What is the difference between a Diplexer and Duplexer?

A Diplexer is a 3-port passive device that allows two signals at different frequency bands to share a common antenna or transmission line. It consists of two frequency-selective filters (low-pass, high-pass, or band-pass) connected to a common port. As seen in the figure below, Signal A at Frequency A passes through Filter A to the antenna, while Signal B at Frequency B passes through Filter B to the same antenna. 

Each filter passes its intended band and rejects the other, providing isolation between the two signal paths. For proper operation, the frequency bands must be sufficiently separated to allow practical filter designs that provide adequate isolation while maintaining low insertion loss and good impedance matching.

A common use case for a diplexer is in a cellular base station where two different frequency bands, such as 700 MHz and 1900 MHz, share the same antenna. Instead of installing separate antennas for each band, a diplexer combines the signals while keeping them isolated through frequency-selective filters. This reduces tower space, cabling, and hardware complexity while enabling multiband operation.

What is a Duplexer?

A Duplexer is a 3-port device that allows the transmitter and receiver to use a single antenna, while operating at the same/similar frequencies. Its primary function is to isolate the receiver from the high-power transmitter signal during transmission, and isolate the transmitter from incoming signals during reception. This isolation prevents receiver overload or damage.

Unlike a diplexer, a duplexer does not rely solely on frequency separation. It typically uses a circulator or switching structure so that:

• Signal from Port 1 is routed to Port 2
• Signal from Port 2 is routed to Port 3
• Port 1 and Port 3 remain isolated

A common use case for a duplexer is in a radar system operating at a single frequency. During transmission, the duplexer isolates the sensitive receiver from the high-power transmit pulse. Immediately after the pulse ends, it routes the returning echo signal from the antenna to the receiver. This allows the system to transmit and receive on the same antenna and frequency without damaging the receiver.