What is a limiter and how does it protect the receiver in a pulsed radar system?
RF Limiters
Key limiter specifications: flat leakage power (the maximum power reaching the LNA during limiting), spike leakage (the energy in the fast transient before the limiter activates), recovery time (time to return to low insertion loss after the high-power signal ends), insertion loss (typically 0.3-1 dB in the non-limiting state, directly adding to the receiver noise figure), and power handling (maximum input power before permanent damage).
| Parameter | Pulsed | CW/FMCW | Phased Array |
|---|---|---|---|
| Range Resolution | c/(2B) | c/(2B) | c/(2B) |
| Velocity Resolution | PRF dependent | Direct from Doppler | Coherent processing |
| Peak Power | High (kW-MW) | Low (mW-W) | Moderate per element |
| Complexity | Moderate | Low | High |
| Typical Application | Surveillance, weather | Altimeter, automotive | Tracking, multifunction |
- 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
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
How does insertion loss affect NF?
The limiter's insertion loss is in the receive path before the LNA, so it adds directly to the system noise figure. A 0.5 dB limiter loss increases the system NF by 0.5 dB. For noise-critical receivers: use a low-loss limiter (< 0.3 dB) or place the limiter after a first-stage LNA (but then the LNA must be robust enough to survive the leakage power).
What is spike leakage energy?
Spike leakage occurs during the turn-on transient of the limiter (1-10 ns). The spike energy (integral of leakage power over the spike duration) determines whether the LNA's gate junction is damaged. GaAs LNAs: damage threshold approximately 1-10 ergs. Modern low-spike limiters achieve < 0.1 ergs spike energy for reliable LNA protection.