The present invention relates to a microwave filter. More particularly, but not exclusively, the present invention relates to a microwave filter comprising input and output microwave resonators and a tube extending therebetween, the tube having a plurality of pucks arranged therein, each puck having a dominant mode which is a doubly degenerate mode, the pucks being spaced apart by iris plates, each iris plates having at least one coupling slot extending therethrough, at least one of the input or output resonators having an extracted pole resonator connected thereto. In a further aspect the present invention also relates to a plurality of such microwave filters connected together in parallel.

Microwave filters comprising a plurality of pucks in a tube are known. W02018/150171 shows an example of such a filter. Such filters however have a number of drawbacks. The number of resonant pucks is equal to the number of transmission zeros. If the filter is required to have a large number of transmission zeros then the filter can be expensive to manufacture. Further, with such an arrangement it can be difficult to provide a sufficient degree of selectivity close to the transmission band edge required by some applications.

The microwave filter according to the invention seeks to overcome the problems of the prior art.

Accordingly, in a first aspect, the present invention provides a microwave filter comprising a hollow tube comprising a hollow tube wall which defines a tube bore, the tube extending along a length axis from a first end to a second end; a single mode input microwave resonator comprising an electrically conductive input cavity wall which defines an input cavity, the first end of the hollow tube extending over a cavity slot which extends through the input cavity wall so as to allow a microwave signal to pass from the input cavity into the hollow tube; a single mode output microwave resonator comprising an electrically conductive output cavity wall which defines an output cavity, the second end of the hollow tube extending over a cavity slot which extends through the output cavity wall so as to allow a microwave signal to pass from the hollow tube into the output cavity; a plurality of dielectric resonant pucks, each puck comprising first and second end faces and a side wall extending therebetween, each puck being dimensioned such that when in the tube its dominant mode is a doubly degenerate mode; the pucks being arranged within the tube bore spaced apart from each other and the first and second tube ends, each puck being centred on the length axis; each puck being separated from the adjacent puck in the tube bore by a coupling gap, each coupling gap having an electrically conductive iris plate therein, each iris plate being arranged substantially normal to the length axis, each iris plate comprising at least one coupling slot extending therethrough; each puck comprising a symmetry breaking structure for modifying the frequency of one of the degenerate modes relative to the other and the coupling between the modes; at least one of the input or output microwave resonators having an extracted pole resonator connected thereto, the extracted pole resonator comprising an extracted pole resonator cavity defined by an electrically conductive extracted pole cavity wall, the extracted pole cavity wall having an aperture therein which overlies an aperture extending through the cavity wall of the input or output microwave resonator to which it is connected; an extracted pole dielectric resonant puck arranged within the extracted pole resonator cavity, the extracted pole resonant puck being dimensioned such that when in the extracted pole resonator cavity its dominant mode is a doubly degenerate mode; the extracted pole dielectric resonant puck comprising a symmetry breaking structure for modifying the frequency of one of the degenerate modes relative to the other and the coupling between the modes.

The microwave filter according to the invention has fewer pucks than transmission zeros. This reduces the cost of manufacture. Further. The microwave filter according to the invention has a high degree of selectivity close to the band edges of the filter. Preferably the extracted pole resonator cavity extends along an extracted pole length axis, the extracted pole dielectric resonant puck comprising first and second end faces and a side wall extending therebetween; the extracted pole dielectric puck being arranged with its end faces centred on the extracted pole length axis.

Preferably the extracted pole length axis and length axis are coaxial.

Preferably the symmetry breaking structure for at least one, preferably each dielectric resonant puck comprises at least one, preferably a plurality of apertures extending at least part way through the puck from one end face to the other parallel to but spaced apart from the length axis passing through the puck.

Preferably at least one aperture extends from one end face to the other of the puck in which it is arranged.

Preferably at least one puck, preferably each puck comprises a central aperture extending from one face to the other along the length axis passing through the puck.

Preferably each central aperture has an electrically conductive insert arranged therein.

Preferably an end face of at least one puck has a normal inclined to the length axis passing through that puck.

Preferably for each puck the end faces of the puck are normal to the length axis passing through that puck. Preferably the pucks are cylindrical.

Preferably the tube bore is circular in a plane normal to the length axis and the extracted pole cavity wall is circular in a plane normal to the extracted pole length axis.

Preferably the pucks in the tube bore are dimensioned such that for each puck the side wall of the puck abuts the tube wall and there is no air gap between the two.

Preferably the puck in the extracted pole resonator cavity is dimensioned such that the side wall of the puck abuts the extracted pole cavity wall and there is no air gap between the two.

Preferably the tube wall has an N fold rotational symmetry about the length axis where 2<N<10, the pucks being spaced apart from the tube wall by a plurality N of spacer blocks.

Preferably the extracted pole cavity wall has an N fold rotational symmetry about the extracted pole length axis where 2<N<10, the extracted pole resonant puck being spaced apart from the extracted pole cavity wall by a plurality N of spacer blocks.

Preferably the single mode input microwave resonator comprises at least one combline resonator.

Preferably the single mode output microwave resonator comprises at least one combline resonator.

Preferably the tube wall is a single electrically conductive piece. Preferably the tube is divided into a plurality of spaced apart tube segments, the periphery of each iris plate being sandwiched between adjacent segments, the tube segments and the portions of the iris plates sandwiched between the tube segments defining the tube wall.

Preferably the microwave filter comprises an extracted pole resonator connected to the input microwave resonator and an extracted pole resonator connected to the output microwave resonator.

In a further embodiment of the invention there is provided a microwave filter comprising a plurality of microwave filters as claimed in any one of claims 1 to 20 connected in parallel.

Preferably the microwave filters connected in parallel share at least one of a common single mode input microwave resonator and single mode output microwave resonator.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which

Figure 1 shows a hollow tube of a microwave filter according to the invention;

Figure 2 shows a single mode input microwave resonator of a microwave filter according to the invention.

Figure 3 shows a single mode output microwave resonator of a microwave filter according to the invention;

Figure 4 shows an extracted pole resonator of a microwave filter according to the invention; Figure 5 shows a microwave filter according to the invention;

Figure 6 shows the reflection and transmission coefficients of the filter of figure 5 as a function of frequency;

Figure 7 shows the tube of the microwave filter of figure 5 in vertical cross section;

Figure 8 shows the tube of an alternative embodiment of a microwave filter according to the invention in vertical cross section;

Figure 9 shows a further embodiment of a microwave filter according to the invention;

Figure 10(a) shows a tube of a further embodiment of a microwave filter according to the invention;

Figure 10(b) shows an extracted pole resonator corresponding to the tube portion of figure 10(a);

Figure 11 shows a further embodiment of a microwave filter according to the invention; and,

Figure 12 shows a further embodiment of a microwave filter according to the invention.

Shown in figure 1 is a hollow tube 1 of a microwave filter according to the invention. The hollow tube 1 comprises a hollow tube wall 2 which defines a tube bore 3. The tube 1 extends along a length axis 4 from a first end 5 to a second end 6. The tube 1 is open at both ends. The tube wall 2 is shown transparent so the components inside the tube 1 can be seen. In this embodiment the tube bore 3 is cylindrical and has a circular cross section in a plane normal to the length axis 4.

Arranged within the tube 1 is a plurality of dielectric resonant pucks 7. Each dielectric resonant puck 7 is cylindrical having first and second end faces 8 and a side wall 9 extending therebetween. Each puck 7 is arranged with its end faces 8 centred on the length axis 4 with the end faces normal 8 to the length axis 4. Each puck 7 is dimensioned such that when in the tube bore 3 its dominant mode is a doubly degenerate HE011 mode. The diameter of each puck 7 is equal to the diameter of the tube bore 3 such that the side wall 9 of each puck 7 abuts the tube wall 2 as shown. There is no air gap between each puck 7 and the tube wall 2.

Each puck 7 comprises a symmetry breaking structure 10 for modifying the frequency of one of the degenerate modes relative to the other and the coupling between them. In this embodiment the symmetry breaking structure 10 for each puck 7 comprises a plurality of apertures 11 extending from one end face 8 to the other of the puck 7, parallel to but spaced apart from the length axis 4. The number of such apertures 11, their diameters and distances from the length axis 4 can be varied between different embodiments of the invention in order to alter the frequency response of the microwave filter.

The pucks 7 are arranged in the tube bore 3 spaced apart from each other and the first and second tube ends 5,6. Each puck 7 is separated from the adjacent puck 7 by a coupling gap 12. Arranged in each coupling gap 12 is an electrically conductive iris plate 13. Each iris plate 13 is circular and arranged normal to the length axis 4. The diameter of each iris plate 13 is equal to the diameter of the tube bore 3 such that the outer edge of each iris plate 13 abuts the tube wall 2. Extending through each iris plate 13 is a coupling slot 14 through which the microwave signal can pass to couple the pucks 7 together. Preferably the coupling slot 14 extends through the length axis 4 although this is not necessarily the case. The number, positions and dimensions of the coupling slots 14 can be varied between embodiments to vary the frequency response of the microwave filter.

Shown in figure 2 is a single mode input microwave resonator 15 of a microwave filter according to the invention. The input microwave resonator 15 comprises an electrically conductive input cavity wall 16 which defines an input cavity 17. The input cavity 17 is cylindrical and extends along the length axis 4. The input cavity wall 16 comprises first and second circular input cavity wall end faces 18,19 arranged normal to the length axis 4 and an input cavity wall side face 20 extending therebetween. One of the end faces 19 has a cavity slot 21 extending therethrough to allow a microwave signal to pass from the input cavity 17 into the hollow tube 1. Extending into the input cavity 17 is an electrically conductive rod 22. The electrically conductive rod 22 and input cavity wall 16 together form a combline resonator 23. The operation of combline resonators 23 is known in the art and so will not be described in detail. Briefly, on providing a microwave signal to the electrically conductive rod 22 a microwave field is generated in the input cavity 17 which then passes through the cavity slot 21 into the hollow tube 1. In an alternative embodiment of the invention the input microwave resonator 15 can comprise a plurality of combline resonators 23, preferably sharing the same cavity wall 16.

Shown in figure 3 is a single mode output microwave resonator 24 of a microwave filter according to the invention. The single mode output microwave resonator 24 comprises an electrically conductive output cavity wall 25 which defines an output cavity 26. The output cavity 26 is cylindrical and extends along the length axis 4. The output cavity wall 25 comprises first and second circular output cavity wall end faces 27,28 arranged normal to the length axis 4 and an output cavity side wall 29 extending therebetween. One output cavity wall end face 27 has a cavity slot 30 extending therethrough to allow a microwave signal to pass from the hollow tube 1 into the output cavity 26. The other output cavity wall end face 28 has an aperture 31 therein through which a microwave signal can pass from the output cavity 26 into an extracted pole resonator cavity as described below. Extending into the output cavity 26 is a plurality (in this case two) of electrically conductive rods 32. The electrically conductive rods 32 and output cavity wall 25 define a plurality of combline resonators 33 sharing the same output cavity wall 25. Also extending into the output cavity 26 is a tuning screw 34 for adjusting the coupling between the combline resonators 33. In alternative embodiments of the invention the output microwave resonator 24 comprises a single combline resonator 33.

Shown in figure 4 is an extracted pole resonator 35 of a microwave filter according to the invention. The extracted pole resonator 35 comprises an electrically conductive extracted pole cavity wall 36 which defines an extracted pole resonator cavity 37. The extracted pole resonator cavity 37 is cylindrical and extends along an extracted pole length axis 38. In this embodiment the extracted pole length axis 38 is coaxial with the length axis 4. In alternative embodiments one axis is inclined relative to the other. The extracted pole cavity wall 36 comprises a circular extracted pole cavity wall end plate 39 arranged normal to the extracted pole length axis 38 and a side wall 40 extending therefrom and surrounding the extracted pole length axis 38 to form a tube of circular cross section. The end 41 of the tube remote from the extracted pole cavity wall end plate 39 is open defining an aperture through which microwaves can enter the extracted pole resonator cavity 37.

Arranged within the extracted pole resonator cavity 37 is a cylindrical extracted pole dielectric resonant puck 42. The extracted pole dielectric resonant puck 42 comprises first and second end faces 43,44 and a side wall 45 extending therebetween. The end faces 43,44 are centred on the extracted pole length axis 38. The diameter of the extracted pole dielectric resonant puck 42 is equal to the diameter of the extracted pole cavity side wall 40 such that the side wall 45 of the extracted pole dielectric puck 42 abuts the extracted pole cavity side wall 40 and there is no gap between the two as shown.

The extracted pole dielectric resonant puck 42 is dimensioned such that its dominant mode is a doubly degenerate mode when in the extracted pole resonator cavity 37. The extracted pole dielectric resonant puck 42 comprises a symmetry breaking structure 46 for modifying the frequency of one of the degenerate modes relative to the other In this embodiment the symmetry breaking structure 46 comprises a plurality of apertures 47 extending from one end face of the extracted pole dielectric resonant puck 42 to the other parallel to but spaced apart from the extracted pole length axis 38.

Shown in figure 5 is a microwave filter 48 according to the invention comprising the component parts described above connected together. The first end 5 of the tube 1 extends over the cavity slot 21in the input cavity wall 16. The second end 6 of the tube 1 extends over the cavity slot 30 in the output cavity wall 25. The aperture in the extracted pole resonator cavity 37 extends over the aperture 31 which extends through the output cavity wall 25. In use a microwave signal is provided to the input microwave resonator 15. This passes through the microwave filter 48 exciting the dielectric pucks 7,42 and then to the output microwave resonator 24. The coupling between the modes both within the pucks 7,42 and between the pucks 7,42 determines the behaviour of the microwave filter 48. Importantly the presence of the extracted pole resonator produces a microwave filter which has a high selectivity close to the transmission band edges and which has a larger number of transmission zeros than pucks.

Figure 6 show the transmission and reflection coefficients for the microwave filter 48 of figure 5 as a function of frequency. There are four transmission zeros but only three pucks.

Figure 7 shows the tube 1 of the microwave filter of figure 5 in vertical cross section. As can be seen, the tube wall 2 is a single piece. Entirely contained within the tube bore 3 is the iris plate 13. Figure 8 shows the tube 1 of an alternative embodiment of a microwave filter 48 according to the invention. In this embodiment the tube 1 comprises a plurality of spaced apart tube segments 49 each having a tube segment wall 50. The iris plates 13 extend out of the tube bore 3 as shown. The tube segment walls 50 and the portions of the iris plates 13 sandwiched between the tube segment walls 50 define the tube wall 2.

Shown in figure 9 in vertical cross section is a further embodiment of a microwave filter 48 according to the invention. This is similar to that of figure 5 except each of the dielectric resonant pucks 7,42 comprises a central aperture 51 extending from one puck end face to the other along the length axis 4,38extending through that puck 7,42. Arranged within each central aperture 51 is an electrically conductive insert 52. As can be seen, for each puck 7,42 the length of the insert 52 measured along the length axis 4,38 is slightly less than the distance between the end faces of the puck 7,42. Such a configuration is particularly useful for low frequency applications, typically below lGHz. A further difference between the embodiment of figure 9 and the embodiment of figure 5 is that for some of the pucks 7,42 at least one end face 8 is slightly inclined to the length axis 4, 38passing through the puck 7,42. This affects the coupling between the modes in the puck 7,42 and so provides a further way in which the behaviour of the microwave filter 48 can be modified. A further difference is that for at least some of the pucks 7 the apertures 11 only extend part way from one puck end face8 to the other. This provides a further way in which the behaviour of the microwave filter 48 according to the invention can be modified. Shown in figure 10(a) is the tube of a further embodiment of a microwave filter 48 according to the invention. In this embodiment the tube bore 3 has an N fold symmetry about the length axis 4 where 2<N<10. In this case N = 3. The iris plate 13 has the same symmetry. The dielectric resonant pucks 7 are cylindrical and are spaced apart from the tube wall 2 by electrically conductive spacer blocks 53. The extracted pole cavity side wall 40 also has an N fold symmetry about the extracted pole length axis 38 where 2<N<10, in this case N=3. The extracted pole dielectric resonant puck 42 is cylindrical and is spaced apart from the extracted pole cavity side wall 40 by electrically conductive spacer blocks 53.

Shown in figure 11 in schematic form is a further embodiment of a microwave filter 48 according to the invention in schematic form. This embodiment comprises a plurality of microwave filters 48 as described with reference to figure 5 connected together in parallel. Figure 12 shows a further embodiment of a microwave filter 48 according to the invention similar to that of figure 11 except the individual microwave filters 48 share a common output microwave resonator 24 and a common extracted pole resonator 35. The common output microwave resonator 24 comprises a plurality of combline resonators 33 sharing the same output cavity 26.

Further embodiments of the invention are possible. In an alternative embodiment of the invention the extracted pole resonator 35 is connected to the input microwave resonator 15. In a further alternative embodiment of the invention each of the input microwave resonator 15 and output microwave resonator 24 has an extracted pole resonator 34 connected thereto.

The input microwave resonator 15 can comprise a single combline resonator 23. In an alternative embodiment of the invention the input microwave resonator 15 can comprise a plurality of combline resonators 23.

The output microwave resonator 24 can comprise a single combline resonator 33. In an alternative embodiment of the invention the output microwave resonator 24 can comprise a plurality of combline resonators 33.