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A circulator is a passive non-reciprocal three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only).
When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports. An isolator is used to shield equipment on its input side from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.
In radar, circulators are used as a type of duplexer, to route signals from the transmitter to the antenna and from the antenna to the receiver, without allowing signals to pass directly from transmitter to receiver. The alternative type of duplexer is a transmit-receive switch (TR switch) that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.
In the future-generation cellular communication, people talk about full-duplex radios, where signals can be simultaneously transmitted and received at the same frequency. Given the currently limited, crowded spectrum resource, full-duplexing can directly benefit the wireless communication by twice of the data throughput speed. Currently, the wireless communication is still performed with "half-duplex", where either the signals are transmitted or received at different time frames, if at the same frequency (typically in radar), or the signals are simultaneously transmitted and received at different frequencies (realized by a set of filters called a diplexer).
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