One AC comparator

Detecting the difference between two periodic, arbitrary waveforms is described. The input signals have the same frequency (in the audio range) and are in phase. The inputs are combined into a composite signal, carrying the information about the difference between the mean values of half-periods of inputs. The required sensitivity is achieved by integrating the composite signal. The instrument can detect an amplitude difference of a few p.p.m. The instrument is microprocessor controlled and can be used to maintain the stable ratio between amplitudes mean values, or other characteristics of the two input signals.<>