A circulator is a passive electronic component with three or more ports in which the ports can be accessed in such a way that when a signal is fed into any port it is transferred to the next port only, the first port being counted as following the last in numeric order. 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. There are circulators for VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "turnstile" or "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behavior from a true 4-port circulator.

In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a 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.

Radio frequency circulators are composed of magnetised ferrite materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.

There have also been investigations into making "active circulators" which use transistors instead of ferrites. However, the power handling capability and linearity and signal to noise ratio of the latter is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies. Because both sender and receiver of a radar or any communication system consist of transistors (or vacuum tubes), the transistor-based circulator merely means that the sent signal is subtracted from the received signal. Nevertheless, most communication systems are simplex, and duplex systems are composed of two simplex systems by means of frequency division multiplexing or time division multiplexing.



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