DIGITAL PHASE LOCKED LOOPS WITH SEQUENTIAL LOOP FILTERS: A CASE FOR COARSE QUANTIZATION

Abstract

A class of digital phase locked loops exists for which performance can be demonstrated to be comparable to that of the linear analog model, yet the use of coarse quantization seems to be favored. This class is characterized by the use of (1) a phase detector with quantized output, (2) a sequential loop filter, and (3) a discrete phase adjustment clock. The sequential filter is an inherently nonlinear device which operates on a sequence of inputs and provides a sequence of outputs, usually fewer in number and with a variable interval between outputs. Operation of these loops is decidedly nonlinear and a conventional linear loop bandwidth cannot be defined. Although higher than first order loops of this class may be constructed, their analysis, so far, has been limited to first order types by random walk techniques. A quasi-bandwidth measure can be defined for these loops in terms of their transient response which serves as an absolute basis for comparison of different digital loops. Using this tool, examples of 3- and 4-level quantization in these loops are found to have no obvious advantages over the binary quantized model and in some cases striking disadvantages.