TRANSMIT/RECEIVE SWITCH AND CONTROL CIRCUIT

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

4-227.

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

attenuation)

4-41