WO2009056813A1

WO2009056813A1 - Tunable filter and method of use thereof

Info

Publication number: WO2009056813A1
Application number: PCT/GB2008/003645
Authority: WO
Inventors: Philip David Sleigh; Elizabeth Francis Phillips
Original assignee: Radio Design Limited
Priority date: 2007-10-30
Filing date: 2008-10-27
Publication date: 2009-05-07
Also published as: GB0721361D0; EP2203953A1; WO2009056813A9; EP2203953B1

Abstract

A tunable filter is provided. The filter includes a housing with two or more cavities defined therein. Primary resonating means are located in each of the cavities and each primary resonating means is capable of resonating at a first frequency. The tunable filter further includes secondary resonating means associated therewith which are capable of resonating at a second frequency. The secondary resonating means are arranged such that each primary resonating means has a secondary resonating means coupled therewith to form a primary-secondary resonator pair. The second frequency of the secondary resonating means is different to the first frequency of the primary resonating means. The tunable filter includes adjustment means for adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonating means.

Description

TUNABLE FILTER AND METHOD OF USE THEREOF

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

Conventionally, filters have comprised a conductive housing defining a plurality of resonant 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 screw 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 impractical once the tuning screws have been locked post deployment in die 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 die resonator frequencies substantially equally or individually, allowing each resonator frequency to be adjusted independendy 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. It is therefore an aim of the present invention to produce a tunable filter which resolves the issues described above.

It is a further aim of the present invention to provide a method of using a tunable filter.

It is a yet further aim of the present invention to provide communication apparatus incorporating a tunable filter and a method of using said 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, said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency, 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, the second frequency of the secondary resonating means being different to the first frequency of said primary resonating means, and wherein said tunable filter includes adjustment means for adjusting the coupling within each primary and secondary resonator pair, and/or the resonant frequency of the secondary resonator means.

Thus, the tunable filter of the present invention allows either or both of: a) the centre operating frequency of the filter to be moved to a different frequency; and /or b) the bandwidth of the filter to be changed (i.e. for the size of the bandwidth to be increased or decreased to cover a greater or reduced frequency range).

This is in contrast to prior art tunable filters which only allow adjustment of the centre operating frequency. In addition, the means of adjusting the centre frequency in prior art tunable filters is different to that of the present invention.

In prior art tunable filters, an adjuster element is introduced into each resonant cavity to influence die frequency of a primary resonator located in said cavity. The primary resonator is fixed and the adjuster element is movable. In the present invention, secondary resonating means are associated with each primary resonating means to influence the frequency of the primary resonating means and the adjustment means are provided to allow adjustment of coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonator. The filter of the present invention therefore has the advantage that the filter is easier to manufacture, it allows complete filter parametric adjustment to be achieved and the filter is less sensitive to adjustments, thereby allowing tuning of the filter to be more consistently reproducible compared to prior art filters. Furthermore, by adjusting the resonant frequency of the secondary resonator the rate of change of adjustment of the coupling between the primary-secondary resonator pair can be adjusted allowing a greater potential tuning range of the filter characteristics.

The secondary resonating means can be located in a cavity of the tunable filter with a primary resonating means or can be located externally of the cavity housing the primary resonating means. In either embodiment 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.

Preferably the filter housing is formed from conductive material. Further preferably the conductive housing has a plurality of cavities defined therein for the location of at least the primary resonating means and preferably the primary-secondary resonating pair.

The primary resonating means is arranged so as to resonate close to the desired operating frequency of the filter. Tuning means are typically associated with each primary resonating means to allowing tuning of the primary resonating means to a pre-determined or first frequency on manufacture or initial set up of said filter. Thereafter, tuning of the primary resonating means takes place via the adjustment means in accordance with the present invention. The tuning means are preferably located direcdy above the primary resonating means in the cavity (i.e. substantially vertically above and within the outer perimeter of the primary resonating means).

In one embodiment the secondary resonating means are provided with tuning means for allowing the secondary resonating means to be tuned to a pre-determined frequency or second frequency. The tuning means of die 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).

The tuning means can be in the form of a rotatable threaded screw and/or the like. Once the pre-determined frequency is achieved, 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 diat 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 one embodiment the secondary resonating means is substantially fixed relative to the housing following set-up of the filter (i.e. once any tuning means, if present, have been locked). Preferably the primary resonating means are substantially fixed relative to the housing following set-up of the filter (i.e. once tuning means if present have been locked). The adjustment means are typically movable relative to the primary and/or secondary resonating means.

Thus, in contract to prior art filters in one embodiment of the present invention, the primary and secondary resonating means are substantially fixed and the adjustment means are movable relative to both said primary and secondary resonating means.

The primary resonating means in each of the two of more cavities of the filter are coupled together to form the required filter response.

In one embodiment the adjustment means for adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonator 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 an alternative embodiment the adjustment means for adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonator are connected via connection means such that each resonator pair and/or secondary resonator means can be adjusted substantially simultaneously.

In a preferred embodiment, in order to achieve adjustment of the centre frequency of the filter, a first adjustment means can be used to influence the coupling within the primary-secondary resonator pair. In order to achieve a change in the bandwidth of the filter, second adjustment means are used in addition to or as an alternative to the first adjustment means to influence the resonating frequency of the secondary resonating means. Both the first and second adjustment means can be controlled remotely without a user having the access the interior of the cavity to make the adjustment.

Thus, in the embodiment wherein the first adjustment means are capable of adjusting the coupling within the primary-secondary resonating pair, the first adjustment means are preferably located laterally of the primary resonating means and between the primary and secondary resonating means.

In one embodiment the first adjustment means of two or more primary- secondary resonating pairs in 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 the embodiment wherein the second adjustment means are capable of adjusting the resonating frequency of the secondary resonating means, the second adjustment means can be located laterally of the secondary resonating means or, in one embodiment, can be located above or integraly with the secondary resonating means. In one embodiment the second adjustment means of two or more secondary resonating means can be connected together via connection means. This allows the frequency of two or more secondary resonating means to be adjusted substantially simultaneously.

The connection means can be any suitable connection means. 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 diode adjuster, 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 die 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 lid of the filter. For example, the connection means can be moved from side to side in a direction substantially parallel to the longitudinal axis of the filter. However, the connection means could be slidably movable, pivotably movable and/or rotatable. For example, the connection means could be a ring or disc member located with die lid of the filter to which the adjustment means are connected at spaced apart intervals therealong. Rotation of the connection means could cause sliding movement of the adjustment means relative to the primary and/or secondary resonating means.

In a preferred embodiment the connection means are located underneath the lid of the filter and movable within the cavities of the filter, thereby substantially 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.

Guide means can be associated with the connection means to help guide the connection means during movement of the same and for supporting the same. 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 manually.

In one embodiment adjustment of the adjustment means can be undertaken automatically using suitable drive means. The drive means can include any or any combination electrical, pneumatic, mechanical, hydraulic means and/or the like and 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.

Control means can be associated with the adjustment means and/or connection means to allow control of the adjustment. Control of the adjustment means can be undertaken remotely.

In one embodiment the adjustment means includes one or more adjustment elements. The adjustment means can be formed at least in part from dielectric material or from conductive material. In the case of di-electric material, the adjustment means can engage or abut with the primary and/or secondary resonating means. In die case of conductive material, the adjustment means are typically a spaced distance apart from the primary and/or secondary resonating means.

The adjustment means can be located internally or externally 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.

The adjustment means are preferably capable of undergoing rotatable, pivotable and/or slideable movement during the adjustment process.

Each associated primary and secondary resonating means are typically located a spaced distance apart and laterally from each other. For example, in one embodiment the primary and secondary resonating means can be located at any suitable position within a cavity of the filter 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 primary resonating means with which it is associated or coupled.

The primary resonating means is preferably located on a base, side wall and /or on a lid of the defined cavity. It can take the form of a solid post, hollow cylindrical post and/or the like. The secondary resonating means can be located on a base, side wall and/or on a lid associated widi the cavity and also take the form of a solid post, hollow cylindrical post and/or the like. In either case the primary and secondary resonating means are of such a form, size, shape and/or material to allow them to resonate at a desired resonating frequency. In one embodiment two adjustment means can be located in or associated with each cavity. Thus, for example, first adjustment means can be provided to adjust the coupling between the primary-secondary resonating pair and at least second adjustment means can be provided to adjust the resonating frequency of the secondary resonating means.

Preferably the difference between the first and second frequencies of the between the primary and secondary resonating means is less than or substantially equal to three times the resonating frequency of the primary resonating means.

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).

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 adjusting the operating frequency and/or bandwidth of 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, said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency, 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, the second frequency of the secondary resonating means being different to the first frequency of said primary resonating means, and wherein said method includes the step of adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonating means.

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.

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, a greater overall tunable frequency range can be achieved and a simpler mechanical implementation can be utilised.

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

Figure 1 shows a simplified circuit diagram of a tunable filter design illustrating the basic principles of the prior art; Figure 2 shows a simplified circuit diagram of a tunable filter design illustrating the basic principle of the present invention in one embodiment;

Figures 3a and 3b show cross sections of a top and a side respectively of part of a filter according to a first embodiment of the present invention showing a primary-secondary resonator pair and coupling adjustment;

Figures 4a and 4b show cross sections of a top and side respectively of part of a filter according to a second embodiment of the present invention showing a primary-secondary resonator pair and coupling adjustment;

Figures 5a, 5b and 5c show cross sections of a top, side and end respectively of a five section tunable filter in a first operating position according to an embodiment of the present invention;

Figures 6a, 6b and 6c show cross sections of a top, side and end respectively of the five section tunable filter in figures 5a-5c in a second operating position according to an embodiment of the present invention;

Figure 7 illustrates a graph of Frequency vs Return/Insertion Loss and shows a change in the operating frequency of the filters in figures 5a-6c following adjustment of the adjustment means.

Figures 8a and 8b show cross sections of a top and side respectively of part of a tunable filter according to a further embodiment of the present invention. Figures 9a, 9b and 9c show cross sections of a top, side and end respectively of a five section tunable filter in a first operating position according to a further embodiment of the present invention; and

Figures 10a, 10b and 10c show cross sections of a top, side and end respectively of the five sectioned tunable filter in figures 9a-9c in a second operating position.

Referring firstly to Figure 1, there is illustrated a simplified circuit diagram of a tunable filter of the prior art. The filter includes a conductive housing defining a plurality of cavities therein. Each cavity has a primary resonator (R) located therein. An adjuster (not shown) is provided in each cavity to allow adjustment of the frequency of each primary resonator. Adjustment of the adjuster has the effect of changing the loading capacitance (Cap) to the resonator R. The adjusters of least two resonators (Rl, R2; R(n-l), R(n)) are connected together so that the frequency of each resonator Rl, R2; R(n-l), R(n) can be adjusted substantially simultaneously. This results in the loading capacitance (Capl, Cap2; Cap(n-l), Cap(n)) of each resonator being adjusted substantially simultaneously by equal amounts.

In Figure 2 there is illustrated a simplified circuit diagram of a tunable filter according to an embodiment of the present invention. The filter includes a conductive housing defining a plurality of cavities therein. Each cavity contains a Primary Resonator (PR) and a Secondary Resonator (AR) electromagnetically coupled (CA) to the PR, to form a primary-Secondary resonator pair. Adjustment means (not shown) are associated with each cavity to allow adjustment of the coupling between said primary-secondary resonator pairs. Adjustment of the adjustment means has the effect of changing the resonant frequency of the resonator PR. The adjusters of least two primary resonators (PRl, PR2; PR(n-l), PR(n)) are connected together so that the frequency of each primary resonator PR can be adjusted substantially simultaneously thus changing the frequency of the filter parameters.

The adjustment means can be arranged so as to adjust the frequency of the secondary resonator and/or the coupling between each Primary-Secondary pair, thereby changing the centre frequency of the filter, and/or bandwidth of the filter.

A more detailed explanation of the present invention will now be given with reference to Figures 3a and 3b. The figures show cross sectional views of a top and side of one section of a tunable filter according to an embodiment of the present invention. More particularly, the section is defined in a housing 4 having a base 6, side walls 8 and a lid 10. The housing is formed from conductive material, such as for example metal. A cavity 2 is defined between the base, side walls and a lid. The cavity is typically one of a plurality of cavities defined in the housing of the filter but only one cavity is shown in these figures for the purposes of clarity.

A primary resonator 12 in the form of a conductive post is located on base 6 of the cavity 2 and protrudes upwardly into the cavity. Tuning means in the form of a frequency adjustment screw 14 is inserted through lid 10 to allow tuning of the primary resonator 12 during manufacture of the filter. Screw 14 is in the form of a threaded screw having locking means in the form of a locking nut 16 located thereon. Once the primary resonator has been tuned to a desired operating frequency on set up of the filter, the locking nut is locked to prevent tuning of the filter via the adjustment screw 14.

A secondary resonator 18 in the form of a conductive post is located on base 6 of the cavity 2 and also protrudes upwardly into the cavity. The size, material and/or position of the secondary resonator 18 is such so as to allow electromagnetic coupling with the primary resonator 12 to form a primary- secondary resonator pair.

Adjustment means in the form of an adjuster element 20 is pivotally mounted in cavity 2 and is located between die primary resonator 12 and the secondary resonator 18. The purpose of die adjuster element is to adjust the electromagnetic coupling between the primary and secondary resonators 12, 18 (i.e. adjust die coupling within die primary-secondary resonator pair). The adjuster element can be formed from di-electric material or conductive material as required. In the illustration, element 20 is pivotable about a substantially horizontal axis. However, it will be appreciated that die adjuster element can be moved dirough any suitable angle and/or about any suitable axis providing that suitable adjustment of the coupling between die primary- secondary resonator pair is achieved.

Pivotal movement of adjuster element 20, as shown by arrow 22, influences the electromagnetic field within die primary-secondary resonator pair. This causes a change in the centre operating frequency of die primary resonator 12. This in turn changes the loading of the primary resonator thus changing the resonating frequency associated with dierewith.

The secondary resonator 18 can be located in any suitable position within or externally to cavity 2 providing it is coupled to primary resonator 12. For example, figures 4a and 4b illustrates a similar filter to that of figures 3a and 3b but secondary resonator 18 is attached to lid 10 in this embodiment. More particularly, tuning means in the form of a screw thread 24 is associated with an external end of secondary resonator 18 to allow the frequency of secondary resonator 18 to be tuned during manufacture of die filter. Locking means in die form of a locking nut 26 can be provided on screw thread 24 to lock the same once initial tuning has taken place. The opposite end of resonator 18 protrudes into cavity 2 and projects downwardly into the cavity (i.e. protrudes in an opposite direction to primary resonator 12).

It will be appreciated that the tuning means could also be used as second adjustment means for adjustment of the resonating frequency of the secondary resonator 18 even after initial set up of the filter. For example, a self locking screw 24 could be used which is integral with the secondary resonator 18 to allow movement of the secondary resonator after initial set up of the filter has been completed.

In this illustrated embodiment, adjuster element 20 is slidably mounted in the cavity 2 for sliding movement between the primary resonator 12 and secondary resonator 18, as shown by arrow 28. Sliding movement of adjuster element 20 acts in a similar manner to the adjuster element in figures 3a and 3b to influence the coupling within the primary-secondary resonator pair, thereby changing the frequency of the primary resonator.

As mentioned previously, a filter typically includes a plurality of cavities defined therein and each cavity typically contains a primary-secondary resonator pair and an adjustment element, as shown in figures 5a-6c. More particularly, figures 5a-5c illustrate a filter housing 100 having five sections or cavities 102 defined therein. Each cavity communicates with an adjacent cavity via an iris or aperture.103.

The filter housing 100 has a base 106, side walls 108 and a lid 110. A primary resonator 112 is located in each cavity 102 and has tuning means in the form of an adjustment screw 114 located in lid 110 and associated with each resonator 112. This allows each of the plurality of primary resonators to be individually and independently tuned during manufacture of the filter. A locking nut 116 is provided to lock the adjustment screw in place once it has been correctly positioned. Transformers 109 are used to electromagnetically couple the signal to/ from the input/output connectors 111 to the cavities.

A secondary resonator 118 is attached to lid 110 and protrudes inwardly of the cavity in a direction towards base 106. A screw thread 124 is associated with an external lid end of resonator 118 to allow tuning of the secondary resonator 118 on manufacture and a locking nut 126 is provided to lock screw 124 following tuning, as previously described in figures 4a-4b.

An adjuster element 120 is located in each cavity 102 and the adjuster elements 120 of all five cavities are connected together via connection means in the form of a common control rod 128. Control rod 128 is slidably mounted within housing 100 to allow all five adjuster elements 120 to be slidably moved substantially simultaneously on actuation of the control rod. More particularly, control rod 128 is slidably mounted in a plurality of guide channels defined in an upper edge of the side walls of the housing. Figures 5a-5c illustrate a first position of the adjuster elements 120 and figures 6a-6c illustrate a second position of the adjuster elements 120 following actuation of control rod 128 in use. This allows the frequency of all the primary resonators 112 to be adjusted by a substantially equal amount substantially simultaneously, thereby creating an accurate and substantially reproducible tuning method for the filter. The provision of the secondary resonator 118 in addition to the adjuster element 120 provides allowing a greater tunable frequency range and simpler mechanical implementation compared to the use of an adjuster element and a primary resonator alone as is used in the prior art.

Actuation of the control 128 can be performed manually or can be performed automatically using for exampled one or more powered motors. In either case actuation of control 128 can be performed directly or remotely. Figure 7 illustrates how the frequency of the filter in figures 5a-6c can be adjusted on moving the adjuster elements 20 from the first to the second position. Lines 'A' and 'B' show the lowest and highest operating frequencies of the filter following bandwidth adjustment. Thus, it can be seen that the centre operating frequency of the filter can be adjusted using the described invention. ^v

Referring to figures 8a-8b, there is illustrated a further embodiment of the present invention in which the range of the bandwidth of the filter can be increased or decreased. This embodiment can be used and/or can be provided independently of the embodiments previously described or can be used and/or can be provided in combination with die previously described figures.

More particularly, a single cavity of a multiple cavity tuner is illustrated in figures 8a-8b. Similar reference numbers have been used to denote similar features in figures 4a and 4b. In addition to first adjuster element 20, a second conductive or dielectric adjuster element 200 is provided between the side wall 8 of the filter housing and die secondary resonator 18. Provision of the second adjuster element 200 in the cavity allows the resonant frequency of the secondary resonator 18 to be adjusted independently of the primary resonator 12 by a sliding movement as indicated by the arrow 222.

As mentioned previously, a filter typically includes a plurality of cavities defined therein and each cavity typically contains a primary-secondary resonator pair and an adjustment element. Figure 9a, 9b and 9c show a similar filter to die one in figure 5a, 5b and 5c with an additional second tunable element 131 enabling the tuning adjustment of the resonant frequency of the secondary resonator 130.

Each of the additional tunable elements are connected together via connection means in the form of a common control rod 128. Control rod 128 is slidably mounted within housing 100 to allow all five additional adjuster elements 131 to be slidably moved substantially simultaneously on actuation of the control rod. More particularly, control rod 128 is slidably mounted in a plurality of guide channels defined in an upper edge of the side walls of the housing. Figures 9a-9c illustrate a first position of the adjuster elements 131 and figures 10a- 10c illustrate a second position of the adjuster elements 131 following actuation of control rod 128 in use. This allows the frequency of all the secondary resonators 130 to be adjusted by a substantially equal amount substantially simultaneously, thereby creating an accurate and substantially reproducible tuning method for the filter bandwidth and frequency.

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, said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency, 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, the second frequency of the secondary resonating means being different to the first frequency of said primary resonating means, and wherein said tunable filter includes adjustment means for adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonating means.

2. A tunable filter according to claim 1 wherein the secondary resonating means is located internally of the filter housing with the primary resonating means.

3. A tunable filter according to claim 1 wherein the secondary resonating means is located externally of the filter housing or cavity in which the primary resonating means is located.

4. The tunable filter according to claim 1 wherein the adjustment means for adjusting two or more primary-secondary resonating pairs in the filter are connected together via connection means, such that the coupling within each primary-secondary resonating pair of the two or more primary-secondary resonating pairs can be adjusted substantially simultaneously.

5. The tunable filter according to claim 1 wherein the adjustment means for adjusting two or more secondary resonating means are connected together via connection means, such that the resonating frequency of each secondary resonating means can be adjusted substantially simultaneously.

6. The tunable filter according to claim 1 wherein the adjustment means for adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonating means are arranged such that each resonator pair and/or secondary resonator can be adjusted substantially independently of any other resonator pair and/or secondary resonating means in said filter.

7. The tunable filter according to claim 1 wherein the adjustment means are adjusted manually.

8. The tunable filter according to claim 4 wherein the adjustment means are driven by any or any combination of electrical, pneumatic, and/or mechanical means.

9. The tunable filter according to claim 1 wherein the adjustment means includes one or more adjustment elements formed, at least in part, from conductive material.

10. The tunable filter according to claim 1 wherein the adjustment means includes one or more adjustment elements formed, at least in part, from dielectric material.

11. The tunable filter according to claim 1 wherein the adjustment means and/or connection means for connecting a number of adjustment means together are capable of undergoing rotatable, pivotable and/or slideable movement during the adjustment process.

12. The tunable filter according to claim 9 wherein the one or more adjustment elements are engaged with or capable of being engaged with the primary and/or secondary resonator means within the primary-secondary resonating pair.

13. The tunable filter according to claim 8 wherein the one or more adjustment elements are a spaced distance apart from the primary and/or secondary resonating means within the primary-secondary resonating pair.

14. A method of adjusting the operating frequency and/or bandwidth of 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, said tunable filter further including secondary resonating means associated therewith which are capable of resonating at a second frequency, 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, the second frequency of the secondary resonating means being different to the first frequency of said primary resonating means, and wherein said method includes the step of adjusting the coupling within each primary and secondary resonator pair and/or the resonant frequency of the secondary resonating means.