AB Amps 980A Operations Instructions download pdf (Page 71)

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-125. AGC CIRCUIT. Refer to figure FO-20. The AGC

circuit consists of an integrator (U6B), three AGC

amplifiers (U7A, U7B, Q17), and two shunt attenuators

(CR6, CR1). In addition, a series/shunt attenuator in rf

filter module A7 receives AGC voltage to form a part of

the overall receiver gain control circuit. This circuit will

be discussed as part of rf filter module A7.

4-126. By nature of the AGC circuit, the carrier-derived

dc output voltage from af amplifier U6A is automatically

held at about 2.5 V dc whenever there is a nonvarying

receive rf signal applied to the rf receiver input. This can

be seen by considering the operation of integrator U6B.

Voltage divider R83-R84 develops an AGC reference

voltage (approximately

2.5 V dc) that is applied through

resistor R85 to the noninverting input of integrator U6B.

The carrier-derived dc voltage from af amplifier U6A is

applied through resistor R81 to the inverting input of

U6B. Integrator U6B compares these two voltages and

integrates the difference to develop the AGC output

voltage. The AGC voltage is inverted and amplified by

AGC amplifier U7A and applied through resistors R64,

R62, and R63 to the gain control inputs (pin 2) of 10.7-

MHz if amplifiers U4 and U5. A portion of the output of

U7A is amplified by AGC amplifiers U7B and Q17 and

applied as 30-MHz AGC voltage to the two shunt

attenuators, CR6 and CR1. Also, the rf attenuator (AGC)

voltage output of AGC amplifier U7B is applied to rf filter

module A7 to control the series and shunt attenuators in

that module.

4-127. To control receiver gain, the integrator output

voltage (AGC voltage) settles at whatever AGC voltage

necessary to give a carrier-derived voltage from the af

amplifier equal to the AGC reference voltage (voltage

divider R83-R84). This is the only stable output from the

integrator (other than end stops). Any other carrier-

derived voltage compared to the AGC reference voltage

will cause the integrator to integrate up or down to

change the receiver gain until the af amplifier output

equals the AGC reference voltage.

4-128. The time constant of integrator U6B determines

the attack time and release time of the AGC circuit.

Resistor R82 and capacitor C82 set the attack time at

about 100 milliseconds. Resistor R81 and capacitor C82

set the release time at about 500 milliseconds. When a

signal is first applied to the receiver, the carrier-derived

voltage will be large until the AGC voltage reduces the

receiver gain. If the carrier-derived voltage exceeds the

zener voltage of VR1, it will be applied to integrator U6B

through resistor R82, diode CR11, and zener diode VR1.

This changes the RC time constant components of

integrator U6B from R81 and C82 to R82 and C82 to

control the attack time. Once the receiver has stabilized

and the carrier-derived voltage settles

back to about 2.5 V dc, integrator U6B components R81

and C82 determine the release time.

4-129. Receiver gain is controlled in stages. Over the

range from about 3 to about 10 microvolts at the

antenna, the 10.7-MHz AGC voltage applied to the 10.7-

MHz if amplifiers controls receiver gain. Once the input

signal exceeds 10 microvolts, the gain of the 10.7-MHz if

amplifiers is varied slower, and the rf attenuator (AGC)

voltage applied to the series attenuator in the rf filter

module controls most of the receiver gain up to

approximately 100-microvolt input signal. At that point,

the series attenuator reaches maximum attenuation.

Above 100 microvolts, the rf attenuator voltage applied to

the shunt attenuator in the rf filter module and the 30-

MHz AGC voltage applied to the two shunt attenuators of

the receiver rf module control receiver gain.

4-130. The 10.7-MHz AGC voltage that controls the first

stage of gain reduction is developed as follows: The

AGC voltage at the output of integrator U6B (test point

10) varies from about -0.65 to about -4.4 V dc for input

signal variations from 1 microvolt to 1 volt. AGC

amplifier U7A amplifies this AGC voltage to produce the

10.7-MHz AGC voltage that is applied to 10.7-MHz if

amplifiers U4 and U5. For AGC voltages that range from

0 to -1 V dc, the gain of amplifier U7A is about 7.5 V/V.

For AGC voltages above -1 V dc, zener diode VR2

begins to conduct to change the gain of U7A to about

0.25 V/V. This nonlinear gain characteristic causes the

10.7- MHz AGC voltage to increase from 0 to about 7.8

V dc for an increase in AGC voltage from 0 to -1 V dc

respectively. Above -1 V dc, the 10.7-MHz AGC voltage

only increases to about 8.6 V dc for a -4.4 V dc AGC

voltage. This results in a 0- to approximately 20-dB

receiver gain reduction by the 10.7-MHz if amplifier for

the first volt of AGC voltage and a nearly constant 20-dB

gain reduction for AGC voltage above -1 V dc.

4-131. The rf attenuator (AGC) voltage that controls the

second stage of gain reduction is developed as follows:

AGC amplifier U7B, which has a gain factor of about 5.2

V/V, amplifies a divided down portion of the 10.7-MHz

AGC voltage from U7A. Through voltage divider action

of resistors R90 and R91 across 7.5 V dc zener diode

VR2, the voltage developed across resistor R90 is about

0.4 V dc whenever the AGC voltage exceeds -1 V dc.

This voltage in conjunction with the reduced gain of U7A

(about 0.25 V/V when AGC voltage exceeds -1 V dc)

results in a positive voltage at the noninverting input of

U7B that varies from about 0.65 to about 0.9 V dc as the

AGC voltage increases from -1 to -2 V dc. When

amplified by U7B, the resulting rf attenuator (AGC)

voltage applied to the rf filter module varies from about

3.5 V dc for -1 V dc of AGC voltage to about 4.7 V dc for

-2 V dc of AGC voltage. This voltage is in the active

range of the series attenuator in the

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AB Amps 980A Operations Instructions Page 71

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