b, Apply 10 ns and 1 s markers from the Time-Mark
Generator through the 20 dB attenuator to the band A RF IN-
PUT. Set the DISPERSION to 500 kHz/div. Preset the DIS-
PERSION BAL R234, to its midrange position and the kHz/div
Cal R368 (Fig. 6-30) approximately 90 counterclockwise
from its fully clockwise position.
c. Adjust C384 and C385 simultaneously for 1 marker/2
div. Keep the IF feedthrough signal centered on the graticule
as this adjustment is made.
d. Adjust the kHz/div Cal R368, for optimum dispersion
linearity. These adjustments interact, so repeat the adjust-
ments until optimum dispersion linearity and accuracy is
obtained.
NOTE
An alternate source of frequency markers may be
obtained by using the Harmonic Modulator (See
equipment list) with a 100 MHz or 200 MHz source
and an accurate audio signal source. Apply the RF
signal to the RF Input and the audio frequency to
the Mod Freq 1 Input of the Harmonic Modulator.
Connect the Modu Harm Out connector through
a 20 dB attenuator to the RF INPUT connector of
the Type 491. This will provide an IF feedthrough
signal modulated by the audio frequency for fre-
quency markers.
25. Check Dispersion of the kHz/div Selector
Positions
a. Test equipment setup is described in step 24.
b. Apply 10 ns and 1 s m a r k e r s f r o m t h e T i m e - M a r k
Generator through the 20 dB attenuator and proper adapter
to band A RF INPUT or through the proper adapter to the
Waveguide Adapter for band C. Set the band selector to
the appropriate band and the DISPERSION selector to 500
kHz/div.
c. Check-The range of the IF CENTER FREQ control by
rotating the control to the limits each side of center. Count
the number of 1 MHz (1 s) markers from the 200 MHz (20
ns) feedthrough signal. Must equal or exceed 2.5 MHz.
Note the dial reading when the control is 2.5 MHz from
center. This reading will be used later in the procedure.
6-27
