A Diplexer separates two different frequency bands in the receive path and combines them in the transmit path. These frequency bands usually will be wide apart in frequency domain for diplexer to work satisfactorily. It is often referred as a RF power combiner/divider with added functionality of filtering. Broadband Filters are used to pass appropriate bands in the Tx and Rx path.
A diplexer is used to separate/combine frequency bands of GSM900 and DCS 1800 systems. It is also used in multiband design of mobile handset with multiple technologies GSM,CDMA,LTE. A diplexer does multiplexing and demultiplexing of wide frequency bands with a large enough difference.
There are two RF ports in a diplexer other than the output port, the device RF triplexer will have 3 ports other than one output port.
A duplexer is a device that allows the use of the single antenna by both transmitter and receiver. In other words a duplexer is a device which couples the transmitter and receiver to the antenna while producing isolation between transmitter and receiver. There are two types of duplexer, one by using PIN diode switches and the other using circulators. Both transmit and receive paths usually will have frequency bands very close, hence narrow band filters are used to separate these frequencies. A Duplexer is often referred as 3-port RF circulator.
It separates an uplink (890-915MHz,Uplink-UL) and downlink (935-960MHz,Downlink-DL) frequency bands in GSM900 system.