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.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
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.