RF isolators and circulators are passive multiport devices constructed of ferrite material. They steer and suppress RF energy between ports due to their non-reciprocal characteristics. Non-reciprocity is caused by the ferrite having magnetic properties that make its behavior dependent upon RF current direction. A ferrite will phase-shift, physically displace or absorb RF energy in relation to the difference between a static magnetic bias field and the rotating electromagnetic field of the RF or microwave energy.

RF isolators are two port devices that present opposing directions of RF currents with different insertion losses. They isolate by suppressing reflected energy or received signals from the output port returning to the input port. Resonance absorbers dissipate reflected RF energy internally and rely upon good heat sinking to remove energy from the device. Field displacement isolators steer energy into the resistive covering of one surface, which dissipates the heat. This reduces the heating in the ferrite material itself.

RF circulators are 3 or 4 port devices that utilize the steering characteristics of statically biased ferrites, directing energy from one port to the clockwise adjacent port. They work on either ‘Faraday rotation’ of waves propagating through the device or cancellation of waves due to different phase shifts of RF travelling in clockwise and anticlockwise directions combining at each port. Devices with higher port numbers are constructed of multiple 3 and 4 port types.

Insertion loss is a very important characteristic of these devices. Insertion loss is a measure of the RF power dissipated by a circulator in the forward direction, typically 1-2dB. This causes loss of output power and efficiency in a transmitter along with additional heating. Insertion loss causes a reduction in the sensitivity in a receiver by affecting the receiver’s overall noise figure (NF).

Isolation is a measure of the attenuation of signals travelling between ports not associated with the forward direction of the circulator. Ports will have different isolation characteristics. This could be energy ‘leakage’ from the input port to ports other than the clockwise adjacent output port, and is a measure of ‘directivity’ of the device. Isolation is important for transmitters because reflected energy due to mismatch (VSWR) of an antenna is shunted away from the power amplifier – usually into a dump load. This reduces the variation of load impedance presented to the transmitter – helping to maintain optimized loading characteristics. This can protect the power amplifier by reducing power dissipation and improving stability.

Circulators are used as duplexers in transceiver systems with shared antennas. They isolate the transmitter and receiver – reducing the transmitter’s output signal power level presented to the receiver. Circulators are also part of a microwave Gunn diode reflection amplifier. They are used to turn a one port diode into a two port device, enabling the negative resistance characteristics of the Gunn diode to be utilized as an amplifier.