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Section III
Paragraphs 3-23 to 3-30
output.
The linear modulation region extends from
approximately +4 volts (where beam de-focusing
occurs) to approximately -15 volts (where the rf out-
put becomes an exponential function of the grid volt-
age). This linear operating region permits up
to
30%
modulation with less than 2.5% envelope distortion
and up to 50% modulation with less than 5% distortion.
Envelope distortion increases rapidly above 50% modu-
lation.
In pulse work the twt may be biased near or
below cut-off and the rf drive adjusted for saturation
output at the peak amplitude of the modulating pulse.
However, the grid voltage at the peak of the modu-
lating signal must not cause de-focusing (approxi-
mately +4 to + 8 volts) nor excessive average electrode
current, see table 3-1. The transient response of the
492A or 494A to a step function applied to the grid is
approximately 15 nsec.
3-23. Amplitude modulation is accompanied by some
incidental phase modul'btion of the rf signal, amount-
ing to approximately 90 phase shift of the rf carrier
for a 10 db change in the modulated rf output level.
In practice this phase modulation is unimportant when
using the conventional square-law crystal detectors,
but is important in detection systems where the out-
put is a function of the rf carrier phase.
3-24. PULSE MODULATION.
3-25. There is considerably latitude in the adjustment
of modulation characteristics when pulse modulating
an rf signal using the 492A or the 494A; see figures
3-5 and 3-6.
The cw input level, the modulation-
pulse amplitude, and the grid bias determine the
characteristics of the rf output pulse as follows:
a. The cw input signal primarily determines the
maximum possible level of the rf output pulse and
whether or not the twt can be operated into saturation.
b. The peak-to-peak amplitude of the modulating
pulse primarily determines the on-off ratio of the rf
output pulse.
c. The grid bias level primarily determines the rf
output levels attained during the pulse-on and pulse-
off times and also, in conjunction with the modulating
pulse, determines the rf input level necessary
to
saturate the twt.
The GRID BIAS control always
should be set so that the twt grid will not draw cur-
rent (approximately 4 volts positive) during the pulse-
on period.
3-26. To pulse modulate the rf signal being amplified
in the 492A or 494A, refer to figures 3-5 or 3-6 and
proceed as follows:
a. Determine if the twt is to be driven into satura-
tion and if the rf output must be at a specific level.
b. Set the GRID BIAS control for zero bias.
c. Connect the rf input signal
to
the twt and adjust
its level to produce the desired rf pulse output level.
d. Determine the on-off voltage ratio required in
the rf output pulse.
3-6
Model 492A/494A
e. Using the graph in figure 3-5 determine the
magnitude of modulation pulse required to produce the
desired on-off ratio.
f. Set the
GRID BIAS control to obtain the voltage
determined in step e., Le., the peak voltage of the
modulating pulse. The bias VOltage may be measured
at the pin jack on the front panel.
g. Connect the modulating pulse to the GRID MOD.
connector and adjust its amplitude to the voltage de-
termined in step e to produce the desired on-off ratio
in the rf output pulse. Since the grid of the twt is con-
nected directly to the GRID MOD. jack, a dc com-
ponent in the modulating signal will affect the grid
bias.
Also, if capacitive coupling is used the modu-
lating signal will drive the grid of the twt above and
below the dc level established by the grid bias, an
amount determined by the duty cycle of the modulating
signal.
The GRID BIAS control must be adjusted to
compensate for both of these effects.
h. To increase the on-off ratio of the rf output
pulse, increase the amplitude of the modulation pulse,
at the same time adjust the grid bias so that the grid
will not be driven beyond 4 volts positive, see the Note
paragraph 3-21.
Note
Large input modulating pulses, above 15
volts, tend
to
shock-excite the helix, produc-
ing ringing on the top of the rf output pulse
and a slow rise time.
If
the traveling-wave
tube is operated near saturation this effect is
minimized and better pulse characteristics
are obtained.
3-27. LIMITED PHASE MODULATION.
3-28. The signal being amplified in the 492A or 494A
can be phase-modulated by applying voltage to the
HELIX MOD. connector.
This voltage varies the
electron-beam velocity by changing the potential be-
tween the cathode and the helix--a positive voltage
change accelerates the electron bunches and advances
the phase of the rf output signal; a negative change
slows them and retards the phase of the output signal.
The resultant phase deviation in the output signal is
directly proportional to the applied voltage. The de-
gree of phase deviation produced is limited by the
range of helix voltages that produces amplification,
and by the amount of incidental amplitude modulation
permissible in the rf output. Phase deviation of 360
0
is possible with the output amplitude held to variations
of approximately 1-1/2 db and is obtained with a helix
voltage variation of less than 50 volts.
The actual
voltage required for a phase shift of 360
0
varies with
the operating frequency and from tube-to-tube.
3-29. UNLIMITED PHASE MODULATION AND
FREQUENCY SHIFTING.
3-30. Although the limited phase deviation described
in paragraph 3-27 is useful in some applications, un-
limited phase deviation has a much wider range of
usage. It is particularly useful because the frequency
00144-2
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