To measure the center frequency and the limits of deviation
of a frequency-modulated carrier, proceed as follows:
a. Follow all instructions in paragraph 2-4 for measur-
ing the frequency of c-w signals, giving special
attention to step f .
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b. Since the beat-frequency will be varying at the rate
of the frequency modulation, it is not possible to
reduce the beat-frequency to a simple zero. -Instead,
the carrier frequency sweeps through a zero-beat
with the 540A twice during each cycle of modulation,
first in one direction then in the other. Consequently,
two zero-beat points will be obtained simultaneously
on the 540A oscilloscope sweep. With the phase
control superimpose the two zero-beats or separate
them so they do not interfere. Figure 9a and d
show the beat superimposed while the 543A isyuned
to the approximate center frequency of the carrier.
Figures 9b and c show the same pattern as it would
appear first witz the 540A tuned to one limit of fre-
quency deviation, then to the opposite limit of de-
viation.
2-6 MEASURING PULSED R-F SIGNALS
The carrier frequency of pulsed r-f signals is measured by ob-
serving the actual carrier beating which occurs during one pulse
of r-f energy. In order to observe a single pulse of r-f energy,
the beat frequency from the video amplifier in the 540A must be
connected to a synchroscope whose sweep is synchronized with the
source of the video pulse. The scope is adjusted to display one
pulse of r-f energy over a large portion of the screen. The pat-
tern within the pulse envelope will then indicate when the beat-
frequency is tuned toward zero. Figure lla shows a typical pat-
tern obtained when the pulsed carrier is close to a zero-beat
and lld shows a typical pattern at zero beat. The lines within
the puEe will be exactly parallel only at zero-beat.
Carriers pulsed as short as 1 microsecond are readily indicated
by this method. If the pulse is shorter than one microsecond it
will be necessary to take the heterodyne signal from the ,MIXER
OUTPUT jack on the 540A and amplify it in an external broadband
amplifier having the required gain and bandwidth for the pulse
being measured and then apply it to the external synchroscope. To
measure the frequency of a pulsed r-f carrier refer to Figure 10
and proceed as follows:
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2-11 |
ACEC
