Duplexer
Understanding Duplexers
A duplexer is critical in any FDD (Frequency Division Duplex) system where the transmitter and receiver operate simultaneously at different frequencies. Without adequate isolation, the high-power transmitter would desensitize or damage the receiver. The duplexer must pass both bands with low loss while providing 40-60 dB of isolation between them.
Duplexer Types
- Filter-based: Two bandpass filters with a common antenna port. Standard for cellular and satellite systems.
- Circulator-based: Uses a ferrite circulator with a bandpass filter to separate TX and RX. Provides moderate isolation.
- Hybrid: Combines circulators and filters for maximum isolation.
TX insertion loss: 0.3-1.5 dB
RX insertion loss: 0.3-1.5 dB
TX-to-RX isolation: 40-60 dB
TX-to-antenna: matched (VSWR < 1.3)
RX-to-antenna: matched (VSWR < 1.3)
Frequently Asked Questions
What is a duplexer?
A duplexer allows a transmitter and receiver to share one antenna simultaneously by providing frequency-selective isolation between them. It passes both TX and RX bands with low loss while preventing transmitter power from reaching the receiver.
What is the difference between a duplexer and a diplexer?
The terms overlap. A diplexer separates any two frequency bands. A duplexer specifically separates transmit and receive paths. In FDD systems, the duplexer is technically a diplexer. In systems where TX and RX share the same frequency, a circulator or switch acts as the duplexer.
How much isolation does a duplexer need?
Isolation must be sufficient to prevent the transmitter from desensitizing the receiver. Typically 50-60 dB is required. For a +40 dBm transmitter and -100 dBm receiver sensitivity, 60 dB isolation still delivers -20 dBm to the receiver, which the LNA must handle linearly.