Real-Time Analog Fourier Analyzer Using PLL and the High-Speed Null Method

Abstract

A harmonic analyzer which furnishes accurate information on the phases of harmonics is proposed. The operating principles of this harmonic analyzer consist in performing compute operations on function f(t) of interest in terms of analog voltage for the Fourier coefficients, using quadrature harmonic sinewaves that are generated in phase with the fundamental of f(t) by two phase-locked oscillators arranged tandem. Thus a d. c. voltage proportional to the real or imaginary part of each harmonic component of f(t) can be obtained. The configuration used in the harmonic analyzer provides a high flexibility in analysis and allows a perfect real-time operation as well. The test run with a prototype consisting of about 35 logic and analog ICs indicated an accuracy of ±0.6% FS over the covered frequency range of 1Hz to 50kHz. A closed-loop version which was implemented by a kind of null method is also discussed. This version not only reduces errors by half but can find use in such applications as filtering devices, e. g., tracking band-pass/notch filters, and high-speed distortion meters.