PIM stands for Passive Inter-Modulation
“Passive IM, similar to Active IM but occurring in passive devices, is present whenever RF signals at two or more frequencies are simultaneously present in a conductor of RF energy. Every passive RF device generates passive IM products when more than one frequency is present in the device. The signals are mixed by the non-linear properties of junctions between dissimilar materials. Typically, it is the odd-ordered products (e.g. IM3=2*F1-F2) that can be very problematic should they fall within an uplink, or receive band of the base station because they appear to the receiver as interference. The result can be a receiver desensitization which is independent of the receiver's random noise floor.”
PIM occurs/is generated when two or more signals are passed through a passive device. Examples of passive devices are antennas, coax, jumpers, connectors, and diplexers. If there is a manufacturing defect, workmanship issue, or an issue with a connection, say in the mating of a jumper and a hard-line connector (i.e., the mating of the center pin of a connector and its receptor), a third signal is created. That third signal is “PIM” and is what the test set measures
There is a great deal of misinformation about PIM, its causes, and PIM testing circulating out in the field these days. According to some crew accounts, wind, trains, low flying planes, the wrong color sunshine, and the occasional scurrying squirrel cause PIM.
In the field, there are two main categories for PIM causes.
Manufacturing defects: weak soldier joints, contamination, mismatched materials, etc.
- These usually result in a pass/fail for a component.
- Failing components are replaced when spares are available.
- These are the tower crews’ nightmare and result in countless hours of troubleshooting, rework, and frustration.
- These issues are probably the source of most myths and misinformation surrounding PIM and PIM testing.
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