A phase detector is disclosed in which means are provided for sensing when two AC signals being subjected to phase difference measurement become close in phase, within a predetermined range, and for inverting one of these signals in phase when such a close phase relationship is sensed. The phase difference...http://www.google.com/patents/US4520321?utm_source=gb-gplus-sharePatent US4520321 - Phase difference detector with phase inversion of one input signal when phase difference is small
