RF circulator with non-conductive preform

Abstract

An RF circulator comprising a ferrite layer, a non-magnetic spacer, and a non-conductive preform layer arranged between the ferrite layer and the spacer, wherein the non-conductive preform comprises spherical glass particles arranged within an epoxy resin.

Description

[0001] RF circulator with non-conductive preform

[0002] Technical Field

[0003] The present invention relates to circulators for use in high power communication systems, and in particular non-conducting preforms used in isolator apparatus, their methods of manufacture and their use in components which require a large amount of thermal cycling to ensure reliability.

[0004] Background

[0005] Circulators are electrical devices used in the processing of RF or microwave signals. They typically comprise three input / output ports and are arranged such that any signal which enters through a given port exits via the port directly adjacent to it. Three-port circulators may be used as electrical isolators in high power communications systems, to ensure that power is only transferred in one direction. In particular, these isolators have designated input, output and termination ports, such that any power entering via the input is directed to the output, and any power reflected at the output is directed to the termination, rather than back towards the input. For example, isolators may be provided between transmission electronics such as an amplifier and an antenna, to protect such amplifiers from reverse power levels during operation.

[0006] Ferrite circulators and isolators are one class of these components, which use a magnet to magnetise a ferrite material in order to provide the functionality described above. Specifically, a permanent magnet produces a static magnetic bias in the ferrite material in order to direct an input signal received at one port to another.

[0007] The electromagnetic field acting on the ferrite therefore needs to be tightly controlled. In addition, during their lifetime, such circulators may go through many thermal cycles and be exposed to a wide range of temperatures. It is therefore also necessary for the circulators to withstand the large thermal and mechanical stresses that occur during its lifetime, for example due to the different thermal expansion coefficients of their constituent parts. Summary

[0008] Embodiments of the present invention aim to address the above problems and others.

[0009] 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.

[0010] In an aspect, there is provided an RF circulator comprising a ferrite layer, a non-magnetic spacer, and a non-conductive preform layer arranged between the ferrite layer and the spacer, wherein the non-conductive preform layer comprises spherical glass particles arranged within an epoxy resin.

[0011] The circulator may comprise three ports. The circulator may comprise an isolator and may comprise an input port an output port and a termination port. The circulator may be a monolithic circulator. It may have a monolithic or micro-strip construction and / or interface. The preform layer may be disc shaped.

[0012] The glass particles may comprise spheriglass (RTM). The preform layer may comprise less than 10% glass particles by weight. The preform layer may comprise between 2% and 5% glass particles by weight, for example 3% glass particles by weight.

[0013] The epoxy may be Loctite Ablestik 958-11 (RTM).

[0014] The preform layer may have a thickness equal to the diameter of the glass particles. The glass particles may have a diameter of between 0.10 and 0.15 mm, for example between 0.11 and 0.14 mm, for example approximately 0.124 mm. The glass particles may be distributed evenly in the epoxy.

[0015] The RF circulator may further comprise a magnet coupled to the spacer, for example via a second non-conductive preform layer. The magnet may be a rare earth magnet. The spacer may comprise alumina (aluminium oxide), e.g. it may be formed substantially of alumina. The ferrite layer may be carried on a magnetic carrier layer. The magnetic carrier layer may be a nickel-cobalt ferrous alloy such as Fernico e.g. Kovar (RTM).

[0016] The circulator may comprise a resonator. The ferrite layer may comprise the resonator. The resonator may be connected to each of the ports of the circulator. One or more of the non-conductive preform layer, the non-magnetic spacer, the second non-conductive preform layer, and the magnet, may be aligned with the resonator, e.g. arranged on top of the resonator.

[0017] The preform layer may have a dielectric constant (relative permittivity) of between 3.96 and 4.8, for example it may have a dielectric constant of approximately 4.6. This may ensure that there is minimal or no detuning in the RF circuit. The preform layer may have a coefficient of thermal expansion of between 5 and 50 parts per million per °C (PPM), for example approximately 44 PPM. This may prevent cracks forming due to differences in the coefficients of thermal expansion of the other components of the circulator.

[0018] The preform layer may be configured not to compress upon expansion and / or contraction of one or more of the non-magnetic spacer, the second non-conductive preform layer, the magnet, the ferrite layer and the magnetic carrier.

[0019] In another aspect, there is provided a method of manufacturing an RF circulator, the method comprising applying a homogenous mixture of spherical glass particles and epoxy to a ferrite layer, curing the mixture to provide a non-conductive preform layer, and laying a non-magnetic spacer on the preform layer.

[0020] The method may further comprise, e.g. before applying the homogenous mixture to a ferrite layer, mixing the glass particles and epoxy to provide the homogeneous mixture. Mixing may comprise using a speed mixer, e.g. for 1 minute or more. Applying the homogenous mixture may comprise applying the mixture within 30 minutes of mixing. Curing may comprise curing at a temperature of over 100°C, e.g. 150°C, fora time of more than 30 minutes e.g. one hour.

[0021] The method may further comprise, e.g. before applying the homogenous mixture to a ferrite layer and / or before mixing the glass particles and epoxy, adding the glass particles to the liquid epoxy. This may comprise adding glass particles to a liquid epoxy in a ratio of less than 1 part glass particles to 9 parts epoxy by weight, for example 3 parts glass particles to 97 parts epoxy by weight. For example this may comprise dispensing an amount of the epoxy (e.g. 1 gram) onto chemical scales and may comprise adding a proportionate amount (e.g. 0.03g) of the glass particles to the epoxy.

[0022] Brief

[0023] Some examples of the present disclosure will now be described with reference to the figures, in which:

[0024] Figure 1 shows a plan view of an example circulator;

[0025] Figure 2 shows a perspective view illustrating the component parts of the example circulator;

[0026] Figure 3 shows a side view of a portion of the example circulator including a nonconducting preform;

[0027] Figure 4 illustrates a method of constructing a circulator;

[0028] Figure 5 illustrates another method of constructing a circulator;

[0029] Figure 6 shows a further method of constructing a circulator.

[0030] In the drawings like reference numerals are used to indicate like elements.

[0031] The present disclosure relates to RF isolators and circulators which include a non- conductive preform layer arranged between a magnet and a ferrite layer. The non- conductive preform layer includes spherical glass particles arranged within an epoxy resin, thereby providing a substantially non-compressible layer. It also has a dielectric constant that enables a stable magnetic field to be applied to the ferrite, and a coefficient of thermal expansion which minimises the mechanical stresses induced by thermal expansion of the components of the circulator. Figure 1 shows a plan view of a circulator 100 with a first port 101 , a second port 102, and a third port 103. The circulator 100 can be configured to act as an isolator, with the first port 101 acting as an input, the second port 102 acting as an output, and the third port 103 acting as a termination to dissipate RF signals. In this case, the circulator ensures that all signals entering the input 101 , which may be connected to an amplifier or power source are directed to the output 102, which may be connected to an antenna, and any signals entering through the output are directed to the termination 103, where they are dissipated . The circulator 100 includes a ferrite layer 107 and a magnet 105, as well as several other layers as discussed in more detail with reference to Figure 2.

[0032] Figure 2 shows a perspective view of the circulator 100, partially disassembled. As shown in Figure 2, the circulator 100 is a layered structure and comprises an input 101 , an output 102, and a termination 103 (hidden) as described above. Each of the input 101 , output 102 and termination 103 are provided on a ferrite plate layer 107. The ferrite layer 107 also comprises at its centre a resonator, coupled to each of the input 101 , the output 102 and the termination 103. The resonator is provided as a specific pattern that is generated directly onto the ferrite layer 107, e.g. in gold using sputtering technology. The size and shape of the resonator is defined by the desired electrical performance. The ferrite plate layer 107 is provided on a magnetic carrier layer 106, which may be made of a nickel iron alloy such as Kovar (RTM). A circular non-conductive preform layer 104a is provided, e.g. affixed, on the ferrite layer 107 at a central portion over the resonator. A non-magnetic spacer 109, which is typically made of alumina, is provided on the preform layer 104a. The spacer is configured to provide a desired spacing between the magnet 105 and the ferrite layer 107, such that an appropriate magnetic field is applied to the ferrite layer 107 for proper functioning of the circulator 100. A second non-conductive preform layer 104b is provided above the non-magnetic spacer layer. The magnet 105, which may be a rare earth magnet, is provided on the second non-conductive preform 104b.

[0033] The structure of the non-conductive preform layer 104a will now be discussed in more detail, with reference to Figure 3. The non-conductive preform layer 104a forms a key component of the RF circuit by having a specific dielectric constant which does not vary significantly. The preform layer 104a is in direct contact with the resonator of the ferrite meaning that any large variance in the dielectric constant of the preform layer may reduce the overall performance of the circulator. In addition, the non-conductive preform layer 104a is configured to minimize the mechanical stresses induced during changes in temperature due to the difference between the coefficient of thermal expansion (CTE) of the ferrite layer 107 and the magnetic circuit, i.e. the interaction of the magnet 105, the magnetic carrier 106 and the ferrite layer 107. If there is a significant mismatch in the relative CTE values between neighbouring layers, the shear stresses induced can result in cracks forming within the ferrite material. In addition, the overall level of the magnetic field generated by the magnet 105 which acts on the ferrite layer 107 is dependent on the thickness of the preform layer 104a. If the thickness of preform layer 104a varies whilst the circulator 100 is in use, the induced internal magnetic field may vary, thereby reducing the overall performance of the circulator 100. Without wishing to be bound by theory, the magnetic field may follow a fourth law relationship, meaning that small changes in the thickness of the spacer 109 and / or the preform layers 104a, 104b, can have a large impact on the applied magnetic field and subsequent performance of the circulator 100.

[0034] Figure 3 shows a schematic side view of a portion of the circulator 100 which illustrates the internal structure of the non-conducting preform layer 104a. The preform layer 104a is provided between the non-magnetic spacer 109, and the ferrite plate 107 which is mounted on the magnetic carrier 106. The preform layer comprises spherical glass particles 121 arranged in an epoxy resin 122. The diameter of the glass particles 121 define the width of the preform layer 104a, and the presence of the glass particles 121 may prevent the preform 104a from being compressed during thermal expansion and contraction of any of the surrounding layers. The preform layer 104a is substantially incompressible, which ensures that the distance between the components either side of the preform layer 104a, namely the non-magnetic spacer 109 and the ferrite layer 107, remains substantially constant.

[0035] The spherical glass particles 121 may be spheriglass (RTM) particles, ABLESTIK 958-11 liquid epoxy (RTM) may be used to form the epoxy resin 122. By weight, the preform layer 104a may comprise between 2% and 5% glass particles 121 , for example 3% glass particles 121 . The preform layer 104a has a dielectric constant of approximately 4.6, which may ensure that there is minimal or no detuning in the RF circuit. The coefficient of thermal expansion of the preform layer 104a is approximately 44 PPM. Figure 4 shows a method 400 of manufacturing a circulator, such as the circulator 100 described above. The method 400 comprises a first step 401 of applying a homogenous mixture of spherical glass particles and epoxy to a ferrite layer. At a second step 402, the mixture is cured to provide a non-conductive preform layer. At a third step 403, a nonmagnetic spacer is laid on the preform layer.

[0036] Figure 5 shows another method 500 of manufacturing a circulator, which may for example be a refinement of the method 400 discussed above. The method 500 comprises a first step 501 of adding glass particles to a liquid epoxy in a ratio of less than 1 part glass particles to 9 parts epoxy, by weight. At a second step 502 the glass particles and epoxy are mixed to provide a homogeneous mixture. At a third step 503, the homogenous mixture of spherical glass particles and epoxy is added to a ferrite layer. At a fourth step 504 the mixture is cured to provide a non-conductive preform. At a fifth step 505 a non-magnetic spacer is laid on the preform.

[0037] Figure 6 shows a method 600 of manufacturing a circulator, which may be a refinement of the methods described above. The first step 601 comprises a first step of removing a quantity of epoxy, e.g. ABLESTIK 958-11 , from a freezerand allowing it to come to room temperature, e.g. over a period of around 30 minutes. At second step 602 an amount of the epoxy, e.g. 1 gram is dispensed onto chemical scales. At a third step 603, solid glass spheres, e.g. 0.03 grams, are added to the epoxy. At fourth step 604 the glass spheres and epoxy are mixed, e.g. using a speed mixer for around 1 minute, such that the mixture is homogenous. At fifth step 605 the mixture is applied to a ferrite layer, e.g. within 30 minutes of the previous step. At a sixth step 606, the mixture is cured, e.g. at 150°C for one hour. A non-magnetic spacer may then be placed / laid onto the cured layer.

[0038] 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.

[0039] 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.

[0040] Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

Claims1 . An RF circulator comprising a ferrite layer, a non-magnetic spacer, and a non- conductive preform layer arranged between the ferrite layer and the spacer, wherein the non-conductive preform comprises spherical glass particles arranged within an epoxy resin.

2. The RF circulator of claim 1 , wherein the glass particles comprise spheriglass (RTM).

3. The RF circulator of any preceding claim, wherein the preform comprises less than 10% glass particles by weight.

4. The RF circulator of claim 3, wherein the non-conductive preform layer comprises between 2% and 5% glass particles by weight.

5. The RF circulator of claim 4, wherein the non-conductive preform layer comprises 3% glass particles by weight.

6. The RF circulator of any preceding claim, wherein the epoxy is Loctite Ablestik 958- 11 (RTM).

7. The RF circulator of any preceding claim, wherein the non-conductive preform layer has a thickness equal to the diameter of the glass particles.

8. The RF circulator of any preceding claim, wherein the glass particles have a diameter of between 0.10 and 0.15 mm.

9. The RF circulator of any preceding claim, wherein the glass particles are distributed evenly in the epoxy.

10. The RF circulator of any preceding claim, further comprising a magnet coupled to the spacer via a second non-conductive preform layer.

11. The RF circulator of any preceding claim, wherein the ferrite layer is carried on a magnetic carrier layer.

12. A method of manufacturing an RF circulator, the method comprising: applying a homogenous mixture of spherical glass particles and epoxy to a ferrite layer; curing the mixture to provide a non-conductive preform layer; laying a non-magnetic spacer on the non-conductive preform layer.

13. The method of claim 12, further comprising mixing the glass particles and epoxy to provide the homogeneous mixture.

14. The method of claim 13, further comprising adding the glass particles to the liquid epoxy.

15. The method of any of claims 11 to 14, comprising adding glass particles to a liquid epoxy in a ratio of less than 1 part glass particles to 9 parts epoxy, by weight.

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