d. Adjust the GAIN control clockwise until the signal
amplitude is again 8 divisions. This checks that the range
of the control equals or exceeds 50 dB.
20. Check INTENSIFIER Control Range
a. Requirement-With the control in the OFF position
(fully counterclockwise) the display, plus the baseline, must
be intensified. With the control fully on, the baseline plus
approximately 30% of the signal should be suppressed.
b. Change the DISPERSION to 100 kHz/div. Uncouple
the RESOLUTION control and turn the control fully clock-
wise.
c. Set the VERTICAL DISPLAY switch to LOG position,
then adjust the GAIN control and/or the Signal Generator
output for a signal display amplitude of 8 divisions.
d. Turn the INTENSIFIER control to the OFF position,
adjust the INTENSITY control for a display of nominal bright-
ness. Set the CONTRAST to midrange. Note that the entire
display is intensified.
e. Turn the INTENSIFIER control full on or clockwise.
Fig. 5-6. Typical display showing incidental frequency modulation.
Measure the horizontal displacement of the signal at the most
vertical slope of the signal.
f. Check-The remaining intensified portion of the signal
should measure between 3 and 5 divisions.
21. Check Signal Amplitude at the TO
RECORDER Connector
a. Requirement-Signal amplitude at the TO RECORDER
output connector should equal or exceed 4 mV per division
of displayed signal amplitude when terminated into a 600
ohm load.
b. Set the VERTICAL DISPLAY switch to LIN, adjust the
Signal Generator output and the Type 491 GAIN control
for a signal amplitude of 6 divisions.
c. Connect the TO RECORDER connector on the back
panel to the Vertical Input of a test oscilloscope through
a
1× probe. Terminate the connector into a 600 Ω load by
connecting a 600
Ω resistor from the TO RECORDER jack
to chassis ground.
d. Check-Signal amplitude at the TO RECORDER con-
nector should equal or exceed 24 mV
(≥4 mV/div).
22. Check Incidental Frequency Modulation
a. Requirement-With the DISPERSION RANGE at kHz/
DIV the IF incidental FM should not exceed 200 Hz. The
incidental FM of the local oscillator plus the IF with phase
lock must not exceed 300 Hz.
NOTE
Signal source must supply a very stable signal
to accurately measure this performance and the
Type 491 must be on a stable, vibration-free plat-
form. Incidental FM measurements for bands B
and C must be performed with the MIXER PEAKING
adjusted to maximize signal amplitude for each
display window.
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 Gen-
erator through a 20 dB attenuator to band A RF INPUT
connector and center the IF feed-through 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 ampli-
tude,
f. Check the amount of signal frequency modulation (see
Fig. 5-6). 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
FREQUENCY control. Center a tunable 200 MHz signal an
screen with the RF CENTER FREQUENCY controls. Adjust the
MIXER PEAKING control for maximum signal amplitude.
h. Turn the INT REF FREQ on and phase lock the dis-
play. 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 amount of frequency modulation in the dis-
play. Must not exceed 1.5 minor divisions (300 Hz).
k. Return the DISPERSION-COUPLED RESOLUTION con-
trols to 500 kHz/div.
23. Check Display Flatness
a. Requirement-Display flatness with
controls centered,
is 3 dB maximum
the IF CENTER FREQ
amplitude variation
5-9
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