ALC Circuit.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-274. The pa power output reference source, U6A,

be varied to set the receiver-transmitter power output to

amplifies (by unity gain) a portion of the reference

the desired rf level. To maintain a constant power output

voltage from U1 applied across voltage divider R56-R57-

from the receiver-transmitter, variations in power output

R58 to produce the pa power output reference voltage.

appear as variations in the forward power signal. These

This reference voltage, as set by potentio-meter R57,

variations, when compared to the pa power output

determines the power output from the receiver-

reference voltage, cause the ALC voltage to change in

transmitter.

such a direction to counteract the power output

variations.

4-275. In the primary ALC control loop, when the

transmitter is keyed (logic 0 key 2 voltage), the ALC

4-280. If a vswr, pa/antenna power, or temperature fault

differential amplifier, U6B, compares the antenna

is detected, the ALC voltage is increased to reduce the

forward power signal applied to its noninverting input

power output to approximately 4 watts.

This is

through low-pass filter C13-R41 to the pa power output

accomplished by the following circuit action: When a

reference voltage applied to its inverting input through

fault is detected, the pa power turndown switch, U2A,

resistor R62. Low-pass filter C13-R41 removes modula-

connects resistor R59 to ground in parallel with

tion from the antenna forward power signal. The ampli-

potentiometer R57 and resistor R58. This decreases the

fied difference signal at the output of U6B is applied to

voltage applied to the pa power output reference source

the noninverting input of U4A through resistor R64.

(U6A) by about 7 dB which, in turn, causes the output of

U6A to decrease by the same amount. The decrease in

4-276. In the circuit of U6B, diode CR7 prevents the

pa power output reference voltage causes an increase in

output from going negative by more than about -0.6 V dc.

the differential input voltage at the input of U4A. This

Capacitor C12 in parallel with resistor R63 in conjunction

increase in voltage, when amplified by the ALC

with U6 forms an active low-pass filter to remove

differential amplifier, U4A, and ALC output amplifier, Q3-

modulation. Diode CR8 provides temperature stability.

Q4, causes the ALC voltage to go to a high positive

During transmit mode, the ALC precharge amplifier,

value to reduce the power output of the receiver-

U4B-Q5, and time constant control Q1-Q6-Q7, have no

transmitter to 0 watt. As a result of this, the antenna

effect on the output of U6B, since the logic 0 key 2

forward power and pa forward power go to zero, but

voltage causes Q5 to be turned off and FET's Q6 and Q7

because of the antenna forward power voltage stored in

to be biased at maximum resistance.

capacitor C13 and the lag in the output of U6B, the

reduction in reference voltage applied to U6B does not

4-277. In the secondary ALC control loop, the pa

cause an instantaneous increase in its output voltage.

forward power amplifier, U3A, amplifies the pa forward

The resulting voltage at the output of U6B, when applied

power signal and applies the result to the noninverting

to U4A, holds the ALC voltage at a high positive value

input of U4A through low-pass filter C10-R44, voltage

until the charge across capacitor C13 discharges. As the

divider R45-R53, buffer U7, and isolation diode CR12.

charge across C13 decreases, the output of U6B

Resistor R67 in the circuit of U3A is test selected to

decreases which, in turn, causes the ALC voltage to

adjust the gain of U3A to compensate for level variations

decrease. After a period of time, power again appears at

in pa forward power that exist between different

the antenna, but because of the reduction in the pa

reflectometer, A8A7, subassemblies. Low-pass filter

power output reference voltage, the new power output

C10-R44 removes modulation from the pa forward

level is approximately 4 watts.

power signal. Under normal circuit conditions, the output

voltage from U6B is greater than the output of U7,

4-281. When the fault is cleared, the pa power turn-

therefore, making it the primary controlling signal. Diode

down switch, U2A, removes resistor R59 from ground

CR12 isolates the two control loops so that they do not

and allows capacitor C11 to charge. The RC time

interfere with each other.

constant of R59 and C11 determines the rise time of the

pa power output reference voltage which, in turn,

4-278. ALC differential amplifier U4A compares the

determines the power up time of the power amplifier

voltage from either U6B or U7 applied to its noninverting

module to the normal power output level.

input to the pa power output reference voltage applied

through resistor R74 to its inverting input. The amplified

4-282. When the receiver-transmitter is unkeyed, the

difference voltage from U4A drives the ALC output

logic 1 key 2 voltage (P1-9) causes the following circuit

amplifier, Q3-Q4, to produce the ALC voltage.

actions to occur to turn off the rf power amplifier and to

precondition the ALC circuit.

4-279. As a result of the above circuit action, ALC

voltage applied to the pin diode ALC attenuator is a

4-283. Logic 1 key 2 voltage applied to U2E of the pa

function of both the pa power output reference voltage

turndown switch causes transistor Q2 to turn off. When

and either the antenna forward power or pa forward

Q2 turns off, +5 V dc is removed from the +5 V dc

power. By adjusting potentiometer R57, the ALC voltage

switched output (P1-2) and from the antenna vswr and

(via the pa power output reference voltage) can

pa/antenna

power

comparators

(U5A,

U5B).

4-48