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Model 492A/494A
Section IV
Paragraphs 4-22 to 4-25
)
time constant circuit.
The discharge tube, V5B, is
connected between the helix of the twt and chassis to
form a low-impedance and therefore a rapid-discharge
circuit for this capacity. A negative-going modulating
signal applied to the HELIX MOD. is coupled to the
helix through the cathode follower, V5A, and simul-
taneously inverted by V5A and applied to the grid of
V5B.
The stray capacity is thus able to discharge
rapidly through V5B, allowing a rapid decay time
when fast pulses are applied to the HELIX MOD. jack.
4-22. CONSTANT-AMPLITUDE, LINEAR SAW-
TOOTH GENERATOR.
4-23. To shift the phase or frequency of the
rf
signal
in a traveling wave tube amplifier, a sawtooth such
as is used in oscilloscope sweep circuits can be
applied to the traveling wave tube helix.
If
the rf out-
put signal is to be a nearly pure offset frequency and
contain a minimum of spurious components, the saw-
tooth waveform applied to the helix must be constant
in amplitude, must be linear, must not contain noise
or ripple and must have a fast flyback time. In addi-
tion to frequency-modulate an
rf
signal, the sawtooth
amplitude and the sawtooth repetition rate must
be
completely independent and separately adjustable and
the repetition rate should be adjustable over a wide
range.
To frequency-modulate an
rf
signal with a
traveling wave tube amplifier it is necessary to mod-
ulate the slope or repetition rate of the sawtooth wave
without affecting the sawtooth amplitude.
4-24. A sawtooth generator having these character-
istics is shown in simplified form in figure 4-5.
This generator consists of an adjustable, regulated
power supply, a charging capacitor, a blocking oscil-
lator to charge the capacitor, a pentode tube to dis-
charge the capacitor, and a cathode follower to isolate
the generator from the output.
4-25. The broken line in figure 4-5 indicates the
capacitor-charging circuit, the solid line the discharge
circuit.
When power is applied to the generator, the
grid bias on the blocking oscillator tube is zero and
the oscillator goes through one cycle of operation.
During the oscillation cycle, the tube conducts heavily
and rapidly charges the capacitor to the B+ voltage,
t----@
SAWTOOTH
OUTPUT
SAWTOOTH
SLOPE
CD
PENTODE DISCHARGES
CAPACITOR AT A RATE
DETER~INED
BY GRID
BIAS
-I-----4~t------@
SLOPE
MODULATION
INPUT
o
VOLTS
B+r-----
I
~
CD
BLOCKING OSCILLATOR
;I
CHARGES CAPACITOR
I
RAPIDLY TO THE Bt
I
VOLTAGE
I
I
I
I
I
I
VARIABLE Bt
ADJUSTS
+
A~PLI
TUDE
OF OUTPUT
SAWTOOTH
B- ...
......-----4....--------J
L 0- L-41
Figure 4-5. Simplified Circuit of a Constant Amplitude, Variable Slope Sawtooth Generator
00144-2
4-5
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