d. Adjust the GAIN control clockwise until the signal
b. Set the VERTICAL DISPLAY switch to LIN, adjust the
amplitude is again 8 divisions. This checks that the range
Signal Generator output and the Type 491 GAIN control
of the control equals or exceeds 50 dB.
for a signal amplitude of 6 divisions.
c. Connect the TO RECORDER connector on the back
20. Check INTENSIFIER Control Range
a 1 probe. Terminate the connector into a 600 Ω load by
connecting a 600 Ω resistor from the TO RECORDER jack
a. Requirement-With the control in the OFF position
(fully counterclockwise) the display, plus the baseline, must
to chassis ground.
be intensified. With the control fully on, the baseline plus
d. Check-Signal amplitude at the TO RECORDER con-
approximately 30% of the signal should be suppressed.
nector should equal or exceed 24 mV (≥4 mV/div).
b. Change the DISPERSION to 100 kHz/div. Uncouple
the RESOLUTION control and turn the control fully clock-
22. Check Incidental Frequency Modulation
a. Requirement-With the DISPERSION RANGE at kHz/
c. Set the VERTICAL DISPLAY switch to LOG position,
DIV the IF incidental FM should not exceed 200 Hz. The
then adjust the GAIN control and/or the Signal Generator
incidental FM of the local oscillator plus the IF with phase
output for a signal display amplitude of 8 divisions.
lock must not exceed 300 Hz.
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
Signal source must supply a very stable signal
display is intensified.
to accurately measure this performance and the
e. Turn the INTENSIFIER control full on or clockwise.
Type 491 must be on a stable, vibration-free plat-
Incidental FM measurements for bands B
and C must be performed with the MIXER PEAKING
adjusted to maximize signal amplitude for each
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-
f. Check the amount of signal frequency modulation (see
Must not exceed 1 minor division.
( ≤ 2 0 0 H z ) .
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.
Fig. 5-6. Typical display showing incidental frequency modulation.
Measure the horizontal displacement of the signal at the most
i. Decrease the DISPERSION to 1 kHz/div, keeping the
vertical slope of the signal.
phase locked signal on screen with the IF CENTER FREQ
f. Check-The remaining intensified portion of the signal
j. Check the amount of frequency modulation in the dis-
should measure between 3 and 5 divisions.
play. Must not exceed 1.5 minor divisions (300 Hz).
k. Return the DISPERSION-COUPLED RESOLUTION con-
21. Check Signal Amplitude at the TO
trols to 500 kHz/div.
a. Requirement-Signal amplitude at the TO RECORDER
23. Check Display Flatness
output connector should equal or exceed 4 mV per division
a. Requirement-Display flatness with the IF CENTER FREQ
of displayed signal amplitude when terminated into a 600
controls centered, is 3 dB maximum amplitude variation