In electrical engineering, an active circulator is an active non-reciprocal three-port device that couples a microwave or radio-frequency signal only to an adjacent

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...

in the direction of circulation. Other (external) circuitry connects to the circulator ports via transmission lines. An ideal three-port active circulator has the following

scattering matrix In physics, the ''S''-matrix or scattering matrix is a matrix that relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory ...

: :

S = \begin
  0 & 0 & 1\\
  1 & 0 & 0 \\
  0 & 1 & 0
\end

An active circulator can be constructed using one of several different technologies. One early technology is the use of transistors as the active devices to perform the non-reciprocal function.

Varactor A varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction. Applications Varactors are used ...

circuits are another technology, relying on a time-varying transmission line structure, driven by a separate pump signal. A third technology utilizes spatiotemporally-modulated rings of coupled

resonator A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a reso ...

s. Another design approach relies on staggered commutation and integrated circuit techniques. Compared to passive (ferrite)

circulator In electrical engineering, a circulator is a passivity (engineering), passive, non-Reciprocity (electrical networks), reciprocal three- or four-port (circuit theory), port device that only allows a microwave or radio frequency, radio-frequency ...

s, active circulators have the advantages of small size, low mass, and simple integration with other circuitry. System designers must weigh these factors with the disadvantages of active circulators: they require DC power and sometimes a separate pump or clock signal, they can be nonlinear, and can introduce significant

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

into the signal path.

References

Electrical components {{Electric-stub