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Generally even all the devices designed as linear circuits and must not generate inter modulation from the ideal viewpoint. Since they always have some non-linearity, however, the intermdolation is measured in practical tests. This undesired inter modulation distortion in liner circuits is called “passive inter modulation: PIM”. The amplitude of PIM is much lower than that of non-linear devices such as power amplifier, however, it is exactly measurable by the high sensitivity receiver such as spectrum analyzer because its amplitude is higher than the noise floor of receiver.
When PIM is considered in the area of wireless communication such as maritime, mobile and space communications, it would be a problem with high transmitting powers in a microwave band that the PIM interferes weak reception signal. On the other hands, from the materialistic viewpoint, PIM can be a powerful tool to detect undesired non-linearity such as generated by electric contacts. In order to detect them, PIM is generally evaluated with low frequencies. It has been already reported that electric non-linearity generated in dissimilar metals can be eliminated by DC bias. This theory would be also proved by the test with PIM.
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