A System for Automatically Recording Third and Higher Order Intermodulation Distortion Components

Abstract

A problem arises in measuring certain intermodulation distortion components when measurements are made at more than one set of input frequencies. When two input frequencies with a constant frequency difference between them are swept through the frequency range of interest, only some intermodulation components remain fixed in frequency. Others. which may be of significance, particularly in push-pull systems, change in frequency as the input frequencies are varied. A system is described in which some of these variable frequencies may be tracked by a slightly detuned reference frequency, which is derived by suitable heterodyning of the radio-frequency signals of two beat frequency audio oscillators. This signal serves as the reference signal in a non-phase-sensitive synchronous detector, the output of which is filtered, detected, and automatically recorded. (This work was done under the sponsorship of the Veterans Administration.)