Cavity Duplexer
Understanding Cavity Duplexer
Protecting Sensitive Receivers
In full-duplex radio systems, such as emergency service repeaters, marine base stations, and cellular transceivers, a system must transmit and receive signals simultaneously. Installing separate antennas for the transmitter and receiver is expensive and often causes spatial coupling problems on towers. The standard solution is a duplexer, a three-port filter network that connects both the transmitter (TX) and receiver (RX) to a single common antenna.
The primary engineering challenge is the massive power difference between the transmit and receive signals. A base station transmitter might output 100 Watts (+50 dBm), while the receiver must detect signals as weak as -110 dBm. If even a tiny fraction of the TX power leaks into the RX port, it will desensitize the receiver (blinding it to weak signals) or burn out the low-noise amplifier (LNA). A cavity duplexer provides the high isolation needed to prevent this.
Cavity Configuration and Selectivity
A cavity duplexer consists of two branches of bandpass or band-reject (notch) cavity filters connected to a common antenna junction. The TX branch contains cavities tuned to pass the TX frequency and reject the RX frequency. The RX branch contains cavities tuned to pass the RX frequency and block the TX frequency. Because the frequency spacing between TX and RX is often narrow (such as 4.5 MHz in UHF repeaters), these filters must have extremely steep roll-off. Resonant cavities, with their high quality factor ($Q_u$), provide this steep selectivity, achieving over 85 dB of isolation with minimal insertion loss.
Key Mathematical Relations
Technical Specifications Comparison
| Duplexer Class | Q Factor Range | Typical Isolation | Insertion Loss | Size and Weight |
|---|---|---|---|---|
| Cavity Duplexer | High (2,000 - 8,000) | Very High (85 - 100 dB) | Low (0.5 - 1.2 dB) | Large, heavy metallic housings |
| SAW / BAW Duplexer | Low to Moderate | Moderate (45 - 55 dB) | Moderate (2.0 - 3.5 dB) | Miniature surface-mount chip |
| Ceramic Duplexer | Moderate (500 - 1,500) | High (65 - 80 dB) | Moderate (1.5 - 2.5 dB) | Medium block footprint |
| Microstrip Duplexer | Low (100 - 250) | Low (< 40 dB) | High (> 3.5 dB) | Flat PCB layout |
Frequently Asked Questions
What is 'receiver desensitization' in a duplex system?
Receiver desensitization occurs when transmitter RF power leaks into the receiver LNA, driving it into saturation. This reduces the amplifier gain and increases its noise figure, blinding the receiver to weak signals.
What is a band-reject (notch) cavity duplexer?
A notch duplexer uses cavities tuned to reject the opposite frequency (e.g., the RX branch has a notch at the TX frequency). This provides high isolation for close frequency spacings, though it offers less broad out-of-band rejection.
Can a cavity duplexer be retuned to different frequencies?
Yes. Unlike solid-state or ceramic duplexers, cavity duplexers can be manually tuned by adjusting the internal rods. This allows operators to reconfigure the duplexer for different channel allocations.