How do I protect a sensitive receiver from nearby high power transmitters?
Receiver Protection from Co-Located TX
Receiver protection is a layered defense strategy. No single technique provides adequate protection; the combination of filtering, limiting, and high-linearity design creates a robust front end that can coexist with high-power transmitters.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
- 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
What is the damage threshold for common RF components?
GaAs pHEMT LNA: +15 to +25 dBm damage threshold (device dependent; check the datasheet for maximum safe input power). GaN LNA: +30 to +40 dBm (GaN is much more robust than GaAs). Schottky diode mixer: +15 to +20 dBm (depends on the diode ring). Si CMOS receiver IC: +5 to +15 dBm (sensitive; requires external protection). The damage threshold is the power level at which permanent degradation occurs. Below the damage threshold but above the linear range: the device is compressed but not damaged (recovers when the signal is removed).
What limiter technology should I use?
PIN diode limiter: the most common. Limits the signal to +5 to +13 dBm flat leakage. Recovery time: 100 ns to 10 us. Can handle CW input power of +30 to +50 dBm. Available as surface-mount components from Skyworks, MACOM, and Microsemi. GaAs or GaN limiter: integrated into the LNA module for compact protection. Passive limiter (varactor or back-to-back diodes): simplest, lowest cost, but limited power handling. For high-power environments: use a multi-stage limiter (a coarse limiter that handles the bulk power followed by a fine limiter that reduces the leakage to a safe level for the LNA).
What about receiver blanking?
For systems where the transmitter and receiver operate on the same platform but not simultaneously (radar, TDD systems): blanking turns off the receiver (or disconnects it from the antenna) during the transmit pulse. This provides absolute protection during transmit. The receiver is re-enabled after the transmit pulse, with a recovery time of 100 ns to 10 us. Blanking is implemented by: a high-isolation T/R switch (20-30 dB isolation), a PIN diode limiter with bias control (the limiter is forward-biased during transmit to provide maximum attenuation), or disconnecting the LNA bias during transmit (the LNA provides no gain when unbiased, effectively blanking the receiver).