Editing Circulator (section)

==Non-ferrite circulators==
Though ferrite circulators can provide good "forward" signal circulation while suppressing greatly the "reverse" circulation, their major shortcomings, especially at low frequencies, are the bulky sizes and the narrow bandwidths.

Early work on non-ferrite circulators includes [[active circulator]]s using transistors that are non-reciprocal in nature.<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> In contrast to ferrite circulators which are passive devices, active circulators require power. Major issues associated with transistor-based active circulators are the power limitation and the signal-to-noise degradation,<ref name="Carchon & Nanwelaers">{{cite journal|title= Power and noise limitations of active circulators|journal= IEEE Transactions on Microwave Theory and Techniques|date= 2000-02-01|issn= 0018-9480|pages= 316–319|volume= 48|issue= 2|doi= 10.1109/22.821785|first1= G.|last1= Carchon|first2= B.|last2= Nanwelaers|bibcode= 2000ITMTT..48..316C}}</ref> 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.

[[Varactor]]s offer one solution. One study employed a structure similar to a time-varying transmission line with the effective nonreciprocity triggered by a one-direction propagating carrier pump.<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> This is like an AC-powered active circulator. The research claimed to be able to achieve positive gain and low noise for receiving path and broadband nonreciprocity. Another study used resonance with nonreciprocity triggered by angular-momentum biasing, which more closely mimics the way that signals passively circulate in a ferrite circulator.<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|author-link3=Andrea Alù}}</ref>

In 1964, Mohr presented and experimentally demonstrated a circulator based on transmission lines and switches.<ref name="Mohr">{{cite journal|last1=Mohr|first1=Richard|title=A New Nonreciprocal Transmission Line Device|journal=Proceedings of the IEEE|date=1964|volume=52|issue=5|page=612|doi=10.1109/PROC.1964.3007}}</ref> In April, 2016 a research team significantly extended this concept, presenting an [[integrated circuit]] circulator based on N-path filter concepts.<ref name="Nordrum">{{cite web|last1=Nordrum|first1=Amy|url=https://spectrum.ieee.org/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> It offers the potential for full-duplex communication (transmitting and receiving at the same time with a single shared antenna over a single frequency). The device uses capacitors and a clock and is much smaller than conventional devices.<ref name="Wang">{{cite web|last1=Wang|first1=Brian|url=https://www.nextbigfuture.com/2016/04/novel-miniaturized-circulator-opens-way.html|title=Next Big Future: Novel miniaturized circulator opens way to doubling wireless capacity|date=April 18, 2016|website=nextbigfuture.com|access-date=2016-04-19}}</ref>