What materials and designs minimize passive intermodulation in connectors and cables?
Low-PIM Design Practices
Unlike active intermodulation from amplifiers and mixers, passive intermodulation arises from the nonlinear behavior of imperfect metal-to-metal contacts and certain materials. When two high-power carriers pass through a nonlinear passive junction, IM products are generated at the same frequencies as active IM (2f1-f2, 2f2-f1, etc.), but at much lower levels. These low-level PIM products become problematic in systems where the transmit and receive bands are closely spaced, such as cellular base stations.
The most common PIM sources are: connector interfaces (especially if under-torqued or contaminated), solder joints with cold joints or flux residue, rusty or corroded metal surfaces, and carbon steel hardware near the signal path. Even a single loose screw in an antenna mounting bracket can generate PIM products that desensitize a nearby receiver.
Low-PIM connector designs use silver-plated white bronze or copper alloy bodies, carefully controlled contact pressure, and non-ferromagnetic materials throughout. 7-16 DIN connectors are the industry standard for low-PIM applications because their larger contact area provides more consistent, lower-resistance junctions than smaller connectors like N-type or SMA.
Frequently Asked Questions
Why is nickel plating bad for PIM?
Nickel is ferromagnetic, and ferromagnetic materials exhibit hysteresis that creates a nonlinear B-H response. Even thin nickel plating (standard on many commercial connectors) can generate PIM. Low-PIM connectors use silver over white bronze or tin-plated copper instead.
What PIM level is acceptable?
For cellular base stations: -150 dBc or better (third-order) with two +43 dBm carriers. For satellite systems: -160 dBc may be required. PIM specifications vary by standard: IEC 62037, 3GPP, and carrier-specific requirements all define acceptable levels.
Can PIM be measured in the field?
Yes. Portable PIM analyzers inject two CW carriers at the specified test power and measure the IM3 product levels. Field testing identifies PIM sources in installed systems, which are typically loose connectors, damaged cables, or corroded hardware. Walking the system while monitoring PIM localizes the source.