Receiver Automatic Gain Control purpose

Question

Receiver AGC objective is to have a constant amplitude signal in order to optimize the receiver performance, so it should respond according to the amplitude of the signal of interest and not to the amplitude of another signal (interferer) next to the signal of interest ( next in frequency domain), or RSSI (in the presence of a strong interferer for example). Is there any application or any case where the AGC operates according to RSSI in the presence of a strong interferer, i.e. it responds practically to the power of the interferer and not to the power of the signal of interest? Would that make any sense at all?

Answer 1

The function of the AGC in a radio receiver is to keep the audio output fairly constant even as the incoming RF signal varies. This avoids annoying changes in the audio level as you change stations or, especially in the case of mobile radios, go in and out of good and bad reception areas. A good AGC can keep the audio level constant within 1 dB as the RF level varies as much as 100 dB. This is especially useful in shortwave receivers where the input RF level is subject to large variations due to changes in station distance, direction, and transmitting power. However AGC is not really intended to handle interference problems. These are better done by filtering, passband tuning, noise limiters, and other techniques.

Answer 2

it [AGC] responds practically to the power of the interferer and not to the power of the signal of interest? Would that make any sense at all?

It makes no sense for the purpose of controlling amplitude of sound for your ears, so that strong signals and weak signals are similarly loud. AGC circuits most commonly have fast attack time constant, and slow decay time constant.

There is a case where interfering signals are used to control amplitude: to reduce impulse noise. Rather than calling it AGC, this function might be called noise blanking.
Unlike AGC, time constants for noise blankers are very fast attack, and fast decay too. Noise blanking bandwidth must be necessarily wide, often much wider than the audio communication channel (and its AGC control).
Noise blankers use a fast-acting gating circuit whose purpose is to completely attenuate for a very short interval, timed so that a short noise spike is killed...so short that a listener's ears don't notice. Also, the attenuated noise spike will now not activate AGC.

Answer 3

Expecting one single purpose for AGC is unrealistic. Glen_geek's answer illustrates one purpose for allowing interfering signals to control gain automatically - noise blanking.

But there are others. If you allow a mixer to be overloaded you can at the very least expect distortion, or worse, intermodulation between multiple interfering signals to drown out the wanted signal.

In this case you must control the gain of an RF stage according to ALL the signals it processes, to stay below the point where a following mixer stage overloads (usually related to its 3rd order intercept point or IP3), so that you can separate wanted from interfering signals in an IF filter following the mixer.

(For SDR, the ADC replaces the mixer; you cannot allow the total output from any front end to exceed the ADC input range, however weak the wanted signal. If the interfering signals clip the ADC, say goodbye to the wanted signal)

So multiple AGC loops are common : one to protect linearity at each mixer or ADC; and one before the final demodulator to maintain a constant audio level.