Automatic Gain Control - Codan NGT 2010 RF Technical & Service Manual

NGT—Technical description (2010)

Automatic gain control

Drawing
ALTERNATIVE TEXT
Drawing
NOTE
An AGC is applied to a number of stages in the receiver. This is used to increase the
dynamic range of the receiver and to maintain an almost constant audio output level for a
large variation of input signals.
When the receiver is at maximum gain, the AGC at the output of IC205/A is nominally
6.4 V. This voltage is set by the voltage reference IC206 and associated components.
The IF signal is rectified by V214 and the resulting DC is amplified by IC205/B. The
signal is then passed to various timing circuits.
The signal is passed through D207 to the fast attack circuit (2 ms) comprising V215,
R274 and C253. The signal also charges the slow decay circuit comprising IC207/D,
R279 and C254. The outputs from these two circuits are summed together at the emitter
junction of V216 and IC207/A, then passed to the amplifier IC205/A.
The decay characteristic of the AGC circuit can be fast, slow or hold. Fast decay time is
set by C253 and R280. Slow decay time is set by C254 and R283 (1 s). Hold decay
timing is set by the discharge constant of C254, R285 and delay circuit C256 and R284.
The hold circuit holds the AGC voltage constant for a short period after the signal
reduces, which can improve the readability of the signal.
If fast decay AGC is required the AGC FAST/SLOW control line is held high by the
microprocessor. This disables the slow charge capacitor C254.
If slow decay AGC is required the AGC FAST/SLOW control line is held low and the
HOLD/TRIGGER line is held high.
If hold decay AGC is required both the AGC FAST/SLOW and the HOLD/TRIGGER
control lines are held low.
Note that the 'trigger' part of the HOLD/TRIGGER control line name is not related to
the AGC circuit at all. This multifunction HOLD/TRIGGER line is used by a built-in test
function during adjustment procedures.
The AGC voltage is passed to the IF amplifier. As the signal increases in level, the AGC
voltage reduces and hence lowers the gain of the IF amplifier, thus keeping the output
level constant.
If the signal level is very large, the anti-log and bias circuit comprising IC203 and
associated components operates. When operating, this circuit causes RF AGC voltage to
reduce, which in turn causes D104 (drawing
conduct. This reduces signal levels at mixer RxMIX1 and helps to keep the level
constant at the 455 kHz IF output.
There is an AGC DUMP input to dump any AGC voltage from a high-level signal on a
previously selected channel when the channel is changed. This allows the new channel to
have maximum gain. The dump is achieved by pulling the AGC DUMP control line low
and the AGC FAST/SLOW control line high for a short period when a channel is
changed.
90
04-03106 (sheet 2), or
04-03378 (sheet 2)
If the PCB issue status is -06 or earlier, see drawing
04-03106 (sheet 1) or 04-03378 (sheet
NGT Transceiver System Technical Service Manual
04-03378 (sheet 2).
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