Phase locked loop system incorporating automatic gain control

Abstract

An automatic gain control (AGC) circuit is described in the context of a phase locking system in a digital television receiver. The signal produced by the phase locking system is used to develop a sampling clock signal that is locked in frequency and phase to the color reference burst signal component of a color television signal. An AGC detector develops a gain control signal by averaging only those samples which would correspond to the peaks of the burst signal if the sampling clock signal were locked to the burst signal. The gain control signal is applied to a multiplier which increases or decreases the magnitude of the television signal samples to bring the calculated peak amplitude value of the burst signal within a predetermined range. The AGC circuitry interacts with the phase locking system to decrease the lock-up time when there is a relatively large difference in phase between the burst signal and the sampling clock signal.