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BBC R&D White Paper WHP106

How does one calculate and measure AGC time-constants?
R.H.M. Poole

Keywords
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Abstract
This White Paper is the result of field trials carried out on some portable radio equipment. During the trials, it became evident that the automatic gain control (AGC) of the receiver was operating more slowly than desirable - a rapid change of incoming signal level caused the receiver output to ‘bounce’. As a consequence, the output could temporarily either clip or sink into the noise.

In this case, alteration of the AGC time-constant proved to be an easy cure for the problem. However, there must be many other cases where an understanding of AGC systems would help in avoiding similar pitfalls. This White Paper describes, in simple terms, how to calculate and measure either the time-constant or its close relation the loop bandwidth. The Paper also introduces an Excel spreadsheet to simulate the effect of the AGC on any amplitude disturbance. Only a single-loop AGC system is considered, but it would be easy to extend the work to higher order systems.