Editing Circulator (section)

===Junction circulators===
====Stripline junction circulators====
[[File:Rotating Modes in Junction Circulator.jpg|thumb|400px|Rotating modes in a junction circulator.]]

A stripline junction circulator contains a resonator, which is located at the central junction of the [[stripline]]s.  This resonator may have any shape that has three-fold [[Rotational symmetry]], such as a disk, hexagon, or triangle.  An RF/microwave signal entering a circulator port is connected via a [[stripline]] to the resonator, where energy is coupled into two counter-rotating circular [[Mode (electromagnetism)|modes]] formed by the [[elliptically polarized]] waves.  These circular modes have different [[phase velocities]] which can cause them to combine constructively or destructively at a given port.  This produces an [[anti-node]] at one port (port 2 if the signal is incident upon port 1) and a [[Node (physics)|node]] or null at another port (port 3 if the microwave energy is coupled from port 1 to port 2 and not reflected back into port 2).

If losses are neglected for simplification, the counter-rotating modes must differ in phase by an integer multiple of <math>2\pi</math> for signal propagation from port 1 to port 2 (or from port 2 to port 3, or from port 3 to port 1):<ref name="Soohoo 85">{{Cite book|title=Microwave Magnetics|first=Ronald F.|last=Soohoo|date= 1985|publisher=Harper & Row|isbn=0-06-046367-8}}</ref>

:<math>2\Gamma_-l - \Gamma_+l = 2m\pi</math>

and similarly, for the remaining port (port 3 if signal propagation is from port 1 to port 2) to be nulled,

:<math>-\Gamma_-l + 2\Gamma_+l = (2n - 1)\pi</math>

where <math>l</math> is the path length between adjacent ports and <math>m</math> and <math>n</math> are integers.  Solving the two preceding equations simultaneously, for proper circulation the necessary conditions are

:<math>\Gamma_-l = \frac{4m + 2n - 1}{3}\pi</math>

and

:<math>\Gamma_+l = \frac{2m + 4n - 2}{3}\pi</math>

Each of the two counter-rotating modes has its own resonant frequency.<ref name="Fay & Comstock" />  The two resonant frequencies are known as the split frequencies.  The circulator operating frequency is set between the two split frequencies.

These circulator types operate based on [[Faraday effect|faraday rotation]]. Wave cancellation occurs when waves propagate with and against the circulator's direction of circulation.  An incident wave arriving at any port is split equally into two waves. They propagate in each direction around the circulator with different phase velocities. When they arrive at the output port they have different phase relationships and thus combine accordingly. This combination of waves propagating at different phase velocities is how junction circulators fundamentally operate.

The geometry of a [[stripline]] junction circulator comprises two ferrite disks or triangles separated by a stripline center conductor and sandwiched between two parallel ground planes.<ref name="Helszajn Stripline Circulator">{{Cite book|title=The Stripline Circulator: Theory and Practice|first=Joseph|last=Helszajn|date=2008|publisher=John Wiley & Sons|isbn=978-0-470-25878-1}}</ref>  A stripline circulator is essentially a stripline center conductor sandwich on ferrite, between ground planes. That is, there is one ferrite disk above the stripline circuit and one ferrite disk below the stripline circuit.  Stripline circulators do not have to be constructed with disk- or triangle-shaped ferrites; the ferrites can have almost any shape that has three-way symmetry.  This is also true of the resonator (the center junction portion of the center conductor)- it can be any shape that has three-way symmetry, although there are electrical considerations.<ref name="Microwave Circulator Design" />

The ferrites are magnetized through their thicknesses, i.e., the static magnetic bias field is perpendicular to the plane of the device and the direction of signal propagation is transverse to the direction of the static magnetic field. Both ferrites are in the same static ad RF magnetic fields.  The two ferrites can be thought of as one continuous ferrite with an embedded stripline center conductor.  For practical manufacturing reasons, the center conductor is not generally embedded in ferrite, so two discrete ferrites are used.  The static magnetic bias field is typically provided by permanent magnets that are located external to the circulator ground planes.  Magnetic shielding incorporated into the circulator design prevents detuning or partial demagnetization of the circulator in the presence of external magnetic fields or ferrous materials, and protects nearby devices from the effects of the circulator's static magnetic field.

<gallery class="center" mode="packed" caption="Internal Construction of Stripline Junction Circulators">
Stripline Junction Circulator.jpg|Internal construction of a [[stripline]] junction circulator having triangular ferrites and an irregular triangle-shaped resonator.
Circulator Disk Ferrite Suspended Stripline with Bullets.jpg|Internal construction of [[stripline]] junction circulator having disk ferrites and a disk-shaped resonator.
BR Circulator Disk F & D Air Stripline with Bullets.jpg|Internal construction of a [[stripline]] junction circulator having disk ferrites and a triangle-shaped resonator.
</gallery>

====Waveguide junction circulators====
<br>
[[File:WG Junction Circulator HP LC cropped.jpg|center|thumb|500px|High-Power Liquid-Cooled Waveguide Junction Circulator.  Image courtesy of [http://www.microwavetechniques.com Microwave Techniques]]]
<br>
[[File:WR-112 Circulator with Bullets.jpg|thumb|400px|Internal construction of a WR-112 (WG 15; R 84) waveguide junction circulator.]]

A waveguide junction circulator contains a magnetized ferrite resonator, which is located at the junction of three [[Waveguide (radio frequency)|waveguides]].<ref name="Helszajn Waveguide Junction Circulators">{{Cite book|title=Waveguide Junction Circulators: Theory and Practice|first=Joseph|last=Helszajn|date=1998|publisher=John Wiley & Sons|isbn=0-471-98252-0}}</ref>  In contrast with a stripline junction circulator, the ferrite itself is the resonator, rather than the metal central portion of a stripline center conductor.  The ferrite resonator may have any shape that has three-fold [[Rotational symmetry]], such as a cylinder or [[Triangular prism]].  The resonator is often just one ferrite, but it is sometimes composed of two or more ferrites, which may be coupled to each other, in various geometrical configurations.  The geometry of the resonator is influenced by electrical and thermal performance considerations.  Waveguide junction circulators function in much the same way as stripline junction circulators, and their basic theory of operation is the same.

The internal geometry of a [[Waveguide (radio frequency)|waveguide]] junction circulator comprises a junction of three waveguides, the ferrite resonator, and impedance matching structures.  Many of these circulators contain pedestals located in the central junction, on which the ferrite resonator is located.  These pedestals effectively reduce the height of the waveguide, reducing its [[characteristic impedance]] in the resonator region to optimize electrical performance.  The reduced-height waveguide sections leading from the resonator to the full-height waveguides serve as impedance transformers.

The ferrite resonator is magnetized through its height, i.e., the static magnetic bias field is perpendicular to the plane of the device and the direction of signal propagation is transverse to the direction of the static magnetic field.  The static magnetic bias field is typically provided by permanent magnets that are external to the waveguide junction.

{{center|'''E-Field Plots Showing Electromagnetic Wave Propagation in Waveguide Junction Circulators'''}}
{|
| [[File:Wg scatter.ogg|thumb|400px|E-field scatter plot of an electromagnetic wave propagating through a waveguide junction circulator.]]

| [[File:Wg ferrite.ogg|thumb|400px|E-field plot of the rotating standing wave pattern in the ferrite of a waveguide junction circulator.  The direction of signal propagation is from bottom to upper right, and the upper left ferrite apex is nulled.]]
|}

====Microstrip junction circulators====

[[File:Microstrip Circulator a.jpg|left|100px|thumb|Microstrip junction circulator.]]

[[File:Tr-module.jpg|thumb|500px|Transmit-Receive (T-R) module used in the [[Euroradar CAPTOR|CAPTOR-E]] [[Active electronically scanned array|active electronically scanned array (AESA)]] airborne radar.  The microstrip junction circulator is visible at the left end of the module.  The left port of the circulator connects to the antenna port of the module and ultimately to an element of the [[phased array]].  The top right circulator port connects to receiver and signal processing circuitry, and the lower right circulator port connects to the transmitter power amplifier near the center of the module.  In this instance, the circulator performs a [[Duplexer|duplexing]] function.]]

The microstrip junction circulator is another widely-used form of circulator<ref name="Fuller">{{Cite book|title=Ferrites at Microwave Frequencies|first=A. J.|last=Baden-Fuller|date=1987|publisher=Peter Peregrinus Ltd.|isbn=0-86341-064-2}}</ref> that utilizes the [[microstrip]] transmission line topology.  A microstrip circulator consists primarily of a circuit pattern on a ferrite substrate.  The circuit is typically formed using [[Thick film technology|thick-film]] or [[Thin film|thin-film]] metallization processes, often including [[photolithography]].  The ferrite substrate is sometimes bonded to a ferrous metal carrier, which serves to improve the efficiency of the magnetic circuit, increase the mechanical strength of the circulator, and protect the ferrite from [[thermal expansion]] mismatches between it and the surface to which the circulator is mounted.  A permanent magnet that is bonded to the circuit face of the ferrite substrate provides the static magnetic bias to the ferrite.  Microstrip circulators function in the same way as stripline junction circulators, and their basic theory of operation is the same.  In comparison with stripline circulators, electrical performance of microstrip circulators is somewhat reduced because of [[Electromagnetic radiation|radiation]] and [[Planar transmission line|dispersion]] effects.

The performance disadvantages of microstrip circulators  are offset by their relative ease of integration with other planar circuitry.  The electrical connections of these circulators to adjacent circuitry are typically made using [[Wire bonding|wire bonds]] or ribbon bonds.  Another advantage of microstrip circulators is their smaller size and correspondingly lower mass than stripline circulators.  Despite this advantage, microstrip circulators are often the largest components in microwave modules.<ref name="Palmer, Kirkwood, Gross, et al.">{{cite journal|title=A Bright Future for Integrated Magnetics|journal=IEEE Microwave Magazine|date=June 2016|issn=1527-3342|pages=36–50|volume=20|number=6|doi=10.1109/MMM.2019.2904381|first1=William|last1=Palmer|first2=David|last2=Kirkwood|s2cid=148572410 |display-authors=etal}}</ref>

====Self-biased junction circulators====

[[File:Self-Biased Circulator 2a.jpg|thumb|100px|Self-biased junction circulator.]]

Self-biased junction circulators are unique in that they do not utilize permanent magnets that are separate from the microwave ferrite.  The elimination of external magnets significantly reduces the size and weight of the circulator compared to electrically-equivalent microstrip junction circulators for similar applications.

Monolithic ferrites that are used for self-biased circulators are M-type [[uniaxial]] (single magnetic axis) [[hexagonal ferrite]]s<ref name="Zeina, How, et al.">{{cite journal|title=Self-Biasing Circulators Operating at K<sub>A</sub>-Band Utilizing M-Type Hexagonal Ferrites|journal=IEEE Transactions on Magnetics|date=September 1992|issn=0018-9464|pages=3219–3221|volume=28|number=5|doi=10.1109/20.179764|first1=N.|last1=Zeina|first2=H.|last2=How|display-authors=etal}}</ref> that have been optimized to have low microwave losses.  In contrast with the [[magnetically soft]] (low-[[coercivity]]) ferrites used in other circulators, the hexagonal ferrites used for self-biased circulators are [[magnetically hard]] (high-[[coercivity]]) materials.  These ferrites are essentially ceramic permanent magnets.  In addition to their high [[magnetic remanence]], these ferrites have very large [[magnetic anisotropy]] fields, enabling circulator operation up to high microwave frequencies.<ref name="Geiler and Harris">{{cite journal|title=Atom Magnetism: Ferrite Circulators - Past, Present, and Future|journal=IEEE Microwave Magazine|date=September–October 2014|issn=1527-3342|pages=66–72|volume=15|number=6|doi=10.1109/mmm.2014.2332411|first1=Anton|last1=Geiler|first2=Vince|last2=Harris|s2cid=46417910 }}</ref>

Because of their thin, planar shape, self-biased circulators can be conveniently integrated with other planar circuitry.  Integration of self-biased circulators with semiconductor wafers has been demonstrated at [[Ka-band|K<sub>A</sub>-band]] and [[V-band]] frequencies.<ref name="Cui, Chen, et al.">{{cite conference|title=Monolithically Integrated Self-Biased Circulator for mmWave T/R MMIC Applications|first1=Yongjie|last1=Cui|first2=Hung-Yu|last2=Chen|display-authors=etal|date=December 2021|pages=4.2.1–4.2.4|location=San Francisco, CA, USA|conference= 2021 IEEE International Electron Devices Meeting (IEDM)|doi=10.1109/IEDM19574.2021}}</ref>