Tunable filter and / or method of use thereof

Abstract

A tuneable filter 102 comprises a housing 6 with two or more cavities 4 defined therein. A primary resonating means 8 is located in each of said cavities 4 and each primary resonating means 4 is capable of resonating at a first frequency in use. A further secondary resonating means 10 is capable of resonating at a second frequency in use, where the second frequency is different to the first frequency of the primary resonating means 8. Each primary resonating means 8 has a secondary resonating means 10 coupled therewith to form a primary-secondary resonator pair. Adjustment means 112 are provided for adjusting the coupling within each primary-secondary resonator pair and / or the resonating frequency of the secondary resonating means 10. The adjustment means 112 are arranged such that adjustment of the adjustment means 112 can selectively move the coupling within each primary-secondary resonator pair of said tuneable filter 102 between coupling enhancement and coupling depletion.

Description

This invention relates to a tunable filter or a radio frequency (RF) tunable filter and / or to a method of use thereof. In addition, the invention relates to communication apparatus incorporating a tunable filter and / or to a method of use of said communication apparatus. The present invention also relates to tunable coupling means for use within a tunable filter and / or to a method of using tunable coupling means. A radio frequency (RF) filter allows one or more radio frequency (RF) signals that fall within a pre determined frequency range to pass through the filter while supressing unwanted signals. They are often used in telecommunication applications. A tunable RF filter allows the pre-determined frequency range to be adjusted at the site of manufacture or prior to deployment in the field. An example of an RF tunable filter is an RF tunable cavity filter. The cavity filter typically comprises an electrically conductive housing defining a plurality of resonant cavities with a resonator located in each cavity. In order to obtain a required RF response from the filter, it is necessary to provide a pre-determined electromagnetic coupling strength between the resonators. Each resonator has a natural resonating frequency and therefore each resonator typically has to be tuned manually due to production build and mechanical part tolerances to achieve a pre-determined frequency for all the resonators. This tuning is typically performed using an adjustment screw located in a Ed of the filter housing above each cavity and typically takes place at the site or manufacture. Figures la and lb show an example of a prior art tunable RF cavity filter arrangement 2 with the tuning screw removed for clarity purposes. Each resonator cavity 4 (only one of which is shown for clarity purposes) defined in the conductive filter housing 6 is provided with a fixed primary resonator 8. The primary resonator is designed to resonate at a pre-determnied frequency. A Ed 18 is provided over the cavity 4. Resonator 8 is attached to a base 14 of the housing 6 and a plurality of cavity resonators of this form provide a comb-line configuration. There is an electromagnetic coupling between two resonators in adjacent cavities due to their close proximity to each other and the interaction between the two adjacent resonators forms a primary-secondary pair. This coupling changes the resonant frequency of the primary (and secondary) resonators and can be arranged to either increase or decrease the primary resonator’s frequency. The amount of coupling between the primary and secondary resonators is referred to as the ‘nominal COUpling ITnom. In addition to tunable filters that are tuned at the point of manufacture via tuning means, it is known to provide tunable filters that can also undergo adjustment of the centre frequency and / or bandwidth of the filter once the filter has been deployed in the field (i.e. post manufacture or post field deployment). An example of a known RF tunable cavity filter arrangement which allows post field deployment adjustment of the centre frequency and / or bandwidth of the filter is shown in figures 2a and 2b. The tuning screws are not shown for clarity purposes. In this prior art RF filter arrangement 2, a secondary resonator 10 is provided in the same cavity 4 as the primary resonator 8 to form the primary-secondary resonator pair and both resonators 8,10 are attached to the base 14 of the cavity. The primary resonator 8 is larger in size than the secondary resonator 10 and is designed to resonate at a different frequency to the secondary resonator. A movably mounted adjustment element or coupling block 12 is provided in the resonator cavity 4 adjacent the base 14 (or low down on the Z-axis) of the cavity for adjusting the coupling within each primary and secondary resonator pair and or the resonator frequency of the secondary resonator 10 post-deployment of the filter. If the coupling block 12 is made of metal and is located in the illustrated position, coupling block 12 mainly interacts with the H-field of the resonators, it does not interact strongly with the E-field and enhances the coupling between the primary and secondary resonators 8, 10. Conversely, if the coupling block 12 is made from a dielectric material, such as plastic or ceramic, it interacts mainly with the E-field and depletes the coupling. The coupling block 12 is arranged to be moved along an x-axis of the filter between the primary and secondary resonators 8, 10, as shown by arrow 20. As the coupling block is moved closer to the primary and secondary resonators, depending on what the coupling block is made from, the effect is to either enhance or deplete the resonator pair coupling and hence adjust the resonator frequencies. Figure 3 is a graph showing the coupling level versus the position of a metal coupling block along the X-axis for the filter arrangement shown tn figures 2a and 2b.When the coupling block 12 is located within the cavity as far away as possible from the resonators 8, 10, the coupling level is close to Knom. As the coupling block 12 is moved towards the resonators 8, 10, die coupling level increases. A maximum coupling level towards Kmax is achieved (maximum enhancement) when the coupling block 12 is directly in between the primary and secondary resonators 8, 10. The coupling level then falls back towards Knom as the coupling block 12 moves away from the primary and secondary resonators 8, 10 to the other side of the cavity 4 from where it started. If the coupling block 12 is made from dielectric material rather than metal, the graph plot is inverted. Thus, for a dielectric material coupling block, the graph plot shows coupling depletion. Figures 4a and 4b illustrate a further example of a known RF tunable filter arrangement 2 (the tuning screws are not shown for clarity purposes) that allows post field deployment adjustment of the centre frequency and / or bandwidth of the filter. In this arrangement the adjustable coupling block 12 is provided a spaced distance above the base 14 of the filter cavity towards a top end of the primary and secondary resonators (i.e. high up the Z-axis). In this region of the cavity, the coupling block 12 interacts mainly? with the E-field and does not interact strongly with the H-field. A coupling block made from either metal or dielectric material will deplete the coupling between the primary and secondary resonators as it moves along the X-axis, as shown by arrow 22. Figure 5 is a graph showing the coupling level versus the position of a metal or dielectric material coupling block along the X-axis for the filter arrangement shown tn figures 4a and 4b. When the coupling block is as far away as possible from the resonators 8, 10, the coupling level is close to KnOm ■ As the coupling block 12 is moved toward the resonators 8, 10, the coupling level decreases towards Kmm (maximum depletion). The coupling levels then rises again towards KnOm as the coupling block 12 moves away from the primary and secondary resonators 8, 10 to the other side of the cavity 4 from where it started. The size of the adjustment element or coupling block 12 can also be varied to adjust the maximum coupling depletion and enhancement that can be achieved in the filter arrangement. For example, when a relatively tall metal coupling block is positioned low on the Z-axis (similar to the filter arrangement shown tn figures 2a and 2b), greater coupling enhancement (maximum enhancement or Kmax) between the primary and secondary resonators can be achieved. When a relatively short metal coupling block is positioned high on the Z-axis (similar to the filter arrangement shown in figures 4a and 4b), a greater coupling depletion (maximum depletion or Kmm) between the primary and secondary resonators can be achieved. It is to be noted that the optimum height of the coupling block for maximum depletion is less than the optimum height for maximum enhancement. EP2203953 discloses a number of the prior art tunable filter arrangements described above. In addition, it discloses embodiments where an adjustable coupling block can be pivotably mounted within the resonator cavity to achieve adjustment of the coupling between the primary and secondary resonators. The commonality with all the prior art tunable filter arrangements described above which allow the centre frequency and / or bandwidth of the filter to be adjusted post field deployment is that the adjustable coupling block within the resonator cavity is arranged to be moved along or about an X-axis within the resonator cavity in use but is fixed in the Z-axis within each working embodiment. Furthermore, the coupling level between the primary and secondary resonator pairs is limited to being adjusted between KnOm and Kmax (maximum enhancement) or between KnOm and Kmm (maximum depletion). Although these types of prior art RF tunable filter arrangements allow the centre operating frequency of the filter to be moved to a different frequency in use and / or the bandwidth (passband or band stop band frequency) of the filter incorporating two or more of these resonator cavities to be changed, the tunable operating frequency or bandwidth over which change can take place is still relatively limited. For example, a typical prior art tunable filter arrangement in one example is able to operate at 2.1GHz, with a tuning range of approximately 40 MHz. Hence, the applications in which this type of filter can be used are relatively limited. It is therefore an aim of the present invention to provide a tunable filter or RF tunable filter that overcomes the abovementioned problems. It is a further aim of the present invention to provide a method of using a tunable filter or RF tunable filter that overcomes the abovementioned problems. It is a yet further aim of the present invention to provide a communication apparatus incorporating a tunable filter or RF tunable filter and / or a method of using communication apparatus incorporating a tunable filter or RF tunable filter. It is a further aim of the present invention to provide coupling means for use in a tunable filter and / or communication apparatus. It is a further aim of the present invention to provide a method of using coupling means for use in a tunable filter and / or communication apparatus. According to a first aspect of the present invention there is provided a tunable filter, said tunable filter including a housing with two or more cavities defined therein, primary resonating means located in each of said cavities and each primary resonating means capable of resonating at a first frequency in use, said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency in use, the second frequency of the secondary resonating means being different to the first frequency of the primary resonating means, said secondary resonating means arranged such that each primary resonating means has a secondary resonating means coupled therewith to form a primary-secondary resonator pair, and adjustment means are provided for adjusting the coupling within each primary-secondary resonator pair and / or the resonating frequency of the secondary resonating means, characterised in that the adjustment means are arranged such that adjustment of the adjustment means can selectively move the coupling within each primary-secondary resonator pair of said tunable filter between coupling enhancement and coupling depletion. Thus, the RF tunable filter of the present invention provides adjustment means that are arranged to selectively move the coupling within each primary-secondary resonator pair between both KnOm and a coupling value greater than KnOm (coupling enhancement) and between Knom and a coupling value below KnOm (coupling depletion). This is in contrast to the prior art tunable filter arrangements where adjustment of coupling adjustment means within a particular tunable filter only allows coupling enhancement or coupling depletion of each primary-secondary resonator pair but not the option to select one of both coupling depletion and coupling enhancement. Preferably the coupling position selected between coupling enhancement and coupling depletion is arbitrary and is typically user selectable. The present invention has the advantage that a five to ten fold increase in the tuning range of the tunable filter is possible, thereby providing a much greater range of industrial applications, particularly for example in the defence communications and national security intelligence gathering sectors. Preferably reference to a tunable filter herein is a RF filter that may or may not already have been tuned using conventional tuning means at the point of manufacture. Preferably reference to adjustment of the adjustment means of the tunable RF filter refers to adjustment of the RF filter post field deployment, post manufacture and / or which is independent of and / or separate to tuning using conventional tuning means at the point of manufacture or pre-deployment in the field. In one embodiment the primary-secondary resonator pair can include or form any or any combination of the following: a) One or more tunable primary-secondary resonator pairs — adjustment of which enables control of the passband or stop band frequency of the tunable filter in use; b) One or more tunable transformer pairs (i.e. one or more of the primary? and second resonator pairs are provided at one or both ends of the filter next to the connectors and resonate at a frequency outside the bandwidth of other resonators within the filter) - adjustment of which enables control of the bandwidth of the tunable filter in use; c) One or more tunable mainline couplings (i.e. two or more primary resonators adjacent each other) — adjustment of which enables control of the bandwidth of the tunable filter in use; d) One or more tunable cross couplings (ie. two or more primary resonators that are non-adjacent to each other) - adjustment of which enables control of the frequency and / or number of transmission zeros. In one embodiment adjustment of the adjustment means can selectively move the coupling within each primary-secondary resonator pair between KnOm and Kmax or a coupling value therebetween, and between KnOm and Kmin or a coupling value therebetween. Thus, it is possible for coupling depletion or coupling enhancement to be selected within a filter arrangement using the same adjustment means. In one embodiment the ability to selectively move the coupling within each primary -secondary resonator pair between coupling enhancement and coupling depletion is achieved by one or more of the following mechanisms: i) Two or more adjustment means can be provided or associated with each primary-secondary resonator pair; one adjustment means for providing coupling enhancement and one adjustment means for providing coupling depletion. The two or more adjustment means can be joined together to form a single adjustment element or the two or more adjustment means could be provided separately and / or a spaced distance apart from each other; ii) The shape and / or dimensions of the adjustment means is such so as to selectively allow coupling enhancement and coupling depletion; and / or iii) Movement of the coupling along or about one of three possible cartesian axes (i.e. X-axis, Y-axis and / or Z-axis), along or about an axis between the cartesian axes, along a non-linear path or along a linear or substantially linear path is such so as to selectively allow coupling enhancement and coupling depletion. Preferably the mechanisms i), ii) and iii) could be used independently and separately of each other within a tunable filter or they could be used in one or more combinations with each other within a tunable filter in order to provide a required filter response or frequency. In one embodiment two or more adjustment means are provided or associated with each primary-secondary resonator pair. Preferably one of the adjustment means is made of, consists of or includes dielectric material, and preferably one of the adjustment means is made of, consists of or includes metal. In one example, the two or more adjustment means are joined together or mechanically joined together to form a single adjustment element. For example, a composite adjustment means comprising or consisting of metal and dielectric material could be provided. The advantage of a single composite adjustment means is that it can be actuated or moved by a single adjustment mechanism, thereby simplifying and reducing the cost of the same. In one example, the dielectric material is provided at one end of the adjustment element and the metal material is provided at an opposite end of the adjustment element. Preferably the single joined or composite adjustment means is arranged to be moved along or about an X-axis of the resonator cavity in use. In one embodiment, the two or more adjustment means are separate to each other, and further optionally / preferably are provided a spaced distance apart from each other with respect to the primary-secondary resonator pair. In this example, each of the two or more adjustment means could be adjustable independently and separately of each other. However, the two or more adjustment means could be arranged to be adjusted simultaneously or substantially simultaneously to each other. Thus, there could be a single adjustment mechanism or at least two separate adjustment mechanisms to allow independent or simultaneous adjustment for each primary secondary resonator pair. In one embodiment the shape and / or dimensions of the adjustment means is such that adjustment of the adjustment means allows selective coupling enhancement or coupling depletion. For example, the shape of the adjustment means could be such that two or more parts of the adjustment means at provided at different and / or separate positions along one or more axes. In one embodiment the adjustment means is S-shaped, Z-shaped, substantially S-shaped or substantially Z-shaped. Preferably the shaped and / or dimensioned adjustment means is arranged to moved along or about an X-axis of the resonator cavity in use. In one embodiment the dimensions of the adjustment means or part(s) of the adjustment means are relatively smaller the further distance the adjustment means or part(s) are above a base of the primary resonator or along the Z-axis of the resonator cavity, and relatively larger the closer the distance the adjustment means or part(s) of the adjustment means are above a base of the primary resonator or along the Z-axis of the resonator cavity. For example, the Z-shaped adjustment means could include a part that is relatively thinner in dimensions the higher up the Z-axis within the resonator cavity and could include a part that is relatively thicker in dimensions the lower down the Z-axis within the resonator cavity. In one embodiment, when the adjustment means is S-shaped, Z-shaped, substantially S-shaped or substantially Z-shaped, or when the adjustment means includes two adjustment means or parts, it includes a first part which is positioned relatively higher up the cavity (i.e. higher up the Z-axis) and a spaced distance apart from a second part that is positioned relatively lower in the cavity (i.e. lower down the Z-axis). Preferably the first and second parts are arranged to be parallel or substantially parallel to each other and / or to a lid or base of the resonator cavity. In one embodiment an intermediate part(s) is provided between the first and second parts and further preferably is provided at an angle between the first and second parts. Thus, m one example, the first and second parts are offset from each other via the sloped or angled intermediate part. In one embodiment the adjustment means are arranged to move along or about a Z-axis of the resonator cavity in use. Preferably the Z-axis is an axis parallel or substantially parallel to a longitudinal axis of the primary and / or secondary resonators. Preferably the X-axis is an axis passing between the primary and secondary resonators and / or is transverse or perpendicular to a longitudinal axis of the primary and / or secondary resonators. Preferably the tunable filter of the present invention allows any or any combination of: a) The centre operating frequency of the filter to be moved to a different frequency or to be controlled; b) The bandwidth of the filter to be changed or to be controlled (ie. for the size of the bandwidth to be increased or decreased to cover a greater or reduced frequency range); and / or c) The number and / or frequency of transmission zeros to be changed or controlled. Preferably the primary resonating means is fixed in location and the adjustment means is movable relative to the primary resonating means. Preferably the secondary resonating means is fixed in location and the adjustment means is movable relative to the primary and / or secondary resonating means. In one embodiment the secondary resonating means is located in the resonant cavity with the primary resonating means. In one embodiment the secondary resonating means is located externally of the resonant cavity housing the primary resonating means. Preferably the secondary resonating means is located in such a position and manner to allow coupling with the primary resonating means with which it is associated in use. Preferably the adjustment means are arranged to move along a movement path when moving between coupling enhancement and coupling depletion. In one embodiment at least part of the adjustment means is located between and / or is arranged to move directly between the primary and secondary resonator pair, for at least some of the movement path of the adjustment means. In one embodiment the adjustment means is movably mounted within or relative to the resonator cavity. In one embodiment the adjustment means is movably mounted outside of the resonator cavity. In one embodiment the adjustment means is arranged to move between coupling depletion and coupling enhancement in use. In one embodiment the adjustment means is arranged to undergo sliding, rotation and / or pivotable movement in moving between coupling depletion and coupling enhancement in use. Preferably the filter housing is formed from conductive material. Further preferably the filter housing has a plurality of cavities defined therein for the location of at least the primary resonating means, and further preferably the primarysecondary resonator pairs. For example, each cavity of the filter can include a primary resonating means or a primary-secondary resonator pair. Preferably the primary resonating means is arranged so as to resonate close to the desired operating frequency of the filter. Preferably the primary and / or secondary resonating means are any means, device or object that exhibits resonance or resonant behaviour in use. Preferably tuning means are provided on and / or associated with the or each primary resonating means to allow tuning of the primary resonating means to a predetermined frequency or the first frequency on manufacture, on initial set up of said filter or on pre-deployment in the field. Thereafter, tuning of the primary and / or secondary resonating means or pairs typically takes place via the adjustment means in accordance with the present invention (i.e. the adjustment is preferably for the purpose of adjustment post manufacture or post deployment in the field). However, it will be appreciated that the adjustment means could also be used to adjust the coupling within each primary and secondary resonator pair and / or the resonating frequency of the secondary resonating means during manufacture, on initial set up of said filter or pre-deployment in the field if required. Preferably the tuning means provided on and / or associated with the or each primary resonating means is located directly above the primary resonating means in the cavity (i.e. vertically or substantially vertically above and within the outer perimeter of the primary resonating means). In one embodiment the secondary resonating means are provided with and / or associated with tuning means for allowing the secondary resonating means to be tuned to a pre-determined frequency or to the second frequency. Further preferably the secondary resonating means is tuned to the pre-determmed frequency or second frequency on manufacture, on pre-deployment in the field or on initial set up of said filter. Thereafter, tuning of the secondary resonating means preferably takes place via the adjustment means in accordance with the present invention. Preferably the tuning means provided with and / or associated with the secondary resonating means can be integrally formed therewith or can be located a spaced distance from the secondary resonating means (i.e. the tuning means could be located a spaced distance above the secondary resonating means in one example). In one embodiment, the tuning means provided with and / or associated with the primary and / or secondary resonating means is in the form of a rotatable threaded screw and / or the like. Preferably once the pre-determined, first and / or second frequency is achieved using the tuning means, the tuning means can be locked via locking means, such as for example by a locking nut and / or the like. It is not essential that separate locking means are used with the tuning means since the tuning means could be self-locking and could be moved manually or automatically? (i.e. without direct user actuation) using drive means and / or the like in some examples. In one embodiment the secondary resonating means is fixed or substantially fixed relative to the filter housing following set-up of the filter (i.e. once any tuning means, if present, have been locked). Preferably the primary resonating means are fixed or substantially fixed relative to the filter housing following set-up of the filter (i.e. once tuning means, if present, have been locked). Preferably the adjustment means are typically movable or movably mounted relative to the primary and / or secondary resonating means. In one embodiment the primary- resonating means in each of the two of more cavities of the tunable filter are coupled together to form the required filter response. In one embodiment the adjustment means are arranged such that each resonator pair and / or secondary resonator means are adjusted substantially independently of each other and / or of other resonator pairs and / or secondary resonator means in the filter. In one embodiment the adjustment means for two or more of the primary-secondary resonator pairs and / or two or more secondary resonator means are connected via connection means such that each resonator pair and / or secondary resonator means can be adjusted simultaneously or substantially simultaneously in use. Preferably the connection means are any suitable device or mechanism that allows two or more adjustment means to be adjusted or moved simultaneously or substantially simultaneously. For example, the connection means could include low loss printed circuit board (PCB) material. In one embodiment, in order to achieve adjustment of the centre frequency of the filter, first adjustment means can be used to influence the coupling within the primary-secondary resonator pair. Preferably the adjustment means can be controlled remotely" without a user having to access the interior of the cavity to make the adjustment. In the embodiment the adjustment means are located laterally of the primary resonating means and between the primary and secondary resonating means. In one embodiment the adjustment means of two or more primary-secondan resonating pairs m the filter can be connected together via connection means. This allows the coupling within each primary-secondary resonating pair of the two or more primary-secondary resonating pairs to be adjusted substantially simultaneously. In one embodiment the connection means can include any suitable linkage means, mechanical means and / or the like. For example, the adjustment means can be joined to control means in the form of an elongate rod, any? other suitably? shaped rod and / or the like. Alternatively, or in addition, the connection means could be electrical means, such as for example a piezoelectric actuator, connected via a circuit board, suitable electrical circuitry and / or the like. Adjustment of any? electrical parameters through the electrical means could result in adjustment of the coupling of the resonator pair and / or the resonating frequency? of the secondary? resonating means. In one embodiment the connection means can be moved in a substantially horizontal plane or in a plane substantially? parallel to the Ed and / or base of the filter. For example, the connection means can be moved from side to side in a direction substantially parallel to a longitudinal axis of the filter or along an X-axis. In one embodiment, the connection means could be slidably movable, pivotably movable and / or rotatable in use. For example, the connection means could be a ring or disc member located with the Ed of the filter to which the adjustment means are connected at spaced apart intervals therealong. In one embodiment, rotation of the connection means could cause sEding movement of the adjustment means relative to the primary and / or secondary resonating means. In one embodiment, the connection means are located underneath a Ed of the filter housing and preferably are movable within the cavities of the filter, thereby substantiahy preventing electrical leakage from the filter. However, the connection means could be located on top of or above the lid or cavity walls of the filter and movable relative thereto if required. In one embodiment guide means can be provided on and / or associated with the connection means to help guide the connection means during movement of the same and for supporting the same. Preferably the guide means could include one or more guide channels defined in the housing of the filter. In one embodiment adjustment of the adjustment means can be undertaken manuaUy. In one embodiment adjustment of the adjustment means can be undertaken automatically using suitable drive means. Preferably the drive means can include any or any combination electrical, pneumatic, mechanical, hydraulic means and / or the Eke, and further preferably can be connected to the adjustment means directly or the connection means. For example, one or more electrically powered motors can be used to drive the adjustment means and / or connection means. In one embodiment control means can be associated with the adjustment means and / or connection means to aUow control of the adjustment hi use. In one embodiment control of the adjustment means can be undertaken remotely. In one embodiment the adjustment means includes one or more adjustment elements, blocks or members. Preferably the adjustment means can be formed at least in part from di-electric material and / or from conductive material. In one embodiment, in the case of di-electric material, the adjustment means can could come into contact with the primary and / or secondary resonating means but it is preferred that the adjustment means is a spaced distance apart from the primary and / or secondary resonating means. In one embodiment, in the case of conductive material, the adjustment means are typically a spaced distance apart from the primary and / or secondary resonating means. In one embodiment the adjustment means can be located internally and / or externally7 of the cavity and preferably in either embodiment movement of the adjustment means relative to the primary and / or secondary resonating means influences the coupling within the primary-secondary pair and / or resonating frequency of the secondary resonating means. Preferably the adjustment means are arranged to undergo rotatable, pivotable and / or slidable movement during the adjustment process. Preferably each associated primary and secondary resonating means are typically located a spaced distance apart from each other, and further preferably are arranged laterally from each other. For example, in one embodiment the primary and secondary resonating means can be located at any suitable position within a cavity7 of the filter or within the filter housing providing the secondary resonating means can influence the primary resonating means in use. In one embodiment the secondary resonating means of a primary-secondary resonating pair can be located in a separate cavity to the primary7 resonating means with which it is associated or coupled. In one embodiment the primary7 resonating means is located on a base, side wall and / or on a Ed of the defined cavity. Preferably the primary7 resonating means can take the form of a solid post, hollow post, cylindrical post and / or the like. In one embodiment the secondary resonating means can be located on a base, side wall and / or on a lid associated with the cavity. Preferably the secondary resonating means can take the form of a solid post, hollow post, cylindrical post and / or the like. Preferably the primary and / or secondary resonating means are of such a form, size, shape and / or material to allow them to resonate at a desired resonating frequency in use. Preferably the difference between the first and second frequencies of the primary and secondary resonating means is less than or substantially equal to three times the resonating frequency of the primary resonating means. Preferably the term coupling used herein refers to any arrangement whereby the electromagnetic fields of two or more parts of the filter, the primary resonating means, the secondary resonating means and / or the adjustment means are electromagnetically connected together or influence each other (i.e. the parts or means are mutually affected by the same electromagnetic field). Preferably the primary and / or secondary? resonating means can take any suitable size, shape and / or form in which they can resonate at one or more desired frequencies and can be coupled to each other. In a preferred embodiment the primary and / or secondary resonating means are in the form of a conductive post like element protruding inwardly into a cavity defined by one or more surfaces of the filter housing. According to a second aspect of the present invention there is provided a method of using a tunable filter, said tunable filter including a housing with two or more cavities defined therein, primary resonating means located in each of said cavities and each primary resonating means capable of resonating at a first frequency in use; said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency in use; the second frequency of the secondary resonating means being different to the first frequency of the primary resonating means; said method including the steps of coupling each primary resonating means with said secondary resonating means to form a primarysecondary resonator pair; and adjusting the coupling within each primary-secondary resonator pair and / or the resonating frequency of the secondary resonating means using adjustment means, characterised in that adjusting the adjustment means selectively moves the coupling within each primary-secondary resonator pair of said tunable filter between coupling enhancement and coupling depletion. Preferably the position which the adjustment means is moved to between coupling enhancement and coupling depletion is user selectable, arbitrary and / or one of a number of different possible positions.According to a third aspect of the present invention there is provided communication apparatus incorporating a tunable filter. According to a fourth aspect of the present invention there is provided a method of using communication apparatus incorporating a tunable filter. According to a further aspect of the present invention there is provided tunable coupling means for adjusting the coupling within a primary and secondary resonating pair and / or the resonating frequency of a secondary resonating means of a RF filter in use, characterised in that the tunable coupling means comprises adjustment means arranged such that adjustment of the adjustment means can selectively move the coupling within each primary and secondary resonator pair of said tunable filter between coupling enhancement and coupling depletion. According to a further aspect of the present invention there is provided a method of using a tunable coupling means for adjusting the coupling within a primary and secondary resonating pair and / or the resonating frequency of a secondary resonating means of a RF filter. Thus, the present invention has the advantages over the prior art in that complete filter parametric adjustment can be achieved; the bandwidth of the filter can be increased or reduced; the frequency and / or number of transmission zeroes can be changed or controlled; a greater overall tunable frequency range can be achieved; and a simpler mechanical implementation can be utilised. The present invention can provide the following: a) A variable coupling means, which has multiple uses, including tuning the resonant frequencies of a resonant cavity containing a primary and secondary resonator. This allows the centre frequency of the filter to be adjusted over a wider range; b) A variable coupling means which can be used to vary the coupling between two primary resonators in adjacent resonator cavities within a filter. This has the effect of altering the bandwidth of the filter over a wider range rather than the centre frequency of the filter; c) A variable coupling means which can be used to vary the coupling between a resonator acting as a trans former input or output to a filter and an associated secondary transformer resonator coupled to the primary transformer resonator. This allows compensation for the required changes in external coupling that are required as the filter to allow a fixed return loss to be maintained; d) A way of introducing tunable or variable transmission zeros in the transmission response of the filter. Transmission zeros are desirable features that are used to improve the stop-band rejection of a filter. Embodiments of the present invention will now be described with reference to the following figures, wherein: Figures la and lb (PRIOR ART) show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line A-A respectively in one example; Figures 2a and 2b (PRIOR ART) show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line B-B respectively in a further example; Figure 3 (PRIOR ART) is a graph showing the coupling level versus the position of the coupling block along the X-axis for the filter arrangement shown in figures 2a and 2b; Figures 4a and 4b (PRIOR ART) show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line C-C respectively in a yet further example; Figure 5 (PRIOR ART) is a graph showing the coupling level versus the position of the coupling block along the X-axis for the filter arrangement shown in figures 4a and 4b; Figures 6a and 6b show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line D-D respectively according to an embodiment of the present invention; Figure 7 is a graph showing the coupling level versus the position of the coupling block along the Z-axis for the filter arrangement shown in figures 6a and 6b; Figures 8a and 8b show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line E-E respectively according to an embodiment of the present invention; Figure 9 is a graph showing the coupling level versus the position of the coupling block along the X-axis for the filter arrangement shown in figures 8a and 8b; Figure 10 is a perspective view of a five cavity tunable filter of a type shown in figures 8a and 8b with the coupling blocks connected together; Figures Ila and 1 lb show a perspective view of a resonator cavity of an RF tunable filter (without the tuning screws shown) and a cross sectional view taken along line F-F respectively according to an embodiment of the present invention; Figure 12 is a perspective view of a resonator cavity of a tunable filter (without the tuning screws shown) according to a further embodiment of the present invention; and Figure 13 is a perspective view of a resonator cavity of a tunable filter (without the tuning screws shown) according to a yet further embodiment of the present invention. Referring firstly to figures 6a and 6b, there is illustrated an RF tunable cavity filter 102 according to an embodiment of the present invention. The same reference numerals are used to define the same features as for prior art figures la 5. The filter 102 comprises a plurality of resonator cavities 4 defined in the conductive filter housing 6. Only one cavity 4 is shown in the figures for the purposes of clarity. The resonator cavity 4 is provided with fixed primary and secondary resonators 8, 10 to form a primary-secondary resonator pair. The primary and secondary resonators 8, 10 are of different sizes and each resonator is designed to resonate individually at different frequencies. A lid 18 is provided over the cavity 4. Both resonators 8, 10 are attached to a base 14 of the housing 6, providing a comb-line configuration. Adjustment means in the form of a metal coupling block 112 is provided in the resonator cavity 4 and is positioned between the primary and secondary resonators 8, 10. Tuning means (not shown) can be provided and / or associated with each of the primary and / or secondary resonators as per a conventional RF tunable filter. In accordance with the present invention, the coupling block 112 is arranged to be movably adjustable within the filter housing for movement along a Z axo of the filter housing, as shown by arrow 104. The Z-axis is typically parallel or substantially parallel to a longitudinal axis of the primary and secondary resonators 8, 10. The coupling block 112 is fixed in position along the X-axis and the Y-axis in this example. Thus, the coupling block 112 is arranged to be movable in use from a position where it is relatively low on the Z-axis (i.e. close to the base 14 and to base 110, 114 of resonators 8 and 10 respectively), to a position which is relatively high on the Z-axis (i.e. close to a top free end 106,108 of the resonators 8,10 respectively and a relatively greater distance from base 14). As the coupling block 112 moves from a position adjacent the base 110,114 of the resonators 8,10 (i.e. relatively low down on the Z-axis) towards the top free end 106, 108 of the resonators 8, 10 (i.e. relatively high up on the Z-axis), the effect of the coupling block 112 on the coupling between the primary and secondary resonator pair changes from coupling enhancement to coupling depletion. This substantially increases the tuning range of the resulting filter compared to prior art filter arrangements. Figure 7 shows a graph of the coupling between the primary and secondary pair of the filter arrangement 102 shown in figures 6a and 6b as the metal coupling block 112 is moved from relatively low down on the Z-axis to relatively high up on the Z-axis. The coupling level starts off at a level above KnOm towards Kmax. As the coupling block 112 moves towards the top free end 106, 108 of the resonators 8, 10, the coupling level moves below Knom towards Kmm. Thus, for a single coupling block 112, the coupling between the primary-secondary resonator pair of the tiler can be moved between enhancement and depletion. In this arrangement, it is not necessary to optimise the height of the coupling block. However, optimisation of the height of the coupling block could take place. Referring to figures 8a and 8b, there is illustrated a further embodiment of a tunable filter 102 according to an embodiment of the present invention wherein the coupling block is shaped in such a manner so as to allow coupling between the primarysecondary resonator pair to be moved between enhancement and depletion in use. More particularly, the adjustment means or coupling block 120 in this embodiment is an S-shaped, Z-shaped or substantially S-shaped or Z-shaped metal block. The coupling block 120 is arranged to be moved along the X-axis of the filter housing in use between the primary and secondary resonators 8, 10. The X-axis in this example is parallel or substantially parallel to the base 14 of the housing or transverse, perpendicular or substantially perpendicular to a longitudinal axis of the resonators 8, 10. The coupling block 120 comprises a relatively thin upper part 122 and a relatively thick lower part 124, which are joined together by an intermediate part 126. The upper and lower parts 122,124 in this illustration are parallel or substantially parallel to each other but are offset from each other (i.e. the lower part 124 is provided a spaced distance in front of the higher part 122). The intermediate part 126 is provided on a slope or angle to provide the offset arrangement of the upper and lower parts 122, 124. The lower thicker part 124 provides coupling enhancement close to Kmax , and the upper thinner part 122 provides coupling depletion close to Kmm. This arrangement has been shown to provide an even greater range of coupling, as illustrated by the graph in figure 9, compared to the embodiment shown in figures 6a 7. Thus, a single coupling block can be moved in such a manner relatively to the primary^ secondary resonator pair so as to selectively provide enhancement or depletion of the coupling. Figure 10 shows an example of a tunable filter 102 in which five resonator cavities 4 are shown, each cavity comprising primary and secondary resonators 8, 10 and adjustment means in the form of a metal S^shaped coupling block 120. Connection means are provided to connect all the S^shaped coupling blocks together so that a single actuation mechanism can be used to move all the coupling blocks 120 simultaneously in use. In this example the connection means is in the form of a low loss tangent Printed Circuit Board (PCB) material. However, other connection arrangements could be used to allow simultaneous movement of the coupling blocks in each resonator cavity in use. Figures Ila and 11b illustrate a further embodiment of the present invention of a tunable filter 102 in which the coupling block 128 is formed of two different materials joined together to form a single composite coupling block. More particularly, the two different materials include a first metal part 130 and a second dielectric part 132. The coupling block is located between the primary and secondary resonators 8, 10 and is arranged to move along the X-axis, as shown by arrow 134. The coupling block is movable low down m the cavity 4 adjacent the base 14 of the cavity (i.e. low down the Z-axis). This combined or composite coupling block allows the coupling level to be selectively moved between enhancement and depletion as required. Figures 12 and 13 show further embodiments of tunable filters according to the present invention m which a coupling block 136 is rotated about an axis rather than moved along an axis. In figure 12, the coupling block is a cruciform shape and is rotated on a rotatable post 138 about the Z-axis. In figure 13, the primary and secondary resonators 8, 10 are provided in an inter-digital arrangement (i.e. one of the resonators 8 is located on a base 14 of the cavity 4 and one of the resonators is located on a lower surface of a lid 18 of the cavity 4) and the coupling block 136 is rotated on a rotatable post 140 about a X-axis. Any or any combination of the abovementioned embodiments could be used independently or in combination to provide greater enhancement or depletion of the coupling levels as required. Although the above illustrated embodiments show the coupling block being moved along or about one of the three cartesian axes, it will be appreciated that the coupling block could be arranged to be moved along or about other axes in between said three cartesian axes, in either linear or non-linear paths and / or the like.

Claims

1. A tunable filter, said tunable filter including a housing with two or more cavities defined therein, primary resonating means located in each of said cavities and each primary resonating means capable of resonating at a first frequency in use; said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency in use; the second frequency of the secondary resonating means being different to the first frequency of the primary resonating means; said secondary resonating means arranged such that each primary resonating means has a secondary resonating means coupled therewith to form a primary-secondary resonator pair; and adjustment means are provided for adjusting the coupling within each primary-secondary resonator pair and / or the resonating frequency of the secondary resonating means, characterised in that the adjustment means are arranged such that adjustment of the adjustment means can selectively move the coupling within each primarysecondary resonator pair of said tunable filter between coupling enhancement and coupling depletion.

2. The tunable filter according to claim 1, wherein the primary-secondary resonator pair can include or form any or any combination of the following: i) One or more tunable primary-secondary resonator pairs — adjustment of which enables control of the passband or stop band frequency of the tunable filter in use;ii) One or more tunable transformer pairs - adjustment of which enables control of the bandwidth of the tunable filter in use; oriii) One or more tunable cross couplings- adjustment of which enables control of the frequency and / or number of transmission zeros.

3. The tunable filter according to any preceding claim, wherein the ability to selectively move the coupling within each primary-secondary resonator pair between coupling enhancement and coupling depletion is achieved by one or more of the following:i) Two or more adjustment means are provided or associated with each primary-secondary pair; one adjustment means for providing coupling enhancement and one adjustment means for providing coupling depletion;ii) The shape and / or dimensions of the adjustment means is such so as to selectively allow coupling enhancement and coupling depletion; oriii) Movement of the coupling along or about one of three possible cartesian axes, along or about an axis between the cartesian axes, along a non-linear path or along a linear path is such so as to selectively allow coupling enhancement and coupling depletion.

4. The tunable filter according to claim 3, wherein when two or more adjustment means are provided or associated with each primary-secondary pair; one of the adjustment means is made of, consists of or includes dielectric material and one of the adjustment means is made or, consists of or includes metal.

5. The tunable filter according to claim 4, wherein the two or more adjustment means are joined together, form a composite or are mechanically joined together to form a single adjustment element.

6. The tunable filter according to claim 5, wherein the single joined or composite adjustment means is arranged to be moved along or about an X-axis of the resonator cavity in use.

7. The tunable filter according to any preceding claim, wherein when two or more adjustment means are provided or associated with each primarysecondary pair, they are provided separate to each other, and optionally are also provided a spaced distance apart from each other with respect to a primary-secondary resonator pair.

8. The tunable filter according to claim 7, wherein each of the two or more adjustment means are adjustable independently and separately to each other; or the two or more adjustment means are arranged to be adjusted simultaneously or substantially simultaneously to each other.

9. The tunable filter according to claim 3, wherein when the shape and / or dimensions of the adjustment means is such that the adjustment means allows selective coupling enhancement and coupling depletion, the adjustment means is S-shaped, Z-shaped, substantially S-shaped or substantially Z-shaped.

10. The tunable filter according to claim 9, wherein the shaped and / or dimensioned adjustment means is arranged to move along or about an X-axis of the resonator cavity in use.

11. The tunable filter according to claims 3 or 9, wherein the dimensions of the adjustment means or part(s) of the adjustment means are relatively smaller the further distance the adjustment means or parts thereof are above a base of the primary resonator or along a Z-axis of the resonator cavity, and relatively larger the closer the distance the adjustment means or parts thereof are above a base of the primary resonator or along the Z-axis of the resonator cavity.

12. The tunable filter according to any preceding claim, wherein the adjustment means are arranged to move along or about a Z-axis of the resonator cavity in use.

13. The tunable filter according to any preceding claim, wherein the adjustment means is arranged to undergo sliding, rotation and / or pivotable movement in moving between coupling depletion and coupling enhancement in use.

14. The tunable filter according to any preceding claim, wherein tuning means are provided on and / or associated with the or each primary resonating means to allow tuning of the primary resonating means to a pre-determined frequency or the first frequency on manufacture, on initial set up of said filter or on pre-deployment of the filter in the field, and thereafter tuning of the primary and / or secondary resonating means takes place via the adjustment means.

15. The tunable filter according to any preceding claim, wherein tuning means are provided on and / or associated with the or each secondary resonating means to allow tuning of the secondary resonating means to a pre-determined frequency of the second frequency on manufacture, on initial set up of said filter or on pre-deployment of the filter in the field, and thereafter tuning of the primary and / or secondary resonating means takes place via the adjustment means.

16. The tunable filter according to claims 14 or 15, wherein once the predetermined, first and / or second frequency is achieved using the tuning means, the tuning means can be locked via locking means or a locking nut or could be self-locking.

17. The tunable filter according to any preceding claim, wherein the adjustment means are arranged such that each resonator pair and / or secondary resonator means are adjusted substantially independently of each other.

18. The tunable filter according to any preceding claim, wherein the adjustment means for two or more of the primary-secondary resonator pairs and / or two or more secondary resonator means are connected via connection means suchthat each resonator pair and / or secondary resonator means can be adjusted simultaneously or substantially simultaneously in use.

19. The tunable filter according to any preceding claim, wherein the adjustment means are located laterally of the primary resonating means and between the primary and secondary resonating means.

20. The tunable filter according to claim 18, wherein the connection means are arranged to be slidably, pivotably and / or rotatably movable in use.

21. The tunable filter according to claim 18, wherein rotation of the connection means results in sliding movement of the adjustment means relative to the primary and / or secondary resonating means in use.

22. The tunable filter according to claims 20 or 21, wherein the connection means are located underneath a lid of the filter housing.

23. The tunable filter according to any of claims 20-22, wherein guide means are provided on and / or associated with the connection means to help guide movement of the connection means in use.

24. The tunable filter according to any preceding claim, wherein adjustment of the adjustment means takes place manually or takes place automatically using drive means.

25. A method of using a tunable filter, said tunable filter including a housing with two or more cavities defined therein, primary resonating means located in each of said cavities and each primary resonating means capable of resonating at a first frequency in use; said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency in use; the second frequency of the secondary resonatingmeans being different to the first frequency of the primary resonating means; said method including the steps of coupling each primary resonating means with said secondary resonating means to form a primary-secondary resonator pair; and adjusting the coupling within each primary-secondary resonator pair and / or the resonating frequency of the secondary resonating means using adjustment means, characterised in that adjusting the adjustment means selectively can move the coupling within each primary-secondary resonator pair of said tunable filter between coupling enhancement and coupling depletion.

Images