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Circulators


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

Optical circulators have similar behaviour.

Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators.

Ferrite circulators are radio frequency circulators which are composed of magnetised ferrite materials. They 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 stripline are of the 3-port type. Two or more Y-junctions can be combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.


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