Excerpt from “HF Radio Systems and Circuits”, Sabin et al

The means for providing AGC for the overall receiver are important aspects of the block diagram design for an HF SSB receiver. The receiver is operationally required to have nearly constant audio output as the signal input ranges from the 1-μV region to perhaps 1 V or more and to do so without having loop oscillation or distorting the received signal. A receiver primarily designed to be used by an operator for signal search or monitoring will typically allow an audio output rise of 6 dB or so over the input signal range. A receiver for data signals, on the other hand, can have a very flat output requirement so that the following data detection circuits have a constant-amplitude input.