GB2571622A - Tunable filter apparatus and method of use thereof - Google Patents

Abstract

A tunable filter apparatus is provided which has a housing with at least one housing cavity defined therein. At least one resonating means is provided in said at least one housing cavity. The tuning means are provided for tuning said at least one resonating means in use to provide a pre-determined resonant frequency of said at least one resonating means. Movement means are provided for allowing movement of the tuning means in use. The at least one resonating means has a resonator cavity defined therein and the movement means are provided in said resonator cavity of said at least one resonating means. The tuning means may be made of a dielectric or metallic material. The movement means may include any combination of pneumatic, mechanical or hydraulic means. The movement means may be an actuator which may be a shape memory alloy (SMA). The tuning means may undergo rotational, sliding and/or linear movement.

Description

This invention relates to tunable filter apparatus and a method of use thereof. In addition, the invention relates to communication apparatus incorporating tunable filter apparatus and method of use of said communication apparatus.

The microwave region of the electromagnetic spectrum is an essential yet finite resource used by both civilian and military applications such as radar, navigation and wireless communications. Mobile radio communications require careful allocation of the spectrum by the relevant national or international authorities to service providers, who in turn may divide their slots into narrower sub bands for allocation to individual operators. Providers must have sufficient flexibility to re-allocate frequencies rapidly, leading to the requirement for tunable filters. Although sub band filters are generally smaller, easier and therefore cheaper to manufacture, without the ability to tune them in the field, providers are forced to buy multiple products to cover the entire band and to physically swap filters out of base stations as circumstances and adjacent operators change. Tunable filters allow operators to purchase one product only and align it to the desired operating frequency and bandwidth in the field.

A modern cell site within a cellular mobile network could operate at seven or more frequency bands. For example, in Europe these could be taken from the 700 MHz, 800 MHz, 900 MHz, 1400 MHz, 1800 MHz, 2100 MHz, 2300 MHz and 2600 MHz bands. Currently, each radio supports a fixed band or pair of bands. For example, a radio would be designed to work in the 1800 MHz band. As an operator acquires licenses for new spectrum bands, trades spectrum resources or wishes to add capacity to a cell site new fixed radios will need to be fitted and sometimes old ones removed. Cell site upgrades would be significantly simplified if radios had the ability to be reconfigured in the field to operate in a new band. In some cases this would allow site upgrades to be carried out without the need for a site visit to swop hardware. A major barrier to remotely reconfiguring radios is that they contain fixed RF filters that have to be tuned in a factory. As space is at a premium on cell sites it is necessary for the radios to be as small as possible and indeed in recent years there has been a strong trend for the radios to decrease in size. Therefore, compact, tunable filters are a prerequisite for a field reconfigurable radio.

Cell sites also contain antenna line devices such as combiners, multiplexers, tower mounted amplifiers and filtered antennas that contain fixed filters. These too are a barrier to remotely reconfiguring a cell site and therefore again, tunable filters are required to allow the desired reconfiguration.

Conventionally, filters have comprised a conductive housing defining a plurality of cavities with a resonator located in each cavity. In order to obtain a required frequency response from the filter, it is necessary to have 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 screw⁷ located in a lid of the filter housing above each cavity. Additional fine adjustment of the electromagnetic coupling strength may also be required and tuning is typically achieved by an adjustment of a tuning screw located within the conductive housing within or around the area at which the coupling takes place. Turning of the resonator adjustment screw changes the capacitance between the resonator and the lid and therefore changes the frequency of the resonator. By tuning each resonator of the filter in this manner, the performance of the filter can be tuned to a required response. A problem with this type of tuning is that it is time consuming, expensive and prevents retuning of the filter once the tuning screws have been locked and the device is deployed in the field.

In order to overcome this problem, it is known to provide filters which allow tuning of all the resonators of the filter substantially simultaneously, thereby allowing post deployment tuning and reducing associated time and costs. Typical examples of such prior art filters are disclosed in US

2007/0052495, and US7180391. The filters contain a plurality of resonant cavities, each cavity comprising a resonator contained within an outer conductor and a dielectric tuner held by a holder. The holders of all the dielectric tuners in the filter are connected together such that they can be moved simultaneously as a group or individually, allowing the dielectrics to adjust the resonator frequencies substantially equally or individually, allowing each resonator frequency to be adjusted independently of the others. As the position and/or angle of the dielectric with respect to each resonator is varied, perturbation is applied to the electric field occurring in the resonant cavity, changing the capacitive loading at the upper end of the resonator, so that the resonant frequency of each cavity resonator may be varied. The amount of capacitive change limits the range of frequency shift. A problem with the above filters is that in connecting the dielectric holders in such a way to allow simultaneous and/or individual movement, it requires a complicated mechanical structure which leads to possible cost and performance issues and significantly increases assembly and alignment times when compared with a fixed frequency band filter.

EP2203953A1 overcomes some of these issues with a simplified mechanical implementation. However, it is difficult to finely set the frequency of each resonator independently. Attempts have been made to overcome this issue by attaching stepper motors to tuning elements that protrude through the filter lid. Although this approach allows independent setting of the tuning screws the stepper motors are bulky and add considerably to the size of the device.

It is therefore an aim of the present invention to produce tunable filter apparatus which resolves the issues described above.

It is a further aim of the present invention to provide a method of using or adjusting the operating frequency and/or bandwidth of tunable filter apparatus.

It is a yet further aim of the present invention to provide communication apparatus incorporating tunable filter apparatus and/or a method of using said communication apparatus.

According to a first aspect of the present invention there is provided a tunable filter apparatus, said apparatus including a housing with at least one housing cavity defined therein, at least one resonating means provided in said at least one housing cavity, tuning means provided for tuning said at least one resonating means in use to provide a pre-determined resonant frequency of said at least one resonating means, movement means provided for allowing movement of the tuning means in use, characterised in that the at least one resonating means has a resonator cavity defined therein and the movement means are provided in said resonator cavity of said at least one resonating means.

Thus, the tunable filter apparatus of the present invention allows individual tuning of each resonating means or resonator within the filter apparatus without adding to the overall size of the filter apparatus.

This is in contrast to prior art tunable filters that allow for individual setting of the resonating means or resonators using electrically powered movement means located outside of the housing cavities or externally of the apparatus housing and therefore add to the size of the filter apparatus.

In the present invention, the movement means are located within or internally of the resonating means and so do not add to the overall size of the apparatus or increase the external dimensions of the apparatus housing. The region on the inside of the resonating means has conventionally been wasted space as the resonant electromagnetic field generated by the resonating means typically resides in the volume bounded by the housing cavity in which the resonating means is located, such as for example by the base, side walls and/or lid of the housing cavity and the outer surface of the resonating means. By locating or concealing the movement means within the resonating means, a significant space saving is achieved.

Preferably the apparatus includes a lid attached to or associated with the at least one housing to cover or substantially cover said at least one housing cavity when the apparatus is fully assembled in use.

Preferably the resonator cavity is located centrally or substantially centrally of said resonating means. Alternatively the resonator cavity can be located non-centrally.

Preferably an opening of said resonator cavity is defined in a external wall of said resonating means.

Preferably an opening of said resonator cavity is located centrally or substantially centrally of a wall of said resonating means. Alternatively, an opening of said resonator cavity is located non-centrally of a wall of said resonating means.

Preferably the movement means are wholly or at least partially located within the resonator cavity of the resonating means.

Preferably the tuning means is located external of the resonator cavity, is located wholly within the resonator cavity, or is located at least partially within the resonator cavity of the resonating means.

For example, the tuning means could be located entirely within the resonator cavity in a start or non-tuned position and/or in a tuned position (i.e. following tuning of the resonating means). Thus, the maximum extent or range of movement of the tuning means relative to the resonating means retains the tuning means within the cavity of the resonating means.

In one example, the tuning means can be located entirely within the resonator cavity in a start or non-tuned position and can be moved to a position where it partially or wholly protrudes from an opening of the resonator cavity in a tuned position (i.e. following tuning of the resonating means).

In one example, the tuning means can protrude outwardly from an opening of the resonator cavity in both a start or non-tuned position and a tuned position. Thus, irrespective of the range of movement of the tuning means during tuning, the tuning means will always protrude from an opening of the resonator cavity.

In one embodiment the tuning means could be located in a position or could be moved to a position where a surface of the same is flush or substantially flush with an external surface of the resonating means.

In one embodiment the tuning means is recessed within the resonator cavity, such that an uppermost surface of the tuning means is sub-flush or set back from an external surface of the resonating means.

In one embodiment the movement means are electrically powered movement means.

Preferably the electrical power to power the movement means is from one or more batteries, rechargeable batteries, a generator, mains electrical power, solar power, wind power and/or the like.

Preferably the movement means are directly or indirectly connected to or in abutting relationship with the tuning means.

In one embodiment the movement means includes any or any combination of electrical, pneumatic, mechanical, hydraulic means or mechanism and/or the like. For example, one or more electrically powered motors can be used to drive the tuning means.

In one embodiment the movement means is an electrically powered motor. For example, the electrically powered motor can be a stepper motor, piezoelectric motor and / or the like.

In one embodiment the movement means can be an actuator. For example, the actuator can be a piezoelectric actuator, shape memory alloy (SMA) actuator, shape memory alloy (SMA) wire actuator and/or the like.

In one embodiment control means are provided with or associated with the movement means to allow control of the movement means and/or the tuning means in use.

In one embodiment the control means are located remotely from the movement means and/or the filter housing. Thus, control of the movement means can be undertaken remotely.

In one embodiment the control means are provided in or associated with the movement means and/or the filter housing. In this embodiment one or more control signals could preferably still be sent to and/or from the control means from a remote control source.

Preferably the control means consists of or includes an electronic control circuit. The electronic control circuit can monitor the position of the movement means and/or tuning means and/or control the movement of the movement means and/or tuning means in use.

Preferably the control means is controlled remotely via wired or wireless means. For example, the control means or electronic control circuit can receive one or more command or control signals from a physically remote device through a wired connection such as Ethernet, Antenna Interface Standards Group (AISG) protocol, RS485 and/or the like, or through a wireless connection such as Bluetooth, Zigbee, cellular radio, Internet of Things protocol and / or the like.

In one embodiment the control means is housed within the filter and/or is located within one or more walls defining the at least one housing cavity of said filter housing, such as for example, in material that forms the walls or floors of the housing cavities.

Preferably one or more, or all, of the inner surfaces of the at least one housing cavity, the exterior surface of the resonating means and/or the surface of a lid covering or substantially covering said housing cavity is electrically conductive, formed from or includes an electrically conductive material and/or layer.

In one embodiment two or more housing cavities are defined in said housing and each of said two or more housing cavities has resonator means provided therein. Preferably each of the two or more resonator means has a resonator cavity defined therein and movement means are provided in each of said resonator cavities of said two or more resonator means.

In one embodiment the tuning means includes one or more tuning elements.

Preferably the tuning means includes any means or member to allow the resonant frequency of the resonating means to be adjusted or changed in use.

Preferably the tuning means can be formed, at least in part, from di-electric material and/or from an electrically conductive material.

In one embodiment the tuning means can engage or abut with a surface, or an interior surface of the resonating means.

In one embodiment the tuning means can be a spaced distance apart from one or more surfaces or interior surfaces of the resonating means.

In one embodiment, in addition to the tuning means provided in or associated with the resonator cavity, a further conventional tuning means is provided on or associated with a lid of the filter apparatus.

In one embodiment the tuning means is a linear or substantially linear element, such as for example, a cylindrical element.

In one embodiment the tuning means includes a first portion and at least a second portion that is perpendicular or substantially perpendicular to the first portion.

Preferably the second portion protrudes outwardly from the first portion.

In one example, one or more flanges are provided on or associated with the second portion. Further preferably the flanges are provided perpendicular or substantially perpendicular to the second portion and/or parallel or substantially parallel to the first portion.

Preferably the flanges protrude outwardly from the second portion and face away from a lid of the housing cavity, towards a base of the housing cavity and/or towards a surface of the housing cavity at which the resonator is attached to or located on.

In one example, the first portion is cylindrical in shape.

In one example, the second portion is a disc or is disc shaped.

In one embodiment the tuning means consists of or includes a rotatable threaded screw and/or the like.

Preferably the tuning screw can be constructed of metallic material, dielectric material, ceramic material or a combination thereof.

Preferably the tuning means and/or movement means is arranged to undergo linear movement, sliding movement, rotational movement and/or pivotable movement in use.

In one embodiment the tuning means overall undergoes linear movement, or linear vertical movement, irrespective of whether the means to achieve the overall linear movement is through rotational, pivotal or linear movement.

Preferably, the tuning means is arranged to protrude from a top of the resonating means or close to an end of the resonating means that is furthest from or opposite to the end of the resonating means attached to or located on a base or wall of the housing cavity or a surface of a lid facing the housing cavity.

The tuning means are arranged to perturb the electromagnetic field within the housing cavity and hence change the resonant frequency of the resonating means in use. In particular, the tuning means can change the capacitance between a top of the resonating means and a lid of the housing cavity in which the resonating means is lcoated.

In one embodiment the filter housing includes a plurality of housing cavities with resonating means provided therein. The resonating means of the plurality of housing cavities are coupled or electromagnetically coupled together to form a required filter response for the filter apparatus.

The required filter response for the filter response and/or the predetermined resonant frequency of the resonating means is typically selected by a user, service provider or manufacturer.

Preferably the movement means are arranged to undergo rotatable, pivotable and/or slideable movement during the tuning process.

In one embodiment the resonating means is an integral part of the housing.

In one embodiment the resonating means is a separate element that is attached to a wall defining a housing cavity via suitable attachment means, such as for example, via one or more screws, solder, adhesive, clips, interengaging members, interference fit and/or the like.

Preferably the filter apparatus is a radio frequency filter apparatus for allowing one or more radio frequency signals to be input and/or output therefrom in use.

The resonating means can be any means or object that allows an electromagnetic resonance within the said at least housing cavity at a desired or user selected radio frequency in use.

Preferably, the resonating means is or includes any or any combination of one or more rods, posts, pucks and/or the like.

Preferably the resonating means is or includes any or any combination of dielectric material, ceramic material, metallic material, or a non-metallic material with a metallic surface or outermost layer.

Further preferably, the resonating means is formed from a metal or a nonmetallic material with a metallic surface applied.

Further preferably, the resonating means is in the form of a protrusion, of any cross section, from a wall or base of the housing cavity or a surface of a lid facing the housing cavity. The protrusion can be an integral part of the housing or lid or can be an additional part that is fixed to the housing or lid via a suitable attachment means such as screw, solder, interference fit and / or the like.

1

The resonating means can be galvanically connected to a base or wall of the housing cavity or the surface of a lid facing the housing cavity.

The resonating means can be of constant cross section or non-constant cross section.

In one embodiment the resonator cavity of the resonating means is a through hole, a blind hole or a recess defined in said resonating means.

For example, the resonating means can have a hole within it that houses the movement means. The hole within the resonating means can be a through hole extending from one face of the resonating means to an opposing face of the resonating means. The hole in the resonating means can overlap or coincide with a hole in a wall or base of the housing cavity or the surface of a lid that faces the housing cavity, and such coinciding holes may allow a path from the outer surface of the filter housing, through the holes to the interior of the housing cavity, such that movement means can be connected to control means located on an outer surface of the filter housing or external to the filter housing.

In one embodiment the total achievable change in resonant frequency of the resonating means using the tuning means is preferably equal to or greater than 1%, further preferably equal to or greater than 5%, further preferably equal to or greater than 10% and further preferably equal to or greater than 25%.

Preferably the tuning means is independent and/or separate to the lid of the filter apparatus.

Preferably the tuning means is located a spaced distance apart from a lid or one or more walls defining the housing cavity.

According to a second aspect of the present invention there is provided a method of adjusting the operating frequency and/or bandwidth of tunable filter apparatus, said apparatus including a housing with at least one housing cavity defined therein, at least one resonating means provided in said at least one housing cavity, said method including the step of moving tuning means to provide a pre-determined resonant frequency of said at least one resonating means using movement means, characterised in that the at least one resonating means has a resonator cavity defined therein and the movement means are provided in said resonator cavity of said at least one resonating means.

Preferably the tuning of the tunable filter apparatus can take place at any time pre- or post- manufacture of the tunable filter apparatus.

According to a one aspect of the present invention there is provided a method of adjusting the operating frequency and/or bandwidth of a tunable filter, said tunable filter including a housing with at least a first cavity defined therein, a lid attached to the housing covering or substantially covering said at least first cavity, and at least first resonator means attached to a base or wall of said at least first cavity or the face of said lid facing said at least first cavity, said tunable filter characterised in that an at least first powered movement means is housed within said at least first resonator means and is connected to an at least first tuning means that protrudes from said at least first resonator means, said powered movement means arranged to allow movement of said at least first tuning means.

According to one aspect of the present invention there is provided a tunable filter, said tunable filter including a housing with at least a first cavity defined therein, a lid attached to the housing covering or substantially covering said at least first cavity, and at least first resonator means attached to a base or wall of said at least first cavity or the face of said lid facing said at least first cavity, said tunable filter characterised in that an at least first powered movement means is housed within said at least first resonator means and is connected to an at least first tuning means that protrudes from said at least first resonator means, said powered movement means arranged to allow movement of said at least first tuning means.

According to a further aspect of the present invention there is provided communication apparatus incorporating tunable filter apparatus. For example, telecommunication apparatus.

According to a yet further aspect of the present invention there is provided a method of using communication apparatus incorporating tunable filterapparatus.

The present invention has the advantages over the prior art in that complete fine independent tuning of the resonators within a filter can be achieved without adding to the volume of the filter.

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

Figure 1 (PRIOR ART) shows a resonator means within a cavity illustrating the basic construction of a resonant cavity in the prior art;

Figure 2 (PRIOR ART) shows a cross section of the resonant cavity of figure 1;

Figure 3 (PRIOR ART) shows a motorised tuning element illustrating the basic principles of the prior art;

Figure 4 shows a resonant means containing a motor illustrating the basic principle of the current invention in one embodiment;

Figure 5 shows a further embodiment of the present invention;

Figure 6 shows a further embodiment of the present invention;

Figure 7 shows a further embodiment of the current invention containing multiple cavities.

Referring firstly to Figures 1 and 2, there is illustrated a housing 1 containing a cavity 3 with a resonator 5 attached to the base of the cavity. The top surface 7 of the resonator has a hole 9 within it. The hole is typically provided in the resonator to allow a tuning screw 13, which is attached to a lid of the cavity, to partially penetrate into the resonator and further increase the capacitance between the top end of the resonator and the lid 11 which has the effect of lowering the resonant frequency of the cavity 3. A filter typically contains multiple cavities and the resonators in each cavity electromagnetically couple together to form a filter response. The filter is tuned during the manufacturing process by rotating the tuning screws on the lid and setting them in the required position. When in position they are locked using a nut 15. It is clear that such a filter cannot change its response in the field as the tuning element is locked in place.

In figure 3 there is illustrated a resonant cavity with remote tuning as per the prior art. The same numerals are used as before to indicate similar features. A stepper motor 17 is provided on an external surface of the filter lid to drive movement of the tuning screw 13. The stepper motor is electrically controlled and control signals can be sent to it while in the field to facilitate retuning the filter. It is clear that the stepper motor is large and adds considerably to the size of the filter which would contain several such cavities with a separate stepper motor provided for each cavity.

In figure 4 there is illustrated an embodiment of the present invention. In accordance with the present invention, resonating means in the form of resonator 5 has a resonator cavity defined in the same and movement means in the form of an electrically powered stepper motor and tuning means in the form of a tuning element 23 are located in the cavity.

In the example provided in figure 4, the resonator cavity is in the form of a hole 9 that is the full length of the resonator 5. The hole 9 contains an electrically powered motor means, in this case a small stepper motor 27. The shaft of the stepper motor 25 engages with tuning element 23. In this embodiment the tuning element 23 has an external screw thread and rotatably engages with a complementary screw thread on an inner surface of the resonator 5 defining the hole 9. Therefore, rotating the shaft 25 causes the tuning element 23 to rotate and overall move vertically up or down in a linear manner depending upon the direction of rotation.

In this embodiment the tuning element 23 is made of a di-electric material, but equally could be made of a metallic material, ceramic material or a combination thereof. Increasing the protrusion of the di-electric tuning element from the resonator has the effect of increasing the capacitance between the top end (i.e. the end closest to the lid) of the resonator 5 and the lid 11 and reducing the associated resonant frequency. A metal, or metallised tuning element both increases the capacitance between the top end of the resonator 5 and the lid 11 and effectively increases the electrical length of the resonator when it protrudes further from the resonator, both of these physical changes reduce the resonant frequency.

The stepper motor 27 is linked by wire 19 to a control means 21 located on an external surface of the filter housing. The control means consists of an electric circuit produced on a printed circuit board. The bottom of the cavity 3 has been stepped 29 to allow the control means to be housed within the original outline of the filter housing. A small, localised step such as 29 has a negligible impact on the performance of the resonant cavity 3. Therefore, the control means do not add to the volume of the filter.

Figure 5 illustrates another embodiment of the present invention. In this embodiment the tuning element is T-shaped, consisting of a first portion 29 and a perpendicular portion 31 that protrudes outwardly from the first portion. In this example, the first portion is a cylindrical portion 29 and the perpendicular portion 31 is a disc. Either portion 29, 31 can be made of dielectric, ceramic, metallic, or metallised material. The two portions need not be made of the same material. The disc allows a greater change in the capacitance between the top of the resonator 5 and the lid 11 than the embodiment shown in the figure 4 for the same change in protrusion of the tuning element from the resonator 5.

Figure 6 illustrates another embodiment of the present invention. In this embodiment the tuning element consists of a first or cylindrical portion 29 and a second protruding portion or disc 31. The second protruding portion or 31 can be provided with flanges 33 that protrude downwardly or towards an opposite end of the resonator 5. Any of the portions can be made of dielectric, ceramic, metallic, or metallised material. The portions need not be made of the same material. The disc and flange arrangement allows a greater change in the capacitance between the top of the resonator 5 and the lid 11 than the embodiment shown in the figure 4 for the same change in protrusion of the tuning element from the resonator 5.

Figure 7 illustrates an embodiment of the current invention that contains two filter housing cavities 3. The control means 21 is shared between the stepper motors 27 provided in each of the cavities. This reduces the component count and allows for coordinated tuning of the resonators within the cavities. The control means is connected to a remote device via a wired or unwired connection allowing remote tuning of the filter apparatus when on a cell site in the field in use. The control means could extend beneath the resonators 5 allowing direct connection / mounting of the stepper motors onto the printed circuit board.

Claims (25)

1. Tunable filter apparatus, said apparatus including a housing with at least one housing cavity defined therein, at least one resonating means provided in said at least one housing cavity, tuning means provided for tuning said at least one resonating means in use to provide a predetermined resonant frequency of said at least one resonating means, movement means provided for allowing movement of the tuning means in use, characterised in that the at least one resonating means has a resonator cavity defined therein and the movement means are provided in said resonator cavity of said at least one resonating means.

2. Apparatus according to claim 1 wherein the tuning means are wholly or at least partially located in the resonator cavity of said at least one resonating means.

3. Apparatus according to claim 1 wherein the movement means are electrically powered movement means.

4. Apparatus according to claim 1 wherein the movement means are directly or indirectly connected to the tuning means.

5. Apparatus according to any of claims 1-4 wherein the movement means include any or any combination of pneumatic, mechanical or hydraulic means.

6. Apparatus according to any of claims 1-5 wherein the movement means is an electrically powered motor.

7. Apparatus according to claim 6 wherein the electrically powered motor is a stepper motor or a piezoelectric motor.

8. Apparatus according to any of claims 1-5 wherein the movement means is an actuator.

9. Apparatus according to claim 8 wherein the actuator is a piezoelectric actuator, a shape memory alloy (SMA) actuator or a shape memory alloy (SMA) wire actuator.

10. Apparatus according to any of claims 1-9 wherein control means are provided with or associated with the movement means to allow control of the movement means and/or tuning means in use.

11. Apparatus according to claim 10 wherein the control means consists of or includes an electronic control circuit.

12. Apparatus according to claims 10 or 11 wherein the control means is controlled remotely via wired or wireless means.

13. Apparatus according to any or claims 10-12 wherein the control means is housed within the housing of said filter housing and/or is located within one or more walls defining the at least one housing cavity of said filter housing.

14. Apparatus according to any preceding claim wherein a lid is attached to or associated with the housing to cover or substantially cover the at least one cavity in use.

15. Apparatus according to any preceding claim wherein the one or more interior facing walls of the filter housing defining said at least one cavity and/or a lid of the at least one cavity is formed from or has a layer of electrically conductive material thereon.

16. Apparatus according to any preceding claim wherein two or more housing cavities are defined in said housing and each of said two or more housing cavities has resonating means provided therein; each of said two or more of said resonating means having a resonator cavity defined therein and movement means are provided in each of said resonator cavities of said two or more resonating means.

17. Apparatus according to any preceding claim wherein the tuning means is arranged to undergo rotational, sliding and/or linear movement when being moved to tune the one or more resonating means in use.

18. Apparatus according to claim 1 wherein the resonator cavity is a through hole, a blind hole or a recess defined in said resonating means.

19. Apparatus according to any preceding claim wherein the total achievable change in resonant frequency using the tuning means is equal to or greater than 1%, equal or greater than 5%, equal or greater than 10%, or equal or greater than 25%.

20. Apparatus according to any preceding claim wherein the resonating means is or includes any or any combination of one or more rods, pucks or posts.

21. Apparatus according to any preceding claim wherein the resonating means is or includes any or any combination of di-electric material, ceramic material, metallic material, or a non-metallic material with a metallic surface or outermost layer.

22. Apparatus according to any preceding claim wherein the resonating means is galvanically connected to a base or wall of the housing cavity or to a surface of a lid facing the housing cavity.

23. Apparatus according to any of claims 1-21 wherein the resonating means is in the form of a protrusion protruding outwardly from a wall defining the housing cavity or from a lid of the housing cavity, the protrusion being integral with the housing cavity wall or lid, or the protrusion being attached to the housing cavity wall or lid via attachment means.

24. A method of adjusting the operating frequency and/or bandwidth of tunable filter apparatus, said apparatus including a housing with at least one housing cavity defined therein, at least one resonating means provided in said at least one housing cavity, said method including the step of moving tuning means to provide a pre-determined resonant frequency of said at least one resonating means using movement means, characterised in that the at least one resonating means has a resonator cavity defined therein and the movement means are provided in said resonator cavity of said at least one resonating means.

25. A method according to claim 24 wherein tuning of the tunable filter apparatus can take place at any time pre- or post- manufacture of the tunable filter apparatus.