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). A port in this context is a point where an external waveguide or transmission line (such as a microstrip line or a coaxial cable), connects to the device. For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, so to within a phase-factor, the scattering matrix for an ideal three-port circulator is
Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "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 behaviour from a true 4-port circulator. 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.
In 2014 researchers at University of Texas at Austin reported the development of a tunable solid state resonator based circulator that does not rely on magnetic materials and thus could be space efficient at all frequencies. With further development, such devices could be embedded directly in conventional integrated circuits and tuned to whatever frequency is required in real-time. 
Circulators exist for many frequency bands, ranging from VHF up to optical frequencies, the latter being used in optical fiber networks. At frequencies much below VHF, ferrite circulators become impractically large. It is however possible to simulate circulator behaviour all the way down to d.c. using op-amp circuits. Unlike ferrite circulators, these active circulators are not lossless passive devices but require a supply of power to run. Also the power handling capability and linearity and signal to noise ratio of transistor-based circulators is not as high as those made from ferrites. Until the development of resonator based circulators it seemed that transistors were the only (space efficient) solution for low frequencies.
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.
A reflection amplifier is a type of microwave amplifier circuit utilizing negative resistance diodes such as tunnel diodes and Gunn diodes. Negative resistance diodes can amplify signals, and often perform better at microwave frequencies than two-port devices. However since the diode is a one-port (two terminal) device, a nonreciprocal component is needed to separate the outgoing amplified signal from the incoming input signal. By using a 3-port circulator with the signal input connected to one port, the biased diode connected to a second, and the output load connected to the third, the output and input can be uncoupled.
- Hogan, C. Lester (January 1952). "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications - The Microwave Gyrator". The Bell System Technical Journal 31 (1): 1–31. in which the four-port Faraday rotation circulator is proposed.
- Hogan, C. Lester (1953), "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications", Reviews of Modern Physics 25 (1): 253–262, doi:10.1103/RevModPhys.25.253
- Wenzel, C. (July 1991), "Low Frequency Circulator/Isolator Uses No Ferrite or Magnet", RF Design
- For a description of a circulator, see Jachowski (1976)
- Bosma, H. (1964), "On Stripline Y-Circulation at UHF", IEEE Transactions on Microwave Theory and Techniques 12 (1): 61–72, doi:10.1109/TMTT.1964.1125753
- Chait, H. N.; Curry, T. R. (1959), "Y-Circulator", Journal of Applied Physics Supplement 30 (4): 1525, 1535, doi:10.1063/1.2185863
- US 3935549, Jachowski, Ronald E., "Ferrite Circulator", issued January 27, 1976
- Ohm, E. A. (1956), "A Broad-Band Microwave Circulator", IRE Transactions on Microwave Theory and Techniques 4 (4): 210–217, doi:10.1109/TMTT.1956.1125064
- Circulators and Isolators