A sampled phase-locked loop

A phase-locked loop is described which can lock onto one half cycle in a repetitive waveform composed of short bursts of frequency of arbitrary, and changing, phase relationships. Such a signal is encountered in pulsed sonar and radar systems and the loop can be used to lock onto, and follow, one target. The technique of locking to a zero crossing has the advantage that it is more tolerant of changes in signal amplitude which are a common source of error in threshold detection systems. Results of two such loops tracking the moving walls of an elastic tube subjected to a pulsatile pressure, and human artery walls are presented.