Digitally controlled crystal-based jitter attenuator

Abstract

A circuit for attenuating phase jitter on an incoming clock signal includes a digitally controlled oscillator, a phase lock loop including a phase detector, and a dithering circuit. The oscillator is capable of generating N discrete frequencies selectable through digitally controlled inputs controlling switched, capacitively-loaded amplifier stages. The phase lock loop provides a total of C.times.N.times.NB frequencies. The phase detector consists primarily of an up/down counter with an overflow/underflow limiter circuit. The dithering circuit modulates the oscillator signal to reduce inadequate rejection behavior when the incoming clock frequency is substantially the same as one of the N selectable frequencies of the oscillator divided down to match the frequency of the incoming clock.