AGC CIRCUIT

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

back to about 2.5 V dc, integrator U6B components R81

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

and C82 determine the release time.

circuit consists of an integrator (U6B), three AGC

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

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

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

range from about 3 to about 10 microvolts at the

filter module A7 receives AGC voltage to form a part of

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

the overall receiver gain control circuit. This circuit will

MHz if amplifiers controls receiver gain. Once the input

be discussed as part of rf filter module A7.

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

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

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

voltage applied to the series attenuator in the rf filter

dc output voltage from af amplifier U6A is automatically

module controls most of the receiver gain up to

held at about 2.5 V dc whenever there is a nonvarying

approximately 100-microvolt input signal. At that point,

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

the series attenuator reaches maximum attenuation.

be seen by considering the operation of integrator U6B.

Above 100 microvolts, the rf attenuator voltage applied to

Voltage divider R83-R84 develops an AGC reference

voltage (approximately 2.5 V dc) that is applied through

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

MHz AGC voltage applied to the two shunt attenuators of

resistor R85 to the noninverting input of integrator U6B.

the receiver rf module control receiver gain.

The carrier-derived dc voltage from af amplifier U6A is

applied through resistor R81 to the inverting input of

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

U6B. Integrator U6B compares these two voltages and

stage of gain reduction is developed as follows: The

integrates the difference to develop the AGC output

AGC voltage at the output of integrator U6B (test point

voltage. The AGC voltage is inverted and amplified by

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

AGC amplifier U7A and applied through resistors R64,

signal variations from 1 microvolt to 1 volt. AGC

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

amplifier U7A amplifies this AGC voltage to produce the

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

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

U7A is amplified by AGC amplifiers U7B and Q17 and

amplifiers U4 and U5. For AGC voltages that range from

applied as 30-MHz AGC voltage to the two shunt

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

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

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

voltage output of AGC amplifier U7B is applied to rf filter

begins to conduct to change the gain of U7A to about

module A7 to control the series and shunt attenuators in

0.25 V/V. This nonlinear gain characteristic causes the

that module.

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

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

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

voltage (AGC voltage) settles at whatever AGC voltage

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

necessary to give a carrier-derived voltage from the af

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

amplifier equal to the AGC reference voltage (voltage

receiver gain reduction by the 10.7-MHz if amplifier for

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

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

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

gain reduction for AGC voltage above -1 V dc.

derived voltage compared to the AGC reference voltage

will cause the integrator to integrate up or down to

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

change the receiver gain until the af amplifier output

second stage of gain reduction is developed as follows:

equals the AGC reference voltage.

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

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

4-128. The time constant of integrator U6B determines

AGC voltage from U7A. Through voltage divider action

the attack time and release time of the AGC circuit.

of resistors R90 and R91 across 7.5 V dc zener diode

Resistor R82 and capacitor C82 set the attack time at

VR2, the voltage developed across resistor R90 is about

about 100 milliseconds. Resistor R81 and capacitor C82

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

set the release time at about 500 milliseconds. When a

This voltage in conjunction with the reduced gain of U7A

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

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

voltage will be large until the AGC voltage reduces the

results in a positive voltage at the noninverting input of

receiver gain. If the carrier-derived voltage exceeds the

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

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

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

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

amplified by U7B, the resulting rf attenuator (AGC)

This changes the RC time constant components of

voltage applied to the rf filter module varies from about

integrator U6B from R81 and C82 to R82 and C82 to

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

control the attack time. Once the receiver has stabilized

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

and

the

carrier-derived

voltage

settles

range

the

series

attenuator

the

4-26