AB Amps 980A Operations Instructions download pdf (Page 93)

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

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

amplifies (by unity gain) a portion of the reference

voltage from U1 applied across voltage divider R56-R57-

R58 to produce the pa power output reference voltage.

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

determines the power output from the receiver-

transmitter.

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

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

differential amplifier, U6B, compares the antenna

forward power signal applied to its noninverting input

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

reference voltage applied to its inverting input through

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

tion from the antenna forward power signal. The ampli-

fied difference signal at the output of U6B is applied to

the noninverting input of U4A through resistor R64.

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

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

Capacitor C12 in parallel with resistor R63 in conjunction

with U6 forms an active low-pass filter to remove

modulation. Diode CR8 provides temperature stability.

During transmit mode, the ALC precharge amplifier,

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

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

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

to be biased at maximum resistance.

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

forward power amplifier, U3A, amplifies the pa forward

power signal and applies the result to the noninverting

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

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

Resistor R67 in the circuit of U3A is test selected to

adjust the gain of U3A to compensate for level variations

in pa forward power that exist between different

reflectometer, A8A7, subassemblies. Low-pass filter

C10-R44 removes modulation from the pa forward

power signal. Under normal circuit conditions, the output

voltage from U6B is greater than the output of U7,

therefore, making it the primary controlling signal. Diode

CR12 isolates the two control loops so that they do not

interfere with each other.

4-278. ALC differential amplifier U4A compares the

voltage from either U6B or U7 applied to its noninverting

input to the pa power output reference voltage applied

through resistor R74 to its inverting input. The amplified

difference voltage from U4A drives the ALC output

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

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

voltage applied to the pin diode ALC attenuator is a

function of both the pa power output reference voltage

and either the antenna forward power or pa forward

power. By adjusting potentiometer R57, the ALC voltage

(via the pa power output reference voltage) can

be varied to set the receiver-transmitter power output to

the desired rf level. To maintain a constant power output

from the receiver-transmitter, variations in power output

appear as variations in the forward power signal. These

variations, when compared to the pa power output

reference voltage, cause the ALC voltage to change in

such a direction to counteract the power output

variations.

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

is detected, the ALC voltage is increased to reduce the

power output to approximately 4 watts. This is

accomplished by the following circuit action: When a

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

connects resistor R59 to ground in parallel with

potentiometer R57 and resistor R58. This decreases the

voltage applied to the pa power output reference source

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

U6A to decrease by the same amount. The decrease in

pa power output reference voltage causes an increase in

the differential input voltage at the input of U4A. This

increase in voltage, when amplified by the ALC

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

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

value to reduce the power output of the receiver-

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

forward power and pa forward power go to zero, but

because of the antenna forward power voltage stored in

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

reduction in reference voltage applied to U6B does not

cause an instantaneous increase in its output voltage.

The resulting voltage at the output of U6B, when applied

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

until the charge across capacitor C13 discharges. As the

charge across C13 decreases, the output of U6B

decreases which, in turn, causes the ALC voltage to

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

the antenna, but because of the reduction in the pa

power output reference voltage, the new power output

level is approximately 4 watts.

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

down switch, U2A, removes resistor R59 from ground

and allows capacitor C11 to charge. The RC time

constant of R59 and C11 determines the rise time of the

pa power output reference voltage which, in turn,

determines the power up time of the power amplifier

module to the normal power output level.

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

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

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

precondition the ALC circuit.

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

turndown switch causes transistor Q2 to turn off. When

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

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

pa/antenna power comparators (U5A, U5B).

4-48

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AB Amps 980A Operations Instructions Page 93

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