Technical Description 4-6 X-2 Technical Service Manual
Publication No: 15-02047
4.2.7 Automatic Gain Control
To increase the dynamic range of the receiver and to maintain an almost
constant audio output for large variation of input levels, an automatic gain
control (AGC) operates on the two stage 455 kHz IF amplifier.
The demodulated output from pin 6 of IC302 is applied to amplifier
IC303/A
[D7]
(set to a gain of 3). At its output (pin 1) the audio connects to a
full wave peak rectifier circuit consisting of:
• D305
[B8]
to rectify the positive component of the audio
• inverter IC307/A and D304 to rectify the negative component of the
audio.
Both diode cathodes are ORed and charge capacitor C330 via R345 to the
peak audio level (less the forward diode drop of D304 and D305).
The DC across C330 is applied to one input of the differential DC amplifier
IC307/B (pin 6). The second input (pin 5) is connected to a reference
voltage of 5.2 V set by divider network R350, D306, R351 and R352.
No Signal Conditions
Under no signal conditions, the DC level at the output of IC307/B is set by
the reference voltage (5.2 V). This is then applied to gate 2 of both FETs
V301 and V302 setting the IF amplifier to maximum gain.
With Signal Conditions
When the receive signal is of sufficient level to cause the peak audio to
charge C330 above the reference of 5.2 V (AGC threshold), the output of
the DC amplifier IC307/B falls. Gate 2 voltage subsequently drops to the
two FETs causing the IF amplifier gain to fall.
The high loop gain of the AGC control network causes any signal above
AGC threshold (about 3 µV EMF) to reduce the IF amplifier gain to a level
required to maintain an almost constant audio output.
The first IF stage (V301) has an additional gain control applied to gate 1.
This consists of V305
[C4]
and associated resistive components. Under normal
signal levels V301 remains saturated by the forward bias applied from the
AGC line via R369
[C4]
and gate 1 voltage is determined by the resistor divider
R302 and R303 (1.8 V).
When the receive signal increases to a level that causes the AGC control
voltage to drop to about 2 V, V305 comes out of saturation allowing gate 1
voltage to commence rising. This increases the dynamic gain control of the
first stage and prevents overloading at very high signal levels.
The AGC fast attack time is set by the time constant of R345/C330
[C8]
and
the slow decay time by R347/C330.
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