View source for Active circulator

{{Short description|Active non-reciprocal three-port device}}
{{about|radio-frequency (RF) or microwave active circulators|other uses|Circulator (disambiguation)}}

[[File:Circulator-symbol-CW.svg|thumb|Symbol for a 3-Port Circulator]]

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 (circuit theory)|port]] 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 parameters|scattering matrix]]:

:<math>S = \begin{pmatrix}
  0 & 0 & 1\\
  1 & 0 & 0 \\
  0 & 1 & 0
\end{pmatrix}</math>

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.<ref name="Tanaka, Simomura et al.">{{cite journal|title= Active circulators - The realization of circulators using transistors|journal= Proceedings of the IEEE|date= 1965-03-01|issn= 0018-9219|pages= 260–267|volume= 53|issue= 3|doi= 10.1109/PROC.1965.3683|first1= S.|last1= Tanaka|first2= N.|last2= Shimomura|first3= K.|last3= Ohtake}}</ref> [[Varactor]] circuits are another technology, relying on a time-varying transmission line structure, driven by a separate pump signal.<ref name="Qin, Xu et al.">{{cite journal|title= Nonreciprocal Components With Distributedly Modulated Capacitors|journal= IEEE Transactions on Microwave Theory and Techniques|date= 2014-10-01|issn= 0018-9480|pages= 2260–2272|volume= 62|issue= 10|doi= 10.1109/TMTT.2014.2347935|first1= Shihan|last1= Qin|first2= Qiang|last2= Xu|first3= Y.E.|last3= Wang|bibcode= 2014ITMTT..62.2260Q|s2cid= 13987504}}</ref> A third technology utilizes spatiotemporally-modulated rings of coupled [[resonator]]s.<ref name="Estep, Sounas et al.">{{cite journal|last1=Estep|first1=N. A.|last2=Sounas|first2=D.L.|last3=Alù|first3=A.|date=2016-02-01|title=Magnetless Microwave Circulators Based on Spatiotemporally Modulated Rings of Coupled Resonators|journal=IEEE Transactions on Microwave Theory and Techniques|volume=64|issue=2|pages=502–518|doi=10.1109/TMTT.2015.2511737|s2cid=17421796 |issn=0018-9480}}</ref> Another design approach relies on staggered commutation and integrated circuit techniques.<ref name="Nordrum">{{cite web|last1=Nordrum|first1=Amy|url=https://spectrum.ieee.org/tech-talk/telecom/wireless/new-full-duplex-radio-chip-transmits-and-receives-wireless-signals-at-once|title=New Full Duplex Radio Chip Transmits and Receives Wireless Signals at Once|website=IEEE Spectrum: Technology, Engineering, and Science News|access-date=2016-07-22|date=2016-04-15}}</ref><ref name="Reiskarimian & Krishnaswamy">{{cite journal|last1=Reiskarimian|first1=Negar|last2=Krishnaswamy|first2=Harish|date=2016-04-15|title=Magnetic-free non-reciprocity based on staggered commutation|journal=Nature Communications|language=en|volume=7|pages=11217|doi=10.1038/ncomms11217|pmc=4835534|pmid=27079524|bibcode=2016NatCo...711217R}}</ref>

Compared to passive (ferrite) [[circulator]]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 (electronics)|noise]] into the signal path.

== References ==
{{reflist}}

[[Category:Electrical components]]


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