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The use of AGC loops at the input and output of the FM link in an AM-FM telemetry system allows the mean-square transmitter deviation to be maintained near maximum value, even though data may be nonstationary. However, errors result because of the inability of the receiver AGC loop to track perfectly gain variations in the transmitter loop. In this paper the general characteristics of AGC are discussed, and a theoretical analysis is performed to determine the time constant, steady-state error, and tracking error for a first-order loop. Also, tracking error for for first-order and second-order loops is investigated by simulation. Curves are presented to illustrate the principal results.