4 Port Devices
Four port devices can be realized by joining two 3 port devices via the appropriate ports. As system performance requirements become more demanding, new design approaches must be adopted. These include Faraday rotation devices and phase shift circulators. The bandwidth and isolation limitations of 3 port devices are overcome through a distributed architecture that enables more ferrite material to be incorporated into the transmission path, thereby achieving an increase in the biasing magnetic field. Phase shift circulators can be designed to handle higher RF power than 3-port devices.
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Circulators
The most common application of a circulator is as a duplexer, allowing the transmitter and receiver in a radio or radar unit to share a common antenna. Energy is transferred from the RF Source to the antenna during transmit, and from the antenna to the receiver during the receive cycle. In each case the circulator functions to isolate the inactive port from the propagated signal. Most importantly, the transmitter output is not passed to the receiver input, preventing the high transmitter power from damaging the sensitive receiver input circuits.
