RF Limiter Function
An RF limiter protects receiver front-ends from high-power RF pulses (e.g., radar TX leakage, jamming, ESD). At low power (small signal), it passes the signal with minimal loss. Above the limiting threshold, it clamps the output power to protect the LNA.
S-Parameter Limitations for Limiters
S-parameters are small-signal linear measurements — they describe limiter performance ONLY in the pass-through (below-threshold) state. The clamping behavior at high power is NOT captured by S21. For complete characterization, you need both S-parameter analysis AND a power sweep test.
Small-Signal S-Parameter Specs
| Parameter | Typical Spec | Note |
|---|---|---|
| S21 (pass-through, low power) | <1.0 dB | Receiver path loss below threshold |
| S11 (input match) | <−15 dB | VSWR at receive frequency |
| S22 (output match) | <−15 dB | Load presented to LNA input |
| Frequency range | DC to rated max | Must cover operating band |
| Recovery time | Not in S-params! | Requires pulse measurement |
Large-Signal Behavior (Not from S-Params)
Key limiter specifications (require power sweep): CW limiting threshold: power at which output saturates (e.g., −20 dBm output for +30 dBm input) Limiting flatness: how flat is the output vs input power in limiting region Recovery time: time to full sensitivity after high-power pulse (critical for radar) Survival power: maximum input power without damage (e.g., +50 dBm, 1 µs pulse)
Analysis in RF View
- Load limiter .s2p (measured at low power, e.g., −30 dBm VNA source)
- S21: verify pass-through IL <1.0 dB across operating band
- S11, S22: verify good port matches to ensure no signal degradation at receive port
- Group Delay: check for any phase distortion that could affect modulation quality
RF View Limiter Analysis: Load limiter .s2p to verify small-signal pass-through insertion loss and port matches. Remember: limiting behavior and recovery time require separate bench tests. Free on Android.