Editing Intermodulation (section)

== Measurement ==
Intermodulation distortion in audio is usually specified as the [[root mean square]] (RMS) value of the various sum-and-difference signals as a percentage of the original signal's root mean square voltage, although it may be specified in terms of individual component strengths, in [[decibel]]s, as is common with [[radio frequency]] work. [[Audio system measurements]] (Audio IMD) include SMPTE standard RP120-1994<ref name="AES"/> where two signals (at 60&nbsp;Hz and 7&nbsp;kHz, with 4:1 amplitude ratios) are used for the test; many other standards (such as DIN, CCIF) use other frequencies and amplitude ratios. Opinion varies over the ideal ratio of test frequencies (e.g. 3:4,<ref name="Cohen_2008"/> or almost — but not exactly — 3:1 for example<!-- where was that article?? -->).

After feeding the equipment under test with low distortion input sinewaves, the output distortion can be measured by using an [[electronic filter]] to remove the original frequencies, or spectral analysis may be made using [[Fast Fourier transform|Fourier transformation]]s in software or a dedicated [[spectrum analyzer]], or when determining intermodulation effects in communications equipment, may be made using the receiver under test itself.

In [[radio]] applications, intermodulation may be measured as [[adjacent channel power ratio]]. Hard to test are intermodulation signals in the GHz-range generated from passive devices (PIM: passive intermodulation). Manufacturers of these scalar PIM-instruments are Summitek and Rosenberger. The newest developments are PIM-instruments to measure also the distance to the PIM-source. Anritsu offers a radar-based solution with low accuracy and Heuermann offers a frequency converting vector network analyzer solution with high accuracy.