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Theory and application of Circulator&Isolator


   Basically, circulators and isolator are ferrite devices that use the properties of ferrite materials to control the transmission of microwave energy. The nonreciproal behavior of ferrite makes microwave energy travel the transmission path in one direction with small loss, but be greatly absorbed when applied from the opposite drection.



    Ferrite circulators and isolatore use ferromagnetic material in the presence of magnetic field to achieve this nonreciprocal operation. When the magnetic field is applied, microwave signal interacts with the lectrons within the ferrite material. This is resonance frequency which is determined by atomic made up of a ferrite component.
Junction Circulators
    The junction circulators provide the nonreciprocal behaveior or ferrite material to transfer microwave energy. The ideal circulator is defined as n-port lossless passive network in which energy introduced at one port is transferred wholly to an adjacent prot while decoupling the energy from all other prots. The basic symbol of circulators if the three port circulator, which is illustrated in figure1, signal applied to prot 1will appear at port 2 will trave to port 3, not back to port 1, Either port 2 or port 3 could be used as an input with the same result. The signal would only travel in a forward direction , because, theoretically, energy cannot flow opposite to the direction of circulation.
Appliction of circulator
    Normally, three prot circulators are used as duplexers. Connection a transmitter to port 1, antenna to port 2, and receiver to port 3 permits simultaneous transmission and reception of signals while protecting the transmitter from reflections due to antenna mismatch. In high power systems, a limiter is commonly used between port 3 and the receiver to prevent recive burnout.
    Another common use of circulators is as coupling elements is reflection amplifier ,such as parametric amplifiers.Which is illustrated in Figure3. In this application three single-juction(3-port) circulators are connected in series with terminations(dummyload)
Application of isolators
    Circulators can be used as isolator with one port terminated with a dummy load. So isolators are most widely used to protect high power RF sources. In this application. an isolator failure could easily destroy the power source. Since a load is not usually well matched to the source, the power is normally reflected from the load back to the source. An isolator in series with the source will absorb most of the reflected energy, elimination these problems. Isolator are often used between the transmitter and the antenna, is several communication systems and radar systems. They are also used on the out put of signal generators.