The invention relates to the area of tunable frequency filters for electronic circuits, for example in the area of wireless and microwave systems and components, e.g. for communication systems.

The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 811232.

Prior Art

[1 ] Smith, David John; Yu, Ming, "microwave resonator and filter assembly”, U.S. Patent US 7,075,392 B2, Jul. 11 , 2006.

[2] Ming Yu, D. Smith and M. Ismail, "Half-wave dielectric rod resonator filter," 2004 IEEE MTT-S International Microwave Symposium Digest (IEEE Cat. NO.04CH37535), 2004, pp. 619-622 Vol.2, doi: 10.1109/MWSYM.2004.1336060.

[3] D. Sh-Asanjan and R. R. Mansour, "A novel coaxial resonator for high power applications," 2014 44th European Microwave Conference, 2014, pp. 295-298, doi: 10.1109/EuMC.2O14.6986428.

[4] Chi Wang, K. A. Zaki, A. E. Atia and T. G. Dolan, "Dielectric combline resonators and filters," in IEEE Transactions on Microwave Theory and Techniques, vol. 46, no. 12, pp. 2501-2506, Dec. 1998, doi: 10.1109/22.739240. [5] Basavarajappa et al, "Tunable filter with minimum variations in absolute bandwidth and insertion loss using a single tuning element”, U.S. Patent US 10,957,960 B2, Mar. 23, 2021.

[6] G. Basavarajappa and R. R. Mansour, "Design Methodology of a High-Q Tunable Coaxial Filter and Diplexer," in IEEE Transactions on Microwave Theory and Techniques, vol. 67, no. 12, pp. 5005-5015, Dec. 2019, doi:

10.1109/TMTT.2019.2937770.

[7] Kwak et al., "variable high frequency filter device and assembly”, U.S. Patent US 9,614.265 B2, Apr. 4, 2017.

[8] Huang et al., "Three dimensional tunable filters with an absolute constant bandwidth and method”, U.S. Patent US 2016/0049710 A1 , Feb. 18, 2016.

[9] S. Fouladi, F. Huang, W. D. Yan and R. R. Mansour, "High-Q Narrowband Tunable Combline Bandpass Filters Using MEMS Capacitor Banks and Piezomotors," in IEEE Transactions on Microwave Theory and Techniques, vol. 61 , no. 1 , pp. 393- 402, Jan. 2013, doi: 10.1109/TMTT.2012.2226601 .

[10] Etienne et al., "Tuneable ultra-high frequency filter with mode TM010 dielectric resonators”, U.S. Patent 4,578,655, Mar. 25, 1986.

[11] Ala-Kojola, "Resonator filter”, U.S. Patent US 2005/0212623 A1 , Sep. 29, 2005.

[12] Han et al., "Frequency-tunable filter”, U.S. Patent US 9,035,727 B2, May 19, 2015.

[13] Piri et al., "Adaptable resonator filter”, U.S. Patent US 9,196,942 B2, Nov. 24, 2015.

[14] Wada, "Tunable filter, duplexer and communication apparatus”, US 7439,828 B2, Oct. 21 , 2008.

[15] Park et al., "Variable radio frequency band filter”, U.S. Patent US 7825,753 B2, Nov. 2, 2010.

[16] Shiroyama et al., "Tunable band - pass filter”, U.S. Patent US 9 , 786 , 974 B2, Oct . 10 , 2017.

[17] G. B. and R. R. Mansour, "A Tunable Quarter-Wavelength Coaxial Filter With Constant Absolute Bandwidth Using a Single Tuning Element," in IEEE Microwave and Wireless Components Letters, vol. 31 , no. 6, pp. 658-661 , June 2021 , doi: 10.1109/LMWC.2021 .3064381 .

[18] Cavey, "Tunable ceramic filters”, U.S. Patent 6,147,577, Nov. 14, 2000.

[19] Tao Shen, K. A. Zaki and Chi Wang, "Tunable dielectric resonators with dielectric tuning disks," in IEEE Transactions on Microwave Theory and Techniques, vol. 48, no. 12, pp. 2439-2445, Dec. 2000, doi: 10.1109/22.898995.

[20] Tihonen, "Tunable resonator”, U.S. Patent US 6,664,873 B2, Dec. 16, 2003.

[21] https://mpgdover.com/

[22] http://www.colemanmw.com/tun_wave_filt.asp

[23] https://www.wainwright-filters.com/standard-filters/digitall y-tunable-band-pass- fi Iters

[24] https://www.smithsinterconnect.com/products/rf-mw-mmw-compon ents/rf- filters/rf-tunable-filters/

[25] http://www.klmicrowave.com/

[26] https://www. pasternack.com/nsearch. aspx?Category=Tunable+Filters&sort=y&view_typ e=grid

[27] https://www.bench.com/lark/tunable-filters

[28] https://www.fairviewmicrowave.com/nsearch. aspx?Category=Tunable+Filters&sort=y&vi ew_type=grid

[29] https://nuwaves.com/rf-microwave-product-solutions-catalog/h ipertuner-rf- broadband-preselector/

[30] TESAT.de/

[31 ] https://www.spinner-group.com/en/

[32] rfmicrotech.com/

[33] htps://www.huawei.com/en/

[34] Y. Kobayashi and S. Yoshida, "Bandpass Filters Using TM/sub 010/ Dielectric Rod Resonators," 1978 IEEE-MTT-S International Microwave Symposium Digest, 1978, pp. 233-235, doi: 10.1109/MWSYM.1978.1123848 Object of the invention

The implementation of high-power compatible, low-loss, compact, and lightweight RF front-end components, including filters, has become very essential to meet the ever- stringent requirements of the evolving high-performance wireless communication systems. Considering these challenging requirements, coaxial and dielectric-loaded waveguide bandpass filters are getting an increasing amount of attention in research and industry due to their ability to handle high-power levels and high unloaded quality factor (Qu), with the additional benefit of miniaturized size when compared to large, bulky air-filled waveguide structures.

These types of filters are commonly realized using the conventional half -wavelength and combline resonator structure as in [1 ]-[4]. Nevertheless, the introduction of more volume-saving, high-Q resonator topologies with enhanced spurious performance is still highly appreciated, especially that filtering components occupy the largest space of the RF front-end units.

Similarly, high-Q frequency tunable filters are a key part of future flexible RF and satellite payloads. Various tunable filter designs were presented based on conventional half-wavelength and combline configurations [5]-[20], A 2.5 GHz tunable A/2 coaxial filter was presented in patent [5] and publication [6] with a 20% tuning range using a common tuning rod. The operation concept is based on the rotation of the halfwavelength resonators inside an el I iptical ly-shaped cavities with an angle varies from 0° to 90°. The space between the resonators and the elliptical cavity changes with the rotation and causes a change in the operation frequency. Apparently, this mechanism of tuning has limited tuning capabilities since it functions only in a specific part of the cavity (0° - 90°). Besides, such tuning mechanism experiences noticeable deterioration in the quality factor when the resonators are rotated closer to the cavity which increases the losses. The bulky, large size and degraded spurious responses are further drawbacks of this half-wavelength configuration. On the other hand, couple combline-based tunable resonators and filters were presented using different loaded tuning elements [7]-[20], A straight-forward approach is through using mechanically, vertically movable tuning screws or disks on the vicinity between the combline resonators and the top part of the metallic cavity [7]-[9], As the tuning element moves towards the combline resonator, more electromagnetic field is perturbed and the resonant frequency decreases accordingly. For example, patent [7] introduced frequency reconfigurable combline filters using frequency tuning posts controlled by a common rotational cam. Also in invention [8] and article [9], A/4 tunable coaxial combline filters were implemented using metallic tuning discs controlled using individual piezomotors. This category of vertically movable tuning elements suffers from severe deterioration in the Qu as the tuning elements approach the combline resonators, and also the possible appearance of unwanted spurious resonances. Excellent electrical contact must be guaranteed between the top part of the cavity and the tuning elements (i.e. using nuts), otherwise the quality factor will be impacted badly, and the tuning range will decrease. Besides, the volume of such filters is relatively large since the height of the cavity must be increased to provide a room for the tuning screws/disks inside the cavities. In another configuration of this vertical movement-based tuning, the combline resonator can be made movable and the frequency can be changed without the need for any further tuning elements as was presented in patent [10] for TM010 mode dielectric resonators (DRs). The change in gap between the moving TM DRs and the top cover of the filter cavity changes the capacitance and reconfigure the resonant frequency. Holes are made in the bottom side of the cavity and the TM DRs are inserted in the cavity through these holes. Similar to the other designs, the quality factor decreases as the TM resonator moves closer to the top cavity. Additionally, such structures are not suitable for tunable filter applications since special arrangements are always needed to fix the resonators in positions which increase the loss, cost and reduce the lifetime of the filters. Besides, the TM dielectric resonators must be elongated, which adds more cost and weight.

Another group of tunable coaxial combline filters is employing horizontally sliding metallic or dielectric elements mounted on a common tuning rod and attached to the top part of the cavity [11 ]-[13], The movement of the common sliding rod changes the effective gap (capacitance) between the combline resonators and their respective tuning elements causing a shift in the resonant frequency. In comparison with the aforementioned group of vertically movable tuning elements, this configuration features easier tuning process. However, the frequency tuning windows in these sliding tuning structures are very narrow since less EM field can be interrupted. A third group of tunable combline filters is similar to the second configuration but uses rotational tuning elements instead of/with the sliding tuning elements [14]-[17], This concept also shares the same major disadvantage of very limited tuning ranges with the sliding-based tuning.

Another tuning mechanism was presented in patent [18] and article [19] for TE mode dielectric resonators based on double-resonator configuration. This design operates similar to earlier mentioned vertically movable tuning elements in combline structures. Here, one of the two TE dielectric resonators is fixed while the other is made movable to change the operation frequency. The advantage of this concept is the high unloaded quality factor thanks to the use of dielectric resonators as tuning components. Nevertheless, besides the increase in the filter size, this kind of designs suffers from the narrow tuning ranges and poor spurious performance. Also, patent [20] presented a tunable resonator, wherein its frequency is tuned by varying the gap between the resonator and the bottom ground of the resonator’s metallic cavity. The resonator is either a dielectric transverse magnetic (TM), transverse electric (TE), or hybrid mode resonator (HE). The resonator is mounted directly on the bottom surface of the metallic cavity. A small change in the position of the resonator will cause a significant change in the resonant frequency. This configuration has degraded quality factor and increasing loss since the resonator is in close proximity to the cavity. Moving the resoantor further away from the bottom surface of the cavity improves the quality factor, but will have less change in frequency. Therefore, the height of the metallic cavity must be increased to extend the tuning range which increases the volume of the structure. Also, the spurious performance will be degraded. In [34] the first TM mode dielectric resonator has been proposed. There, the resonator is clamped at the holes at the top and bottom of the housing. It is not a tunable resonator.

It is therefore an object of the invention to provide for a tunable resonator, a tunable frequency filter and a method for tuning a tunable frequency filter which overcomes at least some of the aforementioned drawbacks.

Embodiments of the invention

The object of the invention is achieved by a tunable resonator arrangement comprising a) a housing having at least two opposite sidewalls and a hollow chamber between the sidewalls, the housing having electrically conductive properties, b) at least one of the at least two opposite sidewalls has an opening extending through the material of the sidewall, c) a sidewall cap on each of the sidewalls which have an opening extending through the material of the sidewall, the sidewall cap covers said opening and protrudes away from its sidewall, wherein the sidewall cap has an inner cavity, d) a resonator located at least partially within the hollow chamber, e) the resonator being displaceably mounted such that the resonator is moveable to different longitudinal positions within the inner cavity of the sidewall cap.

A new so-called “inset” resonator configuration is presented in this invention for fixed and tunable filters applications. The proposed inset topology can advantageously provide substantial volume-saving, better spurious performance, and comparably similar high Qu in comparison with the conventional half-wavelength and combline structures. Additionally, tunable inset filters and components can be designed effectively to maintain high Qu with minimal degradation and without the need for any auxiliary tuning elements and arrangements.

All drawbacks of the aforementioned state-of-the-art designs strongly motivate the introduction of more compact tunable filter designs that able to cover wider frequency ranges with enhanced spurious performance, and less variation in the high quality factor. The tunable resonator arrangement can be a resonator arrangement for the microwave frequency range, in particular for frequencies above 1 GHz.

The resonator can be a three-dimensional body of any shape. The resonator can be displaceably mounted such that the resonator extends through said opening in the sidewall at least in some longitudinal positions. The resonator can be displaceably mounted such that it can be linearly moved to different longitudinal positions. The length of the resonator can be larger than its width and height, or its diameter, whatever is applicable. The length of the resonator is its dimension in the longitudinal direction in which the resonator can be linearly moved.

In this way, the resonator can be moved through the opening in the sidewall into the inner cavity of the sidewall cap. Depending on the construction, it may be possible that the resonator can be moved to a longitudinal position outside the inner cavity of the sidewall cap, or a part of the resonator might always be within the inner cavity of the sidewall cap.

While the electromagnetic resonance in the tunable resonator arrangement of the invention is build up by the fields within the overall structure, the mostly relevant part to build up the intended resonance is the moveable center part of this specific resonator type, namely the part which is named as “resonator”.

The housing shall have electrically conductive properties. For example, the housing can be at least partially covered with electrically conductive metal and or any other suitable electrically conductive material. The housing can be made completely of such metal or other electrically conductive material, for example of a solid block of metal or of thin metal sheets mounted together, or similarly using another electrically conductive material. It is also possible to build the housing using electrically non-conductive material which is covered by electrically conductive material. The sidewall cap can have a cylindrical or conical shape, for example. At least the shape of the inner cavity can be cylindrical or conical. The sidewall cap covers only said opening in the sidewall and a small area of the sidewall surrounding said opening. Generally, the sidewall cap covers less than 50%, less than 20% or less than 10% of the surface of the sidewall. The sidewall cap can be a thin-walled element.

According to an advantageous embodiment of the invention, one, some or all of the sidewall caps have an inner cavity which has the same type of cross-sectional shape as the outer cross-sectional shape of the part of the resonator which is moveable to different longitudinal positions within the sidewall cap, but with larger dimension than the dimension of the part of the resonator which is moveable to different longitudinal positions within the sidewall cap.

According to an advantageous embodiment of the invention, the resonator is not in mechanical and/or electrical contact with the sidewall cap and/or the metallic housing. There is a gap between the resonator and the sidewall cap as well as the material of the sidewall which surrounds the opening in the sidewall. The gap completely circumferentially surrounds the resonator.

According to an advantageous embodiment of the invention, the length of the resonator is larger than the distance between the at least two opposite sidewalls. The length of the resonator is measured in the longitudinal direction, in which the resonator can be moved to different longitudinal positions within the inner cavity.

It is also possible that the resonator is movably mounted with an additional degree of freedom, e.g. such that the resonator can be moved around its longitudinal axis.

According to an advantageous embodiment of the invention, the longitudinal axis of the resonator is generally perpendicular to the least one sidewall having said opening. According to an advantageous embodiment of the invention, one, some or all of the sidewall caps have an end wall which limits the available space for the longitudinal displaceability of the resonator. Therefore, such a sidewall cap can be shaped like a flower pot. At least in the end wall of one sidewall cap there can be a through-opening extending through the end wall. This through-opening allows to guide a support element, which mechanically holds and supports the resonator, through the opening.

According to an advantageous embodiment of the invention, the tunable resonator arrangement comprises a control device which is arranged for tuning the tunable resonator to a desired resonance frequency by changing the longitudinal position of the resonator within the sidewall cap. Such control device can be established, for example, by the aforementioned support element or the control device may be connected to such a support element. The control device can be a manual control device which allows for changing the longitudinal position of the resonator manually. It is also possible that the control device is a motorized control device which allows for changing the longitudinal position of the resonator by an electrical signal.

According to an advantageous embodiment of the invention, the outer shape of the resonator or at least the part of the resonator which is moveable to different longitudinal positions within the sidewall cap is rotationally symmetric. The resonator can have the shape, for example, of a cylinder or a cone.

According to an advantageous embodiment of the invention, the part of the resonator which is moveable to different longitudinal positions within the sidewall cap is located concentrically to the inner cavity of the sidewall cap.

According to an advantageous embodiment of the invention, the resonator is mechanically connected to a support element by which the resonator can be moved to different longitudinal positions within the sidewall cap. The support element holds the resonator mechanically and allows for moving the resonator within the hollow chamber. The support element may extend to the outside of the metallic housing and/or the sidewall cap.

According to an advantageous embodiment, both of the two opposite sidewalls each have an opening extending through the material of the sidewall. Further there is a sidewall cap on each of the two opposite sidewalls which have the opening extending through the material of the sidewall, each sidewall cap covering one of said openings. In such case, the resonator can be displaceably mounted such that the resonator is moveable to different longitudinal positions within both sidewall caps.

The tunable resonator can be a TM mode resonator.

According to an advantageous embodiment, the tunable resonator can be tuned between a first frequency value and a second frequency value, the first frequency value being less than 50% of the second frequency value.

According to an advantageous embodiment, the resonator comprises a metallic material or other electrically conductive material and/or comprises a dielectric material. For example, the resonator may consist completely of metallic material or may have a core dielectric material which is covered by a material having a good electric conductivity, like metal. As a further example, the resonator may consist completely of dielectric material.

The object of the invention is further achieved by a tunable frequency filter having one or more tunable resonator arrangements of the aforementioned kind, wherein the resonance frequency of the frequency filter is tunable to a desired value by changing the longitudinal position of the resonator within the sidewall cap of one, more or all tunable resonator arrangements. For example, the tunable frequency filter can be a bandpass filter. According to an advantageous embodiment of the invention, the support elements of the resonators of several tunable resonator arrangements are mechanically coupled together, such that the resonators be moved simultaneously using a single common moving element. The common moving element may be connected with a control device of the aforementioned kind, for tuning the resonators of the frequency filter to a desired value.

The object of the invention is further achieved by a method for tuning a tunable frequency filter of the aforementioned kind, wherein the resonance frequency of the frequency filter is tuned to a desired value by changing the longitudinal position of the resonator within the sidewall cap of one, more or all tunable resonator arrangements.

Figure 1 shows an exemplary embodiment of a tunable resonator arrangement 1 having a metallic housing 2. The metallic housing 2 has at least two opposite sidewalls 3. The remaining shape of the metallic housing 2 can be of any shape, for example a cuboid shape. The two opposite sidewalls 3 may be parallel sidewalls. In each of the opposite sidewalls 3 there is an opening 4 extending through the material of the sidewall 3. On the outer side of each of the sidewalls 3 there is a sidewall cap 5 which covers the respective opening 4 and protrudes away from its sidewall 3. Further, the tunable resonator arrangement 1 comprises a resonator 6 which is located at least partially within a hollow chamber of the metallic housing 2. The resonator 6 is mechanically held by supporting elements 7 extending away from the resonator 6 in a longitudinal direction L. The resonator 6 is displaceably mounted and can be moved in a linear direction along the longitudinal direction L. In this way, the resonator 6 can be moved to different longitudinal positions within an inner cavity of each sidewall cap 5, depending on a movement exerted on at least one support element 7.

The presented inset resonator configuration, e.g. as depicted in Fig. 1 , is based on the concept of properly modifying either one or both the top and bottom sidewalls 3 of the metallic housing 2 by implementing the openings 4, covering them with the sidewall caps 5 and inserting a resonator 6 inside them partially. Firstly, the height of the original metallic cavity is reduced to be less than the length of resonator 6. Then, both the height and diameter of the modified sidewalls are carefully chosen where the inner diameters of the sidewall caps 5 are set larger than the outer diameter of the resonator 6, and the overall height of the metallic housing 2 (the original cavity height + sidewall caps heights) is larger than the length of the resonator 6. The dimensions of the sidewall caps 5 can be equal or different. The resonator 6 is positioned in the center of the cavity using some support elements 7 with equal insertion inside the modified wall caps. Any sort of low-loss support material or materials (e.g. Teflon) can be used for the sidewall caps 5 under the condition that there is no electrical contact between the resonator 6 and the cavity. The resonator 6 can be either metallic or dielectric, for instance, the TM mode dielectric resonator as it is explained in [10], The dielectric material which may be preferably used shall be a material having a high dielectric constant and low losses.

The operation mode of the proposed inset resonator structure can be the fundamental TM mode and the fields are distributed similarly as in conventional A/2 structures as it was previously shown in patent [1] and publication [2], In this mode, the magnetic field is dominant and it is focused at the center of the structure while the electric field is distributed at the ends of the resonator. In comparison with the conventional combline and half-wavelength configurations, the proposed inset structure features more compact size because the height of the cavity is reduced and parts of the resonator are inserted inside modified portions of the metallic housing (sidewall caps). Also, by reducing the gap between the sidewall caps and the resonator, the resonant frequency can be moved down further providing more size miniaturization. It worth to note that since the majority of the EM fields are in the center of the structure and they are minimal at the sidewall caps, comparably similar high quality factor to the conventional structures can be obtained even when there is a close proximity (> 0.5 mm) between the resonator and sidewall caps. Furthermore, more size reduction can be obtained with smaller gaps but with the cost of lower unloaded Q. In any case, the variation in the quality factor is always better than in the aforementioned state-of-the-art-designs. Another advantage of the presented inset configuration is the enhanced spurious performance as the higher spurious resonances are moved further away from the fundamental mode.

To validate these claims, Table 1 provides a comparison example between the proposed inset-type, conventional half-wavelength and combline resonators (reported in [3]) operating at 2.5 GHz. The length of the resonator in the A/2 structure is 50 mm, and 25 mm in the combline and inset configurations («A/4). As can be seen, the inset resonator structures offer the highest volume-saving (up to 63% of miniaturization), best Qu/volume ratio, and better spurious performance (f _sp ur=8.0 GHz) with a negligible change in the Qu (<5%). The sidewall caps are designed to contain the lowest field intensities, and therefore, the sidewall caps’ heights can be decreased for more compactness without causing any change to the resonant frequency. This can be seen in the second inset-type example (caps height is 8 mm in the first inset example (Inset- 1 ) and 4 mm in the second example (Inset-2) providing more volume-saving, while the resonant frequency (f _r ) is kept fixed at 2.5 GHz).

TABLE 1 Comparison between the conventional half-wavelength and combline resonators, and the proposed inset resonators at 2.5 GHz.

Tunable frequency responses can be ideally obtained through the proposed inset resonator topology. The tuning concept is based on the displacement of the resonator to change the resonant frequency while keeping high Qu without the need for any additional tuning means. As mentioned, the sidewall caps are designed to have the lowest field densities. When considering the rapid decay of the EM fields, this allows designers to increase the caps’ heights with enough room for the movement of the resonators (more tuning can be obtained) without causing any change in the reference (lowest) resonant frequency. When the resonator is centered: this is the reference (fixed) case where the frequency is the lowest. Moving the resonator up or down will move it away from one sidewall and closer to the other one. This means that the resonant frequency will shift up with reference to the first cap sidewall and also, simultaneously want to shift down with reference to the second side. In general, this is correct, but due to the fact that the contained EM fields in the sidewall caps are minimal, the up-shift in the frequency caused by the displacement away from the first sidewall cap is larger than the frequency down-shift caused by the displacement towards the opposite side, which is negligible. Therefore, the net result is that the frequency is always shifted up. Again, since there is small amount of fields in the sidewall caps’ areas, this also means that the quality factor will not be much affected by the tuning process unlike what happens in most of the conventional tuning concepts. The EM fields resonate as half-wavelength waves at the lower tuning states where the resonator is inserted partially at both sidewall caps. Then, the distribution of fields becomes more similar to the combline configuration at higher tuning states where the resonator is inserted only in one sidewall cap. Another key advantage of the presented tunable inset configuration is that there no requirement of electrical contact to the metallic cavity, contrary to the combline-based tuning concepts where excellent electrical contact is always needed and both the tuning capabilities and quality factor will seriously deteriorate. Therefore, tuning process can be applied faster in the inset structure and remote-controlled motorized tuning can be used effectively without no worries.

To validate the inset-type tuning concept, a single inset resonator example is investigated using the Eigen-mode solver in CST Microwave Studio considering copper for the metallic housing and resonator. The corresponding dimensions in mm are: cavity height = 15, cavity width = 30, cavity length = 30, resonator height = 20, resonator outer diameter = 10, resonator inner diameter = 3, caps height = 8, caps diameter = 11. In this case, the term cavity refers to the hollow chamber 14 inside the metallic housing 2 between the at least two opposite sidewalls 3. Fig. 2 demonstrates the simulated frequency and Qu in relation to the resonator displacement. As shown, the frequency is reconfigured from 2.3 GHz (D=0) to 3.6 GHz (D=5.2 mm) providing a wide tunability of 44%. Also, the resonator has maintained a high Qu of 4400±6.5% with minimum variation throughout the tuning window.

Figure 3 shows another embodiment of a tunable resonator arrangement 1 , which differs from the embodiment of figure 1 in a way that only one of the two opposite sidewalls 3 has an opening 4, namely the lower one of the sidewalls 3. The upper sidewall 3 is completely closed. There is no sidewall cap on the upper sidewall 3, but only on the lower sidewall 3. Therefore, the resonator 6 can be moved upwards in the longitudinal direction L only until it reaches the upper sidewall 3.

Since in the configuration at Fig. 3, only the bottom sidewall is modified and the resonator is inserted in it, the frequency decreases when the resonator moves towards the top side of the metallic cavity, and shifts up when it is inserted inside the bottom sidewall cap. The operation principle is the same at the inset configuration at both sidewalls, however, the size miniaturization is less, the next spurious is closer, the tuning is relatively smaller, and the variation in the quality factor is larger. The distribution of the EM fields in this configuration is similar to the conventional combline resonator as in [3] and [4] where the H-field rotates at the centre of the metallic cavity and the E-field mainly resonates in the space between the top end of the movable resonator and the top sidewall cavity.

While in the embodiments of figures 1 and 3, the resonator 6 was shown with a cylindrical shape, figure 4 shows another embodiment, similar to the embodiment of figure 1 , but with a different shape of the resonator 6. In the embodiment of figure 4 a resonator having a conical shape is proposed. As can be seen, the sidewall cap 5 on the upper sidewall 3 can have a shape which is adapted to the outer shape of the conical resonator 6. In the third configuration in Fig. 4, a conical shaped resonator is used. Also, the shape of the top cap has been changed to conical. This configuration is especially useful with dielectric resonators since it has better spurious performance than the common cylindrical shape resonators. Also, the configuration can improve the quality factor and the power handling capabilities. The configuration is similar to the inset configuration in Fig. 1 , but the tuning behaviour is different. At the lowest tuning frequency, the bottom surface of the resonator is positioned at the interface between the original metallic cavity and the bottom sidewall cap or with very small insertion inside the bottom sidewall cap, while the top end of the resonator is inserted inside the top sidewall cap to a distance near to the height of the top sidewall cap. Then, when the resonator is displaced towards the bottom sidewall cap, the gap increases from the top side and the frequency increases. Compared to the inset configuration in Fig. 1 , this conical structure has smaller tuning ranges and larger size.

Fig. 5 and Fig. 6 illustrate a 3D schematic of a tunable frequency filter 8, e.g. a two-pole bandpass filter, using the proposed inset resonator. The tunable frequency filter 8 has two tunable resonator arrangements 1 of the aforementioned kind, which are coupled together via a coupling iris 9. The tunable frequency filter 8 is electrically contacted by wires 10. Further, there can be one or more tuning screws in the tunable resonator arrangements 1 and/or the coupling iris 9.

As can be seen in the structure at Fig. 6, the dielectric support elements 7 of the individual resonators 6 can be moved simultaneously using a single common moving element 12. The filter is designed to have a Chebyshev response and operates at a center frequency of 2.45 GHz with an absolute bandwidth of 50 MHz. Firstly, the corresponding coupling matrix is extracted and optimized following the standard synthesis process. Then, the required physical coupling coefficients (K12) and external quality factor ( Qext) are calculated and realized. The simulated S-parameter responses of the proposed 2nd-order inset BPF are depicted in Fig. 7. The extracted Q factor is 3000 when considering copper metal (a = 5.8 x 10 ⁷ S/m) as the metal of the metallic housing 2. Also, the tuning capabilities of the introduced filter are demonstrated in Fig. 8. In tunable filters, it’s important to maintain a constant bandwidth across the whole tuning range satisfying the following equation:

Where m-12, msi can be obtained from the extracted coupling matrix, and they are fixed regardless of the corresponding frequency. When the resonant frequency (f _r ) increases, the peak group delay (Td) (corresponds to the physical IO coupling) and the interresonator coupling (K12) decrease with a similar amount in order to maintain a constant bandwidth. A wide 1 .41 GHz tuning band is obtained from 2.45 GHz to 3.86 GHz with constant bandwidth of 52 MHz±5%. Similarly, a high unloaded quality factor is obtained with small variation (3200±7%).

Similarly, further tunable filter examples using the presented invention are depicted in Fig. 9 to Fig. 14, respectively. A four-pole 2.54 GHz - 3.94 GHz tunable filter is designed in Fig. 9, 10 with constant bandwidth of 108 MHz±4 MHz. The coupling irises 9 are designed to maintain a constant bandwidth with the aid of two side tuning screws which are used to tune the input coupling strength. A cross-coupled version is presented in Fig. 11 , 12 and 13 in a box configuration where a capacitive coupling probe 15 is employed to introduce two asymmetrical transmission zeros. The filter is tuned from 2.8 GHz to 4.2 GHz with bandwidth of 137 MHz±10%. Figs. 14 and 15 depict a fourth-order tunable filter using the conical inset configuration. The filter operates from 3.18 GHz to 4 GHz with constant absolute bandwidth of 62 MHz±1 MHz.

Figures 16 and 17 show another embodiment of a tunable resonator arrangement 1 which is similar to the embodiment of figure 3, but shown with further constructional details. In the embodiment of figures 16 and 17, the housing 2 is established by a lower housing part 2a, an upper housing part 2b and a number of housing screws 2c. With the housing screws 2c the upper housing part 2b is screwed on the lower housing part 2a. As can be seen in the sectional drawing of figure 17, the housing 2, in particular the lower housing part 2a, can be made of solid metal with a hollow chamber 14 inside which has a significant larger diameter than the diameter of the resonator 6. Further, the sidewall cap 5 is mounted on the upper housing part 2b. As can be seen, the sidewall cap 5 has an inner cavity 13, which is large enough such that the resonator 6 can be moved into the inner cavity 13. However, the dimensions of the inner cavity 13 are significantly smaller than the dimensions of the hollow chamber 14 of the housing 2.

A single inset resonator and a 2nd-order BPF are manufactured, assembled, and measured to validate the above-presented designs. The measured S parameter results of the implemented resonator and filter are depicted in Fig. 18 to Fig. 21 , respectively. Measurements agree very well with simulations. These prototypes were manufactured out of brass metal and then silver-plated to get higher Qu. The resonator dimensions are similar to those of the provided example in the above section in mm: (cavity height = 15, cavity width = 30, cavity length = 30, resonator height = 20, resonator outer diameter = 10, resonator inner diameter = 3, caps height = 8, caps diameter = 11 ). The measured resonant frequency of the fixed resonator configuration is 2.32 GHz with a high extracted Qu of 3909. For the tunable version, the frequency is tuned from 2.32 GHz to 3.59 GHz offering a 43% tunability and a high, stable Qu of 4004±2.4%. The fabricated inset resonator offers 25% volume-saving in comparison with the conventional combline structure and 50% with the half-wavelength one. The measured responses of the proposed two-pole inset filter are shown in Fig. 19 - Fig. 21 . The filter operates at a frequency of 2.51 GHz with 53 MHz bandwidth and extracted Qu of «2000. The filter has a wide spurious-free band up to 8.63 GHz as can be seen in Fig. 20. The measured reconfigurable inset filter has a wide 1 .36 GHz frequency tuning window from 2.51 GHz - 3.87 GHz with a constant BW of 54 MHz±1 MHz and a high Qu of 1950±2.5% throughout the tuning window. Also, a 4th-order filter was manufactured out of copper metal, and then tested. The measured S parameter responses in Fig. 22 and Fig. 23 exhibit a 1 .3 GHz frequency tuning range from 2.66 GHz to 3.96 GHz with a constant bandwidth of 116 MHz±6% and a stable insertion loss of 0.4 dB. It should be noted that those responses were obtained effectively without the use of any tuning screws, advantageously featuring easier and faster tuning process.

In comparison with the all available high Q state-of-the-art and inventions as in [1] - [20], and also to the commercially available tunable bandpass filters as [21] - [29], the presented invention offers more compact size and enhanced spurious performance. Also, the inset configuration is designed to be tuned efficiently without the need for any auxiliary tuning components. A wide tuning range can be effectively obtained with the least variation in the Qu when compared with all available state-of-the-art designs. Furthermore, no electrical contact is required with the metallic housing, therefore, tuning process can be done faster and more efficiently. Such advantages are highly required in most of the current and future communication systems which makes the presented invention really promising candidate in the tunable filters industry.

List of reference signs

1 Tunable resonator arrangement

2 Metallic housing

2a Bottom part of the housing

2b Upper part of the housing

2c Housing screws

3 Opposite sidewalls

4 Gap

5 Sidewall cap

6 Resonator

7 Support element

8 tunable frequency filter

9 Coupling iris

10 Electrical wire

11 Tuning screw

12 Common moving element

13 Inner cavity

14 Hollow chamber

15 Capacitive coupling probe

L Longitudinal direction