Abstract: Electrical signals suffer usually from distortion and noise. This invention gives a very precise and simple digital circuitry as well as basis for software to measure the phase difference between harmonic components of any of two of the input signals. The apparatus may be used also for measuring the voltage, current, impedance, apparent power, active power, reactive power, total harmonic distortion or distortion of preselected harmonics in any two of a plurality of test signals, autocorrelations and cross-correlations for any subset of the input signals. The apparatus may also be used to analyze the amplitude of harmonic components, to filter DC or specific harmonic components of any of the result in an analog or digitized waveform, and to measure object distance for radar, sonar, and telecommunication applications. The apparatus design is based on modern digital signal processor chips and multi-processors operation.