A circulator is a passive non-reciprocal three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only). A port in this context is a point where an external waveguide or transmission line (such as a microstrip line or a coaxial cable), connects to the device. For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, so to within a phase-factor, the scattering matrix for an ideal three-port circulator is
Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators.
Ferrite circulators refer to the radio frequency circulators which are composed of magnetised ferrite materials. They are the classical circulators commonly used even nowadays. They can fall into two main subcategories: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.
Though ferrite circulators can provide good 'forward' signal circulation while suppressing greatly the 'reverse' circulation, their major shortcomings, especially at the frequencies for civilian use, are the bulky sizes and the narrow bandwidths. Despite the popularity of the modern integrated circuit (IC) technology, the circulator is one of the few devices that can still not be integrated on chip. Thus, much effort has been made by researchers to develop nonferrite or solid-state circulators, which can realize the circulation functionality without using ferrite and be compatible with IC technology.
Early work on nonferrite circulators includes active circulators using transistors that are non-reciprocal in nature. In contrast to ferrite circulators which are considered as passive devices, active circulators require a supply of DC power. Major issues associated with the transistor-based active circulator are the power limitation and the signal-to-noise degradation, 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.
Recent work tends to employ varactors to resolve these issues. The representative work includes the independent researches at UCLA and at University of Texas at Austin. The former involves a structure similar to a time-varying transmission line with the effective nonreciprocity triggered by a one-direction propagating carrier pump. It is like an AC-powered active circulator which they claim can achieve positive gain and low noise for receiving path and broadband nonreciprocity. In contrast, the latter is based on resonance with nonreciprocity triggered by angular-momentum biasing, which more mimics the way that signals passively circulate in a ferrite circulator. Nevertheless, both work remains at early stages where IC versions, though possible, have not yet been developed.
When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports. An isolator is used to shield equipment on its input side from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.
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. The alternative type of duplexer is a transmit-receive switch (TR switch) that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.
In the future-generation cellular communication, people talk about full-duplex radios, where signals can be simultaneously transmitted and received at the same frequency. Given the currently limited, crowded spectrum resource, full-duplexing can directly benefit the wireless communication by twice of the data throughput speed. Currently, the wireless communication is still performed with "half-duplex", where either the signals are transmitted or received at different time frames, if at the same frequency (typically in radar), or the signals are simultaneously transmitted and received at different frequencies (realized by a set of filters called a diplexer).
A reflection amplifier is a type of microwave amplifier circuit utilizing negative resistance diodes such as tunnel diodes and Gunn diodes. Negative resistance diodes can amplify signals, and often perform better at microwave frequencies than two-port devices. However, since the diode is a one-port (two terminal) device, a nonreciprocal component is needed to separate the outgoing amplified signal from the incoming input signal. By using a 3-port circulator with the signal input connected to one port, the biased diode connected to a second, and the output load connected to the third, the output and input can be uncoupled.
- Hogan, C. Lester (January 1952). "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications - The Microwave Gyrator". The Bell System Technical Journal 31 (1): 1–31. in which the four-port Faraday rotation circulator is proposed.
- Hogan, C. Lester (1953), "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications", Reviews of Modern Physics 25 (1): 253–262, doi:10.1103/RevModPhys.25.253
- Bosma, H. (1964-01-01). "On Stripline Y-Circulation at UHF". IEEE Transactions on Microwave Theory and Techniques 12 (1): 61–72. doi:10.1109/TMTT.1964.1125753. ISSN 0018-9480.
- Fay, C.E.; Comstock, R.L. (1965-01-01). "Operation of the Ferrite Junction Circulator". IEEE Transactions on Microwave Theory and Techniques 13 (1): 15–27. doi:10.1109/TMTT.1965.1125923. ISSN 0018-9480.
- 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.
- Carchon, G.; Nanwelaers, B. (2000-02-01). "Power and noise limitations of active circulators". IEEE Transactions on Microwave Theory and Techniques 48 (2): 316–319. doi:10.1109/22.821785. ISSN 0018-9480.
- 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. doi:10.1109/TMTT.2014.2347935. ISSN 0018-9480.
- Estep, N.A.; Sounas, D.L.; Alu, A. (2015-05-01). "On-chip non-reciprocal components based on angular-momentum biasing". Microwave Symposium (IMS), 2015 IEEE MTT-S International: 1–4. doi:10.1109/MWSYM.2015.7167003.
- For a description of a circulator, see Jachowski (1976)
- Chait, H. N.; Curry, T. R. (1959), "Y-Circulator", Journal of Applied Physics Supplement 30 (4): 1525, 1535, doi:10.1063/1.2185863
- US 3935549, Jachowski, Ronald E., "Ferrite Circulator", issued January 27, 1976
- Ohm, E. A. (1956), "A Broad-Band Microwave Circulator", IRE Transactions on Microwave Theory and Techniques 4 (4): 210–217, doi:10.1109/TMTT.1956.1125064
- Wenzel, C. (July 1991), "Low Frequency Circulator/Isolator Uses No Ferrite or Magnet" (PDF), RF Design
- Circulators and Isolators