AGC Generators

SL621 Plessey


  • All Time Constants Set Externally
  • Easy Interfacing
  • Compatible with SL610/611/612
  • Supply Voltage: 6V
  • Supply Current: 3mA
  • Operating Temperature: -30°C to +85°C

The SL621C is an audio-operated AGC generator designed for use with the SL610/11/12 RF amplifiers in SSB receivers.
An ideal single sideband AGC generator must set the AGC rapidly when a new signal appears and follow a rising or fading signal but, if the signal disappears altogether (as in pauses in speech), retain the AGC level until the signal recommences. If the signal remains absent for more than a preset time, however, the system should rapidly revert to full gain. The SL621C will perform these functions and will also produce short-lived pulses of AGC to suppress noise bursts.

Circuit Description

The SL621C consists of an input AF amplifier coupled to a DC output amplifier through a voltage back-off circuit and two detectors having short and long rise and fall time constants respectively.
An audio signal applied to the input rapidly establishes an AGC level in time t1. Meanwhile, the long-time constant detector output will rise and after t3 will control the output because this detector is the more sensitive. If the signals at the SL621C input are greater than approximately 4mV RMS, they will actuate the trigger circuits  whose output pulses will provide a discharge current for C2.
By this means the voltage on C2 can decay at a maximum rate which corresponds to a rise in receiver gain of 20dB/sec. Therefore the AGC system will smoothly follow signals which are fading at this rate or slower. However, should the receiver input signals fade faster than this or disappear completely as in pauses in speech, then the input to the AGC generator will drop below the 4mV RMS threshold, and the trigger will cease to operate. As C2 then has no discharge path, it will hold its charge (and hence the output AGC level) at the last attained value. The output of the short time constant detector falls to zero in time t2 after the disappearance of the signal. 
The trigger pulses also charge C3. When the pulses cease, C3 discharges and after t5 turns on TR12, rapidly discharging C2 (in time t4) thus restoring full receiver gain. The hold time, t5, is approximately one second with C3=100 µF. If signals reappear during t5, then C3 will recharge, and normal operation will continue. The C3 recharge time is long enough to prevent prolongation of the hold time by noise pulses. Fig. 12 also shows how a noise burst superimposed on speech will initiate rapid AGC action via the short time constant detector while the long-time constant detector effectively remembers the pre-noise AGC level.
In a receiver for both AM and SSB using an SL623C detector/carrier AGC generator, the AGC outputs of the SL621C and SL623C may be connected provided that no audio reaches the SL621C input while the SL623C is controlling the system.
AGC lines may require some RF decoupling but the total capacitance on the output should not exceed 15000pF, or the impulse suppression will suffer.