junction circulator used as an isolator by placing a
matched load on port 3. The label on the permanent magnet indicates the direction of circulation. Microwave circulators fall into two main classes: differential phase shift circulators and junction circulators, both of which are based on cancellation of waves propagating over two different paths in or near magnetized ferrite material. Waveguide circulators may be of either type, while more compact devices based on
stripline are usually of the junction type. Two or more junction circulators can be combined in a single component to give four or more ports. Typically permanent magnets produce a static magnetic bias in the microwave ferrite material.
Ferrimagnetic garnet crystal is used in
optical circulators.
Junction circulators Stripline junction circulators A stripline junction circulator contains a resonator, which is located at the central junction of the
striplines. 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
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 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 2\pi for signal propagation from port 1 to port 2 (or from port 2 to port 3, or from port 3 to port 1): :2\Gamma_-l - \Gamma_+l = 2m\pi and similarly, for the remaining port (port 3 if signal propagation is from port 1 to port 2) to be nulled, :-\Gamma_-l + 2\Gamma_+l = (2n - 1)\pi where l is the path length between adjacent ports and m and n are integers. Solving the two preceding equations simultaneously, for proper circulation the necessary conditions are :\Gamma_-l = \frac{4m + 2n - 1}{3}\pi and :\Gamma_+l = \frac{2m + 4n - 2}{3}\pi Each of the two counter-rotating modes has its own resonant frequency. 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. 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 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.
Microstrip junction circulators 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
duplexing function. The microstrip junction circulator is another widely used form of circulator 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 or
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
radiation and
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 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.
Self-biased junction circulators 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 ferrites 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. 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 and
V-band frequencies.
Lumped-element circulators Lumped-element circulators are small-size devices that are typically used at frequencies in the
HF through
UHF bands. In a junction circulator, the size of the ferrite(s) is proportional to signal wavelength, but in a lumped-element circulator, the ferrite can be smaller because there is no such wavelength proportionality. In a lumped-element circulator, conductors are wrapped around the ferrite, forming what is typically a woven mesh. The conductor strips are insulated from each other by thin dielectric layers. In some circulators, the mesh is in the form of traces on a
printed wiring board with metallized vias to make connections between layers. The conductive strips can be thought of as non-
reciprocally coupled inductors. Impedance matching circuitry and broad-banding circuitry in lumped-element circulators are often constructed using discrete ceramic capacitors and air-core inductors. This class of circulator offers a considerable size reduction compared with the junction circulators. On the other hand, lumped-element circulators generally have lower RF power handling capacity than equivalent junction devices and are more complex from a mechanical perspective. The discrete lumped-element inductors and capacitors can be less stable when exposed to vibration or mechanical shocks than the simple
distributed impedance transformers in a stripline junction circulator.
Switching circulators Switching circulators are similar to other junction circulators, and their microwave theory of operation is the same, except that their direction of circulation can be electronically controlled. A differential phase shifter provides
non-reciprocal transmission phase shift. That is, the forward phase shift is different from the phase shift in the reverse transmission direction. It is this difference in phase shifts that enables the non-reciprocal behavior of the circulator. A differential phase shifter consists of one or more ferrite slabs, usually positioned on the broad wall(s) of the waveguide. Permanent magnets located outside the waveguide provide static magnetic bias field to the ferrite(s). The ferrite-loaded waveguide is another example of a
transverse-field device as described in . Different microwave propagation constants corresponding to different directions of signal propagation give rise to different phase velocities and hence, different transmission phase shifts. Depending on which circulator port an incident signal enters, phase shift relationships in the hybrid couplers and the differential phase shifts cause signals to combine at one other port and cancel at each of the remaining two ports. Differential phase shift circulators are often used as 3-port circulators by connecting one circulator port to a reflectionless termination, or they can be used as isolators by terminating two circulator ports. ==Non-ferrite circulators==