Passive Intermodulation
Understanding PIM
PIM is one of the most problematic interference sources in cellular networks. Unlike active intermodulation (from amplifiers), PIM comes from passive components that should be perfectly linear but exhibit weak nonlinearity at high power levels.
PIM Sources
- Metal junctions: Loose or corroded connectors create rectifying metal-oxide-metal junctions.
- Ferromagnetic materials: Steel, nickel, and certain alloys have inherently nonlinear magnetic properties.
- Contamination: Metal particles, dirt, or moisture on connector interfaces.
- Cable damage: Crushed or kinked cables create intermittent contact junctions.
PIM Specifications
- 3GPP requirement: PIM < -150 dBc for 2 x 43 dBm (20W) carriers.
- Good PIM: < -155 dBc. Excellent: < -160 dBc.
- Measurement: Two carriers at TX power, measure IM3 product in the RX band.
Frequently Asked Questions
What is PIM?
PIM is intermodulation from passive components (connectors, cables, antennas) that should be linear. Caused by metal-contact nonlinearity, corrosion, ferromagnetic materials, or contamination. Critical in cellular base stations where PIM falls in the RX band.
Why is PIM a problem for cellular?
Two TX carriers at 20W each generate 3rd-order PIM products. If these fall in the RX band, they appear as in-band interference, desensitizing the receiver. Even -150 dBc PIM can be 10-20 dB above the receiver noise floor.
How do you prevent PIM?
Use PIM-rated connectors (7/16 DIN, 4.3-10). Avoid ferromagnetic materials (use brass, copper, aluminum). Proper torque on all connections. Clean connector interfaces. Avoid cable damage. Test with PIM analyzer after installation.