How does PIM testing work and what are the standard test levels?
PIM Testing Methodology
PIM testing requires generating two high-power signals, combining them, passing them through the device under test (DUT), and measuring the intermodulation products at the output. The measurement is challenging because the IM products are 150+ dB below the carrier power, requiring excellent isolation between the transmit and receive paths and very low residual PIM in the test system itself.
| 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 |
Frequently Asked Questions
What test levels does 3GPP specify?
3GPP specifies PIM testing at +43 dBm (20W) per carrier for base station antenna systems. The IM3 products must be below -150 dBc to prevent desensitization of the receive path. Some carriers require -153 or -155 dBc for tighter system margin.
Can I test components at lower power?
Yes, but PIM levels scale with power, and components that pass at low power may fail at full power due to different PIM mechanisms activating at higher RF voltages. Low-power PIM testing is useful for screening but does not guarantee performance at rated power.
What is reverse PIM?
Reverse PIM (reflected PIM) measures IM products traveling back toward the source, as opposed to forward PIM traveling through the DUT. Both must be measured for full characterization. A component can have different forward and reverse PIM levels.