTER FREQ
controls.
j. Check the frequency modulation of the display. Must
not exceed 1.5 minor divisions (300 Hz).
k. Return the DISPERSION-COUPLED RESOLUTION con-
trols to 500 kHz/div.
6-35
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