Abstract:
A multi-mode cavity filter, comprising a resonator body of dielectric material capable of supporting at least two degenerate electromagnetic standing wave modes and having a face, and a conductive pattern on at least part of the face for coupling a radio frequency signal between the pattern and the resonator body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to and claims the benefit of Australian Provisional Patent Application No. 2011903389, filed Aug. 23, 2011 and U.S. Provisional Patent Application No. 61/531,277, filed Sep. 6, 2011, both of whose disclosures are hereby incorporated by reference in their entirety into the present disclosure. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a multi-mode filter, and in particular to a multi-mode filter including a resonator body, for use, for example in frequency division duplexers for telecommunication applications. 
       DESCRIPTION OF PRIOR ART 
       [0003]    The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. 
         [0004]    All physical filters essentially consist of a number of energy storing resonant structures, with paths for energy to flow between the various resonators and between the resonators and the input/output ports. The physical implementation of the resonators and the manner of their interconnections will vary from type to type, but the same basic concept applies to all. Such a filter can be described mathematically in terms of a network of resonators coupled together, although the mathematical topography does not have to match the topography of the real filter. 
         [0005]    Conventional single-mode filters formed from dielectric resonators are known. Dielectric resonators have high-Q (low loss) characteristics which enable highly selective filters having a reduced size compared to cavity filters. These single-mode filters tend to be built as a cascade of separated physical dielectric resonators, with various couplings between them and to the ports. These resonators are easily identified as distinct physical objects, and the couplings tend also to be easily identified. 
         [0006]    Single-mode filters of this type may include a network of discrete resonators formed from ceramic materials in a “puck” shape, where each resonator has a single dominant resonance frequency, or mode. These resonators are coupled together by providing openings between cavities in which the resonators are located. Typically, the resonators provide transmission poles or “zeros”, which can be tuned at particular frequencies to provide a desired filter response. A number of resonators will usually be required to achieve suitable filtering characteristics for commercial applications, resulting in filtering equipment of a relatively large size. 
         [0007]    One example application of filters formed from dielectric resonators is in frequency division duplexers for microwave telecommunication applications. Duplexers have traditionally been provided at base stations at the bottom of antenna supporting towers, although a current trend for microwave telecommunication system design is to locate filtering and signal processing equipment at the top of the tower to thereby minimise cabling lengths and thus reduce signal losses. However, the size of single mode filters as described above can make these undesirable for implementation at the top of antenna towers. 
         [0008]    Multimode filters implement several resonators in a single physical body, such that reductions in filter size can be obtained. As an example, a silvered dielectric body can resonate in many different modes. Each of these modes can act as one of the resonators in a filter. In order to provide a practical multimode filter it is necessary to couple the energy between the modes within the body, in contrast with the coupling between discrete objects in single mode filters, which is easier to control in practice. 
         [0009]    The usual manner in which these multimode filters are implemented is to selectively couple the energy from an input port to a first one of the modes. The energy stored in the first mode is then coupled to different modes within the resonator by introducing specific defects into the shape of the body. In this manner, a multimode filter can be implemented as an effective cascade of resonators, in a similar way to conventional single mode filter implementations. 
         [0010]    Again, this technique results in transmission poles which can be tuned to provide a desired filter response. 
         [0011]    An example of such an approach is described in U.S. Pat. No. 6,853,271, which is directed towards a triple-mode mono-body filter. Energy is coupled into a first mode of a dielectric-filled mono-body resonator, using a suitably configured input probe provided in a hole formed on a face of the resonator. The coupling between this first mode and two other modes of the resonator is accomplished by selectively providing corner cuts or slots on the resonator body. 
         [0012]    This technique allows for substantial reductions in filter size because a triple-mode filter of this type represents the equivalent of a single-mode filter composed of three discrete single mode resonators. However, the approach used to couple energy into and out of the resonator, and between the modes within the resonator to provide the effective resonator cascade, requires the body to be of complicated shape, increasing manufacturing costs. 
         [0013]    Two or more triple-mode filters may still need to be cascaded together to provide a filter assembly with suitable filtering characteristics. As described in U.S. Pat. Nos. 6,853,271 and 7,042,314 this may be achieved using a waveguide or aperture for providing coupling between two resonator mono-bodies. Another approach includes using a single-mode comb-line resonator coupled between two dielectric mono-bodies to form a hybrid filter assembly as described in U.S. Pat. No. 6,954,122. In any case the physical complexity and hence manufacturing costs are even further increased. 
       SUMMARY 
       [0014]    According to some embodiments, the invention provides a multi-mode cavity filter, comprising a resonator body of dielectric material capable of supporting at least two degenerate electromagnetic wave propagation modes and having a face, and a conductive pattern on at least part of the face for coupling a radio frequency signal between the pattern and the resonator body. The body might have more than one face. Using a conductive pattern on the body to couple radio frequency signals to and/or from the body can provide for a relatively simple construction in that the body does not need to be worked to create ports or the like for accommodating conductive connections. Moreover, such a pattern can, in some embodiments, be used to provide both an input for launching a radio frequency signal into the resonator body and an output for receiving a radio frequency signal from the resonator body, meaning that the cavity filter can have a relatively compact construction. 
         [0015]    The pattern may, for example, be a layer. The pattern may, for example, be a coating on the face. The pattern may, for example, form part of a conductive covering over the resonator body. 
         [0016]    The pattern may, for example, include a first part and a second part and the first and second parts are electrically isolated from one another. For example, the first and second parts may be, respectively, an input for launching the signal into the resonator body and an output for recovering the signal from the resonator body. 
         [0017]    The pattern may, for example, include a first part and a second part, where the first part is an input for launching the signal into the resonator body and the second part is an output for recovering the signal from the resonator body. 
         [0018]    The part of the face on which the pattern resides may, for example, be flat. 
         [0019]    The pattern may, for example, be provided on a substrate. The substrate may, for example, be a printed circuit board. 
         [0020]    In some embodiments, the pattern includes an elongate path for launching the signal into the resonator body, the path having an open-circuited end. Such a path may, for example, include first and second parts, each part being for coupling the signal to a standing wave in a respective one of two non-interfering electromagnetic wave modes within the resonator body. Such non-interfering electromagnetic waves are sometimes referred to as ‘orthogonal’, however this does not necessarily imply that they have a 90 degree spatial relationship one with another. The first part may, for example, be elongate and the second part may, for example, be a patch, or the first and second parts may, for example, both be elongate and extend in different, possibly orthogonal, directions. At least one of the parts may, for example, be straight. 
         [0021]    In some embodiments, the pattern includes another elongate path such that there are first and second elongate paths, wherein the first and second paths serve respectively as an input for launching the signal into the resonator body and an output for coupling the signal out of the resonator body. 
         [0022]    According to some embodiments, the invention provides a method of manufacturing a multi-mode cavity filter, the method comprising providing a resonator body of dielectric material capable of supporting at least two degenerate electromagnetic propagation modes and having a face, and providing a conductive pattern on at least part of the face for coupling a radio frequency signal between the pattern and the resonator body. 
         [0023]    Providing the pattern may, for example, involve coating at least part of the face with conductive material and removing part of the coating to form the pattern. 
         [0024]    Providing the pattern may, for example, involve at least one of painting, depositing and printing the pattern on at least part of the face. 
         [0025]    Providing the pattern may, for example, involve providing the pattern on a substrate and offering the substrate to the face. 
         [0026]    According to an aspect of the present invention, there is provided a multi-mode cavity filter, comprising: at least one dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least a second substantially degenerate resonant mode; a layer of conductive material in contact with and covering the dielectric resonator body; and a coupling structure comprising at least one electrically conductive coupling path for at least one of inputting signals to the dielectric resonator body and outputting signals from the dielectric resonator body, the at least one electrically conductive coupling path being arranged for at least one of directly coupling signals to the first resonant mode and the second substantially degenerate resonant mode in parallel, and directly coupling signals from the first resonant mode and the second substantially degenerate resonant mode in parallel. 
         [0027]    The at least one electrically conductive coupling path may, for example, comprise at least one of an input coupling path and an output coupling path for respectively coupling signals to and from the dielectric resonator body. 
         [0028]    The at least one coupling path may, for example, run substantially parallel to a surface of the dielectric resonator body. The at least one coupling path may, for example, lie adjacent the surface of the dielectric resonator body. 
         [0029]    The at least one coupling path may, for example, comprise a first portion primarily for coupling to the first mode and a second portion primarily for coupling to the second mode. The first portion of the at least one coupling path may, for example, be oriented such that at least one of the magnetic field and the electric field generated by said first portion is substantially aligned with the respective magnetic field or electric field of said first mode. The second portion of the at least one coupling path may, for example, be oriented such that at least one of the magnetic field and the electric field generated by said second portion is substantially aligned with the respective magnetic field or electric field of said second mode. 
         [0030]    The first portion and second portion may, for example, be any of the following: a straight or curved elongate track, and a patch. The first portion may, for example, comprise a first straight elongate track and the second portion may, for example, comprise a second straight elongate track arranged substantially orthogonally to the first straight elongate track. 
         [0031]    The at least one coupling path may, for example, comprise a portion for coupling simultaneously to both the first mode and the second mode. The portion may, for example, comprise an elongate track oriented at an angle such that at least one of the magnetic field and the electric field generated by said portion has a first Cartesian component aligned with the respective magnetic field or electric field of said first mode, and a second Cartesian component aligned with the respective magnetic field or electric field of said second mode. 
         [0032]    The coupling structure may, for example, be formed in the layer of conductive material. 
         [0033]    The multi-mode cavity filter may, for example, further comprise a substrate on which the dielectric resonator body is mounted. The coupling structure may, for example, be formed on the substrate. The substrate may, for example, comprise at least one of an input electrically coupled to said coupling structure for providing signals to the coupling structure and an output electrically coupled to said coupling structure for receiving filtered signals from the coupling structure. The substrate may, for example, comprise a printed circuit board. 
         [0034]    The piece of dielectric material may, for example, comprise a substantially planar surface for mounting to the substrate. The coupling structure may, for example, be provided on or adjacent to said substantially planar surface. 
         [0035]    The coupling structure may, for example, be provided on a substantially planar surface of said piece of dielectric material. 
         [0036]    According to another aspect of the present invention there is provided a dielectric resonator body for a multi-mode cavity filter, the resonator including:
       a piece of dielectric material, with at least one substantially flat face for mounting on a substrate layer, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least one substantially degenerate resonant mode;   wherein the shape of the piece of dielectric material is such that the first resonant mode and the at least one substantially degenerate resonant mode are capable of being simultaneously independently excited, and   wherein the piece of dielectric material is at least partially covered with a layer of conductive material.       
 
         [0040]    The dielectric material may have at least two axes and the each resonant mode is at least partially in the direction of a respective axis. Preferably, the dielectric body has three axes and supports three resonant modes that are substantially in the direction of said axes. 
         [0041]    The piece of dielectric material may have at least one axis of symmetry. The axis of symmetry may be in respect of rotational or reflection symmetry. 
         [0042]    The piece of dielectric material may have a shape arranged such that, in conjunction with its associated coupling structures, each resonant mode has a different centre frequency to the remaining resonant modes. Additionally, the piece of dielectric material may have a shape arranged such that each resonant mode has a centre frequency adjacent to another one of the resonant modes. Furthermore, the piece of dielectric material may have a respective major axis corresponding to each resonant mode and is asymmetric about at least one of the major axes. 
         [0043]    The piece of dielectric material may have one or more further surfaces in addition to the flat face, each further surface being substantially even. 
         [0044]    The piece of dielectric material may comprise one of a polyhedron, cuboid, cylinder, a hemisphere (or other portion of a sphere), prism, pyramid or any form of extruded shape. 
         [0045]    The piece of dielectric material may include a ceramic material. 
         [0046]    According to a further aspect of the present invention there is provided a multi-mode cavity filter including:
       a dielectric resonator body for a multi-mode cavity filter, the resonator including:
           a piece of dielectric material, with at least one substantially flat face for mounting on a substrate layer, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least one substantially degenerate resonant mode;   wherein the shape of the piece of dielectric material is such that the first resonant mode and the at least one substantially degenerate resonant mode are capable of being independently excited simultaneously, and   wherein the piece of dielectric material is at least partially covered with a layer of conductive material; and   
           a coupling structure comprising at least one electrically conductive coupling path for inputting signals to and/or outputting signals from the dielectric resonator body, the at least one electrically conductive coupling path being coupled to the substantially flat face.       
 
         [0052]    The dielectric material may have at least two axes and the each resonant mode is at least partially in the direction of a respective axis. 
         [0053]    The piece of dielectric material may have a shape arranged such that, in conjunction with its associated coupling structures, each resonant mode has a different centre frequency to the remaining resonant modes. Additionally, the piece of dielectric material may have a shape arranged such that each resonant mode has a centre frequency adjacent to another one of the resonant modes. Also, the piece of dielectric material may have a respective major axis corresponding to each resonant mode and is asymmetric about at least one of the major axes. 
         [0054]    The piece of dielectric material may have one or more further surfaces in addition to the flat face, each further surface being substantially even. 
         [0055]    The piece of dielectric material may comprise one of a polyhedron, a cuboid, a cylinder, a hemisphere (or other portion of a sphere), prism, pyramid or any form of extruded shape. 
         [0056]    According to various embodiments of another aspect of the present invention, there is provided a multi-mode cavity filter, comprising: at least one dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least a second substantially degenerate resonant mode; and a coupling structure comprising a patterned conductive layer for at least one of coupling signals to the piece of dielectric material and coupling signals from the piece of dielectric material. 
         [0057]    The patterned conductive layer may, for example, be substantially in contact with the dielectric resonator body. 
         [0058]    The patterned conductive layer may, for example, comprise at least one of an input coupling path and an output coupling path for respectively coupling signals to and from the dielectric resonator body. The input coupling path and/or the output coupling path may, for example, be for directly coupling signals to or from the first mode and the second mode in parallel. 
         [0059]    The input coupling path and/or the output coupling path may, for example, comprise a first portion primarily for coupling to the first mode and a second portion primarily for coupling to the second mode. The first portion of the input coupling path and/or the output coupling path may, for example, be oriented such that at least one of the magnetic field and the electric field generated by said first portion is substantially aligned with the respective magnetic field or electric field of said first mode, and the second portion of the input coupling path and/or the output coupling path may be oriented such that at least one of the magnetic field and the electric field generated by said second portion is substantially aligned with the respective magnetic field or electric field of said second mode. 
         [0060]    The first portion and second portion may, for example, be any of the following: a straight or curved elongate track, and a patch. The first portion may comprise a first straight elongate track and the second portion may comprise a second straight elongate track arranged substantially orthogonally to the first straight elongate track. 
         [0061]    The input coupling path and/or the output coupling path may, for example, comprise a portion for coupling simultaneously to both the first mode and the second mode. The portion may, for example, comprise an elongate track oriented at an angle such that at least one of the magnetic field and the electric field generated by said portion has a first Cartesian component aligned with the respective magnetic field or electric field of said first mode, and a second Cartesian component aligned with the respective magnetic field or electric field of said second mode. 
         [0062]    The patterned conductive layer may, for example, form part of a coating covering the piece of dielectric material. 
         [0063]    The multi-mode cavity filter may further comprise a substrate on which the dielectric resonator body is mounted. The patterned conductive layer may be formed on the substrate. The substrate may, for example, comprise at least one of an input electrically coupled to said coupling structure for providing signals to the coupling structure and an output electrically coupled to said coupling structure for receiving filtered signals from the coupling structure. 
         [0064]    The substrate may, for example, comprise a printed circuit board. 
         [0065]    The piece of dielectric material may comprise a substantially planar surface for mounting to the substrate. The patterned conductive layer may, for example, be provided on said substantially planar surface. 
         [0066]    The patterned conductive coating may, for example, be provided on a substantially planar surface of said piece of dielectric material. The patterned conductive coating may comprise an input coupling path and an output coupling path for respectively coupling signals to and from the dielectric resonator body. 
         [0067]    In a further aspect of the present invention, there is provided a method of manufacturing a multi-mode cavity filter, comprising: providing at least one dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least a second substantially degenerate resonant mode; and forming a patterned conductive layer comprising a coupling structure for at least one of coupling signals to the dielectric resonator body and coupling signals from the dielectric resonator body. 
         [0068]    The step of forming a patterned conductive layer may, for example, comprise: coating the piece of dielectric material with conductive material; and etching said coating to form said coupling structure. 
         [0069]    The step of forming a patterned conductive layer may, for example, comprise: printing, depositing or painting said piece of dielectric material with conductive material to form said coupling structure. 
         [0070]    The step of forming a patterned conductive layer may, for example, comprise: forming a patterned conductive layer in a substrate on which the piece of dielectric material is mounted. 
         [0071]    According to an aspect of the present invention, there is provided a multi-mode cavity filter, comprising: a dielectric resonator; a coupling structure for coupling input signals to the dielectric resonator and/or for extracting filtered output signals from the dielectric resonator; 
         [0072]    a covering of conductive material around the dielectric resonator and comprising an aperture; and a printed circuit board structure having at least one ground plane layer arranged over said aperture and electrically coupled to the covering of conductive material. 
         [0073]    The dielectric resonator may, for example, incorporate a piece of dielectric material, the piece of dielectric material having a shape such that it can support at least a first resonant mode and at least a second substantially degenerate resonant mode. 
         [0074]    The coupling structure may, for example, be arranged for at least one of coupling input signals to the dielectric resonator through the aperture and extracting filtered output signals from the dielectric resonator through the aperture. 
         [0075]    The coupling structure may, for example, comprise a first electrical connection on the surface of the dielectric resonator and a second electrical connection in a layer of the printed circuit board structure. The second electrical connection may, for example, be arranged in an outermost layer of the printed circuit board structure. The second electrical connection may, for example, be coupled to an inner signal layer of the printed circuit board structure. 
         [0076]    The coupling structure may, for example, comprise at least one conductive track arranged on the surface of the dielectric resonator. The at least one conductive track may, for example, comprise a first portion for at least one of coupling signals to and extracting signals from a first resonant mode of the dielectric resonator and a second portion for at least one of coupling signals to and extracting signals from a second resonant mode of the dielectric resonator. 
         [0077]    The printed circuit board structure may, for example, comprise a first ground plane layer electrically connected to the covering of conductive material and at least a second ground plane layer electrically coupled to the first ground plane layer. The first and second ground plane layers may, for example, be electrically coupled such that energy leakage from the dielectric resonator is reflected back into the dielectric resonator. The first ground plane layer may, for example, be continuously electrically coupled to the covering of conductive material around the aperture. The coupling structure may, for example, be electrically connected to an inner signal layer of the printed circuit board structure by a connection which passes through said first and second ground plane layers. 
         [0078]    The printed circuit board structure may, for example, comprise a first printed circuit board and a second printed circuit board electrically coupled to each other. 
         [0079]    The dielectric resonator may, for example, comprise a piece of dielectric material having a flat surface, and wherein the aperture is arranged on the flat surface. 
         [0080]    According to another aspect of the present invention there is provided a dielectric resonator body for a multi-mode cavity filter, the resonator body including:
       a piece of first dielectric material, with at least one substantially flat face for mounting on a substrate, the piece of first dielectric material having a shape such that it can support at least a first resonant mode and at least one spurious response; and   a layer of conductive material at least partially coating the resonator body;   wherein the piece of first dielectric material includes at least one region having a different dielectric constant to the first dielectric material, whereby the presence of the region of different dielectric constant alters the frequency separation of the resonant mode and the spurious response.       
 
         [0084]    The region of different dielectric constant may have a lower dielectric constant relative to the first dielectric material, whereby the frequency separation of the first resonant mode and the spurious response is increased. 
         [0085]    The shape of the first dielectric material may include a plurality of surfaces and supports a plurality of resonant modes, the resonator body including at least one of said regions of different dielectric constant on at least one of the surfaces. The region of different dielectric constant may be located at an area of the respective surface at which the field distribution of the spurious response is more concentrated than that of the first resonant mode. The resonator body may be cuboid and the region of different dielectric constant located at the centre of the respective surface. 
         [0086]    The region of different dielectric constant may comprise a piece of second dielectric material secured adjacent to the piece of first dielectric material. The piece of second dielectric material may protrude from the surface of the first piece of dielectric material. Alternatively, the piece of second dielectric material may be located within a recess formed in the first piece of dielectric material. Alternatively, the piece of second dielectric material may encapsulate the first piece of dielectric material. 
         [0087]    The resonator body may further comprise at least one piece of third dielectric material secured adjacent to the piece of second dielectric material, the second and third dielectric materials having different dielectric constants. 
         [0088]    The piece of second dielectric material may be shaped as one of the following: a cylinder, a cuboid, a polyhedron, a portion of a sphere and a prism. 
         [0089]    The piece of second dielectric material may be bonded to the first dielectric material. Alternatively, the piece of second dielectric material may be mechanically secured adjacent to the first dielectric material. 
         [0090]    Alternatively, the region of different dielectric constant may comprise a gas filled space covered by said conductive material. 
         [0091]    The gas filled space may be defined by at least one recess formed in the first dielectric material. Alternatively, the gas filled space may be defined by at least one hollow shaped portion of said conductive material affixed to the surface of the first dielectric material. 
         [0092]    According to a further aspect of the present invention there is provided a method of manufacturing a dielectric resonator body for a multi-mode cavity filter, the method comprising:
       providing a piece of first dielectric material, with at least one substantially flat face for mounting on a substrate, the piece of first dielectric material having a shape such that it can support at least a first resonant mode and at least one spurious response; and   providing a layer of conductive material at least partially coating the resonator body;   wherein the piece of first dielectric material includes at least one region having a different dielectric constant to the first dielectric material, whereby the presence of the region of different dielectric alters the frequency separation of the resonant mode and the spurious response.       
 
         [0096]    The region of different dielectric constant may have a lower dielectric constant relative to the first dielectric material, whereby the frequency separation of the first resonant mode and the spurious response is increased. 
         [0097]    The region of different dielectric constant may comprise a piece of second dielectric material secured adjacent to the piece of first dielectric material. The second dielectric material may be bonded to the surface of the first dielectric material. 
         [0098]    Alternatively, the piece of second dielectric material may be mechanically secured adjacent to the first dielectric material. 
         [0099]    Alternatively, one or more recesses may be formed in the first dielectric material and the second dielectric material is located within the recesses. 
         [0100]    The piece of second dielectric material may encapsulate the first piece of dielectric material. The step of providing the layer of conductive material may include providing a layer of the conductive material coating the first dielectric material; subsequently removing portions of the conductive layer at one or more locations; and adhering respective pieces of the second dielectric material to the first dielectric material at said locations. 
         [0101]    The step of providing the layer of conductive material may alternatively include providing a layer of conductive material in a predefined pattern on the first dielectric material, the pattern including selected regions where no conductive material is provided; and subsequently securing respective pieces of the second dielectric material adjacent to the first dielectric material at said selected regions. 
         [0102]    The respective pieces of the second dielectric material may be partially coated in the conductive material prior to being secured adjacent to the first dielectric material. 
         [0103]    The region of different dielectric constant may be formed by creating one or more recesses in the first dielectric material prior to providing said conductive layer. The recess may be covered with a planar conductive element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0104]    An example of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0105]      FIG. 1A  is a schematic perspective view of an example of a multi-mode filter; 
           [0106]      FIG. 1B  is a schematic side view of the multi-mode filter of  FIG. 1A ; 
           [0107]      FIG. 1C  is a schematic plan view of the multi-mode filter of  FIG. 1A ; 
           [0108]      FIG. 1D  is a schematic plan view of an example of the substrate of  FIG. 1A  including a coupling structure; 
           [0109]      FIG. 1E  is a schematic underside view of an example of the substrate of  FIG. 1A  including inputs and outputs; 
           [0110]      FIGS. 2A to 2C  are schematic diagrams of examples the resonance modes of the resonator body of  FIG. 1A ; 
           [0111]      FIG. 3A  is a schematic perspective view of an example of a specific configuration of a multi-mode filter; 
           [0112]      FIG. 3B  is a graph of an example of the frequency response of the filter of  FIG. 3A ; 
           [0113]      FIGS. 4A to 4F  are schematic plan views of example coupling structures; 
           [0114]      FIG. 5  is a schematic diagram of an example of a filter network model for the filter of  FIGS. 1A to 1E ; 
           [0115]      FIGS. 6A to 6C  are schematic plan views of example couplings illustrating how coupling configuration impacts on coupling constants of the filter; 
           [0116]      FIGS. 7A to 7E  are schematic plan views of example of alternative coupling structures for the filter of  FIGS. 1A to 1E ; 
           [0117]      FIG. 8A  is a schematic side view of an example of a multi-mode filter using multiple resonator bodies; 
           [0118]      FIG. 8B  is a schematic plan view of an example of the substrate of  FIG. 8A  including multiple coupling structures; 
           [0119]      FIG. 8C  is a schematic internal view of an example of the substrate of  FIG. 8A  including inputs and outputs; 
           [0120]      FIG. 8D  is a schematic underside view of an example of the substrate of  FIG. 8A ; 
           [0121]      FIG. 8E  is a schematic diagram of an example of a filter network model for the filter of  FIGS. 8A to 8D ; 
           [0122]      FIG. 9A  is a schematic diagram of an example of a duplex communications system incorporating a multi-mode filter; 
           [0123]      FIG. 9B  is a schematic diagram of an example of the frequency response of the multi-mode filter of  FIG. 9A ; 
           [0124]      FIG. 9C  is a schematic diagram of an example of a filter network model for the filter of  FIG. 9A ; 
           [0125]      FIG. 10A  is a schematic perspective view of an example of a multi-mode filter using multiple resonator bodies to provide filtering for transmit and receive channels; 
           [0126]      FIG. 10B  is a schematic plan view of an example of the substrate of  FIG. 10A  including multiple coupling structures; and, 
           [0127]      FIG. 10C  is a schematic underside view of an example of the substrate of  FIG. 10A  including inputs and outputs. 
       
    
    
     DETAILED DESCRIPTION 
       [0128]    An example of a multi-mode filter will now be described with reference to  FIGS. 1A to 1E . 
         [0129]    In this example, the filter  100  includes a resonator body  110 , and a coupling structure  130 . The coupling structure  130  at least one coupling  131 ,  132 , which includes an electrically conductive coupling path extending adjacent at least part of a surface  111  of the resonator body  110 , so that the coupling structure  130  provides coupling to a plurality of the resonance modes of the resonator body. 
         [0130]    In use, a radio frequency signal, containing, say, frequencies from within the 1 MHz to 100 GHz range, can be supplied to or received from the at least one coupling  131 ,  132 . In a suitable configuration, this allows a signal to be filtered to be supplied to the resonator body  110  for filtering, or can allow a filtered signal to be obtained from the resonator body, as will be described in more detail below. 
         [0131]    The use of electrically conductive coupling paths  131 ,  132  extending adjacent to the surface  111  allows the signal to be coupled to a plurality of resonance modes of the resonator body  110 . This allows a more simplified configuration of resonator body  110  and coupling structures  130  to be used as compared to traditional arrangements. For example, this avoids the need to have a resonator body including cut-outs or other complicated shapes, as well as avoiding the need for coupling structures that extend into the resonator body. This, in turn, makes the filter cheaper and simpler to manufacture, and can provide enhanced filtering characteristics. In addition, the filter is small in size, typically of the order of 6000 mm 3  per resonator body, making the filter apparatus suitable for use at the top of antenna towers. 
         [0132]    A number of further features will now be described. 
         [0133]    In the above example, the coupling structure  130  includes two couplings  131 ,  132 , coupled to an input  141 , an output  142 , thereby allowing the couplings to act as input and output couplings respectively. In this instance, a signal supplied via the input  141  couples to the resonance modes of the resonator body  110 , so that a filtered signal is obtained via the output  142 . However, the use of two couplings is for the purpose of example only, and one or more couplings may be used depending on the preferred implementation. 
         [0134]    For example, a single coupling  131 ,  132  may be used if a signal is otherwise coupled to the resonator body  110 . This can be achieved if the resonator body  110  is positioned in contact with, and hence is coupled to, another resonator body, thereby allowing signals to be received from or supplied to the other resonator body. Coupling structures may also include more couplings, for example if multiple inputs and/or outputs are to be provided, although alternatively multiple inputs and/or outputs may be coupled to a single coupling, thereby allowing multiple inputs and/or outputs to be accommodated. 
         [0135]    Alternatively, multiple coupling structures  130  may be provided, with each coupling structure  130  having one or more couplings. In this instance, different coupling structures can be provided on different surfaces of the resonator body. A further alternative is for a coupling structure to extend over multiple surfaces of the resonator body, with different couplings being provided on different surfaces, or with couplings extending over multiple surfaces. Such arrangements can be used to allow a particular configuration of input and output to be accommodated, for example to meet physical constraints associated with other equipment, or to allow alternative coupling arrangements to be provided. In use, a configuration of the input and output coupling paths  131 ,  132 , along with the configuration of the resonator body  110  controls a degree of coupling with each of the plurality of resonance modes and hence the properties of the filter, such as the frequency response. 
         [0136]    The degree of coupling depends on a number of factors, such as a coupling path width, a coupling path length, a coupling path shape, a coupling path direction relative to the resonance modes of the resonator body, a size of the resonator body, a shape of the resonator body and electrical properties of the resonator body. It will therefore be appreciated that the example coupling structure and cube configuration of the resonator body is for the purpose of example only, and is not intended to be limiting. 
         [0137]    Typically the resonator body  110  includes, and more typically is manufactured from a solid body of a dielectric material having suitable dielectric properties. In one example, the resonator body is a ceramic material, although this is not essential and alternative materials can be used. Additionally, the body can be a multilayered body including, for example, layers of materials having different dielectric properties. In one example, the body can include a core of a dielectric material, and one or more outer layers of different dielectric materials. 
         [0138]    The resonator body  110  usually includes an external coating of conductive material, such as silver, although other materials could be used such as gold, copper, or the like. The conductive material may be applied to one or more surfaces of the body. A region of the surface adjacent the coupling structure may be uncoated to allow coupling of signals to the resonator body. 
         [0139]    The resonator body can be any shape, but generally defines at least two orthogonal axes, with the coupling paths extending at least partially in the direction of each axis, to thereby provide coupling to multiple separate resonance modes. 
         [0140]    In the current example, the resonator body  110  is a cuboid body, and therefore defines three orthogonal axes substantially aligned with surfaces of the resonator body, as shown by the axes X, Y, Z. As a result, the resonator body  110  has three dominant resonance modes that are substantially orthogonal and substantially aligned with the three orthogonal axes. Examples of the different resonance modes are shown in  FIGS. 2A to 2C , which show magnetic and electrical fields in dotted and solid lines respectively, with the resonance modes being generally referred to as TM110, TE011 and TE101 modes, respectively. 
         [0141]    In this example, each coupling path  131 ,  132  includes a first path  131 . 1 ,  132 . 1  extending in a direction parallel to a first axis of the resonator body, and a second path  131 . 2 ,  132 . 2 , extending in a direction parallel to a second axis orthogonal to the first axis. Each coupling path  131 ,  132  also includes an electrically conductive coupling patch  131 . 3 ,  132 . 3 . 
         [0142]    Thus, with the surface  111  provided on an X-Y plane, each coupling includes first and second paths  131 . 1 ,  131 . 2 ,  132 . 1 ,  132 . 2 , extending in a plane parallel to the X-Y plane and in directions parallel to the X and Y axes respectively. This allows the first and second paths  131 . 1 ,  131 . 2 ,  132 . 1 ,  132 . 2  to couple to first and second resonance modes of the resonator body  110 . The coupling patch  131 . 1 ,  131 . 2 , defines an area extending in the X-Y plane and is for coupling to at least a third mode of the resonator body, as will be described in more detail below. 
         [0143]    Cuboid structures are particularly advantageous as they can be easily and cheaply manufactured, and can also be easily fitted together, for example by arranging multiple resonator bodies in contact, as will be described below with reference to  FIG. 10A . Cuboid structures typically have clearly defined resonance modes, making configuration of the coupling structure more straightforward. Additionally, the use of a cuboid structure provides a planar surface  111  so that the coupling paths can be arranged in a plane parallel to the planar surface  111 , with the coupling paths optionally being in contact with the resonator body  110 . This can help maximise coupling between the couplings and resonator body  110 , as well as allowing the coupling structure  130  to be more easily manufactured. 
         [0144]    For example, the couplings may be provided on a substrate  120 . In this instance, the provision of a planar surface  111  allows the substrate  120  to be a planar substrate, such as a printed circuit board (PCB) or the like, allowing the coupling paths  131 ,  132  to be provided as conductive paths on the PCB. However, alternative arrangements can be used, such as coating the coupling structures onto the resonator body directly. 
         [0145]    In the current example, the substrate  120  includes a ground plane  121 ,  124  on each side, as shown in  FIGS. 1D and 1E  respectively. In this example, the coupling paths  131 ,  132  are defined by a cut-out  133  in the ground plane  121 , so that the coupling paths  131 ,  132  are connected to the ground plane  121  at one end, although this is not essential and alternatively other arrangements may be used. For example, the couplings do not need to be coupled to a ground plane, and alternatively open ended couplings could be used. A further alternative is that a ground plane may not be provided, in which case the coupling paths  131 ,  132  could be formed from metal tracks applied to the substrate  120 . In this instance, the couplings  131 ,  132  can still be electrically coupled to ground, for example via vias or other connections provided on the substrate. 
         [0146]    The input and output are provided in the form of conductive paths  141 ,  142  provided on an underside of the substrate  120 , and these are typically defined by cut-outs  125 ,  126  in the ground plane  124 . The input and output may in turn be coupled to additional connections depending on the intended application. For example, the input and output paths  141 ,  142  could be connected to edge-mount SMA coaxial connectors, direct coaxial cable connections, surface mount coaxial connections, chassis mounted coaxial connectors, or solder pads to allow the filter  100  to be directly soldered to another PCB, with the method chosen depending on the intended application. Alternatively the filter could be integrated into the PCB of other components of a communications system. 
         [0147]    In the above example, the input and output paths  141 ,  142  are provided on an underside of the substrate. However, in this instance, the input and output paths  141 ,  142  are not enclosed by a ground plane. Accordingly, in an alternative example, a three layered PCB can be used, with the input and output paths embedded as transmission lines inside the PCB, with the top and underside surfaces providing a continuous ground plane, as will be described in more detail below, with respect to the example of  FIGS. 8A to 8E . This has the virtue of providing full shielding of the inner parts of the filter, and also allows the filter to be mounted to a conducting or non-conducting surface, as convenient. 
         [0148]    The input and output paths  141 ,  142  can be coupled to the couplings  131 ,  132  using any suitable technique, such as capacitive or inductive coupling, although in this example, this is achieved using respective electrical connections  122 ,  123 , such as connecting vias, extending through the substrate  120 . In this example, the input and output paths  141 ,  142  are electrically coupled to first ends of the coupling paths, with second ends of the coupling paths being electrically connected to ground. 
         [0149]    In use, resonance modes of the resonator body provide respective energy paths between the input and output. Furthermore, the input coupling and the output coupling can be configured to allow coupling therebetween to provide an energy path separate to energy paths provided by the resonance modes of the resonator body. This can provide four parallel energy paths between the input and the output. These energy paths can be arranged to introduce at least one transmission zero to the frequency response of the filter, as will be described in more detail below. In this regard, the term “zero” refers to a transmission minimum in the frequency response of the filter, meaning transmission of signals at that frequency will be minimal, as will be understood by persons skilled in the art. 
         [0150]    A specific example filter is shown in  FIG. 3A . In this example, the filter  300  includes a resonator body  310  made of 18 mm cubic ceramic body that has been silver coated on 5 sides, with the sixth side silvered in a thin band around the perimeter. The sixth side is soldered to a ground plane  321  on an upper side of a PCB  320 , so that the coupling structure  330  is positioned against the un-silvered surface of the resonator body  310 . Input and output lines on the PCB are implemented as coplanar transmission lines on an underside of the PCB  320  (not shown). It will therefore be appreciated that this arrangement is generally similar to that described above with respect to  FIGS. 1A to 1E . 
         [0151]    An example of a calculated frequency response for the filter is shown in  FIG. 3B . As shown, the filter  100  can provide three low side zeros  351 ,  352 ,  353  adjacent to a sharp transition to a high frequency pass band  350 . Alternatively, the filter  100  can provide three high side zeros adjacent to a sharp transition to a lower frequency pass band, described in more detail below with respect to  FIG. 9B . When two filters are used in conjunction for transmission and reception, this allows transmit and receive frequencies to be filtered and thereby distinguished, as will be understood by persons skilled in the art. 
         [0152]    Example coupling structures will now be described with reference to  FIGS. 4A to 4F , together with an explanation of their ability to couple to different modes of a cubic resonator, thereby assisting in understanding the operation of the filter. 
         [0153]    Traditional arrangements of coupling structures include a probe extending into the resonator body, as described for example in U.S. Pat. No. 6,853,271. In such arrangements, most of the coupling is capacitive, with some inductive coupling also present due to the changing currents flowing along the probe. If the probe is short, this effect will be small. Whilst such a probe can provide reasonably strong coupling, this tends to be with a single mode only, unless the shape of the coupling structure is modified. For a cubic resonator body, the coupling for each of the modes is typically as shown in Table 1 below. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Mode 
                 H field coupling 
                 E field coupling 
                 Notes 
               
               
                   
               
             
             
               
                 TE 011 
                 Negligible or zero due to 
                 Negligible or zero 
                 Negligible 
               
               
                 (E along X) 
                 tiny and orthogonal field. 
                 due to symmetry. 
                 coupling 
               
               
                 TE 101 
                 Negligible or zero due to 
                 Negligible or zero 
                 Negligible 
               
               
                 (E along Y) 
                 tiny and orthogonal field. 
                 due to symmetry. 
                 coupling 
               
               
                 TM 110 
                 Some for long probe 
                 strong 
                 Strong 
               
               
                 (E along Z) 
                   
                   
                 coupling 
               
               
                   
               
             
          
         
       
     
         [0154]    Furthermore, a probe has the disadvantage of requiring a hole to be bored into the cube. 
         [0155]    An easier to manufacture (and hence cheaper) alternative is to use a surface patch, as shown for example in  FIG. 4A , in which a ground plane  421  is provided together with a coupling  431 . In this example, an electric field extending into the resonator body is generated by the patch, as shown by the arrows. The modes of coupling are as summarised in Table 2, and in general this succeeds in only weakly coupling with a single mode. Despite this, coupling into a single mode only can prove useful, for example if multiple couplings are to be provided on different surfaces to each couple only to a single respective mode. This could be used, for example, to allow multiple inputs and or outputs to be provided. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Mode 
                 H field coupling 
                 E field coupling 
                 Notes 
               
               
                   
               
             
             
               
                 TE 011 
                 none 
                 Negligible or zero 
                 Negligible 
               
               
                 (E along X) 
                   
                 due to symmetry 
                 coupling 
               
               
                 TE 101 
                 none 
                 Negligible or zero 
                 Negligible 
               
               
                 (E along Y) 
                   
                 due to symmetry 
                 coupling 
               
               
                 TM 110 
                 none 
                 Medium 
                 Medium 
               
               
                 (E along Z) 
                   
                   
                 coupling 
               
               
                   
               
             
          
         
       
     
         [0156]    Coupling into two modes can be achieved using a quarter wave resonator, which includes a path extending along a surface of the coupling  431 , as shown for example in  FIG. 4B . The electric and magnetic fields generated upon application of a signal to the coupling are shown in solid and dotted lines respectively. 
         [0157]    In this example, the coupling  431  can achieve strong coupling due to the fact that a current antinode at the grounded end of the coupling produces a strong magnetic field, which can be aligned to match those of at least two resonance modes of the resonator body. There is also a strong voltage antinode at the open circuited end of the coupling, and this produces a strong electric field which couples to the TM110 mode, as summarised below in Table 3. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Mode 
                 H field coupling 
                 E field coupling 
                 Notes 
               
               
                   
               
             
             
               
                 TE 011 
                 Weak or zero 
                 Weak or zero 
                 Negligible 
               
               
                 (E along X) 
                   
                   
                 coupling 
               
               
                 TE 101 
                 strong 
                 Weak or zero 
                 Strong 
               
               
                 (E along Y) 
                   
                   
                 coupling 
               
               
                 TM 110 
                 strong 
                 medium 
                 Strongest 
               
               
                 (E along Z) 
                   
                   
                 coupling 
               
               
                   
               
             
          
         
       
     
         [0158]    In the example of  FIG. 4C , the coupling  431  includes an angled path, meaning a magnetic field is generated at different angles. However, in this arrangement, coupling to both of the TE modes as well as the TM mode still does not occur as eigenmodes of the combined system of resonator cube and input coupling rearrange to minimise the coupling to one of the three eigenmodes. 
         [0159]    To overcome this, a second coupling  432  can be introduced in addition to the first coupling  431 , as shown for example in  FIG. 4D . This arrangement avoids minimisation of the coupling and therefore provides strong coupling to each of the three resonance modes. The arrangement not only provides coupling to all three resonance modes for both input and output couplings, but also allows the coupling strengths to be controlled, and provides further input to output coupling. 
         [0160]    In this regard, the coupling between the input and output couplings  431 ,  432  will be partially magnetic and partially electric. These two contributions are opposed in phase, so by altering the relative amounts of magnetic and electric coupling it is possible to vary not just the strength of the coupling but also its polarity. 
         [0161]    Thus, in the example of  FIG. 4D , the grounded ends of the couplings  431 ,  432  are close whilst the coupling tips are distant. Consequently, the coupling will be mainly magnetic and hence positive, so that a filter response including zeros at a higher frequency than a pass band is implemented, as will be described in more detail below with respect to the receive band in  FIG. 9B . In contrast, if the tips of the couplings  431 ,  432  are close and the grounded ends distant, as shown in  FIG. 4E , the coupling will be predominantly electric, which will be negative, thereby allowing a filter with zeros at a lower frequency to a pass band to be implemented, similar to that shown at  350 ,  351 ,  352 ,  353  in  FIG. 3B . 
         [0162]    In the example of  FIG. 4F , two coupling structures  430 . 1 ,  430 . 2  are provided on a ground plane  421 , each coupling structure defining  430 . 1 ,  430 . 2  a respective coupling  431 ,  432 . The couplings are similar to those described above and will not therefore be described in further detail. The provision of multiple coupling structures allows a large variety of arrangements to be provided. For example, the coupling structures can be provided on different surfaces, of the resonator body, as shown by the dotted line. This could be performed by using a shaped substrate, or by providing separate substrates for each coupling structure. This also allows for multiple inputs and/or outputs to be provided. 
         [0163]    In practice, the filter described in  FIGS. 1A to 1E  can be modelled as two low Q resonators, representing the input and output couplings  131 ,  132  coupled to three high Q resonators, representing the resonance modes of the resonator body  110 , and with the two low Q resonators also being coupled to each other. An example filter network model is shown in  FIG. 5 . 
         [0164]    In this example, the input and output couplings  131 ,  132  have respective resonant frequencies f A , f B , whilst the resonance modes of the resonator body  110  have respective resonant frequencies f 1 , f 2 , f 3 . The degree of coupling between an input  141  and output  142  and the respective input and output couplings  131 ,  132  is represented by the coupling constants k A , k B . The coupling between the couplings  131 ,  132  and the resonance modes of the resonator body  110  are represented by the coupling constants k A1 , k A2 , k A3 , and k 1B , k 2B , k 3B , respectively, whilst coupling between the input and output couplings  131 ,  132  is given by the coupling constant k AB . 
         [0165]    It will therefore be appreciated that the filtering response of the filter can be controlled by controlling the coupling constants and resonance frequencies of the couplings  131 ,  132  and the resonator body  110 . 
         [0166]    In one example, a desired frequency response is obtained by configuring the resonator body  110  so that f 1 &lt;f 2 &lt;h and the couplings  131 ,  132  so that f 1 &lt;f A , f B &lt;f 3 . This places the first resonator f 1  close to the desired sharp transition at the band edge, as shown for example at  353 ,  363  in  FIG. 3B . The coupling constants k A1 , k A3 , k 1B , k 2B , k 3B , are selected to be positive, whilst the constant k A2  is negative. If the zeros are to be on the low frequency side of the pass band, as shown for example at  351 ,  352 ,  353  and as will be described in more detail below with respect to the transmit band in  FIG. 9B , the coupling constant k AB  should be negative, while if the zeros are to be on the high frequency side as will be described in more detail below with respect to the receive band in  FIG. 9B , the coupling constant k AB  should be positive. The coupling constants k AB , k A1  generally have similar magnitudes, although this is not essential, for example if a different frequency response is desired. 
         [0167]    The strength of the coupling constants can be adjusted by varying the shape and position of the input and output couplings  131 ,  132 , as will now be described in more detail with reference to  FIGS. 6A to 6C . 
         [0168]    For the purpose of this example, a single coupling  631  is shown coupled to a ground plane  621 . The coupling  631  is of a similar form to the coupling  131  and therefore includes a first path  631 . 1  extending perpendicularly away from the ground plane  621 , a second path  631 . 2  extending in a direction orthogonal to the first path  631 . 1  and terminating in a conductive coupling patch  631 . 3 . In use, the first and second paths  631 . 1 ,  631 . 2  are typically arranged parallel to the axes of the resonator body, as shown by the axes X, Y, with the coordinates of  FIG. 6C  representing the locations of the coupling paths relative to a resonator body shown by the dotted lines  610 , extending from (−1,−1) to (1,1). This is for the purpose of example only, and is not intended to correspond to the positioning of the resonator body in the examples outlined above. To highlight the impact of the configuration of the coupling  631  on the degrees of coupling reference is also made to the distance d shown in  FIG. 6B , which represents the proximity of patch  631 . 3  to the ground plane  621 . 
         [0169]    In this example, the first path  631 . 1  is provided adjacent to the grounded end of the coupling  631  and therefore predominantly generates a magnetic field as it is near a current anti-node. The second path  631 . 2  has a lower current and some voltage and so will generate both magnetic and electric fields. Finally the patch  631 . 3  is provided at an open end of the coupling and therefore predominantly generates an electric field since it is near the voltage anti-node. 
         [0170]    In use, coupling between the coupling  631  and the resonator body can be controlled by varying coupling parameters, such as the lengths and widths of the coupling paths  631 . 1 ,  631 . 2 , the area of the coupling patch  631 . 3 , as well as the distance d between the coupling patch  631 . 3  and the ground plane  621 . In this regard, as the distance d decreases, the electric field is concentrated near the perimeter of the resonator body, rather than up into the bulk of the resonator body, so this decreases the electric coupling to the resonance modes. 
         [0171]    Referring to the field directions of the three cavity modes shown in  FIGS. 2A to 2C , the effect of varying the coupling parameters is as summarised in Table 4 below. It will also be appreciated however that varying the coupling path width and length will affect the impedance of the path and hence the frequency response of the coupling path  631 . 
         [0172]    Accordingly, these effects are general trends which act as a guide during the design process, and in practice multiple changes in coupling frequencies and the degree of coupling occur for each change in coupling structure and resonator body geometry. Consequently, when designing a coupling structure geometry it is typical to perform simulations of the 3D structure to optimise the design. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Mode 
                 Coupling Strength to Quarter Wave Resonator 
               
               
                   
               
             
             
               
                 TE 011 
                 Maximum coupling when the first path 631.1 is long 
               
               
                 (E along X) 
                 and at y = 0. 
               
               
                   
                 Negligible coupling from the second path 631.2. 
               
               
                   
                 Negligible coupling from the patch 631.3 when 
               
               
                   
                 positioned at x = 0, y = 0. 
               
               
                 TE 101 
                 Negligible coupling from the first path 631.1. 
               
               
                 (E along Y) 
                 Maximum coupling when the second path 631.2 is long 
               
               
                   
                 and at x = 0. 
               
               
                   
                 Negligible coupling from the patch 631.3 when 
               
               
                   
                 positioned at x = 0, y = 0. 
               
               
                 TM 110 
                 Maximum coupling when the first path 631.1 is long 
               
               
                 (E along Z) 
                 and at x = −1, y = 0. 
               
               
                   
                 Maximum coupling when the second path 631.2 is long 
               
               
                   
                 and at x = 0, y = +1 or −1. 
               
               
                   
                 Maximum coupling when the patch 631.3 is large and 
               
               
                   
                 at x = 0, y = 0. 
               
               
                   
                 Decreased coupling when the distance d is small. 
               
               
                   
               
             
          
         
       
     
         [0173]    It will be appreciated from the above that a range of different coupling structure configurations can be used, and examples of these are shown in  FIGS. 7A to 7E . In these examples, reference numerals similar to those used in  FIG. 1D  are used to denote similar features, albeit increased by  600 . 
         [0174]    Thus, in each example, the arrangement includes a resonator body  710  mounted on a substrate  720 , having a ground plane  721 . A coupling structure  730  is provided by a cut-out  733  in the ground plane  721 , with the coupling structure including two couplings  731 ,  732 , representing input and output couplings respectively. In this example, vias  722 ,  723  act as connections to an input and output respectively (not shown in these examples). 
         [0175]    In the example of  FIG. 7A , the input and output couplings  731 ,  732  include a single straight coupling path  731 . 1 ,  732 . 1  extending from the ground plane  721  at an angle relative to the X, Y axes. This generates a magnetic field at the end of the path near the ground plane, with this providing coupling to each of the TE fields. 
         [0176]    In the example of  FIG. 7B , the input and output couplings  731 ,  732  include a single curved coupling path  731 . 1 ,  732 . 1  extending from the ground plane  721 , to a respective coupling patch  731 . 2 ,  732 . 2 . As shown the path extends a distance along each of the X, Y axes, so that magnetic fields generated along the path couple to each of the TE and TM modes, whilst the patch predominantly couples to the TM mode. It will be noted that in this example the patch  731 . 2 ,  732 . 3  has a generally circular shape, highlighting that different shapes of patch can be used. 
         [0177]    In the examples of  FIGS. 7C and 7D , the input and output couplings  731 ,  732  include a single coupling path  731 . 1 ,  732 . 1  extending from the ground plane  721  to a patch  731 . 2 ,  732 . 2 , in a direction parallel to an X-axis. The paths  731 . 1 ,  732 . 1  generate a magnetic field that couples to the TE101 and TM modes, whilst the patch predominantly couples to the TM mode. 
         [0178]    In the example of  FIG. 7D  the grounded ends of the couplings  731 . 1 ,  732 . 1  are close whilst the coupling tips are distant. Consequently, the coupling will be mainly magnetic and so the coupling will be positive, thereby allowing a filter having high frequency zeros to be implemented. In contrast, if the tips of the couplings  731 . 1 ,  732 . 1  are close and the grounded ends distant, as shown in  FIG. 7C , the coupling will be predominantly electric, which will be negative and thereby allow a filter with low frequency zeros to be implemented. 
         [0179]    In the arrangement of  FIG. 7E , this shows a modified version of the coupling structure of  FIG. 1D , in which the cut-out  733  is modified so that the patch  731 . 3 ,  732 . 3  is nearer the ground plane, thereby decreasing coupling to the TM field, as discussed above. 
         [0180]    In some scenarios, a single resonator body cannot provide adequate performance (for example, attenuation of out of band signals). In this instance, filter performance can be improved by providing two or more resonator bodies arranged in series, to thereby implement a higher-performance filter. 
         [0181]    In one example, this can be achieved by providing two resonator bodies in contact with each other, with one or more apertures provided in the silver coatings of the resonator bodies, where the bodies are in contact. This allows the fields in each cube to enter the adjacent cube, so that a resonator body can receive a signal from or provide a signal to another resonator body. When two resonator bodies are connected, this allows each resonator body to include only a single coupling, with a coupling on one resonator body acting as an input and the coupling on the other resonator body acting as an output. Alternatively, the input of a downstream filter can be coupled to the output of an upstream filter using a suitable connection such as a short transmission line. An example of such an arrangement will now be described with reference to  FIGS. 8A to 8E . 
         [0182]    In this example, the filter includes first and second resonator bodies  810 A,  810 B mounted on a common substrate  820 . The substrate  820  is a multi-layer substrate providing external surfaces  821 ,  825  defining a common ground plane, and an internal surface  824 . 
         [0183]    In this example, each resonator body  810 A,  810 B is associated with a respective coupling structure  830 A,  830 B provided by a corresponding cut-out  833 A,  833 B in the ground plane  821 . The coupling structures  830 A,  830 B include respective input and output couplings  831 A,  832 A,  831 B,  832 B, which are similar in form to those described above with respect to  FIG. 1D , and will not therefore be described in any detail. Connections  822 A,  823 A,  822 B,  823 B couple the couplings  831 A,  832 A,  831 B,  832 B to paths on the internal layer  824 . In this regard, an input  841  is coupled via the connection  822 A to the coupling  831 A. A connecting path  843  interconnects the couplings  832 A,  831 B, via connections  823 A,  822 B, with the coupling  823 B being coupled to an output  842 , via connection  823 B. 
         [0184]    It will therefore be appreciated that in this example, signals supplied via the input  841  are filtered by the first and second resonator bodies  810 A,  810 B, before in turn being supplied to the output  842 . 
         [0185]    In this arrangement, the connecting path  843  acts like a resonator, which distorts the response of the filters so that the cascade response cannot be predicted by simply multiplying the responses of the two cascaded filters. Instead, the resonance in the transmission line must be explicitly included in a model of the whole two cube filter. For example, the transmission line could be modelled as a single low Q resonator having frequency f C , as shown in  FIG. 8E . 
         [0186]    A common application for filtering devices is to connect a transmitter and a receiver to a common antenna, and an example of this will now be described with reference to  FIG. 9A . In this example, a transmitter  951  is coupled via a filter  900 A to the antenna  950 , which is further connected via a second filter  900 B to a receiver  952 . 
         [0187]    In use, the arrangement allows transmit power to pass from the transmitter  951  to the antenna with minimal loss and to prevent the power from passing to the receiver. Additionally, the received signal passes from the antenna to the receiver with minimal loss. 
         [0188]    An example of the frequency response of the filter is as shown in  FIG. 9B . In this example, the receive band (solid line) is at lower frequencies, with zeros adjacent the receive band on the high frequency side, whilst the transmit band (dotted line) is on the high frequency side, with zeros on the lower frequency side, to provide a high attenuation region coincident with the receive band. It will be appreciated from this that minimal signal will be passed between bands. It will be appreciated that other arrangements could be used, such as to have a receive pass band at a higher frequency than the transmit pass band. 
         [0189]    The duplexed filter can be modelled in a similar way to the single cube and cascaded filters, with an example model for a duplexer using single resonator body transmit and receive filters being shown in  FIG. 9C . In this example, the transmit and receive filters  900 A,  900 B are coupled to the antenna via respective transmission lines, which in turn provide additional coupling represented by a further resonator having a frequency f C , and coupling constants k C , k CA , k CB , determined by the properties of the transmission lines. 
         [0190]    It will be appreciated that the filters  900 A,  900 B can be implemented in any suitable manner. In one example, each filter  900  includes two resonator bodies provided in series, with the four resonator bodies mounted on a common substrate, as will now be described with reference to  FIGS. 10A to 10C . 
         [0191]    In this example, multiple resonator bodies  1010 A,  1010 B,  1010 C,  1010 D can be provided on a common multi-layer substrate  1020 , thereby providing transmit filter  900 A formed from the resonator bodies  1010 A,  1010 B and a receive filter  900 B formed from the resonator bodies  1010 C,  1010 D. 
         [0192]    As in previous examples, each resonator body  1010 A,  1010 B,  1010 C,  1010 D is associated with a respective coupling structure  1030 A,  1030 B,  1030 C,  1030 D provided by a corresponding cut-out  1033 A,  1033 B,  1033 C,  1033 D in a ground plane  1021 . Each coupling structure  1030 A,  1030 B,  1030 C,  1030 D includes respective input and output couplings  1031 A,  1032 A,  1031 B,  1032 B,  1031 C,  1032 C,  1031 D,  1032 D, which are similar in form to those described above with respect to  FIG. 1D , and will not therefore be described in any detail. However, it will be noted that the coupling structures  1030 A,  1030 B, for the transmitter  951  are different to the coupling structures  1030 C,  1030 D for the receiver  952 , thereby ensuring that different filtering characteristic are provided for the transmit and receive channels, as described for example with respect to  FIG. 9B . 
         [0193]    Connections  1022 A,  1023 A,  1022 B,  1023 B,  1022 C,  1023 C,  1022 D,  1023 D couple the couplings  1031 A,  1032 A,  1031 B,  1032 B,  1031 C,  1032 C,  1031 D,  1032 D, to paths on an internal layer  1024  of the substrate  1020 . In this regard, an input  1041  is coupled via the connection  1022 A to the coupling  1031 A. A connecting path  1043  couples the couplings  1032 A,  1031 B, via connections  1023 A,  1022 B, with the coupling  1023 B being coupled to an output  1042 , and hence the antenna  950 , via a connection  1023 B. Similarly an input  1044  from the antenna  950  is coupled via the connection  1022 C to the input coupling  1031 C. A connecting path  1045  couples the couplings  1032 C,  1031 D, via connections  1023 C,  1022 D, with the coupling  1022 D being coupled to an output  1046 , and hence the receiver  952 , via a connection  1023 D. 
         [0194]    Accordingly, the above described arrangement provides a cascaded duplex filter arrangement. The lengths of the transmission lines can be chosen such that the input of each appears like an open circuit at the centre frequency of the other. To achieve this, the filters are arranged to appear like 50 ohm loads in their pass bands and open or short circuits outside their pass bands. 
         [0195]    It will be appreciated however that alternative arrangements can be employed, such as connecting the antenna to a common coupling, and then coupling this to both the receive and transmit filters. This common coupling performs a similar function to the transmission line junction above. 
         [0196]    Accordingly, the above described filter arrangements use a multimode filter described by a parallel connection, at least within one body. The natural oscillation modes in an isolated body are identical with the global eigenmodes of that body. When the body is incorporated into a filter, a parallel description of the filter is the most useful one, rather than trying to describe it as a cascade of separate resonators. 
         [0197]    The filters can not only be described as a parallel connection, but also designed and implemented as parallel filters from the outset. The coupling structures on the substrate are arranged so as to controllably couple with prescribed strengths to all of the modes in the resonator body, with there being sufficient degrees of freedom in the shapes and arrangement of the coupling structures and in the exact size and shape of the resonator body to provide the coupling strengths to the modes needed to implement the filter design. There is no need to introduce defects into the body shape to couple from mode to mode. All of the coupling is done via the coupling structures, which are typically mounted on a substrate such as a PCB. This allows us to use a very simple body shape without cuts of bevels or probe holes or any other complicated and expensive departures from easily manufactured shapes. 
         [0198]    The above described examples have focused on coupling to up to three modes. It will be appreciated this allows coupling to be to low order resonance modes of the resonator body. However, this is not essential, and additionally or alternatively coupling could be to higher order resonance modes of the resonator body. 
         [0199]    The above examples include coupling structures including conductive coupling paths. It will be appreciated that, in practice, the degree of coupling between such a path (or an element of one) and its associated resonator body will vary as a function of the frequency of the electrical signal that is conveyed by the path (or the element) and that there will be a resonant peak in the degree of coupling at some frequency that is dependent on the shape and dimensions of the path (or the element). If such a path (or element) is arranged to convey an electrical signal at that resonant frequency, then it is reasonable to term the path (or element) a “resonator”. Indeed, the path  431  in  FIG. 4B  is referred to a quarter wave resonator, the resonant frequency being determined by the length of the path  431 . 
         [0200]    Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.