WO2015079227A1

WO2015079227A1 - Ceramic waveguide filter apparatus and method of manufacture and use thereof

Info

Publication number: WO2015079227A1
Application number: PCT/GB2014/053504
Authority: WO
Inventors: Muhammad SANDHU; Ian Hunter; Philip SLEIGH; Richard PARRY
Original assignee: Radio Design Limited
Priority date: 2013-11-28
Filing date: 2014-11-26
Publication date: 2015-06-04
Also published as: GB2539564A; GB201320995D0; GB201608989D0

Abstract

Ceramic waveguide filter apparatus is provided. The apparatus includes a ceramic body portion with two or more resonating means contained therein and/or associated therewith. One or more electrically conductive apertures, channels, holes and/or blind holes are defined in the ceramic body portion at a location between said two or more resonating means.

Description

Ceramic Waveguide Filter Apparatus and Method of Manufacture and Use Thereof

This invention relates to ceramic waveguide filter apparatus and to a method of manufacture and use thereof.

Although the following description refers almost exclusively to ceramic waveguide filter apparatus for use as part of a telecommunications system or network, it will be appreciated by persons skilled in the art that the ceramic waveguide filter apparatus could be used in any suitable system or network as required.

Filters are electronic devices which allow an electromagnetic wave to be transmitted therethrough. They are designed in such a manner so as to allow signals at one or more pre-determined frequencies to pass through the device (passband frequencies) and to substantially prevent signals at frequencies other than the one or more pre-determined frequencies from passing through the device (stop band frequencies) . The frequency selectivity of a filter can be optimised by locating transmission zeros at both or either side of the passband frequency at finite, non-zero frequencies. A transmission zero can be defined as one or more frequencies at which attenuation of a filter is infinite. This improves the performance of the filter to stop interference and prevent blocking. However, conventional filters that provide the required frequency selectivity are typically large and there is a requirement in the industry to optimise both the size of the filter and the frequency selectivity of the filter.

One example of a conventional filter is a hollow air-filled rectangular waveguide filter. An example of a single resonator contained in a conventional hollow air-filled rectangular waveguide is shown in figure 1. A plurality of such resonators would be combined together to form the conventional filter. This t pe of filter resonator supports a fundamental electromagnetic wave or mode transmission (in the direction as shown by arrow 10), TE₁₀ along the "z" axis. The axis "b" is often referred to as the narrow dimension. The axis "a" is often referred to as the broad dimension. Walls 2, 4 are often referred to as the broad walls, walls 6, 8 are often referred to as the narrow side walls and end walls 10 are often referred to as the narrow end walls. The resonator is typically in the form of a metal tube wherein the air contained in the filter housing is acting as the dielectric. By selecting appropriate values for the dimensions a, b and 1, the resonator will resonate in the TE₁₀₁ mode at a specific frequency in the microwave spectrum. A filter using this type of resonator is significantly larger than a conventional TEM (co-axial) Transverse Electromagnetic resonator filter at the frequency that commercial cellular communication systems operate at.

A further example of a conventional filter is a ceramic rectangular waveguide filter. This takes a similar form to the filter shown in figure 1 , but rather than the filter housing containing air, the rectangular filter block is formed from a solid block of high permittivity low-loss ceramic, the exterior of which is coated in a conducting material, such as metal. The ceramic is typically formed by pres sing and firing, and is then coated in a high conductivity adhesive paint. As the relative permittivity of the ceramic material is increased, the guide wavelength will reduce, thus also reducing the physical size o f the filter for a specific resonant frequency, but this is offset by a reduction in resonator Q factor. This is particularly advantageous if an aim is to reduce the space required for the filter apparatus and/or apparatus with which the filter is associated in use. For example, if the permittivity of the ceramic material is chosen such that the ceramic waveguide resonator filter has the same Q factor as a conventional TEM or combline filter, then it will be physically les s than half the volume of the conventional TEM filter. However, it is not possible, or at least it is very difficult, to cros s-couple the resonators within a conventional ceramic waveguide filter. In order to locate transmis sion zeros at finite, non-zero frequencies it is necessary to cros s couple resonators. As such, the frequency selectivity of conventional ceramic waveguide filters is relatively poor.

The network topology for an "nth" order microwave bandpass filter with an all-pole (Chebyshev) response is shown in figure 2. The numbers in the circles represent the resonators, the lines and lettering Kxy (where x-y are integers representing the resonator numbers between which the coupling takes place) represent the coupling between resonators. The couplings between sequentially numbered resonators in a filter are referred to herein as mainline couplings and the couplings between non- sequentially numbered resonators are referred to as cross couplings. This is a so called "ladder network" realisation enabling filters with bandpass characteristics to be constructed with arbitrary bandwidth and centre frequency. It is to be noted that there are no cros s couplings between resonators and therefore no transmission zeros at finite, non-zero frequencies . Typical frequency responses for the filter shown in figure 2 are illustrated in figure 3 for a 5^th degree (i. e. N = 5) filter and a 6^th degree (i. e. N = 6) Chebyshev filter, wherein N = the number o f resonators in the filter. It is to be noted that the frequency response is substantially symmetrical around the centre frequency and the selectivity of the filter increases as N increases . Such all-pole frequency responses are achievable using well documented techniques to construct a conventional ceramic waveguide bandpass filter. The physical realisation of the resulting filter is shown in figures 4a and 4b. Slots are provided on the sides of the filter and form part of the outer perimeter of the filter. An opening of each slot is continuous with the outer narrow wall and the slot is directed inwardly of the filter housing in the direction ^ca'. Thus, the filter housing (or the distance between the two outer side or narrow walls) is narrow at the location of the slots in the ^ca' axis. The slots pass all the way from the top broad wall to the bottom wall with openings in each of said walls respectively. Although the provision of side slots in the ceramic filter housing allows inter-coupling between adjacent resonators (i. e. the sections of the filter housing between the side slots that appear rectangular when viewed in plan view from the top and are labelled 1 -4), with the dimensions of the slots determining the extent of the coupling, this ceramic filter does not allow cros s coupling between resonators within the filter wherein transmission zeros at finite, non-zero frequencies can be produced.

It is pos sible to produce cros s-coupling between resonators within a filter which give transmis sion zeros at finite, non-zero frequencies, but such filters for commercial cellular telecommunication networks are conventionally non-ceramic TEM co-axial bandpass filters. Thus, the resulting filters are large and/or suffer from the problems associated with conventional bandpass filters. For example, bandpas s filters with highly asymmetric frequency responses are often required in telecommunications systems. These have conventionally been realised by introducing cros s coupling acros s "triplets" of 3 resonators, as shown in figure 5 and 6.

Figure 5 shows a filter network for a 6^th degree bandpass filter with two triplets (i. e. the 1 ^{s t} triplet having resonators labelled 1 - 3 and the 2^nd triplet having resonators labelled 4-6) . The triplets allow transmission zeros to be located at finite, non-zero frequencies. The form of the various couplings determine whether the transmission zeros are located above or below the passband. The form of the couplings usually take one of two types and are typically referred to by any of the following terms : positive and negative (in reference to the sign of circuit elements), capacitive and inductive (in reference to the type of circuit element), in phase and out of phase, or electrical and magnetic (in reference to which electromagnetic field is coupled) . As in the earlier illustrations, the numbers in the circles represent the resonators, the lines and lettering Kxy (where x-y are integers representing the resonator numbers between which the coupling takes place) represent the coupling between resonators. The frequency response of the filter network in figure 5 is shown in figure 6. The transmission zero for triplet A is shown by 'A' and the transmission zero for triplet B is shown by 'Β'. The couplings have been chosen such that the transmission zeros are located at finite, non-zero frequenciesare on the high frequency side of the pas sband.

Similarly, a filter network for a 6^th degree bandpass filter with two triplets (the 1 ^{s t} triplet having resonators labelled 1 -3 and the 2^nd triplet having resonators labelled 4-6)is shown in figure 7. The frequency response of the resonator network in figure 7 is shown in figure 8. The couplings have been chosen such that transmission zeros having finite, non-zero frequencies 'A' and 'Β' are produced on the low frequency side of the pas sband.

The ceramic waveguide filter shown in figures 4a and 4b is not able to produce transmission zeros at finite, non-zero frequencies as these are not cross coupled resonators.

It is therefore an aim of the present invention to provide ceramic waveguide filter apparatus that allows cros s coupling between filter resonators to produce transmission zeros that are at finite, non-zero frequencies, thereby optimising the frequency selectivity of the filter.

It is a further aim of the present invention to provide a method of manufacturing and/or using ceramic waveguide filter apparatus that allows cros s coupling between filter resonators to produce transmission zeros that are at finite, non-zero frequencies, thereby optimising the frequency selectivity of the filter.

According to a first aspect of the present invention there is provided ceramic waveguide filter apparatus, said apparatus including a ceramic body portion with two or more resonating means contained therein and/or associated therewith, and wherein one or more electrically conductive apertures, channels, holes and/or blind holes, channels and/or aperture are defined within the ceramic body portion at a location between said two or more resonating means.

Thus, the applicants have found that by providing one or more electrically conductive apertures, channels, holes and/or blind holes between two or more resonating means in ceramic waveguide filter apparatus, this can allow cros s coupling between different, non-adj acent or non-sequentially numbered resonating means within the ceramic filter apparatus, thereby producing one or more transmis sion zeros at finite, non-zero frequencies. The provision of these transmission zeros allows optimisation of the frequency selectivity of the ceramic filter apparatus. Use of a ceramic filter also optimises the size of the filter apparatus and therefore fulfils the requirement for a filter which is optimised in both size and frequency selectivity.

Preferably the ceramic body portion is formed from a continuous or substantially continuous and/or solid or substantially solid block of ceramic material. The one or more electrically conductive apertures, channels, holes and/or blind holes are typically made in the block of solid and/or continuous or substantially solid and/or continuous ceramic material.

Preferably the one or more electrically conductive apertures, channels, holes and/or blind holes, channels and/or apertures are defined entirely within the exterior or external boundaries or edges of the ceramic body portion. Typically one or more open ends of the apertures, channels, holes and/or blind holes, channels and/or apertures are defined in or on one or more surfaces extending between the exterior edges of the ceramic body portion. In one example, the channel, aperture and/or blind hole is elongate, has a length or has a longitudinal axis which is located entirely within the ceramic body portion.

It is to be noted that two or more solid or substantially solid and/or continuous or substantially continuous blocks of ceramic material could be joined or arranged together to produce a filter having the same function and/or performance as a single block ceramic filter if required.

The body portion of ceramic material is designed to operate as a dielectrically loaded waveguide filter where the provision of electrically conductive through apertures, channels, holes and/or blind holes, apertures and/or channels at appropriate positions and/or of appropriate dimensions within the body portion enables the realisation of all the desired resonator (i. e. adj acent resonator or main-line coupling) and inter-resonator (i. e. non- adjacent resonator or cros s coupling) couplings neces sary to realise an nth order waveguide bandpass filter (where n = the number of resonators) with arbitrary located transmission zeros in the complex frequency domain. Preferably two or more resonators or resonating means within the apparatus couple via their electromagnetic fields.

Preferably substantially the entire exterior surface of the ceramic material, or at least the external area of the filter in which the resonating means are provided, is metallised and/or is provided with an electrically conductive material, layer(s) and/or coating(s) thereon.

Preferably the entire surface (i. e. the interior walls defining) or substantially the entire surface of the electrically conductive through apertures, channels, holes and/or blind holes are provided with an electrically conductive material, layer(s) and/or coating(s) and/or are metallised thereon. Thus, the interior walls of the body portion defining the apertures, channels, holes and/or blind holes, channels and/or apertures have one or more metal layers provided thereon in one example. The provision of the electrically conductive through apertures, channels, holes, and/or blind holes, channels and/or apertures allows the same to act in an equivalent manner to metallic rods provided in conventional air filled waveguide filters .

Preferably the electrically conductive material, layer(s) and/or coating(s) used on the exterior of the ceramic body and/or the interior walls of the apertures, channels, holes and/or blind holes, channels and/or apertures is metallic material, one or more metallic layers and/or one or more metallic coatings .

Preferably the ceramic material used in the filter body is a high permittivity ceramic material.

Preferably the high permittivity ceramic material has a permittivity of approximately between 10- 100ε. Typically any number of electrically conductive through apertures, holes, channels and/or blind holes are defined in the ceramic body portion between the resonating means . However, it is to be noted that the greater the number of apertures, holes, channels and/or blind holes defined between two or more resonating means, the weaker the coupling between the resonating means typically is. The electrically conductive through apertures, holes, channels and/or blind holes can be of any size and/or arranged in any suitable location between the resonating means so as to produce the transmission zeros at the desired finite, non-zero frequencies.

Preferably at least one, preferably two, and further preferably three electrically conductive through apertures, holes, channels and/or blind holes are defined between two adjacent resonating means in the ceramic body portion.

In one embodiment the electrically conductive apertures, channels and/or holes are or include through apertures, channels and/or holes in that they pas s from one side of the body portion to an opposite side of the body portion. The openings of the apertures, channels and/or holes are typically defined in the opposite side surfaces or broad walls of the filter apparatus. In this embodiment, if a triplet arrangement of resonating means or resonators is formed where both the mainline couplings and cros s couplings have this through aperture/hole form, then a transmis sion zero is produced at a finite, non-zero frequency on the high frequency side (i. e. the right hand side of a graphic illustration with increasing frequency) of the pass band filter response.

Couplings produced with electrically conductive through channels/apertures/holes typically couple predominantly between the magnetic fields of the resonating means. Preferably the electrically conductive through apertures, channels and/or holes have an opening defined in each side surface of the ceramic body. Further preferably these side surfaes are the broadwalls of the filter or the top and bottom o f the filter apparatus in use.

In one embodiment the one or more electrically conductive apertures, channels and/or holes are or include blind apertures, channels and/or holes in that they do not pas s all the way from one side of the ceramic body to an opposite side of the ceramic body. The blind end of the aperture, channel or hole is typically located within or a spaced distance from the boundary or surfaces of the external walls of the ceramic body. An open end of the blind aperture, channel and/or holes is typically defined in a surface of the body portion, such as for example the broadwall of the filter or the top or bottom of the filter apparatus in use. In this embodiment, if a triplet arrangement of resonating means or resonators is formed where two mainline couplings are formed from electrically conductive through channels, apertures and/or holes and one cros s coupling is formed from an electrically conductive blind hole, channel and/or aperture then a transmis sion zero is produced at a finite, non-zero frequency on the lowside (i. e. the left hand side on a graphical illustration with increasing frequency) of the pass band filter response.

Although triplet arrangements of resonating means or resonators are described herein in that three resonating means or resonators are coupled via mainline or cros s couple couplings, it will be appreciated that any shape or arrangement of resonating means can be provided to produce a desired frequency response including mainline and/or cros s couplings, such as a quad arrangement and/ or the like. Couplings produced with electrically conductive blind holes typically couple predominantly between the electric fields of the resonating means.

Appropriate selection of coupling types (i. e. main line or cross coupling), typically as a result of the selection and arrangement of the through apertures, channels and/or holes and/or the blind end apertures, channels and/or holes, can provide transmission zeros on either side of the passband filter response allowing the selectivity of the filter to be optimised as required.

In one embodiment only a single electrically conductive blind hole, channel or aperture is provided at any particular location or in a particular plane or axis .

In one embodiment each electrically conductive blind hole, channel or aperture is provided as a pair of electrically conductive blind holes, channels or apertures in that one electrically conductive blind hole/aperture/channel is provided on one side of the ceramic body and one electrically conductive blind hole/aperture/channel is provided on an opposite side o f the ceramic body, and the position of the pair of blind holes / channels/apertures is such so as to generate a capacitive response therebetween.

Preferably the electrically conductive blind holes, channels or apertures of each pair are aligned or substantially aligned with each other, co-axially arranged or are opposite or substantially opposite to each other. However, the longitudinal axes of the electrically conductive blind holes, channels or apertures within a pair could be offset from each other and still provide the neces sary coupling response. Preferably an or each opening of said one or more electrically conductive apertures, channels, holes and/or blind holes are defined in a broad wall of the filter housing/ceramic body portion rather than a narrow wall. Thus, the longitudinal axis of the apertures, channels and/or holes is substantially parallel to a ^cb' axis of the filter housing/ceramic body portion.

Preferably an or each opening of said one or more apertures, through apertures, holes, channels and/or blind holes are a spaced distance from the outer edges and/or perimeter of the ceramic body (i. e. are provided within the walls defined by the outer edges or perimeter of the ceramic body) .

Preferably the filter apparatus includes input coupling means and output coupling means . The input coupling means is /are typically coupled to the first resonating means of the filter and allows for the input of an electromagnetic wave therethrough. The output coupling means is/are typically coupled to the last resonating means of the filter and allows for the output of an electromagnetic wave therefrom.

In one embodiment, one or more electrically conductive blind holes, channels or apertures in the filter can form a structure that resonates at a particular frequency. Below this particular frequency the electrically conductive blind hole couples such that if it forms a cros s coupling in a triplet when the mainline couplings are provided by electrically conductive through holes a transmission on the lowside of the passband can be realised. Above this particular frequency, there is modification in the electromagnetic fields such that if it forms a cros s coupling in a triplet when the mainline couplings are provided by electrically conductive through holes a transmission zero on the highside of the passband can be realised. This provides an advantage as a single triplet can provide two transmission zeros and further improves the selectivity of the filter. The location of the two transmission zeros can be independently controlled by appropriate selection of the blind hole dimensions. Preferably the electrically conductive apertures, holes, blind holes and/or channels are substantially linear in form but could be angled or non-linear if required.

Preferably each resonating means is a portion of the ceramic body of the filter which is capable of undergoing resonance at a particular electromagnetic frequency or frequencies. Preferably the resonating means are cuboid or substantially cuboid in shape.

Preferably the sequentially numbered resonating means are arranged in a substantially planar arrangement in the a-z plane (i. e. they are not stacked vertically but are arranged substantially horizontally in a side by side arrangement) . For example, the narrow side walls or the narrow end walls of adj acent resonating means are provided in a side by side or abutting relationship.

According to a second aspect of the present invention there is provided a method of using a ceramic waveguide filter apparatus.

According to a third aspect of the present invention there is provided a method of manufacturing ceramic waveguide filter apparatus, said apparatus including a ceramic body portion with two or more resonating means contained therein and/or associated therewith, and wherein said method includes the step of forming one or more electrically conductive apertures, channels, holes and/or blind holes within the ceramic body portion at a location between said two or more resonating means. The electrically conductive through apertures, channels, holes and/or blind apertures, channels or holes can be formed by machining or by pressing using a pres sing tool.

Preferably the resonating means of the apparatus are arranged to resonate in such a way so as to produce the passband frequency or range of frequencies of the filter apparatus.

According to further independent aspects of the present invention there is provided a telecommunication system including a ceramic waveguide filter apparatus and a method o f use thereof.

It is to be noted that the electrically conductive through apertures, channels, holes and/or blind holes of the present invention are distinguished from the side slots o f a conventional ceramic waveguide filter due to the location of the same. In the present invention, the electrically conductive apertures, channels, holes and/or blind holes are discreet and distinct openings within the boundaries of an external perimeter of the filter housing or ceramic body. Thus the openings of the apertures, channels, holes and/or blind holes are a spaced distance from the external perimeter or edges and do not form part of the same. In the prior art, the slots form part of the external perimeter or edges of the filter housing. In the present invention, the diameter or longest planar axis of the opening of the through apertures or blind end apertures is substantially perpendicular to the longitudinal axis of the apertures. In the prior art, the diameter or longest planar axis of the opening of the slot is substantially parallel to the longitudinal axis of the slot.

It is to be further noted that the couplings produced by side slots of a conventional ceramic waveguide filter are of all the same type (i. e. all magnetic couplings), whereas the present invention allows for both types of couplings to be produced (i. e. magnetic and electric couplings) . This allows the physical realisation of filter networks with transmission zeros provided on the low side of the passband or arbitrarily located in the complex frequency domain.

It is to be further noted still that the side slots that provide the couplings in a conventional ceramic waveguide filter do not allow the resonators to be physically arranged in a manner that would allow for cros s couplings to be provided. The present invention provides flexibility in the physical arrangement of the resonators, such that cross couplings can be provided.

Embodiments of the present invention will now be described with reference to the following figures, wherein:

Figure 1 (PRIOR ART) shows a resonator arrangement of a conventional hollow air-filled rectangular waveguide filter;

Figure 2 (PRIOR ART) shows the network topology for an "nth" order microwave bandpass filter with an all-pole (Chebyshev) response;

Figure 3 (PRIOR ART) shows the frequency response for an all- pole bandpas s filter;

Figures 4a and 4b (PRIOR ART) show a top plan view and a side view of the physical realisation of a ceramic waveguide all- pole bandpass filter shown in figure 3 respectively;

Figure 5 (PRIOR ART) shows the network topolo

degree filter with two triplets; Figure 6 (PRIOR ART) shows the frequency response of the filter resonator network shown in figure 5;

Figure 7 (PRIOR ART) shows the network topology of a 6^th degree filter with two triplets;

Figure 8 (PRIOR ART) shows the frequency response of the filter resonator network shown in figure 7;

Figures 9a and 9b show a top plan view and a cros s sectional side view of the physical realisation of a ceramic waveguide filter according to an embodiment of the present invention respectively;

Figure 10 shows an equivalent circuit for the filter shown in figures 9a-9b;

Figure 1 1 shows the network topology for the filter in figures 9a- 10 comprising a pair of resonators with coupling provided by a set of electrically conductive through apertures therebetween;

Figure 12 shows a top plan view of the physical realisation o f an 8^th degree ceramic waveguide filter according to a second embodiment of the present invention with metallised through apertures provided between each of the resonators;

Figure 13 shows the network topology for the filter in figure 12 comprising two triplets;

Figure 14 shows the frequency response of the filter resonator network shown in figure 13;

Figure 15 shows a top plan view of a ceramic waveguide filter according to a third embodiment of the present invention with metallised blind apertures provided between the pair of resonators;

Figure 1 6 shows a side cros s sectional view of the filter in figure 15;

Figure 17 shows an equivalent circuit for the filter shown in figures 15- 16;

Figure 18 shows the network topology for the filter in figures 15- 17 comprising a pair of resonators;

Figure 19 shows a top plan view of a ceramic waveguide filter according to a fourth embodiment of the present invention with a combination of metallised through apertures and blind end apertures provided between resonators;

Figure 20 shows a side cros s sectional view of the filter in figure 19;

Figure 21 shows the network topology for the filter in figures 19-20;

Figure 22 shows the frequency response of the filter resonator network shown in figure 21 ;

Figure 23 shows a top plan view of a 7^th degree ceramic waveguide filter according to a fifth embodiment of the present invention with a combination of metallised through apertures and blind apertures provided between resonators;

Figure 24 is a side cross sectional view of the filter in figure Figure 25 shows the network topology for the filter in figures 23-24;

Figure 26 shows the frequency response of the filter resonator network shown in figure 25;

Figure 27 shows a top plan view of a filter according to a further embodiment of the present invention; and

Figure 28 shows a frequency response for the filter in figure 27.

Referring to figures 9a-28, there is illustrated different embodiments of the present invention in which the provision of metallised through apertures and/or blind end apertures in a solid ceramic waveguide filter can realise the mainline coupling and/or cros s coupling required to produce transmission zeros on the low side and the high side of the passband frequency response. The dimensions and locations of the metallised through apertures and/or blind end apertures allow the desired couplings necessary to realise an nth order waveguide bandpass filter (where n = the number of resonators) with transmis sion zeros located at arbitrary finite, non-zero frequencies in the frequency domain. Use of a ceramic waveguide filter allows optimisation of the filter size and use of the metallised through apertures and/or blind end apertures allow optimisation of the frequency selectivity of the filter.

Referring firstly to figures 9a- l l , there is illustrated a solid ceramic single block filter 100. In this example, the filter is rectangular in shape, with a height or narrow axis ^cb' and a width or broad axis ^ca'. The top surface 102 and the base surface 104 are defined as the broadwalls which are perpendicular or substantially perpendicular to the electrical field passing through the filter. The end walls 106, 108 are defined as the narrow end walls which are parallel or substantially parallel to the electrical field passing through the filter. The length of the filter and/or resonating means is defined by T and is provided along the Z axis of the filter. The longitudinal axis of the filter is defined parallel to the length of the filter.

Resonating means in the form of resonating portions or resonators are defined by the numbers Ί ' and '2' in circles on the filter. Each resonator has the ability to undergo electromagnetic resonance at a particular frequency or frequency range and is typically defined by the dimensions of a cuboid.

Input coupling means 1 10 are provided and are coupled to the first resonator Ί '. Output coupling means 1 12 are provided and are coupled to the second resonator '2'. The input coupling means 1 10 and the output coupling means 1 12 allow an electromagnetic wave to enter and leave the filter 100 respectively. Input and output couplings could be in the form o f an electrically conductive blind hole that is electrically isolated from the conductive coating on the outer surface of the ceramic.

The entire outer surface of the filter 100 is substantially covered in a metallised outer layer.

In accordance with the present invention, a plurality o f metallised through apertures or channels 1 14 are provided between resonators Ί ' and '2'. The channels 1 14 are defined as "through" channels in that they pass all the way from the top broad surface 102 to the bottom broad surface 104 and have openings in said surfaces respectively. The openings of the through channels are a spaced distance from the external perimeter or edges of the filter body. The metallised through channels 1 14 interact with the electromagnetic field between the pair of resonators to allow coupling between the resonators, as shown in figure 1 1. In particular, the metallised through channels 1 14 allow the resonators, as represented by shunt inductor 1 1 6 in the circuit representation in figure 10. The applicants have found that the metallised through channels behave in a similar manner to the provision of metallic posts in a conventional air filled waveguide.

In a similar manner, metallised through apertures can provide cross couplings within triplets of a ceramic waveguide filter 1 18, as shown in figures 12- 14. In filter 1 1 8, there are 8 resonators as defined by the numbers 1 -8 in the circles. Metallised through apertures or channels 120 are provided between sequentially numbered resonators to provide mainline couplings. The channels 120 pass all the way through the filter between the top surface 122 and the bottom surface (not shown) . The equivalent network topology is shown in figure 13, with mainline couplings shown between the resonators 1 -8. This is an 8^th degree filter (in that there are 8 resonators) with two triplets each with cross couplings (resonators 2-4 and resonators 5-7 respectively) providing, by metallised through apertures, two transmis sion zeros (transmission zero 1 and transmission zero 2) on the high side of the passband 124, as shown in the frequency response o f the filter in figure 14.

Figures 15-1 8 show a ceramic filter arrangement 126 according to a further embodiment of the present invention for realising a coupling between a pair of resonators (resonators 1 and 2) with blind and metallised apertures. The filter 126 takes a similar form to the filter in figures 9a- l l and the same reference numerals are used to define the same features. However, in this embodiment, rather than metallised through apertures or channels 1 14 being provided between the two resonators, two blind end metallised apertures or channels 128 are provided in the top and bottom surfaces 102, 104 respectively. In the example the metallised blind end channels 128 are aligned or substantially aligned with each other and have openings in opposite broadwall surfaces. Thus, as with the metallised through apertures in the previous embodiment, the openings of the metallised blind end channels are a spaced distance from the exterior perimeter or edges of the ceramic body.

The metallised blind end channels 128 interact with the electromagnetic field between the pair of resonators to allow a coupling between the resonators, as shown in figure 18. In particular, the metallised blind end channels 128 allow the resonators to couple predominantly through the electric field, as represented by capacitor 130 in the circuit representation in figure 1 7.

The metallised blind end channels 128 can be applied in conjunction with electrically conductive through holes 1 14 to produce a filter with an arbitrary number of triplets that produce transmission zeros on the low side of the passband, as shown by filter 131 in figures 1 9-22. The filter 131 is a 5^th degree filter comprising 5 resonators (labelled 1 -5 in circles) . In this embodiment, two metallised blind end channels 128 are provided between resonators 2-4 to provide a coupling, as shown by figure 21. The metallised through apertures 1 14 are provided between each of resonators 1 -2, 2-3, 3-4 and 4-5 to provide couplings between these resonators. This arrangement of couplings produces a transmis sion zero on the low side of the passband 124.

It will be appreciated that a filter can be produced with any number and arrangement of couplings provided by metallised through apertures and blind end metallised apertures to provide a filter with a required frequency response. A further example of a filter 132 according to the present invention is shown in figures 23-26. The filter 132 is a 7^th degree filter comprising 7 resonators (labelled 1 -7 in the circles) . An arrangement o f metallised through channels 1 14 and blind end channels 128 produce a triplet 'A' and a triplet 'Β'. In particular, metallised through channels 1 14 are provided between each of resonators 1 -2, 2-3, 3-4, 4-5, 5-6, 5-7 and 6-7. Two metallised blind end channels 128 are provided between resonators 2-4. This arrangement results in the production of two transmis sion zeros; transmission zero 'A' produced to the low side of the pass band 124, and transmission zero 'Β' produced on the high side of the pass band 124.

It will be appreciated that many different combinations of couplings provided by metallised through apertures and blind apertures can be provided and the illustrations herein are examples only. In addition, although the illustrations have only shown two or three metallised through apertures or blind end apertures between resonators for the purposes of clarity, any number of metallised through apertures or blind end apertures could be provided as required. The dimensions and locations of the metallised through apertures and blind end apertures can also vary as required.

Referring back to figures 1 5- 16, the applicant has also found that at a certain frequency, the electromagnetic field within the ceramic body portion, as a result of the metallised blind holes 128, can self resonate, the frequency this phenomena occurs at is referred to as the resonant frequency. The resonant frequency is typically at least partially determined by the location of the blind hole, the dimensions of the blind hole and/or the like. Below the resonant frequency of the metallised blind holes they couple as previously described. Above the resonant frequency there is a modification in the electromagnetic field and they couple in such a way that if they provide the cros s coupling within a triplet with the mainline couplings being provided by metallised through holes the resulting transmission zero is on the high side of the pas sband. The frequency response for the filter arrangement 134 shown in figure 27 is illustrated in figure 28. Filter 134 has three resonators 1 -3 forming a triplet. There is one metallised blind hole 128 between resonators 1 -3 and three metallised through holes 1 14 between resonators 1 -2 and 2-3. At a frequency below the resonant frequency of the electrically conductive blind hole 128, transmis sion zero 138 is formed on the low side of the passband 124. At a frequency above the resonant frequency of the electrically conductive blind hole 128, transmission zero 136 is formed on the high side of the passband 124.

Claims

Claims:

1. Ceramic waveguide filter apparatus, said apparatus including a ceramic body portion with two or more resonating means contained therein and/or as sociated therewith, and wherein one or more electrically conductive apertures, channels, holes and/or blind holes, channels and/or apertures are defined within the ceramic body portion at a location between said two or more resonating means.

2. Apparatus according to claim 1 wherein the ceramic body portion is formed from at least one substantially continuous and/or solid block of ceramic material.

3. Apparatus according to claim 1 wherein the body portion is arranged to operate as a dielectrically loaded waveguide filter, wherein the provision of one or more electrically conductive apertures, channels, holes and/or blind holes, channels and/or apertures at appropriate positions and at appropriate dimensions within the body portion enables the realisation of resonator and inter-resonator couplings to realise an nth order waveguide bandpas s filter with arbitrary located transmission zeroes in the complex frequency domain.

4. Apparatus according to claim 1 wherein the entire exterior surface or substantially the entire exterior surface of the ceramic material, or at least the external area of the filter in which the resonating means are provided, is metallised and/or is provided with an electrically conductive material, layer(s) and/or coating(s) thereon.

5. Apparatus according to claim 1 wherein the entire interior surface or substantially the entire interior surface of the body portion defining the one or more electrically conductive apertures, channels, holes and/or blind holes, channels and/or apertures are provided with an electrically conductive material, layer(s) and/or coating(s) thereon and/or are metallised.

6. Apparatus according to claim 1 wherein the ceramic material used in the filter body has a high permittivity of between 10-100ε.

7. Apparatus according to claim 1 wherein one, two or three electrically conductive apertures, holes, channels and/or blind holes, channels and/or apertures are defined between two adjacent resonating means within the ceramic body portion.

8. Apparatus according to claim 1 wherein the electrically conductive apertures, channels and/or holes are or include through apertures, channels and/or holes in that they pas s from one side of the body portion to an opposite side of the body portion.

9. Apparatus according to claim 8 wherein a triplet arrangement of resonating means is provided, where both mainline couplings and cros s couplings have the electrically conductive through apertures, channels and/or holes, a transmis sion zero is produced at a finite, non-zero frequency on the high frequency side of the pass band filter response.

10. Apparatus according to claim 1 wherein the electrically conductive apertures, channels and/or holes are or include blind apertures, channels and/or holes in that they do not pas s all the way from one side of the ceramic body portion to an opposite side of the ceramic body portion.

1 1. Apparatus according to claim 8 and 10 wherein a triplet arrangement of resonating means is provided, where two mainline couplings are provided from electrically conductive through channels, apertures and/or holes and one cros s coupling is provided from an electrically conductive blind channel, aperture and/or hole, a transmission zero is produced at a finite, non-zero frequency on the low side of the pass band filter response.

12. Apparatus according to claim 8 wherein the couplings produced with the electrically conductive through channels, apertures and/or holes are arranged so as to couple predominantly between the magnetic fields o f the resonating means.

13. Apparatus according to claim 10 wherein the couplings produced with the electrically conductive blind channels, apertures and/or holes are arranged so as to couple predominantly between the electric fields of the resonating means.

14. Apparatus according to claim 10 wherein only a single electrically conductive blind hole, channel or aperture is provided between two resonating means .

15. Apparatus according to claim 10 wherein each electrically conductive blind hole, channel or aperture is provided as a pair of electrically conductive blind holes, channels or apertures in that one blind hole/aperture/channel is provided on one side of the ceramic body portion and one blind hole/aperture/channel is provided on an opposite side of the ceramic body portion, and the position of the pair is such so as to generate a capacitive response therebetween.

16. Apparatus according to claim 1 5 wherein the electrically conductive blind holes, channels or apertures of each pair are aligned or substantially aligned with each other, co-axially arranged or are opposite or substantially opposite to each other.

17. Apparatus according to claim 1 wherein an or each opening of said one or more electrically conductive apertures, channels, holes and/or blind apertures, channels, holes are defined in a broad wall of the filter body portion rather than a narrow wall of the filter body portion, the longitudinal axis of the apertures, channels and/or holes being parallel or substantially parallel to the cb' axis of the filter body portion.

18. Apparatus according to claim 1 wherein the apparatus includes input coupling means and output coupling means.

19. Apparatus according to claim 18 wherein the input coupling means are coupled to the first resonating means of the filter apparatus and the output coupling means are coupled to the last resonating means of the filter apparatus.

20. Apparatus according to claim 1 wherein the resonating means are a portion of the ceramic body of the filter capable of undergoing resonance at a particular electromagnetic frequency or frequencies.

21. Apparatus according to claim 1 wherein the resonating means are sequentially numbered resonating means and are arranged in a substantially planar arrangement in the ^cb' axis .

22. Apparatus according to claim 1 wherein the one or more electrically conductive blind holes are formed such that the electromagnetic field generated in the ceramic body portion is capable of self resonating at a predetermined frequency or frequencies, such that at least two transmission zeros are produced; at least one below and at least one above the pas sband frequency.

23. A method of manufacturing ceramic waveguide filter apparatus, said apparatus including a ceramic body portion with two or more resonating means contained therein and/or associated therewith, and wherein said method includes the step of forming one or more electrically conductive apertures, channels, holes and/or blind holes within the ceramic body portion at a location between said two or more resonating means .

24. A telecommunication system including ceramic waveguide filter apparatus according to claim 1 .