Contents - Index

InterModulation Distortion (IMD)

InterModulation Distortion (IMD) is a measure of the distortion caused by the interaction (mixing) of two tones.  

When multiple signals are injected into a device, undesired modulation or mixing of these two signals can occur.  The IMD level is calculated by first computing the frequencies and amplitudes of the two strongest tones in the spectrum.  The total power in each of the intermodulation product frequencies is then computed.  IMD is the ratio of the intermodulation power to the rms sum of the tone power.

A typical IMD measurement will use tones at 250 Hz and 8020 Hz with the higher frequency tone reduced by 12dB (4:1 ratio) below the low frequency tone.  It is often useful to employ long term averaging to cancel the random noise components and enhance the measurement accuracy.  

This utility window can be resized as required by the user and its contents will update anytime a new spectrum is computed.  If the analyzer is configured to display the spectrum of both the left and right channels, this utility window will also show the value for both channels.  The left channel value will appear above the right channel value.

  • Hanning or Blackman smoothing window is recommended because of its low noise quality.  
  • An FFT size of 4096 or greater is recommended in order to to provide adequate spectral resolution for the IMD measurements
  • The contents of any open utility window will be printed on the right margin of the spectrum view.

    See Also:  IMD test tones