GB2354884A

GB2354884A - Microwave circulator / isolator with separate lumped element resonators

Info

Publication number: GB2354884A
Application number: GB0031717A
Authority: GB
Inventors: William Terence Nisbet
Original assignee: Racal MESL Ltd
Current assignee: Garrett Motion UK Ltd
Priority date: 1996-12-12
Filing date: 1996-12-12
Publication date: 2001-04-04
Anticipated expiration: 2016-12-12
Also published as: GB0031717D0; GB2320369B; GB2354884B; GB9625823D0; GB2320369A

Abstract

A microwave circulator or isolator comprises a circular centre conductor 33, which is sandwiched between two ferrite disc resonators 31, 32. Separate lumped element resonators 34 are attached to the perimeter of the centre conductor. The input / output ports P1-3 may be attached to the conducter via lumped element transformers 35, at connection points diametrically opposite from the resonators 34.

Description

2354884 MICROWAVE CIRCULATORS AND ISOLATORS This invention relates to

microwave circulators and isolators. These devices have the same basic structure; however, an isolator has at least one port which is terminated.

Microwave circulators are widely used in applications requiring a duplexing function. Microwave isolators, on the other hand, are often used in wireless communications systems to isolate unwanted signals which might otherwise give rise to system interference; for example, microwave isolators are used in portable telephone handsets and their associated base station combiners to reduce reflected signals carrying intermodulation products. In this case, the isolator should have a compact, low cost design, giving high reverse isolation, low forward insertion loss with the capability to operate at moderate power levels. Furthermore, the level of self-generated intermodulation products should be low and the isolator should be capable of operating substantially independently of temperature over a typical temperature range of from 400C to 8SOC.

Figure 1 of the accompanying drawings is a diagrammatic, perspective view of a known microwave circulator/ isolator.

2 This construction comprises a resonant structure 1 sandwiched between a pair of ground planes 2,21. In use, the resonant structure 1 is magnetically biassed by a magnetic field H produced by permanent magnets (not shown).

The resonant structure 1 defines a 3-way junction comprising a strip-line centre conductor 3 and pair of ferrite disc resonators 4,41 mounted on opposite sides of the centre conductor. Suitable ferrite materials include ferrimagnetic spinels such as nickel iron oxide (NiFe2ol) and garnets such as yttrium iron garnet (Y3Fe1O12) The centre conductor 3 has a central region 31 and three symmetrically configured legs 3,,32.33 each for connection to a respective input/output port P-, P2 i P3 At the junction resonant frequency, the magnetic field component of microwave radiation interacts with the magnetic moment of the ferrite material causing a change in the microwave permeability of the ferrite material. The effect of this change is to rotate the electric and magnetic field vectors of the microwave radiation causing a change in propagation direction along the centre conductor; however, this process will only take place if the microwave radiation is circularly polarised. In the case of a microwave

3 circulator, microwave power entering port P1, say, will be transferred to port P, microwave power entering port P, will be transferred to port P3, and so on cyclically. In the case of a microwave isolator, one of the ports is terminated using a microwave power absorbing material.

For a classically, weakly magnetised resonator the radius R of the disc is determined by the relationship, kR=1.84, where k= (11."e') (2) C and where w is the resonant frequency i.e. the required microwave operating frequency, c is the speed of light and A,,ff is the effective permeability and ef is the dielectric constant of the ferrite material.

As will be apparent from the above relationship, the required disc size will increase as the resonant frequency falls. Since wireless communications equipment, such as hand-portable telephone equipment, usually operates at a relatively low microwave frequency, typically in the range 400 - 2000 MHz, the disc resonators may be prohibitively large. Furthermore, 4 in most practical implementations, each leg of the centre conductor will incorporate a quarter-wave transformer region T sandwiched between dielectric material 5, as shown in Figure 2, and this adds further to the overall size of the resonant structure.

Some reduction in disc size can be achieved using a so-called "WYEshaped" centre conductor, of the form shown in Figure 3. Each leg 311 321 33 of the centre conductor extends on diametrically opposite sides of the central region 31, thereby enabling the transformer region T to be accommodated within the radius R of the ferrite disc. However, even this structure has significant drawbacks.

Firstly, the magnetic parameters of the ferrite material are temperature dependent, resulting in a degradation in performance at the extremes of the desired temperature range. Hitherto, this problem has been alleviated by magnetically biassing the resonant structure using a temperaturecompensated magnetic field generated by a composite magnetic circuit including temperature dependent magnetic alloys. However, this is difficult and costly to implement in practice.

Secondly, there is a tendency for self-generated intermodulation products to be produced when two signals are simultaneously present at an input port of the device. These self-generated intermodulation products are related to second order terms in the permeability tensor of the ferrite material, and so their magnitude depends upon the volume of ferrite material present. As already explained, intermodulation products can give rise to system interference and are undesirable.

According to one aspect of the invention there is provided a microwave circulator/ isolator including a centre conductor having a plurality of legs each for connection to a respective input/output port and a pair of microwave resonators disposed on opposite sides of the centre conductor, wherein said legs are arranged symmetrically around a central region of the centre conductor, the central region being capable, in use, of supporting propagation of circularly polarised microwave radiation, each said leg has a main section and a further section extending on the diametrically opposite side of said central region from said main section, and each said microwave resonator has a composite structure comprising a ferrite material which overlies said central region of the centre conductor and a non-magnetic dielectric material overlying 6 another region of the centre conductor which lies outside said central region and is substantially incapable of supporting propagation of circularly polarised microwave radiation.

According to a further aspect of the invention there is provided a microwave circulator/ isolator including a centre conductor having a plurality of legs and a pair of microwave resonators disposed on opposite sides of the centre conductor, wherein the centre conductor has a WYEshaped configuration, and each said microwave resonator has a composite structure comprising ferrite material overlying a central region of the centre conductor, the central region being capable, in use, of supporting propagation of circularly polarised microwave radiation, and a nonmagnetic dielectric material overlying a region of the centre conductor which lies outside said central region and is substantially incapable of supporting propagation of circularly polarised microwave radiation.

According to a yet further aspect of the invention there is provided a microwave circulator/ isolator having a plurality of input/output ports, including a circular centre conductor and a pair of ferrite disc resonators disposed on opposite sides of the centre conductor, wherein each input/output port is connected to the centre conductor and a plurality of lumped 7 element resonators is also connected to the centre conductor.

Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings in which: - Figure 1 is a diagrammatic perspective view of a known microwave circulator/isolator, Figure 2 shows the centre conductor of a known microwave circulator/ i sol ator including a transformer region, Figure 3 illustrates a known, WYE-shaped centre conductor, Figure 4a is a diagrammatic sectional view through a microwave circulator/ isolator according to the invention, Figure 4b is a plan view showing the centre conductor and resonator discs in the microwave circulator/ isolator of Figure 4a, and Figures 5a and 5b show plan and sectional views of another microwave circulator/ isolator according to the invention.

8 Referring now to Figures 4a and 4b, the microwave circulator/ isolator comprises a resonant structure 10 sandwiched between ground planes 11,11, and a pair of permanent magnets 12,12, for magnetically biassing the resonant structure 10. The resonant structure 10 defines a 3-way junction comprising a strip-line centre conductor 13 and a pair of disc resonators 14,141 disposed on opposite sides of the centre conductor. The centre conductor 13 has a so-called WYE-shaped configuration comprising a circular central region 15 and three legs 16,17,18 arranged symmetrically around the central region 15, each for connection to a respective input/output port P1,P2,P3 of the microwave circulator/ isolator. Each leg has a main section 161,171,181 which contains a quarter-wave transformer region T and a further section 1611,1711,1811 extending on the diametrically opposite side of the central region 15 from the associated main section 16 1,17 1,181. As already explained in relation to Figure 3, this configuration is particularly compact and enables the quarter-wave transformer regions T to be accommodated within the radius of the disc resonators 14, 141.

Each disc resonator contains a ferrite material (e.g. a ferrimagnetic material such as nickel iron oxide (NiFe2ol) or a garnet such as calcium vanadium garnet or aluminium 9 substituted yttrium iron garnet. As already explained, the ferrite material is only effective to change the propagation direction of circularly polarised microwave radiation; however, only the central region 15 of the centre conductor is capable of supporting propagation of circularly polarised microwave radiation, there being a transition from circularly polarised radiation to linearly polarised radiation at the junction of the central region with each leg.

The inventors have appreciated that the ferrite material contained in the disc resonators need only overlie that part of the centre conductor which is capable of supporting propagation of circularly polarised microwave radiation, i.e. the central region 15, and that a non-magnetic dielectric material, such as magnesium titanate or barium tetratitanate can be used elsewhere. Accordingly, each disc resonator 14,141 has a composite structure comprising an inner part 19 which contains the ferrite material and overlies the central region 15 of the centre conductor, and an outer part 20 containing non-magnetic dielectric material. This outer part 20 is annular in shape and overlies the further section 1611,1711,181- of each leg 16,17,18 and the transformer region T in the main section 161, 171,181 of each leg.

A composite structure of this kind offers significant advantages. It contains a reduced amount of the relatively expensive, non-linear, temperature- sensitive ferrite material thereby lowering production costs and giving improved temperature stability and reduced interference from selfgenerated intermodulation products. There is also a reduction in the amount of temperature-dependent magnetic compensation material needed to generate an effective temperaturecompensated magnetic field. Furthermore, because the amount of ferrite material is reduced the magnet biassing field can be generated using smaller, less costly magnets.

In addition to these advantages, the non-magnetic dielectric material can be selected to have a dielectric constant Ed significantly greater than the value (Peffed' of the ferrite material. Accordingly, as will be seen from equations (1) and (2) above, the overall radius of each composite disc resonator can be made significantly smaller than that of a disc resonator containing only ferrite material. A preferred combination of ferrite and non-magnetic dielectric materials is calcium vanadium garnet or aluminium substituted yttrium iron garnet and magnesium titanate or barium tetratitanate.

In another embodiment, shown in Figures 5a and 5b, non- magnetic dielectric material is dispensed with; instead, resonance at the required microwave operating frequency is attained by the combination of a pair of ferrite disc resonators 31,32 disposed on opposite sides of a circular centre conductor 33, and separate lumped element resonators 34 connected to the centre conductor.

Each input/output port P1 1 P21 P3 of the m-Licrowave circulator/ isolator is connected to the centre conductor at a respective connection point C1 I C2,C3 and each such connection is made via a respective lumped element transformer 35. In this embodiment, the connection points C1,C2, C3 are arranged symmetrically around the periphery of the centre conductor 33 and each lumped element resonator 34 is connected to the centre conductor on the diametrically opposite side of the centre conductor to the respective connection point, Cj'C2,C3. This structure gives a yet further size reduction and. further reduces the temperature dependency of the device.

12

Claims

1. A microwave circulator/isolator having a plurality of input/output ports including a circular centre conductor and a pair of ferrite disc resonators disposed on opposite sides of the centre conductor, wherein each input/output port is connected to the centre conductor and a plurality of lumped element resonators is also connected to the centre conductor.

2. A microwave circulator/ isolator as claimed in claim 1, wherein each said input/output port is connected to the centre conductor at a respective connection point, and a respective said lumped element resonator is connected to the centre conductor on the diametrically opposite side of the centre conductor from each said connection point.

3. A microwave circulator/isolator as claimed in claim 1 or claim 2, wherein each input/output port is connected to the centre conductor via a lumped element transformer.

4. A microwave circulator/isolator as claimed in claims 1 to 3 having three input/output ports.

5. A microwave circulator/ isolator substantially as herein 13 described with reference to Figures 5a and 5b of the accompanying drawings.