Circulator

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APPLICATION NOTE

Circulators and Isolators, unique passive devices
AN98035

Philips Semiconductors

Circulators and Isolators, unique passive devices

Application Note AN98035

0. Contents 1. 2. 3. 4. Introduction Definitions Behaviour of ferrites Principles of operation and construction 4.1 Resonance isolator 4.2 Field displacement circulator 4.3 Phase shift circulator 4.4 Faraday rotation circulator 4.5 Ring circulator 4.6 Junction circulator 4.7 Edge guided mode circulator 4.8 Lumped element circulator 5. Charcteristics 6. Correlation of parameters of a symmetrical 3-port-circulator 6.1 The lossless symmetrical 3-port-circulator 6.2 The lossy symmetrical 3-port-circulator 7. Applications 7.1 Decoupling of generator and load 7.2 Decoupling of amplifier stages 7.3 Decoupling of a transmitter or receiver from its antenna 7.4 Transmitter and receiver on the same antenna 7.5 Combiner for 2 or more transmitters in the VHF and UHF 7.6 Combiner for radio links 7.7 Combining amplifier stages in a solid-state transmitter 7.8 Operation of one-port-amplifiers 7.9 Locking and priming of oscillators 7.10 Variable attenuator and phase shifter Bibliography 1 1 2 7 7 8 9 10 11 11 13 14 16 17 17 19 21 21 22 22 23 23 26 26 27 27 28 29

1998 Mar 23

2

Philips Semiconductors

Circulators and Isolators, unique passive devices

Application Note AN98035

1. Introduction Circulators and isolators are passive devices used in modern rf and microwave equipment since some decades. By using them the stability, performance, and reliability of the systems can be improved, and often better and cheaper solutions are possible. In addition, in certain applications, e.g. one-port-amplifiers, the use of circulators is a must. This booklet will help you to understand these important devices and give you some hints to use them effectively.

2. Definitions

Fig.1: Energy flow in

a. a 3-port-circulator b. a 4-port-circulator

The circulator is defined as a passive device with 3 or more ports, where power is transferred from one port to the next in a prescribed order. That means for a 3-portcirculator (see fig.1a): power entering port 1 leaves port 2, port 3 is decoupled; power entering port 2 leaves port 3, port 1 is decoupled; and power entering port 3 leaves port 1, 1998 Mar 23 3

Philips Semiconductors

Circulators and Isolators, unique passive devices

Application Note AN98035

port 2 is decoupled. For a 4-port-circulator it is similar (see fig.1b): power entering port 1 leaves port 2, port 3 and 4 are decoupled etc. The isolator is defined as a passive twoFig.2: Energy flow in an isolator port, where power is transmitted in one direction and absorbed in the other direction. That means power entering port 1 leaves port 2, but power entering port 2 is absorbed (see fig.2). An isolator can be a specially developed item. But we get also an isolator if we connect a matched load to port 3 of a 3-port-circulator. Figure 3 gives the symbols used for circulators and isolators in circuit drawings. By these definitions circulators and isolators are non-reciprocal devices, that means, their behaviour in one direction is very different from that in the other direction.

Fig.3: Symbols for a. 3-port-circulator b. 4-port-circulator c. isolator 3. Behaviour of ferrites The way of operation of circulators and isolators is based on the unique properties of microwave ferrites. Therefore we will have a look on the behavior of ferrites under static and alternating fields at first. Ferrites are magnetic material with very high ohmic resistance. Therefore they have nearly no eddy currents and are suitable for the operation at rf and microwave frequencies. Like ferromagnetic material 1998 Mar 23

Fig.4: Planar model of Weiss domains

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Philips Semiconductors

Circulators and Isolators, unique passive devices

Application Note AN98035

they consist of magnetic domains called Weiss domains, for Pierre Weiss discovered them...
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