Radio frequency isolator

Abstract

An RF isolator is disclosed, comprising: a circulator; a termination connected to a terminal of the circulator; a housing surrounding the circulator and the termination. The housing comprises: a first housing piece made from a first material, the first housing piece arranged to inhibit leakage of magnetic field from the circulator, and a second housing piece made from a second material, the second housing piece arranged to direct heat generated by the termination away from the circulator. The second material has a greater thermal conductivity than the first material.

Description

[0001]Ref: P73964:2 - 1 - Radio Frequency Isolator Technical Field The present invention relates to RF isolators for use in high power communication systems, and in particular to components used to house such isolators and their method of manufacture. Background Radio and microwave frequency isolators are devices used in the transmission of microwave or radio frequency power, to ensure power transmission in only one direction. Terminated circulators are one class of isolator which may be used in various systems. For example, these isolators may be used in high-powered communication systems to isolate an antenna from an amplif ier or another RF power source, e.g. by preventing signal reflection. These devices include an input, an output, and a termination, and are configured such that any signal received at the input is directed solely towards the output, whilst any signal received at the output (e.g. via reflection due to an impedance mismatch) is directed solely to the termination where power is dissipated, rather than back toward the input. Such functionality is provided due to the magnetic field induced by permanent magnets interacting with the ferromagnetic properties of e.g. a ferrite material contained within the circulator. However, during operation, stray magnetic fields may be generated which extend outside of the isolator and adversely interfere with the system. In addition, when power is dissipated at the termination a considerable amount of heat can be generated. This heat may flow back towards the circulator and interrupt the proper functioning of the magnetic components. Existing attempts to address this problem have sought to position the termination further from the circulator, to reduce the amount of heat transferred. However, doing this increases the overall mass and volume of the isolator. In many implementations, such as high-power communication systems, it is desirable to keep the system components as small and lightweight as possible. Ref: P73964:2 - - Summary Embodiments of the present invention aim to address the above problems and others. Aspects of the disclosure are set out in the independent claims and optional features are set out in the dependent claims. Aspects of the disclosure may be provided in conjunction with each other, and features of one aspect may be applied to other aspects. In an aspect, there is provided an RF isolator comprising a circulator, a termination connected to a terminal of the circulator, a housing surrounding the circulator and the termination, the housing comprising a first housing piece made from a first material, the first housing piece arranged to inhibit leakage of magnetic field from the circulator, and a second housing piece made from a second material, the second housing piece arranged to direct heat generated by the termination away from the circulator wherein the second material has a greater thermal conductivity than the first material. A (substantially) adiabatic boundary, e.g. plane, may be provided between the first housing piece and the second housing piece, and / or between the first portion and the second portion. The first housing piece may comprise a first portion surrounding the circulator, and a second portion surrounding the termination. The second housing piece may be disposed in the second portion of the first housing piece. The first housing piece may consist essentially of the first material. The first material may be martensitic stainless steel. The second housing piece may consist essentially of the second material. The second material may be copper, for example oxygen free, high conductivity (OFHC) copper. The second housing piece may be provided within a complementary opening or cavity in the first housing piece. The second housing piece may be swaged into the first housing piece. Ref: P73964:2 - 3 - The second housing piece may comprise a first surface proximal to the termination, and a second surface at an external face of the isolator. At least one of the first surface and the second surface may be flush with a surface of the first housing piece. The termination of the isolator may be attached, e.g. soldered, to the second housing piece, e.g. to the first surface. The circulator may be provided in an opening or cavity of the first housing piece. The first housing piece and the second housing piece may be coupled together via an interference fit. The first housing piece may comprise an opening and / or a cavity for receiving the second housing piece. The second housing piece may be shaped to engage with complimentary shapes on a surface of the first housing piece, e.g. a surface of the cavity and / or opening. The first housing piece may have a surface, e.g. a surface of the cavity and / or opening which may be shaped e.g. comprise grooves for engaging with complimentarily shaped features of the second housing piece. The second housing piece may comprise a tapered portion arranged to inhibit movement of the second housing piece relative to the first housing piece. The tapered portion may comprise a first tapered portion arranged to inhibit movement of the second housing piece relative to the first housing piece in a first direction. The second housing piece may further comprise a second tapered portion arranged to inhibit movement of the second housing piece relative to the first housing piece in a second direction opposite to the first direction. The second housing piece may comprise a first portion and a second portion wherein the first portion has a diameter greater than the second portion. The first and / or second portion may be substantially cylindrical. The first and / or second portion may comprise a taper, e.g. they may have a truncated conical shape. The first portion may adjoin the second portion. The first portion may adjoin the second portion via a lip region. The lip region may be tapered. The lip region may be tapered in the opposite direction to the first and / or second portion. In an aspect, there is provided a method of manufacturing a housing for an RF isolator comprising a circulator and a termination connected to a terminal of the circulator, the method comprising providing a first housing piece made from a first material, the first housing piece arranged to inhibit leakage of magnetic field from the circulator, providing a Ref: P73964:2 - 4 - second housing piece within the first housing piece, the second housing piece made from a second material and arranged to direct heat generated by the termination away from the circulator wherein the second material has a greater thermal conductivity than the first material. Inserting the second housing piece into the first housing piece may comprise swaging the first housing piece and the second housing piece together. The first material may be martensitic stainless steel. The second material may be copper, for example oxygen free, high conductivity copper (OFHC). Providing the second housing piece into the first housing piece may comprise inserting the second housing piece within a complementary opening in the first housing piece. Providing the second housing piece into the first housing piece may comprise providing an interference fit between the first housing piece and the second housing piece. The method of manufacture may further comprise machining the second housing piece such that at least one surface of the second housing piece may be flush with a surface of the first housing piece. The method may further comprise gold plating the housing. Brief Description of Figures Some examples of the present disclosure will now be described with reference to the figures, in which: Figure 1 shows a plan view of an example isolator; Figure 2 shows a side view of the isolator; Figure 3a shows a perspective view of an example of a housing piece for use in an isolator; Figure 3b shows a cross-sectional side view of the housing piece; Figure 4 shows a cross-sectional perspective view of a portion of the isolator; Figure 5 illustrates an example method of constructing an isolator; Figure 6 illustrates another example method of constructing an isolator. Ref: P73964:2 - 5 - In the drawings like reference numerals are used to indicate like elements. Detailed Description of Figures The present disclosure relates to an RF isolator with a housing that inhibits heat flow from the termination to the circulator whilst minimizing stray magnetic fields generated by the circulator. The housing is formed from two pieces, with one piece made of a material that inhibits leakage of magnetic field from the circulator. The other piece is arranged next to the termination and is made of a material with a higher thermal conductivity than the first piece. This housing arrangement provides an adiabatic plane between the termination and the circulator, such that heat generated at the termination is directed away from the circulator. Figure 1 shows a plan view of an isolator 100 comprising an input port 106, an output port 105 and a termination port 114. The circulator 101 may comprise ferrite and is configured in conjunction with the applied magnetic field to direct RF signals from the input 106 to the output 105. For example, communication signals may be provided from an amplif ier (not shown) at the input 106, and directed to the output 105 from where it is provided to an antenna (also not shown) for transmission. Any signal received at the output 105, e.g. that is reflected back at the antenna due to an impedance mismatch, is directed to the termination 114 where the power is dissipated. The amplif ier, e.g. a solid state amplif ier, is therefore protected from any reverse power levels (whilst maintaining a constant voltage standing wave ratio (VSWR)). The circulator 101 is surrounded by a first housing piece 108, which is made from martensitic stainless steel. The properties and functionality of this housing piece is described in more detail below, with reference to the subsequent figures. The first housing piece 108 also substantially surrounds the termination 114, although not entirely as discussed in more detail below. The first housing piece 108 is also shaped such that a portion extends away from the circulator 101 and surrounds the output 105 A plurality of holes 130a-e are also provided in the first housing piece 108, which extend from one surface of the piece 108 through to an opposite surface. The holes 130a-e are provided Ref: P73964:2 - 6 - for attachment of the isolator 100 to a base plate (not shown). The base plate may be a temperature controlled base plate which provides a heat conduction path, to direct heat generated by the isolator away from the device. The termination 114 is configured to receive a signal from the output and to dissipate large amounts of power. The termination 114 is disposed upon a second housing piece 200, and there is an adiabatic plane between the first housing piece 108 and the second housing piece 200. The second housing piece 200 is constructed from oxygen free high conductivity copper (OFHC), which has a very high thermal conductivity, and is not magnetic, so does not affect the magnetic field of the circulator 101. Figure 2 shows a side view of the isolator 100. The internal arrows shown within the isolator in Figure 2 illustrate how the magnetic field generated by the circulator 101 is directed by the first housing piece 108 when in use. As discussed above, the first housing piece 108 is made of martensitic stainless steel which has magnetic properties which enable the first housing piece 108 to act as a magnetic return path. In particular, the first housing piece 108 is configured to act as an outer yoke, e.g. a “C” yoke, on a conventional magnet assembly that is provided by the circulator 101. The first housing piece 108 is thus configured to channel the magnetic flux generated by the circulator 101 inwards, reducing any unwanted stray magnetic field almost to zero. The first housing piece 108 also substantially surrounds the termination 114. However, a cavity 110 is provided in the first housing piece 108 proximal to the termination 114, e.g. proximal to an under surface as shown, into which a second housing piece 200 can be inserted, discussed in more detail below. Figures 3a and 3b show an example of such a second housing piece 200 in perspective and cross-sectional views respectively. The second housing piece 200 is arranged to be inserted into the cavity 110 in the first housing piece 108 shown in Figure 2. The second housing piece 200 is made from a material with a higher thermal conductivity than the first housing piece 108, e.g. having a higher thermal conductivity than martensitic steel. For example, the second housing piece may be made of copper, such as oxygen free, high conductivity copper (OFHC copper). The second housing piece is shaped to engage with Ref: P73964:2 - 7 - a complimentarily shaped surface of the cavity 110. In this way the first housing piece 108 and the second housing piece 200 can be coupled together via an interference fit. The second housing piece 200 is a unitary piece of OFHC copper comprising a base portion 201 that is substantially cylindrical but which tapers outwards from a first end face 206 of the second housing piece 200 towards the centre of the piece 200. The second housing piece 200 further comprises a central portion 202 adjoining the base portion 201, which is substantially cylindrical, and which has a diameter less than the diameter of the base portion 201. The second housing piece 200 further comprises a truncated conical section 203 which tapers inwardly from the central portion 202 to a second end face 208 of the piece 200, opposite the first end face 206. The base portion 201 adjoins the central portion 202 via a lip region 204, which tapers inwardly from the base portion 201 to the central portion 202. Figure 4 shows a cross section of a portion of the isolator 100 with the second housing piece 200 inserted into the cavity 110 of the first housing piece 108. The electrical components of the isolator 100 are not shown, but it will be appreciated that a circulator 101 is provided within the first housing piece 108, and that the termination 114 is provided proximal to the second housing piece 200. In particular, the termination is attached to the end face 208 of the second housing piece 200, for example the termination 114 can be soldered to the surface 208. The shape of the second housing piece 200 discussed above with reference to figures 3A-B, in combination with the complementary shape of the cavity 110 of the first housing piece aid in securing the housing piece 200 in the cavity 110. In particular, the taper of the base portion 201 inhibits movement of the second housing piece 200 relative to the first housing piece in a first direction (upwards in Figure 4), and the taper of the lip region 204 inhibits movement of the second housing piece 200 relative to the first housing piece in a second direction opposite the first direction (downwards in Figure 4). An interference fit is thus provided between the two housing pieces. The first end face 206 of the second housing portion is aligned, e.g. f lush, with the outer surface of the first housing piece. The first housing piece 108 and the second housing piece 200 are forged e.g. swaged together to form a single unitary housing as discussed below. The housing further comprises a gold plating (not shown) which extends over both the first housing piece 108 and the second housing piece 200. Ref: P73964:2 - 8 - In operation, as power is dissipated at the termination 114, heat is transferred to the second housing piece 200. The second housing piece 200 has a much greater thermal conductivity that the first housing piece 108. For example, OFHC copper has a thermal conductivity of 400 W / mK, and martensitic steel has a thermal conductivity of 16 W / mK. Therefore, the heat is preferentially transferred through the second housing piece 200, from the second surface 208 to the first surface 206, away from the circulator 101 and out of the isolator 100. The housing can thus be thought of as a thermal circuit within the isolator 100, with electrical current analogous to heat flow. The first housing piece 108 and the second housing piece 200 can be considered as first and second resistors respectively. Taking the thermal conductivity values of each housing piece above, the thermal conductivity of the second housing piece 200 is approximately 25 times that of the first housing piece 108. The first housing piece 108 can thus be considered to have a resistance approximately 25 times that of the second housing piece 200, and so by analogy to ohm’s law, for a given amount of power dissipated as heat at the termination 114, upwards of 96% of said power is passed through the second housing piece 200 and out of the device 100, with less than 4% entering the first housing piece 108. In this way, a substantially adiabatic plane is provided between the termination 118 and the circulator 101. Figure 5 illustrates an example method 400 of manufacturing a housing for an RF isolator comprising a circulator and a termination connected to a terminal of the circulator, such as the isolator 100 and housing discussed above. A first step of the method comprises providing 401 a first housing piece made from a first material, where the first housing piece is arranged to inhibit leakage of magnetic field from the circulator. A second step comprises providing 402 a second housing piece within the first housing piece, where the second housing piece made from a second material and is arranged to direct heat generated by the termination away from the circulator. The second material has a greater thermal conductivity than the first material. Figure 6 shows another method 500 for the construction of an isolator, which may be consider a refinement of the method 400 described above. A first step of the method comprises providing 501 a first housing piece made from a first material, where the first Ref: P73964:2 - 9 - housing piece is arranged to inhibit leakage of magnetic field from the circulator. A second step comprises providing 502 a second housing piece within the first housing piece, where the second housing piece made from a second material and is arranged to direct heat generated by the termination away from the circulator. The second material has a greater thermal conductivity than the first material. The two housing pieces have complementary surface shapes to provide an interference fit. At a third step 503, the first housing piece and the second housing piece are swaged together. At a fourth step 504, the first housing piece and / or the second housing piece are machined such that at least one surface of the second housing piece is flush with a surface of the first housing piece. At a fifth step 505 the housing is gold-plated. It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any feature of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention. Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

Claims:

1. An RF isolator comprising: a circulator; a termination connected to a terminal of the circulator; a housing surrounding the circulator and the termination, the housing comprising: a first housing piece made from a first material, the first housing piece arranged to inhibit leakage of magnetic field from the circulator, and a second housing piece made from a second material, the second housing piece arranged to direct heat generated by the termination away from the circulator; wherein the second material has a greater thermal conductivity than the first material.

2. The isolator of claim 1, wherein the first housing piece comprises a first portion surrounding the circulator, and a second portion surrounding the termination.

3. The isolator of claim 1 or 2, wherein the second housing piece is disposed in the second portion of the first housing piece.

4. The isolator of any previous claim, wherein the first housing piece consists essentially of the first material.

5. The isolator of any previous claim, wherein the first material is martensitic stainless steel.

6. The isolator of any previous claim, wherein the second material is copper.

7. The isolator of claim 6, wherein the second material is oxygen free, high conductivity (OFHC) copper.

8. The isolator of any previous claim, wherein the second housing piece is swaged into the first housing piece.

9. The isolator of any previous claim, wherein the second housing piece is provided within a complementary opening in the first housing piece.

10. The isolator of any previous claim, wherein the second housing piece comprises a first surface proximal to the termination, and a second surface at an external face of the isolator.

11. The isolator of claim 10, wherein at least one of the first surface and the second surface is flush with a surface of the first housing piece.

12. The isolator of any previous claim, wherein the first housing piece and the second housing piece are coupled together via an interference fit.

13. The isolator of any previous claim, wherein the second housing piece comprises a tapered portion arranged to inhibit movement of the second housing piece relative to the first housing piece.

14. The isolator of claim 13, wherein the tapered portion is a first tapered portion and is arranged to inhibit movement of the second housing piece relative to the first housing piece in a first direction, and wherein the second housing piece further comprises a second tapered portion arranged to inhibit movement of the second housing piece relative to the first housing piece in a second direction opposite to the first direction.

15. A method of manufacturing a housing for an RF isolator comprising a circulator and a termination connected to a terminal of the circulator, the method comprising: providing a first housing piece made from a first material, the first housing piece arranged to inhibit leakage of magnetic field from the circulator; providing a second housing piece within the first housing piece, the second housing piece made from a second material and arranged to direct heat generated by the termination away from the circulator; wherein the second material has a greater thermal conductivity than the first material.

16. The method of claim 15, wherein inserting the second housing piece into the first housing piece comprises swaging the first housing piece and the second housing piece together.

17. The method of claim 15 or 16, wherein the first material is martensitic stainless steel.

18. The method of any of claims 15 to 17, wherein the second material is copper, for example oxygen free, high conductivity copper (OFHC).

19. The method of any of claims 15 to 18, wherein providing the second housing piece into the first housing piece comprises inserting the second housing piece within a complementary opening in the first housing piece.

20. The method of any of claims 15 to 19, wherein providing the second housing piece into the first housing piece comprises providing an interference fit between the first housing piece and the second housing piece.

21. The method of any claims 15 to 20, further comprising machining the second housing piece such that at least one surface of the second housing piece is flush with a surface of the first housing piece.

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