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Title:
A MULTI-MODE CAVITY FILTER
Document Type and Number:
WIPO Patent Application WO/2019/102326
Kind Code:
A1
Abstract:
A multi-mode cavity filter is disclosed comprising a resonant cavity, formed of an electrically conductive material, having an interior chamber, and at least one dielectric resonator body comprising a piece of dielectric material having a shape that can support two or more resonant modes corresponding to different predetermined resonant frequencies, the dielectric resonator body being located within the interior chamber of the resonant cavity such that the resonator body is substantially enclosed by an interior surface of the resonant cavity. A coupling structure may be provided within, or forming part of, the resonant cavity for transferring signals to or from the two or more resonant modes corresponding to the different predetermined resonant frequencies of the dielectric resonator body in parallel.

Inventors:
VUOPPOLA ESA (FI)
ILMONEN MATIAS (FI)
KEJONEN PASI (FI)
ERVASTI KIMMO (FI)
Application Number:
PCT/IB2018/059049
Publication Date:
May 31, 2019
Filing Date:
November 16, 2018
Export Citation:
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Assignee:
NOKIA TECHNOLOGIES OY (FI)
International Classes:
H01P1/208
Foreign References:
EP3217469A12017-09-13
EP1858109A12007-11-21
US20110133862A12011-06-09
EP0759645A21997-02-26
Other References:
None
Download PDF:
Claims:
CLAIMS

1. A multi-mode cavity filter, comprising:

a resonant cavity, formed of an electrically conductive material, having an interior chamber;

at least one dielectric resonator body comprising a piece of dielectric material having a shape that can support two or more resonant modes corresponding to different predetermined resonant frequencies, the dielectric resonator body being located within the interior chamber of the resonant cavity such that the resonator body is substantially enclosed by an interior surface of the resonant cavity; and

a coupling structure within, or forming part of, the resonant cavity for transferring signals to or from the two or more resonant modes corresponding to the different predetermined resonant frequencies of the dielectric resonator body in parallel,

wherein the resonant cavity is dimensioned and arranged such that an air gap remains between an outer surface of the dielectric resonator body and the interior surface of the resonant cavity.

2. The multi-mode cavity filter of claim 1, wherein the resonant cavity is at least partially formed of a homogenous conductive material which comprises a lower wall and at least two substantially upstanding side walls defining a recess within which the dielectric resonator body is located.

3. The multi-mode cavity filter of claim 2, wherein the homogenous conductive material further comprises an upper wall connected to the side walls, above the lower wall, to substantially enclose the dielectric resonator body.

4. The multi-mode cavity filter of claim 2, wherein the resonant cavity is further formed of a separate upper wall, attached to the side walls, above the lower wall, to substantially enclose the dielectric resonator body.

5. The multi-mode cavity filter according to any preceding claim, wherein the coupling structure within the resonant cavity is located between an input signal path, extending into the resonant cavity, and the dielectric resonator body and/or between the dielectric resonator body and an output signal path, extending from the resonant cavity.

6. The multi-mode cavity filter of claim 5, wherein the coupling structure comprises one or more irises.

7. The multi-mode cavity filter according to any of claim 6, farther comprising one or more dividing walls between the input and/or output signal path and the dielectric resonator body, the dividing walls having an aperture or recess which defines the shape of the one or more irises.

8. The multi-mode cavity filter according to claim 7, wherein the one or more dividing walls comprise an upper edge having one or more teeth thereby to define two or more irises either side of the one or more teeth.

9. The multi-mode cavity filter according to any of claims 5 to 8, further comprising one or more further resonators connected to the input signal path and/or to the output signal path, within the resonant cavity.

10. The multi-mode cavity filter according to claim 9, wherein the one or more further resonators are single mode resonators.

11. The multi-mode cavity filter according to claim 10, wherein the single mode resonators are coaxial or single mode ceramic resonators.

12. The multi-mode cavity filter according to any preceding claim, further comprising one or more adjustable tuning elements, extending through the electrically conductive material of the resonant cavity, into the air gap between the outer surface of the dielectric resonator body and the interior surface of the resonant cavity.

13. The multi-mode cavity filter according to claim 12, wherein a plurality of spaced-apart, adjustable tuning elements are provided on an overcoupling arm which is located within the resonant cavity.

14. The multi-mode cavity filter according to any preceding claim, wherein the dielectric resonator body is a three-mode resonator.

15. A filter system comprising a plurality of multi-mode cavity filters according to any preceding claim, wherein the resonant cavity for each said filter is at least partially formed within a single piece of homogeneous conductive material.

Description:
A MULTI-MODE CAVITY FILTER

Field

[0001] This invention relates to a multi-mode cavity filter.

Background

[0002] Filters are important components in many electrical systems. In general terms, filters are signal processing circuits or functions for removing unwanted frequency components. A stopband refers to the range of frequencies which are rejected or attenuated, and a passband refers to the range of frequencies that are allowed. A low-pass filter is one which presents less attenuation to low-frequency signals than high- frequency signals. A high-pass filter presents less attenuation to high frequency signals than low frequency signals. A band-pass filter is one which comprises low and high-pass filtering characteristics to produce a passband between two cut-off frequencies.

[0003] In the field of mobile communication networks, base transceiver stations (BTS) use RF filters for reducing interference by rejecting out-of-band signals that may interfere with transmission and/or reception. For example, a low-pass RF filter is commonly used in BTS filter units for removing or attenuating harmonic interference signals from the stopband. For example, a BTS duplexer may comprise a filter, able to separate transmit and receive channels. Filtering systems may employ a plurality of different filters.

[0004] In order to achieve a desired performance, cavity filters may be employed, which typically comprise a resonator within a conducting box defining an interior cavity which acts as a waveguide.

Summary

[0005] A first aspect provides a multi-mode cavity filter, comprising:

a. a resonant cavity, formed of an electrically conductive material, having an interior chamber; b. at least one dielectric resonator body comprising a piece of dielectric material having a shape that can support two or more resonant modes corresponding to different predetermined resonant frequencies, the dielectric resonator body being located within the interior chamber of the resonant cavity such that the resonator body is substantially enclosed by an interior surface of the resonant cavity; and

c. a coupling structure within, or forming part of, the resonant cavity for transferring signals to or from the two or more resonant modes corresponding to the different predetermined resonant frequencies of the dielectric resonator body in parallel,

d. wherein the resonant cavity and/or the dielectric resonator body is dimensioned and arranged such that an air gap remains between an outer surface of the dielectric resonator body and the interior surface of the resonant cavity. [0006] The resonant cavity may be at least partially formed of a homogenous conductive material which comprises a lower wall and at least two substantially upstanding side walls defining a recess within which the dielectric resonator body is located. Upstanding end walls may also be provided.

[0007] The homogenous conductive material may further comprise an upper wall connected to the side walls, above the lower wall, to substantially enclose the dielectric resonator body.

[0008] The resonant cavity may further be formed of a separate upper wall, attached to the side walls, above the lower wall, to substantially enclose the dielectric resonator body.

[0009] The coupling structure within the resonant cavity may be located between an input signal path, extending into the resonant cavity, and the dielectric resonator body and/or between the dielectric resonator body and an output signal path, extending from the resonant cavity.

[0010] The coupling structure may comprise one or more irises. In other embodiments, one or more other types of coupling structure may be used, for example comprising tracks, ridges, probes, links and openings.

[0011] The filter may further comprise one or more dividing walls between the input and/or output signal path and the dielectric resonator body, the dividing walls having an aperture or recess which defines the shape of the one or more irises.

[0012] The one or more dividing walls may comprise an upper edge having one or more teeth thereby to define two or more irises either side of the one or more teeth. The lower edge of the one or more dividing walls may comprise one or more apertures. The dividing walls may be formed homogenously with the cavity or may be separate, possibly removable components.

[0013] The filter may further comprise one or more further resonators connected to the input signal path and/or to the output signal path, within the resonant cavity.

[0014] The one or more further resonators may be single mode resonators.

[0015] The single mode resonators may be coaxial or single mode ceramic resonators. [0016] The filter may further comprise one or more adjustable tuning elements, extending through the electrically conductive material of the resonant cavity, into the air gap between the outer surface of the dielectric resonator body and the interior surface of the resonant cavity. The tuning elements may be screws.

[0017] A plurality of spaced-apart, adjustable tuning elements may be provided on an overcoupling arm which is located within the resonant cavity.

[0018] The dielectric resonator body may be a three-mode resonator. The dielectric resonator body may be physically asymmetric, in order to provide the two or more predetermined resonant modes at respective resonant frequencies. For example, a cruciform cross-sectional profile may be employed, comprising first and second pairs of opposed arms. A first pair of arms may comprise a different amount of dielectric material than a second pair of arms. This may be arranged by providing recesses or holes within one pair of arms and not in the other, or alternatively by forming different sized holes in the different pairs. The arrangement of the arms within the cavity may be such that they are diagonally oriented, but they could be in any direction, e.g. parallel to the cavity walls.

[0019] A second aspect provides a filter system comprising a plurality of multi-mode cavity filters according to any preceding definition, wherein the resonant cavity for each said filter is at least partially formed within a single piece of homogenous conductive material.

[0020] For example, the resonant cavity can be formed of a body of cast aluminium alloy. The cavity may alternatively be formed of pure aluminium, copper, zinc, zinc alloy etc. The cavity may be plated, for example with silver, or another metal with properties suitable for good surface conductivity and corrosion resistance.

Brief Description of the Drawings

[0021] The present disclosure will now be described, by way of non-limiting example, with reference to the drawings in which:

[0022] Figure 1 is a schematic diagram of a cellular base station, including a base transceiver station;

[0023] Figure 2A is a perspective view of a dielectric resonator mounted on a substrate;

[0024] Figure 2B is a plan view of one side of the Figure 2A substrate;

[0025] Figure 2C is a schematic diagram of an example filter network model of the Figure 2A and 2B arrangement, useful for understanding embodiments;

[0026] Figure 3 is a perspective view of a cavity filter according to an embodiment;

[0027] Figure 4 is a perspective view of a dielectric multi-mode resonator for use in the Figure 3 cavity filter; [0028] Figures 5A and 5B are, respectively, a partial top plan view and a close-up view of a filter system comprising one or more of the Figure 3 cavity filters;

[0029] Figure 6 is a perspective view of the Figure 5A filter system with a connected lid; and

[0030] Figure 7A is a perspective view of a dielectric multi-mode resonator with an over coupling arm carrying tuning elements ; and

[0031] Figure 7B is a close-up view of the Figure 7A over coupling arm carrying tuning elements.

Detailed Description

[0032] Embodiments described herein relate to filters and also to filter systems that may comprise one or more of said filters.

[0033] Embodiments particularly, though not exclusively, relate to radiofrequency (RF) filters and filter systems.

[0034] Embodiments particularly, though not exclusively, relate to RF filters for use in base transceiver stations (BTS) of mobile communications networks.

[0035] Growth in the mobile telecommunications industry has brought about advances in filter technology as new communications systems emerge, requiring more stringent filter characteristics, for example in terms of high-Q (low loss) characteristics and/or sharp cut-off. More compact filters are also desirable.

[0036] Figure 1 shows a simplified cellular BTS 1 which may be part of, or associated with, an antenna tower 3 carrying one or more RF antennas 5 in signal communication with the BTS 1 using one or more conductors 7. The BTS 1 is usually housed in an enclosure located at or near the base of the antenna tower 3, but it is also known to provide the BTS or at least the radio head towards the top of the antenna tower, closely coupled to the antenna, to minimise feeder cable loss which may increase with higher frequencies, and may be a driving factor for lower size and weight of such equipment. The BTS 1 is in signal communication with a backhaul communications system 11 which provides intermediate links to a core network. Within the BTS 1 are provided various analogue and digital signal processing modules. For example, one or more RF filter units 9 may be provided.

[0037] A plurality of RF filter units 9 may be provided, serving different purposes. These may be low- pass, high-pass and/or band-pass filter units. [0038] For example, a RF filter unit 9 may comprise one or more low-pass filters for removing or attenuating spurious signals from the stopband. Such spurious signals may, for example, result from harmonic interference.

[0039] For example, the RF filter unit 9 may comprise one or more band-pass filters for passing a selected range of frequencies whilst rejecting out-of-band frequencies. The RF filter unit 9 may for example comprise a duplexer for microwave telecommunication applications. Duplexers are provided at base stations, as represented in Figure 1, for enabling both transmit and receive channels to use the same filter unit.

[0040] The RF filter unit 9 may comprise an enclosure housing or providing one or more filters of one or more of the low-pass, high-pass and band-pass types.

[0041] Embodiments herein primarily concern multi-mode cavity filters. Cavity filters typically comprise one or more resonators within a conducting box which defines an interior cavity within which signals propagate. The cavity provides an internal waveguide for RF signals. Cavity filters offer a high-Q (low loss) characteristic and sharp cut-off, particularly when used with one or more dielectric resonators.

[0042] Multi-mode filters typically implement two or more resonators in a single physical body, such that reductions in filter size can be obtained. Thus, a multi-mode filter may have two resonant peaks at different predetermined frequencies. Dielectric resonators, which may be comprised within the cavity of the cavity filter, may be used to provide the different modes at respective resonant frequencies, which may be determined by the dimensions of the dielectric resonator. A ceramic block is an example dielectric that is typically coated in a metallic layer, for example silver, to provide the cavity and prevent leakage of RF energy which will adversely affect the filter performance.

[0043] A problem with this arrangement is that post-assembly tuning of the resonances to achieve a more precise filter performance is very difficult. Manufacturing dielectric resonators, for example from ceramic material, involves relatively high tolerances and hence the resulting performance may not be precisely as desired. It is therefore desirable to be able to tune the resulting filter after assembly. Further, materials having different thermal properties may mean that their expansion and contraction due to heat is different, and hence attachment and electric grounding between the dielectric resonator and of the structure in which it is located may be affected.

[0044] To realise a bandpass filter, it is known to couple resonances in series, one after the other, with couplings between the separate resonators and to input and output ports which feed respective input and output signals to and from the filter. In the context of dielectric resonators, these can be recognised by the presence of multiple dielectric blocks, connected one after the other. [0045] Embodiments herein provide a multi-mode cavity filter which comprises at least one dielectric resonator body comprising a piece of dielectric material having a shape that can support two or more resonant modes corresponding to different predetermined resonant frequencies. The dielectric resonator body may be located within the interior chamber of the resonant cavity such that the resonator body is substantially enclosed by an interior surface of the resonant cavity, wherein they are dimensioned and arranged such that an air gap remains between an outer surface of the dielectric resonator body and the interior surface of the resonant cavity.

[0046] In this way, expansion and contraction of the two different materials will have less impact on the filter structure due to the presence of the air gap. Further, the presence of the air gap enables post-assembly tuning by means of, for example, one or more tuning elements that may pass within the air gap to affect coupling of the modes.

[0047] In this respect, embodiments herein also provide a coupling structure within, or forming part of, the resonant cavity for transferring signals to or from the two or more resonant modes corresponding to the different predetermined resonant frequencies of the dielectric resonator body in parallel.

[0048] In this respect, reference is made to Figures 2A - 2C which is useful for understanding parallel coupling in this context.

[0049] Figure 2A shows a filter 12 comprised of a resonator body 13 mounted on a substrate 14. The resonator body 13 may be formed of a ceramic dielectric, for example, but any other dielectric having suitable dielectric properties may be used. The substrate 14 may be planar and may comprise a printed circuit board (PCB) or the like to allow coupling paths to be provided to the resonator body. The shape and material of the resonator body 13 supports at least two resonant modes at respective predetermined frequencies, and in this case three resonant modes are supported by virtue of the three-dimensional shape. Figure 2B shows an example coupling structure which in this case is provided by conductive tracks. The substrate 14 is shown from its top side, without the resonator body 13 present. The coupling structure comprises, on the underside (and hence shown with dotted lines) conductive input and output paths 15a, 15b which may be defined by cut-outs in the ground plane. On the shown side, the input and output paths 15a, 15b are connected using via connections to respective coupling paths 16a, 16b, although any suitable coupling technique such as capacitive or inductive coupling can be used. Each coupling path 16a, 16b comprises two sections, i.e. parallel to the X and Y axes respectively. This allows the first and second sections of each coupling path 16a, 16b to couple to first and second resonant modes of the resonator body 13. To complete the filter 12, the resonator body 13 may be coated in electrically conductive material, e.g. silver, to provide the cavity filter. [0050] Figure 2C is a filter network model 20 representing the Figure 2A and 2B filter 12. The filter 12 may be modelled as two low Q resonators representing the input and output paths 15a, 15b, coupled to three high Q resonators representing the resonant modes of the resonator body and with the two low Q resonators also being coupled to each other. The input and output paths 15a, 15b have respective resonant frequencies Rl, R5 whilst the resonant modes of the resonator body 13 have respective resonant frequencies R2, R3, R4. Reference numerals Knm represent coupling constants, e.g. K12 represents the coupling constant between Rl and R2. The filtering response of the filter 12 may be controlled by controlling the coupling constants Knm and the resonant frequencies Rl - R5. The strength of the coupling constants Knm can be adjusted by varying the shape and position of the coupling paths, e.g. as shown in Figure 2B.

[0051] By virtue of the parallel coupling between, for example, Rl and R2, R3, R4, and between R2,

R3, R4 and R5, a much simpler and more compact sized filter 12 can be produced. The filter 12 in this example acts as a RF bandpass filter at frequencies of interest.

[0052] The shown boxes 21, 23, 25 represent the separate parts of the filter 12 each of which may be coated in the electrically conductive material to create the cavity and minimise signal leakage. However, coating may reduce the possibility of effective post-assembly tuning as openings in the coating that may be created post-assembly for elements such as tuning screws may increase signal leakage and interference.

[0053] Figure 3 is a perspective, partially-cut view of a multi-mode cavity filter 31 according to an embodiment.

[0054] The multi-mode cavity filter 31 (hereafter“filter”) comprises a cavity 33, being a casing defining a hollow interior chamber 34. The cavity 33 may be formed of an electrically conductive material, which may for example be aluminium. The cavity 33 may be formed of one or more pieces of homogenous material to minimise signal leakage, and in the shown example the cavity comprises a generally rectangular casing having a longitudinal base wall 35, substantially parallel side walls 37 (only one of which is shown), and substantially parallel end walls 39, 41. The side walls 37 and end walls 39, 41 are upstanding from the base wall 35 and all are formed as a uni-body structure, i.e. using a homogenous material, which thereby minimises signal leakage and is relatively straightforward to manufacture.

[0055] As will be explained later on, the cavity 33 is completed by means of placing a lid, or upper wall, on top of the side walls 37 and end walls 39, 41 to enclose the interior chamber 35 and prevent or minimise signal leakage. The lid may be formed of the same material as the remainder of the cavity 33. [0056] In some embodiments, the lid may also be formed as part of the uni-body structure, i.e. so that the entire cavity is a one-piece metallic unit.

[0057] The interior chamber 34 may comprise one or more sections 55, 56, 57. In the shown example, the filter 31 comprises a central section 56 divided from first and second outer sections 55, 57 by means of first and second dividing plates or walls 45, 51, upstanding from the base wall 35. This is provided by way of example, but in other embodiments, fewer or more sections may be used without departing from the scope.

[0058] At or near the base wall 35, the dividing walls 45, 51 may extend across the cavity 34, between the side walls 37. The upper edges of the dividing walls 45, 51, however, may not extend all of the way up to the upper edge of the side walls 37; rather, they may be shaped in such a way as to provide irises 58, 59. As will be appreciated, irises 58, 59 are specially shaped apertures or recesses which couple waveguide cavity sections either-side of them in a manner that is determined by the discontinuities introduced by means of their shape. Therefore, the shape of the irises 58, 59 may determine how resonant elements in each of the above-mentioned sections 55, 56, 57 are coupled, and may dictate the coupling constants referred to in respect of Figure 2C.

[0059] It should be appreciated, however, that iris coupling is not the only way of providing coupling between resonances, and that other methods such as using tracks, ridges, probes, links and openings may be used.

[0060] Within the central section 56 of the filter 31 may be provided a dielectric resonator 47. In the shown example, the dielectric resonator 47 comprises a ceramic body dimensioned and arranged to support at least two, and in this case three, resonant modes. The dielectric resonator 47 has a cross-sectional profile that is generally cruciform in shape, although other profiles may be used. The dielectric resonator 47 may be located over a protruding stud 49 upstanding from the base wall 35 of the cavity 33, and is substantially equidistant from the dividing walls 45, 51. The vertical extent of the dielectric resonator 47, measured from its base and parallel to the Z axis, is less than the internal height of the interior chamber 34 measured from its base wall 35 to the upper edges of the end and side walls 37, 39, 41. Thus, an air gap 60 remains between the upper surface of the dielectric resonator 47 and the upper edges of the end and side walls 37, 39, 41 and therefore there will be a gap between the lid or upper wall that will be mounted on these upper edges. This permits thermal expansion of the dielectric resonator 47 relative to the metallic material of the structure and one or more tuning elements to pass within the air gap, for post-assembly tuning, as also mentioned below.

[0061] The dielectric resonator 47, by virtue of providing three resonant modes, corresponds to the resonators R2, R3, R4 shown in the Figure 2C network model 20. [0062] Within each of the first and second outer sections 55, 57 may be provided one or more further resonators 43, 53.

[0063] In the shown example, each further resonator 43, 53 comprises an air coaxial, single mode resonator, the structure of which will be known and understood. In some embodiments, other types of resonator may be employed, for example, a dielectric resonator may be provided in one or both of the first and second outer sections 55, 57.

[0064] The single mode resonators 43, 53, by virtue of them each providing a single resonant mode, correspond to the resonators Rl, R5 shown in the Figure 2C network model 20.

[0065] Similar to the dielectric resonator 47 provided in the central section 56, the single mode resonators 43, 53 are dimensioned and arranged such that their vertical extent from the base and parallel to the Z axis, is less than the internal height of the interior chamber 34 measured from its base wall 35 to the upper edges of the end and side walls 37, 39, 41. Thus, the air gap 60 remains also between the upper surface of each single mode resonator 43, 53 and the upper edges of the end and side walls 37, 39, 41. The lid or upper wall may be mounted on these upper edges.

[0066] The air gap 60 may have the same dimension across the sections 55, 56, 57 or different respective dimensions may be used.

[0067] The provision of this air gap 60 permits:

(i) thermal expansion of the dielectric resonator 47 relative to the metallic material of the structure in which it is located, which might affect attachment and electrical grounding of the filter; and

(ii) one or more tuning elements to pass within the air gap, permitting post-assembly tuning.

[0068] For example, the tuning elements may comprise screws which pass through holes formed in the lid or upper wall (not shown in Figure 3) extending through the cavity wall between the outside and the interior chamber 34. In this way, a user may adjust, from the exterior, the axial extent to which a distal end of a screw extends within the chamber 34 and therefore how it will affect signal propagation within the waveguide formed by the chamber.

[0069] Figure 3 shows a plurality of tuning screws 64 extending downwards, parallel to the Z axis, from different positions above the triple-mode dielectric resonator 47. A different single tuning screw 65 extends downwardly, parallel to the Z axis, at a position above the single mode resonator 43. Another single tuning screw 66 extends downwardly, parallel to the Z axis, at a position above the other single mode resonator 53. [0070] Each of the tuning screws 64, 65, 66 passes through a respective hole with which the screw closely conforms, to minimise signal leakage. The tuning screws 64, 65, 66 may be formed of the same material as the cavity 33, e.g. aluminium, or may be formed of a material based on thermal compensation considerations, e.g. brass screws which tend to have advantages in terms of manufacture, electrical conductivity and reliability when turned. The allowed downwards movement for the tuning screws 64, 65,

66 is mechanically limited, e.g. by virtue of their length and/or a retaining head located at their distal ends, such that the proximal ends cannot make mechanical contact with the underlying resonators 43, 47, 53.

[0071] Accordingly, post-assembly tuning of the filter 31 can be achieved by passing a signal to the input of the filter, monitoring the output signal, and adjusting one or more of the tuning screws 64, 65, 66 until a desired response is achieved.

[0072] The single mode resonator 43 in first outer section 55 is coupled to a first track 61 which passes through one end wall 39 of the cavity 33. Similarly, the other single mode resonator 53 in the second outer section 57 is coupled to a second track 63 which passes through the opposite end wall 41 of the cavity 33.

The first and second tracks 61, 63 may represent respective input and output conductors for connection to, for example, an antenna and subsequent filtering or processing elements of a filtering system. Other mechanisms for coupling the first and second tracks 61, 63 to the single mode resonators 43, 53 may be employed.

[0073] In order to realise parallel coupling between the single mode resonator 43 and the triple-mode dielectric resonator 47, and similarly between the triple-mode dielectric resonator and the single mode resonator 53 (for example, to realise the network topology shown in Figure 2C, if required) a coupling structure is provided.

[0074] As indicated previously, any type of suitable coupling structure may be used. Such methods may comprise using tracks, ridges, probes, links and openings.

[0075] In embodiments herein, iris coupling is used. The dividing walls 45, 51 are shaped and dimensioned so as to provide the irises 58, 59 to provide parallel coupling between the resonators within the waveguide provided by the interior chamber 34 of the cavity 33. The shape of the irises 58, 59 introduces a discontinuity in the chamber 34 to provide the desired coupling. For example, an iris which reduces the width of a rectangular waveguide has an equivalent circuit of a shunt inductance. An iris which restricts the height of a rectangular waveguide has an equivalent circuit of a shunt capacitance. An iris which restricts in both directions is equivalent to a parallel FC resonant circuit. [0076] Thus, it will be appreciated that the shape and dimensions of the irises 58, 59 has an influence on the parallel coupling between the single mode and dielectric resonators 43, 53, 47.

[0077] Referring now to Figure 4, an alternative dielectric resonator 68 is shown. The dielectric resonator 68 is similar to that shown in Figure 3 in that it has a generally cruciform cross-sectional profile and is asymmetrical. Specifically, the dielectric resonator 68 has holes 69, which can be of any selected shape or size, formed within one pair of opposed cruciform arms 70, the holes extending generally parallel to the Z axis. No holes are provided in the other pair of arms 70. In some embodiments, holes may be provided in both sets of arms 70, the holes for one set of diagonally opposed arms being of a different diameter to those of the other set of diagonally opposed arms. In either case, the electrical field paths of the diagonal corner-to-corner resonances involve different amounts of dielectric material, and result in different resonant frequencies for the different modes. The dimensions are such that the lowest and the highest resonances are at, or close to, the desired passband edges and such that the centre frequency is in-between, depending on the required stopbands.

[0078] Referring to Figures 5 A and 5B, part of a filter system 71 is shown. The filter system 71 comprises a uni-body electrically-conductive chassis 73 which may provide the cavity for a plurality of such filters as described above. A lid formed of corresponding electrically-conductive material is not shown in this Figure. The chassis 73 may be formed by casting or by milling (or laser cutting) a block of suitable material, such as aluminium.

[0079] A plurality of recesses may be formed within the chassis 73 to provide the respective sections 55, 56, 57 referred to above with reference to Figure 3.

[0080] For example, a first recess 77 may house the first, single mode resonator 43. A second recess 79 may house the multi-mode dielectric resonator 47 or 68. A third recess 81 may house the other, single mode resonator 53. The dividing walls 83, 85 may be permanently formed as part of the chassis 73 or may be separate components which can be removably located, e.g. by sliding within opposed receiving slots, between the sections 55, 56, 57. The shape of the dividing walls 83, 85 may dictate the parallel coupling characteristics for reasons already explained.

[0081] In the shown example, the first recess 77 is coupled to a coaxial connector 75 on the exterior of the chassis 73 by means of an input conductor, similar to that referenced by numeral 61 in Figure 3. An output conductor extending from the second recess 81 may connect to an output terminal or to another filter provided within the chassis 73. [0082] Referring to the close-up view of Figure 5B, it will be seen that one or more of the dividing walls 83, 85 may have a complex shape to achieve the required parallel coupling, in this case between the first single mode resonator 43 and the multi-mode dielectric resonator 53. For example, the upper edge of the dividing wall 85 may have one or more teeth. In the shown example, a plurality of teeth 78, 79 are provided which define irises, including a central iris 89. A further iris 91 may be provided in a lower part of the dividing wall 85, by means of an aperture or recess. Any manner of complex iris arrangement may be used to provide the required coupling between adjacent sections 55, 56, 57 within the waveguide.

[0083] Referring to Figure 6, the Figure 5B filter system 71 is shown again with a lid 103 connected to the chassis 71, thereby enclosing the various recesses forming the filter sections, including the three sections 77, 79, 81 for the above-described filter. The lid 103 is screwed to the chassis 71 by means of screws or bolts which pass through the various holes 104 distributed around the perimeter of the lid, and which pass into corresponding holes 72 in the underlying chassis 71.

[0084] Tuning elements 105, e.g. screws, are also shown in Figure 6, which pass through the lid 103 and into the interior cavity of each respective filter to enable post-assembly tuning.

[0085] Referring to Figures 7A and 7B, in some embodiments, but which is by no means essential, two or more tuning elements 105 may be mounted on a single over coupling arm 107 which may locate within the chassis. The tuning elements 105 pass through closely conforming holes in the lid to permit post-assembly tuning. As shown in Figures 7A and 7B, the over coupling arm 107 may comprise an elongate, substantially planar arm having two or more apertures for receiving respective tuning elements, e.g. screws 108. The spaced-apart distance between the screws 108 is therefore predetermined and Figure 7A shows how the over coupling arm 107 can be placed, for example, relative to the dielectric multimode dielectric resonator 68 shown in Figure 4. In some embodiments, but again by no means essential, the screws 108 may have plastic, clip-on tips, which allow the screws to turn and thus alter the distance of the coupling strip from the lid which adjusts the strip’s impedance and hence coupling.

[0086] Embodiments therefore provide a cavity filter which comprises one or more dielectric resonators constructed and arranged to support two or more predetermined resonant modes, and a coupling structure which provides parallel coupling to the two or more modes of the dielectric resonator(s.) This, together with the provision of an air gap between the cavity interior and the dielectric resonator(s) provides a high performance filter which can be tuned after assembly, and which reduces or minimises problems caused by thermal expansion due to the different materials used for the cavity and the dielectric resonator. [0087] It will be appreciated that the above described embodiments are purely illustrative and are not limiting on the scope of the invention. Other variations and modifications will be apparent to persons skilled in the art upon reading the present application. [0088] Moreover, the disclosure of the present application should be understood to include any novel features or any novel combination of features either explicitly or implicitly disclosed herein or any generalization thereof and during the prosecution of the present application or of any application derived therefrom, new claims may be formulated to cover any such features and/or combination of such features.