Passband Timing Recovery in an All-Digital Modem Receiver

The performance of conventional modem receivers, where adaptive equalization is achieved by a digital transversal filter with tap gains spaced at the symbol interval, depends critically on the choice of the sampling phase. In this paper, a digital timing recovery loop is described and analyzed in the case of passband quadrature amplitude modulated data signals. Under conditions likely to be encountered on actual voiceband communication channels, the clock phase derived is shown to prevent spectral nulls and to accurately approximate the optimum timing phase for an infinite equalizer. Computer simulations show that the proposed system is capable of fast timing acquisition.