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Frequency Modulation - HP 492A Operating And Service Manual

Traveling-wave
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)
Model 492A/494A
limit is determined by the crystal and IF amplifier
noise and is approximately -100 dbm.
ote
When a swr indicator (such as the <[j) Model
4158) calibrated for use with a square-
law detector is used in place of the tuned
amplifier and voltmeter, the db readings
must be doubled.
3-37. FREQUENCY MODULATION.
3-38. Narrow-band frequency modulation can be ob-
tained by applying the modulation signal directly to
the helix of the twt; however, to frequency modulate
with an appreciable frequency deviation it is first
necessary to produce an offset frequency as described
in paragraph 3-29.
The deviation of the offset fre-
quency should be slightly greater than 1/2 the total
frequency deviation desired. The offset frequency is
then varied by varying the slope of the sawtooth.
3-39. A sawtooth waveform produced by the special
generator described in paragraph 4-20 can be slope-
00144-2
Section 1II
Paragraphs 3-37 to 3-40
modulated by any waveform before being applied to
the traveling wave tube helix. In this manner complex
signals can be used to frequency-modulate the signal
applied to the twt and the center of the output fre-
quency will be fixed by the sawtooth repetition rate
without slope modulation. In no case should the
p~ase
shift due to a single sawtooth cycle exceed 360 so
that the amplification properties of the traveling wave
tube amplifier will not be adversely affected regard-
less of the magnitude of the apparent phase deviation
when the sawtooth wave is modulated.
3-40. Figure 3-10 is a block diagram of a system for
the generation of frequency-modulated offset signals.
In this arrangement, the slope-modulated sawtooth
voltage which is applied to the twt helix produces an
offset frequency, the instantaneous frequency of which
is proportional to the slope of the sawtooth. Varying
the slope of the sawtooth voltage varies the offset fre-
quency.
Thus the output signal from the twt is an
fm signal.
The detected fm signal (the difference
between the signal generator frequency and the fre-
quency-modulated offset frequency) is shown on the
oscilloscope.
3-9

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