Duplexer: Advantages, Disadvantages, and Applications

This page explores the advantages and disadvantages of duplexers. It covers the benefits and drawbacks of RF duplexers, along with an introduction to their basic function.

What is an RF Duplexer?

A duplexer is an RF (Radio Frequency) device that routes two nearby frequency bands along two distinct paths. Essentially, it allows a single antenna to be used for both transmitting and receiving signals.

There are two primary types of duplexers based on their construction:

• PIN Diode Switch Based: These use PIN diode switches to direct the signals.
• Circulator Based: These utilize circulators to achieve the desired signal routing.

RF duplexers also incorporate narrow-band filters to separate these closely spaced frequency bands between ports and achieve good isolation. This isolation prevents the transmitter from interfering with the receiver during reception and vice versa. It acts as a microwave switch between the antenna and the transmitter/receiver (Tx/Rx).

RF duplexer Example

Application Examples

GSM 900 System

In a GSM 900 system, the frequencies 890 MHz to 915 MHz are used for transmitting, and 935 MHz to 960 MHz are used for receiving. A diplexer is employed to allow the transmitter and receiver to share a single antenna for both frequency bands.

Figure 1 depicts the use of a 3-port circulator as an RF duplexer. Due to its clockwise operation, the input at port 1 leads to the output at port 2, the input at port 2 leads to the output at port 3, and so on. Because the frequencies are different, simultaneous communication is possible in both directions.

LTE System

In LTE (Long-Term Evolution) systems, duplexers separate the uplink frequency band (1710-1785 MHz) and the downlink frequency band (1805-1880 MHz) in the transmit and receive directions. This enables the use of the same antenna for both transmitter and receiver in FDD (Frequency Division Duplexing) mode.

Common Types of Duplexers

Here are the common types of duplexers based on design, each with its own advantages and disadvantages:

• Branch Type Duplexer: Uses TR (Transmit-Receive) and ATR (Anti-Transmit-Receive) switches, often implemented with gas-discharge tubes.
• Circulator Duplexer: A three-port or four-port device that uses a circulator for duplexing operation.
• Balanced Duplexer: A hybrid junction coupled through a short-slot. It uses two waveguide sections coupled through slots, along with TR tubes. The TR tube is used to “fire” and reflect incident power. It functions as a broadband directional coupler.

Benefits (Advantages) of Duplexers

Here are the advantages of using duplexers:

• Shared Antenna: Enables the sharing of a single antenna for both transmit and receive modes, minimizing space and cost in wireless systems.
• Compact Design: Allows for a very compact duplexer design due to reduced space requirements.
• Receiver Protection: Protects the receiver from high-power transmitters through isolation.
• (Branch Duplexer): Low cost.
• (Circulator Duplexer): Used in monostatic radar systems. Refer to the advantages and disadvantages of RF circulators for more information.
• (Balanced Duplexer): Higher power handling capability compared to branch duplexers and a wider bandwidth of operation.

Drawbacks (Disadvantages) of Duplexers

Here are the disadvantages of using duplexers:

• Increased Noise Figure: The noise figure of the receiver increases due to the insertion of the duplexer in the receiver chain. This is due to the duplexer’s insertion loss. Appropriate gains and noise figures must be considered to meet the link budget at the receiver.
• (Branch Duplexer): Limited bandwidth and power handling.
• (Balanced Duplexer): Does not allow simultaneous transmit and receive.