Digital frequency measurement of a square wave

A fully digital and simple to implement method to measure the frequency difference between two square waves is presented. The new frequency detector can detect small phase movements of much less than one cycle between the two square waves, even though the frequency detector is clocked at a low rate. Existing digital techniques can only detect phase slips of more than one cycle, or require clock rates much higher than the frequency of the square waves. Operating limits are given and demodulation of a PM signal is demonstrated