How do I design the transmit/receive switch timing for a pulsed radar to avoid receiver damage?
T/R Switch Timing Design
The T/R switch (duplexer) is a critical protection component. A timing error of microseconds can destroy a $500-5000 LNA or receiver module, and potentially the entire receiver chain.
| 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
What T/R switch technologies are used?
Gas-discharge T/R tube (duplexer): traditional technology for high-power pulsed radar. Uses a gas-filled cavity that ionizes during the transmit pulse, creating a short circuit that reflects the TX power and protects the receiver. Recovery time: 1-10 us (the gas must deionize). Power handling: MW peak. Circulator + switch: a ferrite circulator directs TX power to the antenna and the RX signal to the receiver. The circulator provides 20-30 dB isolation. Additional switches or limiters provide the remaining isolation. Solid-state T/R module: PIN diode or GaN switches integrated into each element of a phased array. Very fast switching (10-100 ns). Isolation: 20-30 dB per stage; cascade for higher isolation.
What is the limiter?
A limiter is a passive protection device that clips (limits) the signal level at its output to a safe value, regardless of the input power. PIN diode limiter: the most common RF limiter. At low signal levels: the PIN diode is reverse-biased and transparent. At high signal levels: the diode self-biases forward, presenting a low impedance that reflects and absorbs the excess power. Limiting level: typically +5 to +15 dBm (adjustable by design). Recovery time: 0.1-1 us after the high-power pulse ends. The limiter is placed immediately before the LNA as the last line of defense.
What about phased array T/R modules?
In a phased array: each element has its own T/R module containing: a transmit amplifier (PA), a receive amplifier (LNA), a T/R switch (GaN or PIN diode), and a phase shifter. The T/R switch in each module: switches between TX and RX modes on a pulse-by-pulse basis. The switching time must be less than 100 ns to minimize the blind range. The module provides: transmit power amplification (1-10 W per element), receive low-noise amplification (NF less than 3 dB), and protection (the T/R switch and limiter protect the LNA from the TX leakage of adjacent elements as well as its own TX path).