Circulator
Encyclopedia
A circulator is a passive
non-reciprocal
three- or four-port
device, in which microwave
or radio frequency
power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix
for an ideal three-port circulator is
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.
There are circulators for LF, VHF, UHF
, microwave frequencies and for light
, the latter being used in optical fiber
networks. Circulators fall into two main classes: 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 stripline
s 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.
In radar
, circulators are used to route outgoing and incoming signals between the antenna, the transmitter
and the receiver
. In a simple system, this function could be performed by a 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.
Radio frequency circulators are composed of magnetised ferrite
materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet
crystal is used in optical circulators.
There have also been investigations into making "active circulators" which are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
non-reciprocal
Reciprocity (electromagnetism)
In classical electromagnetism, reciprocity refers to a variety of related theorems involving the interchange of time-harmonic electric current densities and the resulting electromagnetic fields in Maxwell's equations for time-invariant linear media under certain constraints...
three- or four-port
Two-port network
A two-port network is an electrical circuit or device with two pairs of terminals connected together internally by an electrical network...
device, in which microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
or radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix
Scattering parameters
Scattering parameters or S-parameters describe the electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical signals....
for an ideal three-port circulator is
When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator
Isolator (microwave)
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...
, since a signal can travel in only one direction between the remaining ports.
There are circulators for LF, VHF, UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
, microwave frequencies and for light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, the latter being used in optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
networks. Circulators fall into two main classes: 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 stripline
Stripline
Stripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
s 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.
In radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, circulators are used to route outgoing and incoming signals between the antenna, the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
and the receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. In a simple system, this function could be performed by a 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.
Radio frequency circulators are composed of magnetised ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
crystal is used in optical circulators.
There have also been investigations into making "active circulators" which are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.