User manual

Reference172

Intermodulation Distortion (IMD). A measure of the distortion caused by the

nonlinear mixing of two tones. When multiple signals are injected into a device,

modulation or nonlinear mixing of these two signals can occur. For input signals at

frequencies f1 and f2, the two second-order distortion signals will be found at

frequencies: f3 = (f1 + f2) and f4 = (f1 - f2).

IMD is expressed as the dB ratio of the RMS sum of the distortion terms to the RMS

sum of the two input tones. IMD can be measured for distortion terms of any order,

but the second-order terms are most commonly used. In the second-order case, the

intermodulation distortion is given by:

where

F3 and F4 are the amplitudes of the two second-order distortion terms (at

frequencies f3 and f4 defined above)

and

F1 and F2 are the amplitudes of the input tones (at frequencies f1 and f2, as

marked by the frequency rulers in the spectrum window).

For reference, the third-order terms are at frequencies (2F1 + F2), (2F1 - F2), (F1 +

2F2) and (F1 - 2F2).

Note: Hanning or Blackman windows are recommended because of their low noise. An

FFT size of 4096 or greater is recommended in order to provide adequate spectral

resolution for the IMD measurements.

Mask Failures. See Mask Limit Testing.