Reader Question: What is Intermodulation Distortion and is it Important?

Like Total Harmonic Distortion, Intermodulation Distortion, also known as IM distortion is another type of frequency distortion. However, unlike THD, in which the unwanted frequencies are harmonically related to the fundamental frequency, IM distortion results when non-harmonically related frequencies are generated by an amplifier. THD occurs when multiples of the original fundamental frequency are generated. IM distortion is generated when the sum and difference of two frequencies are reproduced by an amplifier. For example, assume that two frequencies, 700Hz and 1100Hz are reproduced by an amplifier. IM distortion results when the sum and the difference of those two frequencies are generated, or 1800Hz and 400Hz. Those new frequencies are not harmonically related because they are not multiples of 700Hz and 1100Hz .

IM distortion is produced by all amplifiers, and is expressed as a percentage of the total signal, for example, 0.05% IM distortion. A lower number is better. It is a more important specification than THD, however, in most cases the level of IM distortion is so small that it is generally considered insignificant and it should not be used as the only method of comparing amplifiers. For more terms and definitions, visit the glossary of terms.