Isolator (high frequency technology)

In high-frequency technology, an isolator is a passive two-port which only allows a microwave guided in a waveguide to pass in one direction. It is used, among other things, in waveguides to block reflected wave components due to a mismatch, for example in the case of an antenna coupling.

General

Isolator based on resonance absorption with a ferrite and a permanent magnet attached to the outside. The TE wave in the waveguide can only run in the direction of the arrow marked on the right sign

Due to its asymmetrical nature, an isolator is a non-reciprocal element with a non-symmetrical scattering matrix . As an ideal isolator, it only allows the signal to pass from gate 1 to gate 2; in the opposite direction from gate 2 to gate 1, the signal is absorbed and not passed through. The parameters of the descriptive scattering matrix of the isolator then have the following form: ${\ displaystyle S_ {i}}$

{\ displaystyle S_ {i} = {\ begin {pmatrix} 0 & 0 \\ 1 & 0 \ end {pmatrix}}}

In the case of microwaves, the non-reciprocal property can be produced with the aid of a ferrite , which is additionally magnetically biased by a constant magnetic field , usually in the form of a permanent magnet . The ferrite is arranged in a piece of the waveguide, the transverse electrical waves (TE waves) in the TE ₁₀ mode used in technically used rectangular waveguides have a rotation in the magnetic field in the plane normal to the external DC magnetic field. Depending on the direction of rotation of the magnetic field component, the direction of rotation depends on the direction of propagation, resonance absorption occurs in the ferrite. This absorption does not occur in the opposite direction and the TE wave can pass through the isolator.

With higher powers in the insulation direction, the cooling of the ferrite can be a constructive problem, since the absorbed wave is converted into heat in the ferrite .

Circulator with three ports and wave sump (right), operated as an isolator

Another way to build an isolator is to use a circulator . Circulators with three symmetrical ports have the following scatter matrix : ${\ displaystyle S_ {z}}$

{\ displaystyle S_ {z} = {\ begin {pmatrix} 0 & 0 & 1 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \ end {pmatrix}}}

By terminating any gate with the wave impedance , for example with a wave sump , the above scatter matrix and the function of the isolator result. ${\ displaystyle S_ {i}}$

literature

Arthur J. Baden Fuller: Ferrites at Microwave Frequencies (= IEE Electromagnetic Waves Series. 23). Peregrinus on behalf of the Institution of Electrical Engineers, London 1987, ISBN 0-86341-064-2 .
Holger Heuermann: high frequency technology. Linear components of highly integrated components, 1st edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 2005, ISBN 978-3-528-03980-6 .
Rainer Geißler, Werner Kammerloher, Hans Werner Schneider: Calculation and design methods of high frequency technology 2. Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Wiesbaden 1994, ISBN 978-3-528-04943-0 .

Web links

Circulators and Isolators, unique passive devices. Application Note, AN98035. Philips Semiconductor, 1998, accessed September 10, 2015 .