Phase Intermodulation Distortion - Instrumentation and Measurements

Journal of the Audio Engineering Society
Volume 31, March 1983

Conventional SMPTE IM distortion applies a combination of 60 and 6000 Hz (or 4000 Hz) in an amplitude ratio of 4:1 to the amplifier under test. The output of the amplifier is then measured by filtering out the 60 Hz part of the signal and then measuring the amount of amplitude modulation on the remaining 6 kHz "carrier". This test measure the effect of the larger 60 Hz signal on the small-signal gain of the amplifier. Changes in small-signal gain as a function of the signal excursion of the larger 60 Hz signal results in modulation of the 6 kHz signal.

Phase Intermodulation (PIM) distortion instead measures the phase modulation of the 6 kHz carrier that is caused by the larger 60 Hz signal. Phase modulation can occur if, for example, certain capacitances in the amplifier are a function of voltage (such as the collector-base capacitance of the amplifier's voltage amplification (VA) stage).

This paper describes instrumentation for measuring PIM and shows typical PIM measurement results for several operational amplifiers and a power amplifier. The instrument uses phase-locked loop and synchronous detection techniques to provide highly sensitive measurements. Indeed, the instrument can detect changes in signal delay as a function of the 60 Hz signal on the order of nanoseconds. The application of this test to assessing so-called "time smear" is obvious.

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