Intermodulation
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Intermodulation or intermod is the result of two radio signals of different frequencies being mixed together, forming additional signals at frequencies that are not at harmonic frequencies (integer multiples) of either. The largest intermodulation products appear at f1 + f2 or f1 − f2 (second-order intermodulation), and less so at 2f1 − f2 or 2f2 − f1 (third order intermodulation).
The cause for intermodulation is the existence of non-linear characteristics of the according equipment. The theoretical outcome of these nonlinearities can be calculated by conducting a Volterra series of the characteristic, while the usual approximation of those nonlinearities is obtained by conducting a Taylor series. According to the summands in those series, the above numbering of orders is counted.
Intermodulation is rarely desirable in radio, as it essentially creates spurious emissions, which can create minor to severe interference to other operations on the resulting frequency. It may be desirable in audio if the intent is to create specific sound effects.