Isolator (microwave)

The behaviour of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions.

Depending on the exact operating conditions, the signal travelling in one direction may either be phase-shifted, displaced from the ferrite or absorbed.

In all these kinds of devices, the observed non-reciprocity arises from the wave-material interaction which depends on the direction of propagation.

However, to allow heat from the absorbed power to be conducted away, the ferrite does not usually extend from one broad-wall to the other, but is limited to a shallow strip on each face.

Conversely for the backward signal, the electric field is strong over this surface and so its energy is dissipated in driving current through the film.

In rectangular waveguide the ferrite block will typically occupy the full height from one broad-wall to the other, with the resistive film on the side facing the centre-line of the guide.

So to within a phase-factor, the scattering matrix for a three-port circulator is A two-port isolator is obtained simply by terminating one of the three ports with a matched load, which absorbs all the power entering it.

The biased ferrite is part of the circulator and causes a differential phase-shift for signals travelling in different directions.

The bias field is lower than that needed for resonance absorption, and so this type of isolator does not require such a heavy permanent magnet.

Resonance absorption isolator consisting of WG16 waveguide containing two strips of ferrite (black rectangle near right edge of each broad wall), which are biased by a horseshoe permanent magnet external to the guide. Transmission direction is indicated by an arrow on the label on the right
Resonance isolator in rectangular waveguide topology.
Resonance isolator in rectangular waveguide topology. The forward magnetic field (solid line) is circularly polarized in the ferrite slab and FMR absorption is induced therein. The backward field (dashed) is not circularly polarized and flows normally along the guide.
Field-displacement isolator in rectangular waveguide topology
Field-displacement isolator in rectangular waveguide topology. The ferrite slab deforms the electric field so that the forward field is maximal at the border of the ferrite where a resistive sheet has been placed. This sheet decreases the intensity of the electric field. The backward field is minimal at this same place so that it experiences no loss because of the resistive sheet.
Circulator-based isolator.
Circulator-based isolator. The circulation mechanism induced by the ferrite in the cavity constrains the signal to flow from port 1 to port 2 and from port 2 to port 3. However, the port 3 is connected to a matched load. All the incoming signal is then absorbed and no signal can be emitted from port 3.