In an FDD mobile phone, which transmits and receives simultaneously on different frequencies, which component separates the transmit and receive signals using their frequency difference so that one antenna can be shared for transmitting and receiving?

Answer: A. Duplexer

FDD transmits and receives at the same time, so a single antenna must be shared while keeping the transmitter's strong signal out of the receiver. A duplexer separates the two by combining a filter that passes the transmit band with one that passes the receive band. In mobile phones, SAW and BAW filters, which are small and have steep characteristics, are used in duplexers, and many filters are needed depending on the number of bands supported. In TDD, which alternates transmission and reception in time, a switch can be used instead of a duplexer.

• A. Correct. Transmit and receive filters are combined to separate the signals by frequency.
• B. This component converts between balanced (differential) and unbalanced signals.
• C. This component extracts part of the signal traveling on a line, distinguishing its direction, and is used for power monitoring and the like. It does not separate transmit and receive frequencies.