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RF AMPLIFIERS A8A5 AND A8A6
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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Figure 4-9. Power Amplifier Modulator, Simplified Schematic Diagram


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
rf amplifiers A8A5Q1 or A8A5Q2.  In a like manner,
differential input voltage to develop the base drive signal
isolation resistors A8R2 and A8R6 dissipate reflected
to A8Q1. Potentiometer A8A1R11 is adjusted to set the
power from rf amplifiers A8A6Q1 or A8A6Q2.
collector voltage of A8Q1 to about 6 V dc.
4-266.
RF  AMPLIFIERS  A8A4Q1,  ASA5Q1/Q2,
4-271.
When transmit audio is applied, capacitor
A8A6Q1/Q2. The rf amplifier (A8A4Q1 of figure FO-29)
A8A1C3 couples the audio signal to the noninverting
of rf driver A8A4 and the rf amplifier pair (A8A5Q1/Q2
input of A8A1U1. The audio signal, when amplified by
and A8A6Q1/Q2 of figure FO-30) of rf amplifiers A8A5
A8A1U1, causes the collector voltage of A8Q1 to vary at
and A8A6 are all identical. Therefore, only rf amplifier
the audio rate which, in turn, causes the collector voltage
ASA5Q1 of figure FO-30 will be discussed in detail.
of A8A2Q2 and A8A2Q3 to vary at the audio rate. By
Transistors Q1 and Q2 or rf amplifiers A8A5 and A8A6
varying the collector voltage, the gain of A8A2Q2 and
are a matched quad so that the gain through each of the
A8A2Q3 is modulated at the audio rate which causes the
four parallel paths is identical.
transmit rf signal to be amplitude modulated.
4-267. Refer to figure FO-30. The output of hybrid HY1
4-272. Ac gain of the modulator is controlled by negative
is coupled to the base of rf amplifier transistor Q1
feedback of the modulation signal applied to the inverting
through the impedance matching network composed of
input of ABA1U1. The ac component of the modulation
inductors L1 and L11 and capacitors C1, C2, C3, and
feedback signal (P1-5) is applied to the inverting input of
C4.  Transistor Q1 is biased by a reference voltage
A8A1U1 through voltage divider A8A1R7-R15, resistor
developed across diode CR1 of bias network CR1-R2-
A8A1R9, and capacitor A8A1C9
C1 and applied to the base of Q1 through resistor R1.
Capacitor A8A1C7 provides an ac ground for resistor
The amplified output of Q1 is coupled to hybrid HY2
A8A1R10. The ac component of the pa forward power
through the impedance matching network composed of
signal detected at reflectometer A8A7 and amplified by
inductors L12, L4, L3, and L5 and capacitors C5 through
A8A1U3A is fed back as envelope feedback to the
C9.  Capacitors C12, C13, and C10 and inductor L2
inverting input of A8A1U1 through voltage divider
decouple the rf amplifier from the +26-V dc line. The
A8A1R19-R20, (A8A1R20 grounded through A8A1U2D),
amplifier power gain is approximately 8 dB.
resistor A8A1R18, and capacitor A8A1C9. When a vswr,
pa/antenna power, or temperature fault is detected, the
4-268. REFLECTOMETER A8A7. Refer to figure FO-
envelope  feedback  control  switch,  A8A1U2F  and
31.  The reflectometer senses forward and reflected
A8A1U2D, opens the ground circuit to resistor A8A1R20
power at the output of the power amplifier module. The
to increase the level of the envelope feedback signal.
forward power sensor consists of resistor R2, capacitors
The ac gain of the modulator circuit is such that
C1 and C3, and detector diode CR1.  The reflected
approximately 0.85 volt of transmit audio modulates the
power sensor consists of resistor R1, capacitors C2 and
transmit rf signal to 90 percent.  Envelope feedback
C4, and detector diode CR2.  Both sensors develop
improves the audio quality (reduces distortion) of the
approximately 1 volt for 20 watts (0.5 volt at 5 watts) of
transmitted audio signal.
forward or reflected power.
4-273. ALC Circuit. Refer to figure FO-26. Reference
4-269. ALC/MODULATOR A8A1. ALC/modulator A8A1
designators apply to A8A1. The ALC circuit consists of
contains the modulator circuit, ALC circuit, antenna vswr
the following circuits of ALC/modulator A8A1: pa forward
monitor, pa/antenna power monitor, and the temperature
power amplifier U3A, buffer U7A, ALC differential
monitor.
amplifier U6B, ALC differential amplifier U4A, ALC output
amplifier Q3-Q4, pa power output reference source U6A,
4-270. Modulator Circuit. Refer to figures 4-9, FO-25,
ALC precharge amplifier U4B-Q5, and time constant
FO-26, and FO-27.  With no transmit audio applied,
control Q1-Q6-Q7. In the primary control loop, the ALC
modulation  amplifier  A8A1U1  sets  the  quiescent
circuit receives the antenna forward power signal (P1-1)
modulation input voltage applied to rf preamplifier/
from the rf filter module and provides ALC voltage (P1-
modulator A8A2 at about 6 V dc. In A8A2, this voltage is
11) to the pin diode ALC attenuator of rf predriver/ALC
applied as collector voltage to the high-level modulated rf
attenuator A8A3 to control power output at the antenna.
amplifiers, A8A2Q2 and A8A2Q3. To set the quiescent
In the secondary control loop, the ALC circuit receives
modulation input level, modulation amplifier ASA1U1
the pa forward power signal (P1-6) from reflectometer
compares a reference voltage to a modulation feedback
A8A7 and provides ALC voltage (P1-11) to the pin diode
voltage (P1-5) to control base drive to modulation
ALC attenuator (A8A2) to control power output from the
transistor A8Q1.
The reference voltage developed
rf power amplifier module. The secondary control loop
across resistor A8A1R6 of voltage divider A8A1R5-R6 is
takes over control if, for some reason, the rf filter module
applied to the noninverting input of ASA1U1.  The
fails to provide power to the antenna or has a high
feedback voltage developed at the arm of potentiometer
insertion loss.  The ALC circuit functions as follows.
A8A1R11 of voltage divider A8A1R8-R11-R12 is applied
through resistor A8A1R10 to the inverting input of
A8A1U1.
A8A1U1
amplifies
the
4-46


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