Design and Operation of Automatic Gain Control Loops for Receivers in Modern Communications Systems

Figure 1 is a general block diagram for an AGC loop. The input signal passes through the VGA to produce a signal output whose level will be stabilized. The output level is measured by a detector, whose output is compared against a setpoint voltage to produce an error signal. The error signal is then integrated to produce a gain control voltage that is applied to the control input of the VGA. It may be desirable to align the maximum output level of the VGA with the maximum input level of the detector with the attenuator shown between the VGA and the detector.