voltage in accordance with audio content of the transmit
dc component that represents the average forward
audio signal which, in turn, varies the gain of the high-
Capacitor C33 couples the audio
level modulated rf amplifiers to amplitude modulate the
component to peak detector circuit CR15-C34 where the
transmit rf signal.
Modulation feedback from the
negative peak voltage of the modulation envelope is
modulation transistor and envelope feedback taken from
developed across capacitor C34. The negative voltage
the power amplifier forward power signal are fed back as
is applied to the input of U2B where it is amplified by a
negative feedback to control ac gain of the modulator
voltage gain of about 2.4 V/V (inverting). The output of
and to reduce distortion in the transmitted rf signal.
U2B is applied to a voltage divider consisting of resistor
R60 and FET Q5 of the variable attenuator.
The output rf signal from rf preamplifier/
modulator A8A2 is applied to the pin diode ALC
FET Q5 operates in the voltage-controlled
attenuator of rf predriver/ALC attenuator A8A3. Under
Negative voltage developed across
the control of ALC voltage, the pin diode ALC attenuator
potentiometer R59 and applied to the gate of Q5 through
varies circuit gain to maintain a constant forward power
R64 biases Q5 to a high resistance. As forward power
output from the receiver-transmitter. ALC voltage is
increases, the positive dc output voltage from U2A is
developed from the ALC circuit which consists of an
applied to the gate of Q5 through resistor R58.
inner (secondary) control loop and outer (primary) control
Capacitor C35 filters out the audio component. The
loop. In the secondary control loop, power amplifier
positive dc voltage makes the gate voltage less negative
forward power from reflectometer A8A7 is processed by
to reduce the drain to source resistance of Q5 which
increases the attenuation of the output signal from U2B.
forward power from the rf filter module is processed.
For a constant modulation percentage, this circuit action
Under normal circuit conditions, the primary control loop
compensates for the increase in peak-to-peak amplitude
controls the power output from the receiver- transmitter.
of the modulation envelope as the power level increases.
Variations in the antenna forward power signal caused
The result is a nearly constant voltage applied across
by changes in loading will cause the ALC voltage to
R61 and the meter circuit of chassis A10 to indicate the
affect the pin diode ALC attenuator in such a manner as
to counteract the variations to maintain a constant power
output. The rf output signal of the pin diode ALC
4-240. Potentiometer R59 allows calibration of the
attenuator is amplified by the two linear rf amplifiers and
percent modulation monitor.
applied to rf driver A8A4.
4-241. POWER AMPLIFIER MODULE A8.
The output signal from rf predriver/ALC
attenuator A8A3 Is applied to the gain slope pad of rf
4-242. Power amplifier module A8 consists of seven
driver A8A4. The gain slope pad provides frequency-
subassemblies A8A1 through A8A7 and a chassis
versus-gain compensation for the rf power amplifier
(finned heat sink). Each subassembly has an individual
module. The output of the gain slope pad is amplified by
schematic diagram (figures FO-26 through FO- 31) and
the rf amplifier (no 6) to provide about 12-watt output
the chassis (A8) an interconnect diagram (figure FO-25)
that ties all the subassemblies together. A block diagram
of the overall rf power amplifier module is shown in figure
4-247. The output signal from rf driver A8A4 is applied
to the 3-dB, 90-degree hybrid coupling circuit (A8HY1)
where the signal is split into two signals. Each signal is
4-243. GENERAL. Refer to figure FO-10. The transmit
then applied to a 3-dB, 90-degree hybrid coupler in each
rf signal (+12 to +16 dB mW) from the synthesizer
of the amplifiers, A8A5 and A8A6, where each signal is
module to be transmitted is applied to the resistive tee
again split into two signals. Through the two tiers of
attenuator of rf preamplifier/modulator ABA2.
hybrid couplers, the rf signal is applied to four broadband
resistive tee attenuator compensates for gain variations
rf amplifiers that are isolated from one another and
in the three rf amplifiers of A8A2. The amount of
operate in parallel to share one-fourth the load. The rf
attenuation is determined through a test select process
signals are amplified by the four rf amplifiers and
to provide about 0.3- watt carrier level output from A8A2
combined into one rf signal by two tiers of 3-dB, 90-
for +14-dB mW input.
degree hybrid couplers to produce about 30-watt carrier
level at the output of hybrid A8HY2.
4-244. The transmit audio signal from the audio module
to be transmitted is applied to the modulator of
4-248. The output signal from hybrid A8HY2 is applied
ALC/modulator A8A1. The modulator amplifies the
to reflectometer A8A7 where the level of power amplifier
transmit audio signal to provide a modulation base drive
forward power and reflected power is detected and
signal to the modulation transistor that controls power
applied to ALC/modulator A8A1. The rf output signal
supply voltage applied to the high-level modulated rf
from the reflectometer is applied to the rf filter module.
amplifiers. The modulation transistor varies the supply