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Passive Intermodulation (PIM) occurs when two or more signals are present in a passive device (cable, connector, isolator, switch, etc.) that exhibits a nonlinear response. The nonlinearity is caused by dissimilar metals, dirty interconnects, or other anodic effects. Loose connections are another source. Often times the nonlinearity does not manifest itself at low input signal levels either because not enough stress is placed on the dielectric or the PIM levels are at thermal noise level.
Basically, the device is acting like a frequency mixer with a local oscillator (LO) and RF input. All of the spurious mixing products are generated (±j*f1 ± k*f2); however, most are either at levels too low to be detected or are far enough out of the band of interest to be of any consequence. 3rd-order products (2*f1 - f2, and 2*f2 - f1) are generally the most troublesome since the are most likely to fall inband and at higher levels (levels generally decrease as the product order increases).
If a PIM is generated from a transmitter channel that falls within an adjacent receiver channel, desensitization will occur. A typical PIM requirement is -110 dBc or more. Many vendors offer RF cables with PIM specifications of -140 dBc.
Here are some sites that feature application notes: