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In article <59881@aurs01.UUCP> gingell@aurw90.UUCP (Mike Gingell) writes:
>In article <1991May23.070428.23505@funet.fi> varri@cs.tut.fi  writes:
]>What is the name of the distortion which does not appear at the
]>harmonic frequencies of the fundamental component but at some
]>other frequencies? This type of distortion is often introduced
]>by nonlinear filters.
]
]I would call it parametric distortion or intermodulation i.e a
]distortion due to the modulation of some transfer parameter of
]the network by the signal being transferred.

In RF Engineering, intermodulation means the mixing of different
frequency components of a signal. This mixing occurs through ANY
nonlinearity in the system, and produces signals with frequency
components as follows:

   Fd =  nF1 +/- mF2

where n = 1....   and m = 1...

This can be derived by observing that the transfer function with a
nonlinearity can be represented by a power series:
   H = a + bf(x) + cf^2(x) ...

If one substitutes the fourier representation of the signal into
this, one ends up with terms where the signals are multiplied by
powers of each other. This yields the classic sum and difference
mixing formulae.

Note that a system with nonlinearities, if presented with a pure
sinusoid (single frequency signal), with produce only harmonic distortion.
It takes more than one frequency in the input in order to observe
intermodulation distortion.

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