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MICROWAVEISOLATORS

23/07/2010

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BASIC DEFINITION

23/07/2010

“The isolator is a two-port device with a very low insertion loss (less than 0.5 dB) in forward direction and very high insertion loss (greater than 20 dB) in the reverse direction”. It is also known as uniline.

It is depends on various parameter like ferrite materials, design topology, operating frequency and temperature.

Isolators are used to protect other RF components from excessive signal reflection.

This is generally occurs due to the problem of the impedance matching at the interfacing of two RF components.

Isolators are commonplace in laboratory applications to separate a device under test (DUT) from sensitive signal sources.

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BASIC DEFINITION

23/07/2010

Signal source

Isolator

Device Under Test

Low forward loss (<0.5 dB)

High isolation loss (>20dB)

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BASIC PARAMETERS

23/07/2010

Insertion loss: It is defined as the ratio of power received to the power transmitted at the channel. It is generally expressed in dB.

Insertion loss (dB)= 10*log(Pr/Pt)

The ratio of reverse (isolation) to forward insertion loss may also be used and is called the figure of merit.

The most commonly used isolators are related to the resonance and field displacement isolators.

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ISOLATOR TYPES

23/07/2010

Basically there are two types of the isolator based on device used:

• Passive(ferrite) isolators.• Active isolators.

Passive(ferrite) isolators can further classified as follows:

• Terminated circulators.• Faradays rotation isolators.• Resonance isolators.• Field displacement isolators.

Active isolator is generally contain MMIC amplifier(Monolithic Microwave Integrated Circuit).

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TERMINATED ISOLATOR

23/07/2010

As shown in figure it is made up from three port circulator in which port C is terminated by resistive load.

Hence if any reflection from port B is comes at port C, so reflected signal does not effect to input port A.

Figure shows principle of field rotation isolator (courtesy of online source www.e-MECA.com )

3 port circulator Isolator

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FARADAY ROTATION ISOLATOR

23/07/2010

The plane in which the field is present in wave at the input side of ferrite rode is displaced clockwise by 45° at the output of the ferrite rode. The degree of the rotation is depend on the length of the ferrite rode .

Figure shows principle of field rotation isolator (courtesy of online source http://en.wikipedia.org/wiki)

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RESONANCE ISOLATOR

23/07/2010

The reflected wave is strongly interaction with the wave guide material hence it is absorbed due to high loss.

Figure: Resonance curve shows basic regions of circular operation.(courtesy of www.m2global.com)

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FIELD DISPLACEMENT ISOLATOR

23/07/2010

The field displacement isolators are based on the fact that field profiles of forward and reverse waves can be quite different in transmission lines, such as waveguides, when loaded with a biased ferrite.

Figure shows structure of field displacement isolators.