duplexer

A duplexer is an RF device that enables simultaneous transmission and reception on a single antenna by separating signals on two different frequencies or bands. It is connected between the transmitter, receiver, and antenna and provides isolation so that the high-power transmit signal does not overwhelm the receiver.

Most common duplexers are filter-based, consisting of two band-pass networks connected to the same antenna port. The transmitter feeds one filter that passes its transmit band to the antenna, while the receiver feeds another filter that passes its receive band but rejects the transmit frequency. The filters are designed to provide high attenuation of the opposite band, along with some insertion loss on the desired paths. Alternative implementations use circulator-based duplexers, where a three-port circulator routes transmit power to the antenna and directs residual energy away from the receiver, or a ceramic/ resonant-cavity design that achieves compact, high isolation.

Applications and considerations: Duplexers are essential in systems that must share an antenna for both transmitting and receiving, such as two-way radios and cellular base stations. They help protect sensitive receivers from strong transmit signals and determine the practical limits of transmitter power, receiver sensitivity, and overall system performance. Key specifications include isolation (TX-to-RX), insertion loss, passband bandwidth, out-of-band rejection, power handling, and temperature stability. In some systems that do not require simultaneous transmission and reception on the same frequency, such as certain time-division schemes, duplexers may not be used.