Distortion measurement system

摘要

A type of Total-Harmonic-Distortion (THD) distortion analyzer is described in which the use of the State Variable (SV) circuit in both the oscillator and the filter sections confers several benefits and results in performance superior to previously available analyzers. In the SV oscillator, the traditional compromise in the automatic level control function between settling time, component tolerance and distortion is alleviated by the use of polyphase level detection, made possible by the SV circuit. The doubly-integrated output available from the SV circuit also minimizes distortion, especially that contributed by the level-setting AGC. The other property of the SV circuit, which is particularly valuable, is the connection of frequency-setting elements to virtual-ground points, which allows great simplification of wide-range tuning circuitry. Exploitation of this property is described both for manual and programmable cases. The SV circuit is also superior to many previously employed circuits in its tolerance of component inaccuracies. The simultaneous presence of high-pass, band-pass and low-pass outputs from the SV filter is shown to be advantageous also for its use as the filter on a distortion analyzer.