Active circulator

This article is about radio-frequency (RF) or microwave active circulators. For other uses, see Circulator (disambiguation).

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 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:

<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.">Tanaka, S.; Shimomura, N.; Ohtake, K. (1965-03-01). "Active circulators - The realization of circulators using transistors". Proceedings of the IEEE. 53 (3): 260–267. doi:10.1109/PROC.1965.3683. ISSN 0018-9219.</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.">Qin, Shihan; Xu, Qiang; Wang, Y.E. (2014-10-01). "Nonreciprocal Components With Distributedly Modulated Capacitors". IEEE Transactions on Microwave Theory and Techniques. 62 (10): 2260–2272. Bibcode:2014ITMTT..62.2260Q. doi:10.1109/TMTT.2014.2347935. ISSN 0018-9480. S2CID 13987504.</ref> A third technology utilizes spatiotemporally-modulated rings of coupled resonators.<ref name="Estep, Sounas et al.">Estep, N. A.; Sounas, D.L.; Alù, A. (2016-02-01). "Magnetless Microwave Circulators Based on Spatiotemporally Modulated Rings of Coupled Resonators". IEEE Transactions on Microwave Theory and Techniques. 64 (2): 502–518. doi:10.1109/TMTT.2015.2511737. ISSN 0018-9480. S2CID 17421796.</ref> Another design approach relies on staggered commutation and integrated circuit techniques.<ref name="Nordrum">Nordrum, Amy (2016-04-15). "New Full Duplex Radio Chip Transmits and Receives Wireless Signals at Once". IEEE Spectrum: Technology, Engineering, and Science News. Retrieved 2016-07-22.</ref><ref name="Reiskarimian & Krishnaswamy">Reiskarimian, Negar; Krishnaswamy, Harish (2016-04-15). "Magnetic-free non-reciprocity based on staggered commutation". Nature Communications. 7: 11217. Bibcode:2016NatCo...711217R. doi:10.1038/ncomms11217. PMC 4835534. PMID 27079524.</ref>

Compared to passive (ferrite) circulators, 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 into the signal path.