Antenna VSWR and PA/Antenna Power Temperature Monitor

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

When the +5-V dc switched voltage goes to 0 volt, bias

4-286.

Antenna VSWR and PA/Antenna Power

voltage is removed from the linear rf amplifiers of the rf

Temperature Monitor. Refer to figure FO-26. The

predriver/ALC attenuator, A8A3. Removing +5 V dc from

antenna vswr comparator, U5A, detects antenna voltage

U5A and U5B causes the output of the antenna vswr and

standing-wave ratios that exceed about 3 to 1. Voltage

pa/antenna power comparators to remain low,

developed across resistor R24 of voltage divider R17-

independent of their inputs.

R30-R24 sets a positive reference voltage at the

inverting input of U5A that is a function of the antenna

4-284. Logic 1 key 2 voltage applied to the base of time

forward power voltage (about 4 volts at 20 watts).

constant control transistor Q1 through resistor R85

Voltage developed across resistor R32 of voltage divider

causes Q1 to turn on. When Q1 turns on, the +12-V dc

R31-R43-R32 and applied to the noninverting input of

gate voltage that biased FET's Q6 and Q7 off is shunted

U5A is a function of the antenna reflected power voltage

to ground causing Q6 and Q7 to turn on. When on, Q6

(about 4 volts at 5 watts). Whenever the antenna vswr

provides a low resistance path in parallel with resistor

exceeds about 3 to 1, the reflected power voltage across

R63 and capacitor C12 (fast discharge path for C12).

R32 increases above the forward power reference

When on, Q7 provides a low resistance path across

voltage across R24. This causes the output of U5A to go

resistor R62. Together, the low resistance of Q6 and Q7

from low to high whenever the vswr exceeds 3 to 1.

changes the gain and time constant characteristics of the

Resistor R33 provides positive feedback (hysteresis) to

ALC differential amplifier, U6B.

the circuit. This prevents the antenna vswr comparators

from tripping more than once whenever the vswr is at the

4-285. The logic 1 key 2 voltage applied to the cathode

trip threshold. Resistor-capacitor networks R31-C15 and

of CR18 of the ALC precharge amplifier (U4B-Q5)

R17-C14 form low-pass filters to filter out high-frequency

reverse biases CR18 and allows the pa power output

interference (radar pulses) conducted into the circuit

reference voltage to be applied to the noninverting input

from the antenna. When the antenna forward and

(through R87) of U4B. This causes the output of U4B to

reflected power outputs are both zero, positive voltage

go positive, turn on Q5, and apply a positive voltage to

(+12 V dc applied through voltage divider R16-R17-R30-

the noninverting input of U6B. ALC differential amplifier

R24) applied to the inverting input of U5A holds the

U6B compares the output of Q5 to the pa power output

output of U5A low.

reference voltage and develops an output voltage that

applied to the voltage divider formed by resistors R64,

4-287. The pa,/antenna power comparator, U5B, detects

R69, and R72. The voltage across R72 is applied

pa to antenna power ratios that exceed about 2 dB.

through resistor R55 to the inverting input of U4B. The

Voltage developed across resistor R66 of voltage divider

closed-loop gain of U4B, Q5, U6B, and the voltage

R34-R66 sets a positive reference voltage at the

divider (R64-R69-R72) is such that the final voltage

noninverting input of U5B that is a function of the pa

(precharge voltage) at the noninverting input of U6B is

forward power voltage (approximately 1 volt at 20 watts).

just slightly above the level present when the receiver-

Voltage developed across resistor R40 of voltage divider

transmitter is keyed. The precharge voltage represents

R39-R40 and applied to the inverting input of U5B is a

a forward power slightly above the normal forward power

function of the antenna forward power voltage

output from the power amplifier. When the receiver-

(approximately 4 volts at 20 watts). Resistor R66 is test

transmitter is keyed, the ALC circuit takes the precharge

selected so that whenever the pa power output exceeds

voltage as being the antenna forward power output even

the antenna forward power by more than about 2 dB, the

through the actual antenna forward power output is zero

pa forward power voltage across R66 just exceeds the

at that instance.

The ALC circuit processes the

antenna forward power reference voltage across R40.

precharge voltage to develop an ALC voltage that

This causes the output of U5B to go from low to high

causes the pin diode ALC attenuator to reduce the power

whenever the antenna power decreases such that pa

output at the same time as the rf power amplifier is

power output exceeds the antenna power by more than 2

turning on. The net result of this circuit action is to bring

dB.

Resistor R35 provides positive feedback

the power output up in minimum key-on time (120

(hysteresis) to the circuit. This prevents the pa/antenna

milliseconds) and very little overshoot. The logic 1 key 2

power comparator from tripping more than once

voltage applied to the receive mode pa shutdown switch,

whenever the pa/antenna power ratio is at the trip

U2C, causes resistor R50 to be connected to ground

threshold. When the pa and antenna forward power

through U2C to increase the gain of U4A-Q3- Q4. When

voltages are both zero, negative voltage (-12 V dc

this occurs, U4A amplifies the positive output voltage

applied through voltage divider R65-R66) applied to the

from U6B and drives the ALC voltage at the output of the

noninverting input of U5B holds the output of U5B to low.

ALC output amplifiers, Q3-Q4, to a high positive value.

This voltage, when applied to the pin diode ALC

4-288. The overtemperature detector, U7B, detects

attenuator, causes maximum attenuation in the transmit

power amplifier heat-sink temperatures that exceed

rf signal path of the rf predriver/ALC attenuator, A8A3.

about 100 degrees Celsius. Thermistor A8RT1 (figure

FO-25) connected from P1-14 to ground forms a voltage

divider with resistor R78.

For low heat-sink

4-49