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.
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. It seems that transistors are the only (space efficient) solution for low frequencies.
Read more about this topic: Circulator
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