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.
Notes
References
- C. L. Hogan, "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications" Rev. Mod. Phys. 25, 253–262 (1953)
- E. A. Ohm, "A Broadband Microwave Circulator", IRE Trans. on Microwave Theory and Techniques, MTT-4 210-217 (1956)
- H. N. Chait & T. R. Curry, "Y-Circulator," J. Appl. Phys., Suppl. to 30, 1525 and 1535 (1959)
- H. Bosma, "On Stripline Y-Circulation at UHF", IEEE Trans. Microwave Theory & Techniques V12 N1 61-72 (1964)
- Federal Standard 1037C and MIL-STD-188
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Last updated on Sunday July 20, 2008 at 10:07:09 PDT (GMT -0700)
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