Mechanism for interpolating among samples of received communication signal using asynchronous high speed clock which is a nominal multiple of recovered signalling baud rate

Abstract

An analog input to an interpolation scheme for a digital signal processing receiver is digitized by an oversampling sigma-delta modulator running at a clock frequency that is a multiple of the nominal baud rate. The digitized signal sample stream and a delayed version thereof are shifted through finite impulse response (FIR) filters. The outputs of the FIR filters are coupled to a linear interpolator at a time determined by a carry-out of a divide-by-M counter, so as to decimate the digitized inputs to the FIR filters by the ratio of the oversampling clock to the baud clock. The control path to the interpolator includes a digital phase locked loop containing a linear canceler, to which data from an echo canceler and data decisions from an equalizer are supplied, and from which a timing error input is supplied to a loop filter. The output of the loop filter is supplied to a threshold comparator which outputs an add/delete count signal to a modulo N up/down counter that controls the coupling of samples to the interpolator. At baud time, the two decimated FIR filter samples A and B are subjected to a linear interpolation operator A+(B−A)*n/N, where n is a count value provided by the up/down counter of the total number of times that the comparator has initiated an add or delete command, and serves as a software pointer to the signal value to be interpolated.