# Circulator

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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 up to a phase-factor, the scattering matrix for an ideal three-port circulator is

Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems. A passive component, depending on field, may be either a component that consumes but does not produce energy or a component that is incapable of power gain.

In classical electromagnetism, reciprocity refers to a variety of related theorems involving the interchange of time-harmonic electric current densities (sources) and the resulting electromagnetic fields in Maxwell's equations for time-invariant linear media under certain constraints. Reciprocity is closely related to the concept of Hermitian operators from linear algebra, applied to electromagnetism.

In electrical circuit theory, a port is a pair of terminals connecting an electrical network or circuit to an external circuit, a point of entry or exit for electrical energy. A port consists of two nodes (terminals) connected to an outside circuit, that meets the port condition; the currents flowing into the two nodes must be equal and opposite.

## Contents

${\displaystyle S={\begin{pmatrix}0&0&1\\1&0&0\\0&1&0\end{pmatrix}}}$

Optical circulators have similar behavior.

## Types

Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators.

### Ferrite

Ferrite circulators are radio-frequency circulators which are composed of magnetized ferrite materials. They fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetized material, [1] [2] and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetized material. Waveguide circulators may be of either type, while more compact devices based on stripline are of the 3-port type. [3] [4] Two or more Y-junctions can be combined in a single component to give four or more ports, but these differ in behavior from a true 4-port circulator. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.

A ferrite is a ceramic material made by mixing and firing large proportions of iron(III) oxide (Fe2O3, rust) blended with small proportions of one or more additional metallic elements, such as barium, manganese, nickel, and zinc. They are both electrically non-conductive, meaning that they are insulators, and ferrimagnetic, meaning they can easily be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their resistance to being demagnetized (magnetic coercivity).

Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line.

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

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.

### Nonferrite

Early work on nonferrite circulators includes active circulators using transistors that are non-reciprocal in nature. [5] 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, [6] 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.

Varactors 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. [7] 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. [8]

In 1964, Mohr presented and experimentally demonstrated a circulator based on transmission lines and switches. [9] In April, 2016 a research team significantly extended this concept, presenting an integrated circuit circulator based on N-path filter concepts. [10] [11] 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. [12]

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

## Applications

### Isolator

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. [13] 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.

An isolator is a two-port device that transmits microwave or radio frequency power in one direction only. It 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.

### Duplexer

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).

### Reflection amplifier

A reflection amplifier is a type of microwave amplifier circuit utilizing negative differential resistance diodes such as tunnel diodes and Gunn diodes. Negative differential 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.

## Related Research Articles

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, generates a time-varying electromagnetic field, which transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thus increasing the mobility, convenience, and safety of an electronic device for all users. Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are used as feed antennas for larger antenna structures such as parabolic antennas, as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers. Their advantages are moderate directivity, low standing wave ratio (SWR), broad bandwidth, and simple construction and adjustment.

A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal passive semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1962 by physicist J. B. Gunn. Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers.

Finite-difference time-domain or Yee's method is a numerical analysis technique used for modeling computational electrodynamics. Since it is a time-domain method, FDTD solutions can cover a wide frequency range with a single simulation run, and treat nonlinear material properties in a natural way.

Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, with the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Microstrip was developed by ITT laboratories as a competitor to stripline.

An orthomode transducer (OMT) is a waveguide component. It is commonly referred to as a polarisation duplexer. Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths. One of the paths forms the uplink, which is transmitted over the same waveguide as the received signal path, or downlink path. Such a device may be part of a VSAT antenna feed or a terrestrial microwave radio feed; for example, OMTs are often used with a feed horn to isolate orthogonal polarizations of a signal and to transfer transmit and receive signals to different ports.

Distributed amplifiers are circuit designs that incorporate transmission line theory into traditional amplifier design to obtain a larger gain-bandwidth product than is realizable by conventional circuits.

Wilmer Lanier Barrow was an American electrical engineer, inventor, teacher, industrial manager, and a counselor to government agencies. He obtained a BSEE degree in 1926 from Louisiana State University, and a doctorate from the Technical University of Munich in 1931. During the pre-World War 2 development of radar at Massachusetts Institute of Technology, Barrow performed research on microwaves, inventing waveguide in 1936 and the horn antenna in 1938.

George Clark Southworth, who published as G. C. Southworth, was a prominent American radio engineer best known for his role in the development of waveguides in the early 1930s.

RF microwave CAE CAD is computer-aided design (CAD) using computer technology to aid in the design, modeling, and simulation of an RF or microwave product. It is a visual and symbol-based method of communication whose conventions are particular to RF/microwave engineering.

A zero-mode waveguide is an optical waveguide that guides light energy into a volume that is small in all dimensions compared to the wavelength of the light.

Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power.

A tunable metamaterial is a metamaterial with a variable response to an incident electromagnetic wave. This includes remotely controlling how an incident electromagnetic wave interacts with a metamaterial. This means the capability to determine whether the EM wave is transmitted, reflected, or absorbed. In general, the lattice structure of the tunable metamaterial is adjustable in real time, making it possible to reconfigure a metamaterial device during operation. It encompasses developments beyond the bandwidth limitations in left-handed materials by constructing various types of metamaterials. The ongoing research in this domain includes electromagnetic materials that are very meta which mean good and has a band gap metamaterials (EBG), also known as photonic band gap (PBG), and negative refractive index material (NIM).

A beam waveguide antenna is a particular type of large steerable parabolic antenna in which the radio waves are transported in a beam between the movable dish and the stationary transmitter or receiver using multiple reflective surfaces.

Coplanar waveguide is a type of electrical planar transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. On a smaller scale, coplanar waveguide transmission lines are also built into monolithic microwave integrated circuits. Conventional coplanar waveguide (CPW) consists of a single conducting track printed onto a dielectric substrate, together with a pair of return conductors, one to either side of the track. All three conductors are on the same side of the substrate, and hence are coplanar. The return conductors are separated from the central track by a small gap, which has an unvarying width along the length of the line. Away from the central conductor, the return conductors usually extend to an indefinite but large distance, so that each is notionally a semi-infinite plane.

Real-time Analog Signal Processing (R-ASP), as an alternative to DSP-based processing, might be defined as the manipulation of signals in their pristine analog form and in real time to realize specific operations enabling microwave or millimeter-wave and terahertz applications.

A reconfigurable antenna is an antenna capable of modifying its frequency and radiation properties dynamically, in a controlled and reversible manner. In order to provide a dynamic response, reconfigurable antennas integrate an inner mechanism that enable the intentional redistribution of the RF currents over the antenna surface and produce reversible modifications of its properties. Reconfigurable antennas differ from smart antennas because the reconfiguration mechanism lies inside the antenna, rather than in an external beamforming network. The reconfiguration capability of reconfigurable antennas is used to maximize the antenna performance in a changing scenario or to satisfy changing operating requirements.

Distributed element circuits are electrical circuits composed of lengths of transmission lines or other distributed components. These circuits perform the same functions as conventional circuits composed of passive components, such as capacitors, inductors, and transformers. They are used mostly at microwave frequencies, where conventional components are difficult to implement.

## References

1. 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.
2. Hogan, C. Lester (1953), "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications", Reviews of Modern Physics , 25 (1): 253–262, Bibcode:1953RvMP...25..253H, doi:10.1103/RevModPhys.25.253
3. Bosma, H. (1964-01-01). "On Stripline Y-Circulation at UHF". IEEE Transactions on Microwave Theory and Techniques. 12 (1): 61–72. Bibcode:1964ITMTT..12...61B. doi:10.1109/TMTT.1964.1125753. ISSN   0018-9480.
4. 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. Bibcode:1965ITMTT..13...15F. doi:10.1109/TMTT.1965.1125923. ISSN   0018-9480.
5. 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.
6. 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. Bibcode:2000ITMTT..48..316C. doi:10.1109/22.821785. ISSN   0018-9480.
7. 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.
8. 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.
9. Mohr, Richard (1964). "A New Nonreciprocal Transmission Line Device". Proceedings of the IEEE. 52 (5): 612.
10. "New Full Duplex Radio Chip Transmits and Receives Wireless Signals at Once". IEEE Spectrum: Technology, Engineering, and Science News. 2016-04-15. Retrieved 2016-07-22.
11. 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  . PMID   27079524.
12. "Next Big Future: Novel miniaturized circulator opens way to doubling wireless capacity". nextbigfuture.com. April 18, 2016. Retrieved 2016-04-19.
13. For a description of a circulator, see Jachowski (1976)