![Isolator-(resonance-absorption-type)-in-WG16](https://upload.wikimedia.org/wikipedia/commons/7/7a/Isolator-%28resonance-absorption-type%29-in-WG16.jpg)
An isolator is a
two-port
A two-port network (a kind of four-terminal network or quadripole) is an electrical network ( circuit) or device with two ''pairs'' of terminals to connect to external circuits. Two terminals constitute a port if the currents applied to them sat ...
device that transmits
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
or
radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
power in one direction only. The non-reciprocity observed in these devices usually comes from the interaction between the propagating wave and the material, which can be different with respect to the direction of propagation.
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.
Non-reciprocity
An isolator is a non-
reciprocal
Reciprocal may refer to:
In mathematics
* Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal''
* Reciprocal polynomial, a polynomial obtained from another pol ...
device, with a non-
symmetric
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
scattering matrix
In physics, the ''S''-matrix or scattering matrix relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory (QFT).
More forma ...
. An ideal isolator transmits all the power entering
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
1 to port 2, while absorbing all the power entering port 2, so that to within a phase-factor its S-matrix is
:
To achieve non-reciprocity, an isolator must necessarily incorporate a non-reciprocal material. At microwave frequencies, this material is usually a
ferrite which is
biased by a static magnetic field but can be a self-biased material.
The ferrite is positioned within the isolator such that the microwave signal presents it with a rotating magnetic field, with the rotation axis aligned with the direction of the static bias field. The behaviour of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions. Depending on the exact operating conditions, the signal travelling in one direction may either be phase-shifted, displaced from the ferrite or absorbed.
Types
![Circulator-based isolator](https://upload.wikimedia.org/wikipedia/commons/d/d9/Circulator-based_isolator.svg)
Most common types of ferrite-based isolators are classified into four categories: terminated circulators, Faraday rotation isolators, field-displacement isolators, and resonance isolators. In all these kinds of devices, the observed non-reciprocity arises from the wave-material interaction which depends on the direction of propagation.
Resonance absorption
In this type the ferrite absorbs energy from the microwave signal travelling in one direction. A suitable rotating magnetic field is found in the dominant TE
10 mode of rectangular
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
. The rotating field exists away from the centre-line of the broad wall, over the full height of the guide. However, to allow heat from the absorbed power to be conducted away, the ferrite does not usually extend from one broad-wall to the other, but is limited to a shallow strip on each face. For a given bias field, resonance absorption occurs over a fairly narrow frequency band, but since in practice the bias field is not perfectly uniform throughout the ferrite, the isolator functions over a somewhat wider band.
Field displacement
This type is superficially very similar to a resonance absorption isolator, but the magnetic biasing differs, and the energy from the backward travelling signal is absorbed in a resistive film or card on one face of the ferrite block rather than within the ferrite itself.
The bias field is weaker than that necessary to cause resonance at the operating frequency, but is instead designed to give the ferrite near-zero
permeability for one sense of rotation of the microwave signal field. The bias polarity is such that this special condition arises for the forward signal; the backward signal sees the ferrite as an ordinary
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
material (with little permeability, as the ferrite is already
saturated
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
** Saturated and unsaturated compounds
**Degree of unsaturation
** Saturated fat or fatty ac ...
by the bias field). Consequently, for the electromagnetic field of the forward signal, the ferrite has very low
characteristic wave impedance The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). For a transverse-electric-magnetic ...
, and the field tends to be excluded from the ferrite. This results in a null of the electric field of the forward signal on the surface of the ferrite where the resistive film is placed. Conversely for the backward signal, the electric field is strong over this surface and so its energy is dissipated in driving current through the film.
In rectangular waveguide the ferrite block will typically occupy the full height from one broad-wall to the other, with the resistive film on the side facing the centre-line of the guide.
Terminated circulator
A
circulator
A circulator is a passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where an ...
is a non-reciprocal three- or four-port device, in which power entering any port is transmitted to the next port in rotation (only). So to within a phase-factor, the
scattering matrix
In physics, the ''S''-matrix or scattering matrix relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory (QFT).
More forma ...
for a three-port circulator is
:
A two-port isolator is obtained simply by terminating one of the three ports with a
matched load, which absorbs all the power entering it. The biased ferrite is part of the circulator and causes a differential phase-shift for signals travelling in different directions. The bias field is lower than that needed for resonance absorption, and so this type of isolator does not require such a heavy permanent magnet. Because the power is absorbed in an external load, cooling is less of a problem than with a resonance absorption isolator.
Faraday rotation isolator
A last physical principle useful to design isolators is the
Faraday rotation
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the ...
. When a linearly polarized wave propagates through ferrite having a magnetization aligned with the direction of propagation of the wave, the polarization plane will rotate along the propagation axis. This rotation may be used to create microwave devices as isolators, circulators, gyrators, etc. In rectangular waveguide topology, it also requires the implementation of circular waveguide sections which come out of the device plane.
See also
*
Power dividers and directional couplers
Power dividers (also power splitters and, when used in reverse, power combiners) and directional couplers are passive devices used mostly in the field of radio technology. They couple a defined amount of the electromagnetic power in a transmiss ...
*
Circulator
A circulator is a passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where an ...
*
Gyrator
A gyrator is a passive, linear, lossless, two-port electrical network element proposed in 1948 by Bernard D. H. Tellegen as a hypothetical fifth linear element after the resistor, capacitor, inductor and ideal transformer. Unlike the four conventio ...
References
External links
* Circulators and Isolator
{{DEFAULTSORT:Isolator (Microwave)
Telecommunications engineering
Microwave technology