Constant Gain Or Constant Output; Amplification; Buffer Amplifications; Amplitude Modulation - HP 492A Operating And Service Manual

Model 492A/494A
Section III
Paragraphs 3-14 to 3-22
is developed which is then applied to the twt grid to
hold its amplification constant.
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pOSltlOn. To obtain the most nearly constant amplifier
gain over the full frequency range set the HELIX con-
trol to 5, the setting which yields the optimum broad-
band helix voltage. Since noise power is directly
proportional to bandwidth, it may be desirable to
limit the bandwidth and therefore noise by maximizing
the gain at a particular frequency with the HELIX
control and by installing suitable filters at the output.
3-14. CONSTANT GAIN OR CONSTANT OUTPUT
AMPLIFICATION.
SIGNAL
GENERATOR
TRAVEL ING -WAVE
TUBE ANPLIflER
TO
GRID MOD
INPUT
) (
DIRECTIONAL
COUPLER
Rf
OUTPUT
":>--------o------=<o-----+-- OUT
LO-l-66
00
L=J-=-~/~
_ _ , """",,---_--,
BALANCE CONTROL
(SET TO OBTAIN 0 VOLTS TO TWT
GRID MOD. CONNECTOR, AT DESIRED
AMPLIfiCATION)
3-19. The 492A and 494A serve as very effective
buffers to isolate a microwave signal source from a
load. Mismatches, changes in external circuitry, or
the introduction of modulation do not affect the con-
stant 50-ohm input impedance of the twt and thus will
not affect a signal source connected to the input. The
attenuation between the output and input terminals is
60 db due to attenuators placed along the helix. How-
ever, when the output signal is reflected from a mis-
matched load back to the input, the effective signal
isolation is the 60 db minus the gain of the amplifier.
For example: with an amplifier gain of 25 db, an open
or short circuit on the twt output can result in a maxi-
mum reflected signal 35 db below the input level
(approximately 1/56 of the input signal), which corre-
sponds to a swr of less than 1.04.
Note
The GRID MOD. connector is direct-coupled
to the grid of the twt amplifier. If a dc poten-
tial accompanies a modulating voltage applied
to this connector, the grid-bias voltage will
be altered. The GRID BIAS control may be
used to compensate for the change in grid bias
voltage due to a dc component at the input.
Figure 3-4. Block Diagram of an Automatic
Gain Control to Maintain Constant Amplifica-
tion from a TWT Amplifier
3-18. BUFFER AMPLIFICATIONS.
3-20. AMPLITUDE MODULATION.
3-21. To amplitude modulate an rf signal applied to
the twt amplifier with a minimum of envelope distor-
tion in the output signal, carefully establish the opti-
mum rf drive, grid bias, and modulating signal
amplitude for a given setup.
3-22. For minimum distortion, the twt grid voltage
must not leave the linear region of the grid voltage vs
rf output characteristic (see figures 3-5 and 3-6).
Also, the rf drive must be adjusted so that the modu-
lation peaks are at least 2 db below saturation power
DIRECTIONAL
COUPLER
LO-L-'2
00
TRAVELING-WAVE
TUBE AMPLIfiER
TO
AMPLITUDE
MODULATION
SIGNAL INPUT
GENERATOR
!
3-15. Although the traveling wave tube amplifier's
saturated power output characteristic can be used to
provide nearly constant output power, installing suit-
able feedback circuitry provides a constant output for
input signal variations as great as 20 db.
Figure 3-3. Block Diagram of a Circuit
used to Maintain Constant-Level Output
Power from a TWT Amplifier
3-16. An arrangement for obtaining a constant-level
output signal from the twt, in spite of variations in
input signal level or variations in amplifier gain, is
illustrated in figure 3-3. In this circuit a portion of
the rf signal is coupled from the traveling-wave tube
output, through a directional coupler to a detector
such as a crystal rectifier. The rectified voltage is
then amplified in a dc coupled amplifier and applied
to the GRID MOD. connector on the twt. Any tendency
for the output level from the twt to increase is immed-
iately detected, amplified, and fed back to reduce the
gain of the traveling wave tube amplifier in proportion.
Conversely, any reduction in output level increases
the gain of the amplifier to hold the output level con-
stant. The flatness of the rf output power level will
be affected by the frequency response of the detector,
directional coupler and the amplifier gain. The band-
width of the amplifier must be great enough to pass
any rate of change at which the output level may vary.
3-17. A variation of the basic automatic power level
control circuit can be used to obtain constant ampli-
fication with a twt even though the gain changes with
frequency, power line voltage and tube characteris-
tics. This circuit is illustrated in figure 3-4. The
circuit operates as follows: The rf input and output
signals are sampled, rectified, and the resulting dc
voltages amplified and compared. The gain or output
from each half of the circuit can be adjusted to estab-
lish the desired ratio of input to output level. If the
rf input to output ratio changes, a difference voltage
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00144-2
3-3