​Circulators Overview

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.

Isolators are a derivative of the circulator specifically used to protect other RF components from excessive signal reflection. When a circulator is configured as an Isolator its 3rd port is terminated by a matched load, and the device functions as a unidirectional gateway / transmission line. Energy will propagate from the device’s input port to the output port, while energy travelling in the reverse direction is passed to the load where it is dissipated as heat.

Teledyne Lincoln Microwave waveguide circulators and isolators offer very low insertion loss combined with average power capability of several kilowatts. They are available for supply in a range of different packages; planar microstrip, coaxial and waveguide.

Operation

A signal applied to a ferrite disk resonator can be considered as two equal, circularly polarized counter-rotating waves. The velocity of a circularly polarized wave as it propagates through a magnetically biased ferrite material is dependent on its direction of rotation. By selecting the proper ferrite material, and biasing magnetic field, the phase velocity of the wave traveling in one direction can be made greater than the wave traveling in the opposite direction so that when a signal is applied at Port 1 the two waves will arrive in phase at Port 2, but cancel at Port 3. Bandwidths of 10% are typical.

At higher frequencies, where the size of the ferrite material becomes constrained by waveguide dimensions, acceptable insertion loss and isolation is generally achieved at the expense of bandwidth.

Where >10% operating bandwidth is essential, 4 Port phase shift circulators offer a solution. Their distributed design enables sufficient ferrite material to be incorporated to achieve the required phase change, albeit there is a trade-off in respect of the increased space envelope required.

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