A digital phase-locked loop for generating frequency discriminating codes and frequency multiplication

A new digital phase-locked loop system realizale by a few off-the shelf digital ICs is described. The system is locked by tracking the input square wave and produces an output binary code whose value is proportional to the input frequency. It is characterized by wide locking range and fast capture time down to very low frequencies (<1 Hz) and suited to low frequency multiplication. Asymmetry in capture time, however, limits its use as an FM demodulator to only slowly varying signals.