This instrument measures the distortion of a circuit when two sinusoids of different frequencies (Fa and Fb) are applied at the input.

In the case of a single pure sinusoidal input, we find distortion at harmonics of the fundamental frequency, so we expect to find distortion at 2*Fa, 3* Fa, etc and 2*Fb, 3*Fb, etc. With Fa and Fb components to the input, however, the output also has frequency components at sums and differences of the two sinusoid’s frequencies, e.g. Fb-Fa, 2*Fb-Fa, 2*Fa-Fb, Fa+Fb, 2*Fa+Fb, etc.

The Intermodulation distortion meter has three set-up modes, SMPTE (Society of Motion Picture and Television Engineers), CCIF, and manual setup.

The SMPTE test uses a signal made up of two separate sin waves. The first sin component has a high amplitude and low frequency (60Hz, Fl), and the second has a high frequency (7kHz, Fh) and low amplitude (¼ strength of the 60Hz component’s amplitude). Sidebands appear in the frequency response at 60Hz intervals around the 7KHz tone. The percent intermodulation distortion is defined as the percentage of amplitude modulation represented by the 2nd and 3rd sidebands, i.e. Fh +/- Fl, and Fh +/- 2Fl).

The CCIF test uses a source made up of two high frequency, equal amplitude components whose frequencies are separated by a small frequency (e.g. 1Kh). The program defaults to source signal components at frequencies of 13kHz and 14kHz. With this test, the percent intermodulation distortion is defined as the amplitude of the frequency component at Fh-Fl as a percentage of the source amplitude.

The single frequency distortion test and the intermodulation distortion test are often performed on amplifier circuits because with this class of circuit you want to minimize the distortion of the signal during the amplification process.