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Figure 4-8. +12-V Series Regulator, Simplified Schematic Diagram
TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual

TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
current is supplied to transistor Q3 through resistors R27
milliamperes and CR2 and CR3 at about 10 milli-
and R29. This forward biases Q3 causing it to turn on
amperes.  The control circuits process rf attenuation
and enable receive attenuator Q4. The positive voltage
(AGC) voltage and key-line voltage to control dc current
(about +25 V dc) developed by voltage divider action of
flow through the diodes.  Chokes L1, L2, L3, and L4
R29, R27, and the base circuit of Q3 reverse biases
isolate the rf path from the dc control circuits.
diode CR8. This enables shunt diode control amplifier
U1A. When enabled, U1A and Q4 control receive shunt
In the control circuit, series diode control
diode CR3 and receive series diode CR2 respectively.
amplifier U1B and receive attenuator Q4 control dc
The positive voltage developed by voltage divider action
current flow through receive series diode CR2.  The
of R29, R27, and the base circuit of Q3 is also applied
current path is from the 5.1-V dc line (P4-9) through
through resistor R15 to the cathode of transmit series
current shaping network R1-R2-R3-CR4-CR5, capacitor
diode CR1. This reverse biases CR1 and causes it to
C6, choke L2, series receive diode CR2, choke L4,
provide maximum attenuation to the rf signal between
capacitor C9, and receive attenuator Q4 and to ground
the transmitter and antenna ports of the transmit/receive
through receive enable switch Q3. Shunt diode control
switch. Rf attenuation (AGC) voltage determines the rf
amplifier U1A controls dc current flow through receive
resistance of the receive series and receive shunt diodes
shunt diode CR3. The current path is from the 5.1-V dc
which, in turn, determines the rf signal attenuation
line through current shaping network R1-R2-R3-CR4-
between the receiver and antenna ports.
CR5, capacitor C6, choke L2, receive shunt diode CR3,
capacitor C7, and resistor R8 and to ground through
4-230. In receive mode, rf attenuation occurs in stages.
U1A.  The transmit-to-receive and receive-to-transmit
Rf attenuation (AGC) voltage (P4-11) from receiver rf
transfer function of the transmit/receive switch is
module A3 increases in a positive direction as the rf
controlled by the combined circuit action of key off delay
signal strength at the antenna increases. For rf signals
transistor Q1, transmit enable switch Q2, receive enable
that develop an rf attenuation (AGC) voltage of less than
switch Q3, and diodes CR8, CR10, and CR16.
2.5 V dc, the transmit/receive switch provides minimum
attenuation to the receive rf signal.  As the signal
4-228.  The transfer function of the transmit/receive
strength increases and the rf attenuation voltage
switch operates as follows. To switch from receive to
increases above 2.5 V dc, series diode control amplifier
transmit mode, the logic 0 key 1 voltage (P4-13) turns
U1B and receive attenuator Q4 begin to reduce the
transistor Q1 off which, in turn, turns transmit enable
current flow through receive series diode CR2 to series
switch Q2 on. When on, transistor Q2 provides a dc
attenuate the receive rf signal.  At an rf attenuation
current path to ground for receive shunt diode CR3 and
(AGC) voltage of approximately 4.8 V dc, CR2 reaches
transmit series diode CR1. This forward biases CR3 and
maximum signal attenuation.  Above 4.8 V dc, shunt
CR1 (minimum rf resistance) and causes the rf signal to
diode control amplifier U1A begins to increase the
be shunted to ground through capacitors C7-C2-C16 and
current flow through receive shunt diode CR2 to shunt
C8. Transistor Q2 also shunts to ground base drive to
attenuate the receive signal.
transistor Q3 causing Q3 to turn off. This opens the dc
current path through receive attenuator Q4 and allows a
4-231. The following describes circuit action of shunt
voltage of about +75 V dc to be developed across
diode control amplifier U1A, series diode control amplifier
resistor R14 by voltage divider action of resistor R28 and
U1B, and receive attenuator Q4 as the rf attenuation
R14. This voltage, when applied through choke L4 to the
(AGC) voltage varies from 2.5 to 8 V dc (active range of
cathode of receive series diode CR2, reverse biases
rf attenuation). Positive 5.1-V dc power supply voltage
CR2. The combination of CR2 being reverse biased and
supplied through resistor R5 to the noninverting input of
CR3  being  forward  biased  provides  maximum
U1A biases the output of U1A at about 7 V dc when AGC
attenuation to the rf signal between the receiver and
voltage applied through resistor R4 to its inverting input
antenna ports of the transmit/receive switch. Forward-
is 0 volt. This reverse biases receive shunt diode CR3.
biased transmit series diode CR1 provides minimum
At 2.5-V dc AGC voltage, the output of U1A decreases to
attenuation to the rf signal between the power amplifier
about +5.6 V dc which is still sufficient to reverse bias
and antenna ports.
CR3 (minimum shunt rf atten-uation).  Series diode
control amplifier U1B receives the +2.5-V dc voltage at
4-229.  To switch from transmit to receive mode, the
its inverting input through resistor R9. The voltage at its
logic 1 key 1 voltage turns transistor Q1 on which, in
noninverting input (slightly greater than +2.5 V dc) is a
turn, turns transmit enable switch Q2 off. Resistors R16,
function of the current flow through CR2 and the voltage
R17, capacitor C15, and diode CR7 provide about 100-
drop across the current shaping network (R1, R2, R3,
microsecond delay to the turn-on time of Q1. This delay
CR4, CR5). Under these conditions, the output of U1B is
assures the power amplifier output power level has
positive which forward biases Q4 on and causes current
dropped completely to zero before series transmit diode
flow through series diode CR2 (minimum series rf
CR1 is biased off.
When Q2 turns off, base

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