Syllabus Sections:-

Automatic Gain Control

Please have in front of you 

3L1  28 Understand the source and use of an AGC voltage.

The automatic gain control or AGC is a voltage controlled part of the receiver with the voltage coming from the link between the second IF and the detector stage. The level of the voltage is directly related to the strength of the signal.

Now looking at   Page 61 fig 9.12  and you will see the comment AGC bias line.

As a strong signal alters the bias on the base of the transistor by reducing the voltage and thereby reduces the gain of the transistor. This voltage is used to control the gain of the receiver and so enable the receiver to handle a wide range of signal strengths with little or no change in output audio level.

This same voltage can be used to drive a Signal strength meter or "S" meter.

Please have in front of you  Page 63 fig 9.17  and refer to it for most of this section of text.
3L1  28 continued Recall that the speed of the AGC response can be adjusted on both attack and decay

If your Receiver has AGC and the input amplitude increase the term "Attack time" or "Fast Attack" refers to that time which the AGC take to respond and lower the output amplitude. the fast attack is required to stop unwanted large increases in signal from reaching the speaker or head phnes where hearing could be damaged.

Similarly the length of time the AGC responds to a decrease in input amplitude is called "decay time" and is a measure to the time to boost the input amplitude.

Thus according to how the amplitude of the input signal is varying will depend as to how you want the settings e.g. a slow fading can require longer response time than quick fading.