Circulator

In electrical engineering, a circulator is a passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency (RF) signal to exit through the port directly after the one it entered.

This resonator may have any shape that has three-fold Rotational symmetry, such as a disk, hexagon, or triangle.

An RF/microwave signal entering a circulator port is connected via a stripline to the resonator, where energy is coupled into two counter-rotating circular modes formed by the elliptically polarized waves.

These circular modes have different phase velocities which can cause them to combine constructively or destructively at a given port.

If losses are neglected for simplification, the counter-rotating modes must differ in phase by an integer multiple of

Solving the two preceding equations simultaneously, for proper circulation the necessary conditions are and Each of the two counter-rotating modes has its own resonant frequency.

This combination of waves propagating at different phase velocities is how junction circulators fundamentally operate.

The ferrite resonator may have any shape that has three-fold Rotational symmetry, such as a cylinder or Triangular prism.

A microstrip circulator consists primarily of a circuit pattern on a ferrite substrate.

The circuit is typically formed using thick-film or thin-film metallization processes, often including photolithography.

The ferrite substrate is sometimes bonded to a ferrous metal carrier, which serves to improve the efficiency of the magnetic circuit, increase the mechanical strength of the circulator, and protect the ferrite from thermal expansion mismatches between it and the surface to which the circulator is mounted.

The performance disadvantages of microstrip circulators are offset by their relative ease of integration with other planar circuitry.

[10] Self-biased junction circulators are unique in that they do not utilize permanent magnets that are separate from the microwave ferrite.

Integration of self-biased circulators with semiconductor wafers has been demonstrated at KA-band and V-band frequencies.

[13] Lumped-element circulators[14] are small-size devices that are typically used at frequencies in the HF through UHF bands.

[15] In a lumped-element circulator, conductors are wrapped around the ferrite, forming what is typically a woven mesh.

In some circulators, the mesh is in the form of traces on a printed wiring board with metallized vias to make connections between layers.

On the other hand, lumped-element circulators generally have lower RF power handling capacity than equivalent junction devices and are more complex from a mechanical perspective.

The discrete lumped-element inductors and capacitors can be less stable when exposed to vibration or mechanical shocks than the simple distributed impedance transformers in a stripline junction circulator.

The ferrites that are used in switching circulators have square magnetic hysteresis loops and often sub-Oersted coercivities.

The coil is connected to electronic driver circuitry[8] that sends current pulses of the correct polarity through the magnetizing coil to magnetize the ferrite in the polarity to provide the desired direction of circulation.

Differential phase shift circulators are mainly used in high power microwave applications.

A differential phase shifter consists of one or more ferrite slabs, usually positioned on the broad wall(s) of the waveguide.

The ferrite-loaded waveguide is another example of a transverse-field device as described in Circulator § Theory of operation.

Major issues associated with transistor-based active circulators are the power limitation and the signal-to-noise degradation,[18] which are critical when it is used as a duplexer for sustaining the strong transmit power and clean reception of the signal from the antenna.

One study employed a structure similar to a time-varying transmission line with the effective nonreciprocity triggered by a one-direction propagating carrier pump.

The research claimed to be able to achieve positive gain and low noise for receiving path and broadband nonreciprocity.

[20] In 1964, Mohr presented and experimentally demonstrated a circulator based on transmission lines and switches.

In radar, circulators are used as a type of duplexer, to route signals from the transmitter to the antenna and from the antenna to the receiver, without allowing signals to pass directly from transmitter to receiver.

Negative differential resistance diodes can amplify signals, and often perform better at microwave frequencies than two-port devices.