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Abstract

The present invention is a method and apparatus for implementing a two stage AGC circuit. In the preferred embodiment, the present invention is used as part of a receive channel in a modem. The first stage of the AGC is a 'coarse' AGC and is used to track large signal transients of an input signal. The coarse AGC locks on to transient signals without excessive settling time. In operation, the coarse AGC acquires a new signal by using a nonlinear clipped feedback loop technique supported by a linearized feedback loop. The coarse AGC stage uses an error signal derived from the noncoherent power fluctuations of the incoming signal. The second stage of the AGC circuit is a 'fine' AGC using a decision-directed coherent amplitude error signal and a quick linear feedback loop to correct for finer signal level fluctuations. The fine AGC has a high pass characteristic which decouples its response from that of the equalizer for stability reasons. The present invention avoids performance and response limitations of prior art AGC's in that the coarse stage is not required to have wide band response and associated noisy tracking response. Further, when the input signal is in quadrature amplitude modulation (QAM) the coarse stage tracks by using a nonlinear power detection algorithm which removes the effect of data power modulation. The fine AGC stage utilizes a decision-directed (coherent) error signal and a linear feedback loop with zero excess delay so that wideband response can be achieved without introducing amplitude modulation error in the signal path. As a result, the two stage scheme of the present invention can track rapid gain changes and restore correct data detection within a few baud, limiting error corruption to only one data block.

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First Claim