Graphical Determination of Third Order Intermodulation Frequencies in RF Systems
AuthorJeske, Harold O.
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AbstractA simple discussion is given on the origin of intermodulation distortion in nonlinear devices concluding with reasons for placing primary interest on third order products. Nearby third order products are located at 2f(x) – f(y) and f(x) + f(y) – f(z) where f(x), f(y), and f(z) represent all frequencies in a group. The number of nearby IM products of this type is equal to N(N - 1) (1 + (N - 2 / 2)) where N is the number of input frequencies to the nonlinear device. For eight input signals, for example, a total of 224 intermodulation products are produced that are located in a bandwidth that is three times the bandwidth between the highest and lowest input frequencies. The graphical method of locating intermodulation products is rapid and simple. Basically, interfering frequencies are found by the alignment of two pieces of paper. On each piece of paper, the spacings between all input frequencies are plotted on a linear scale. Frequencies which may cause interference are easily identified. A guide which will allow the selection of frequencies to avoid third order intermodulation interference is also given.
SponsorsInternational Foundation for Telemetering