WO2016185196A1

WO2016185196A1 - A microwave resonator and a microwave filter

Info

Publication number: WO2016185196A1
Application number: PCT/GB2016/051414
Authority: WO
Inventors: David Rhodes
Original assignee: David Rhodes
Priority date: 2015-05-17
Filing date: 2016-05-17
Publication date: 2016-11-24
Also published as: GB2540030A; GB201608670D0; GB201508457D0

Abstract

A microwave resonator comprising a resonator cavity defined by an electrically conducting cavity wall, the cavity wail comprising first and second spaced apart end faces and a side wall extending therebetween; the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity having a length A along the length axis between the first and second end faces; the cavity having a width axis normal to the length axis, the width of the cavity from the side wall through the length axis to the side wall on the opposite side of the length axis being C; a dielectric plate arranged with in the cavity normal to the length axis; the dielectric plate being spaced apart from the first and second end faces, so defining first and second gaps therebetween; the width of the dielectric plate normal to the length axis being at least 90% of C, the thickness B of the dielectric plate along the length axis being at least 60% of A; and, wherein C is at least four times A.

Description

A microwave resonator and a microwave filter

The present invention relates to a microwave resonator. More particularly, but not exclusively, the present invention relates to a microwave resonator comprising a resonator cavity defined by an electrically conducting cavity wall, the cavity wall comprising first and second spaced apart end faces and a side wall extending therebetween, the resonator further comprising a dielectric body arranged within the cavity spaced apart from the first and second end faces, the width of the cavity being at least four times larger than the length. In a further aspect the present invention relates to a microwave filter. More particularly, but not exclusively, the present invention relates to a microwave filter comprising a plurality of such resonators, the resonators being electrically coupled together.

Microwave resonators often find application in mobile telecommunications systems. Either alone or in combination with other resonators they can form microwave filters. A microwave signal can often comprise signals at a plurality of different frequencies. It is desired to filter such a signal to extract the signal at one frequency only. Similarly, before transmitting a microwave signal it is typically desired to pass the signal through a filter so that the signal is tightly constrained to one frequency band only. In modern telecommunications systems different signals are very closely spaced together in frequency and so filters having a very narrow bandpass are required. This in turn typically requires the use of resonators having very high Q values. Such resonators can be difficult and expensive to manufacture and are also large structures.

The present invention seeks to overcome the problems of the prior art.

In a first aspect the present invention provides a microwave resonator comprising a resonator cavity defined by an electrically conducting cavity wall, the cavity wall comprising first and second spaced apart end faces and a side wall extending therebetween; the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity having a length A along the length axis between the first and second end faces; the cavity having a width axis normal to the length axis, the width of the cavity from the side wall through the length axis to the side wall on the opposite side of the length axis being C; a dielectric plate arranged within the cavity normal to the length axis; the dielectric plate being spaced apart from the first and second end faces, so defining first and second gaps therebetween; the width of the dielectric plate normal to the length axis being at least 90% of C, the thickness B of the dielectric plate along the length axis being at least 60% of A; and, wherein C is at least four times A.

The microwave resonator according to the invention has a very high Q value compared to known microwave resonators of a similar volume at the same frequency. Further, it is relatively compact compared to known resonators, in particular it is very thin. This is of particular importance in modem telecoms systems where size is at a premium.

Preferably C is at least five times A, more preferably at least six times A.

Preferably normal to the length axis the shape of the dielectric plate is the same as the shape of the cavity.

Preferably the width of the dielectric plate normal to the length axis is at least 95% of C, the dielectric plate being spaced apart from the side wall.

Preferably normal to the length axis the dielectric plate is the same size as the cavity and abuts the side wall. Preferably B is at least 70% of A, more preferably at least 80% of A, more preferably at least 90% of A.

Preferably the relative permittivity of the dielectric plate is in the range 10 to 90, more preferably 30 to 60, more preferably 45.

Preferably the cavity has a circular cross section normal to the length axis.

Preferably the cavity has a fourfold rotational symmetry about the length axis, is preferably square.

Preferably the thickness of the first air gap along the length axis is equal to the thickness of the second air gap along the length axis.

Preferably the dielectric plate has an aperture extending therethrough, the microwave resonator further comprising a tuning screw extending through an end plate into the aperture.

Preferably the microwave resonator further comprises at least one electrically conducting tuning plate arranged in one of the first and second gaps and an electrical conductor extending from the tuning plate through an aperture in the end face proximate to the tuning plate.

Preferably the microwave resonator further comprises a switch connected in series with the electrical conductor, preferably a MEMS switch or a Gallium Nitride PIN diode.

Preferably the microwave resonator further comprising a microwave filter connected to the resonator cavity. Preferably the microwave resonator further comprises a microwave source, the microwave source being adapted to provide a microwave signal suitable for exciting either the ΕΗ_01Δ or ΕΗ_ηΔ mode of the resonator.

Preferably C is in the range 10 to 60mm, more preferably 20mm to 30mm.

Preferably B is in the range 2mm to 6mm, more preferably 4 to 5mm.

In a further aspect of the invention there is provided a microwave filter comprising a plurality of resonators as claimed in any one of claims 1 to 17, the microwave resonators being electrically coupled together.

Preferably at least some of the resonators are connected together in cascade.

Preferably the coupled resonators share a common side wall, the common side wall having an aperture extending therethrough.

In a further aspect of the invention there is provided a method of filtering a microwave signal comprising the steps of providing a microwave resonator as claimed in any one of claims 1 to 16; and, providing a microwave signal to the resonator, the microwave signal having a frequency suitable for exciting either the ΕΗ₀ι_Δ or EH_m mode of the 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(a) shows a microwave resonator according to the invention in cross section;

Figure 1(b) shows the microwave resonator of figure 1(a) from above;

Figure 1(c) shows an alternative embodiment of a microwave resonator according to the invention;

Figure 2 shows an alternative embodiment of a microwave resonator according to the invention in vertical cross section;

Figures 3(a) to 3(c) show plots of resonant frequency and Q value as a function of cavity height for a resonator according to the invention showing the desired modes and non-desired modes;

Figure 4 shows the electric field within the resonator cavity of a resonator according to the invention in vertical cross section;

Figure 5 shows the magnetic field in the dielectric plate of the resonator according to the invention in a plane normal to the length axis;

Figure 6 shows the magnitude contours for the magnetic field in the resonator cavity of a resonator according to the invention in vertical cross section;

Figure 7 shows and end face of a microwave resonator according to the invention;

Figure 8 shows a microwave filter according to the invention further comprising a filter and microwave source; and, Figure 9 shows two microwave resonators of a filter according to the invention coupled together.

Shown in figure 1(a) is a microwave resonator 1 according to the invention. The microwave resonator 1 comprises a resonator cavity 2 defined by an electrically conducting cavity wall 3. The cavity wall 3 comprises first and second spaced apart end faces 4,5 and a side wall 6 extending therebetween.

A length axis 7 extends from the centre of the first end face 4 to the centre of the second end face 5. The length of the resonator cavity 2 along the length axis 7 is A.

Arranged in the resonator cavity 2 is a dielectric plate 8. The dielectric plate 8 has a thickness B along the length axis 7. The dielectric plate 8 is spaced apart from the first and second end faces 4,5 as shown. B is at least 60% of A, more preferably at least 70% of A, more preferably at least 80% of A, more preferably at least 90% of A.

Between the plate 8 and the first end face 4 is a first gap 9. Between the plate 8 and the second face 5 is a second gap 10. Preferably the gaps 9,10 are filled with air. In this embodiment the thickness of each gap 9,10 along the length axis 7 is the same. In alternative embodiments one gap 9,10 is larger than the other 9,10.

The dielectric of the dielectric plate 8 is typically a high dielectric constant, low loss ceramic. The relative permittivity of the dielectric plate 8 is typically in the range 10 to 90, more preferably 30 to 60, more preferably 45.

Figure 1(b) shows the microwave resonator 1 of figure l(a() from above. In this embodiment the first and second end faces 4,5 are circular and the side wall 6 has a circular cross section normal to the length axis 7. The cross section of the cavity 2 is constant along the length axis 7. The dielectric plate 8 is the same size and shape as the cross section of the cavity 2 and abuts against the cavity side wall 6 as shown. The width C of the cavity 2 is the same as the diameter of the plate 8.

For this cavity C is at least four times A, more preferably at least six times A.

In practice the dielectric plate 8 is positioned in the cavity 2 by firstly heating the cavity 2 so that it expands slightly. The plate 8 is then inserted into the cavity 2. The cavity 2 is then allowed to cool and contract, so fixing the plate 8 in place.

Figure 1(c) shows a further embodiment of a microwave resonator 1 from above. In this embodiment the cross section of the cavity 2 normal to the length axis 7 is square. The dielectric plate 8 is also square. Unlike the embodiment of figure 1(b) as one rotates about the length axis 7 the distance between opposite sides of the cavity 2 through the length axis 7 varies. The width of the cavity 2 is the maximum value of this distance which in this case is the distance across the square as shown. Similarly, the width of the dielectric plate 8 is the distance between opposite corners of the plate 8.

In alternative embodiments the plate 8 and the cross section of the cavity 2 have other shapes. The shape of the plate 8 is the same as the shape of the cross section of the cavity 2. The plate 8 and cross section of the cavity 2 have a fourfold rotational symmetry about the length axis 7, that is to say if one rotates the plate 8 or cavity 2 by 90 degrees about the length axis 7 its final orientation is indistinguishable from its original orientation.

In the embodiment of figure 1(c) the plate 8 is slightly smaller than the cross section of the cavity 2 so resulting in a small air gap 11 between the plate 8 and cavity side wall 6. Preferably the width of the dielectric plate 8 normal to the length axis 7 is at least 90% of C, more preferably at least 95% of C. Shown in figure 2 is a further embodiment of a microwave resonator 1 according to the invention. In this embodiment an aperture 12 extends through the dielectric plate 8. A tuning screw 13 extends through the first end face 4 into the aperture 12. By turning the tuning screw 13 one can adjust the frequencies of the resonant modes of the microwave resonator 1.

By way of specific example, the resonator 1 is the resonator of figures 1(a) and 1(b). The width of the plate 8 (and hence the cavity 2) is 24mm and the thickness B of the plate 8 is 4mm. More generally, the width C is in the range 10 mm to 60mm, more preferably 20mm to 30mm. Preferably B is in the range 2mm to 6mm, more preferably 4mm to 5mm.

Shown in figure 3(a) is a plot of the resonant frequencies of different resonant modes of the resonator 1 described above as the dimension A is reduced to a level where ΕΗ₀ι_Δ and ΕΗ_11Δ become the dominant modes. Figure 3(b) is a table of the resonant frequencies of the different modes as a function of cavity length A. Figure 3(c) is a table of Q values at resonance for the different modes as a function of cavity length A. For this particular microwave resonator 1 the region of particular interest is A between 4.5mm and 5.5mm as is explained in more detail below. It should be appreciated that it is not the absolute values of the dimensions that are important, rather the relative values. If all the dimensions of the resonator 1 were scaled by the same scaling factor the resonator 1 would operate in the same manner but at a different frequency. These particular dimensions are chosen as with these dimensions the resonator 1 shows particularly desirable characteristics in the 3GHz to 4GHz range commonly employed in modern telecommunications systems

As can be seen from figure 3(a) for A between 4.5mm and 5.5mm and between 3 and 4 GHz the dominant mode is the ΕΗ₀ι_Δ mode. In known low loss microwave resonators the mode used is the ΤΕ mode. The resonator 1 according to the invention is therefore smaller than known microwave resonators. Further, in known microwave resonators the ΤΕ₀ι_Δ mode is not the dominant mode which can complicate use of the resonator. The ΕΗ₀ΙΔ mode has a further advantage in terms of the accessibility to the field structure to perform electronic tuning of the resonator. Shown in figure 4 is the electric field at resonance through a cross section of the resonator 1. The figure is not drawn to scale to as to enable the relative strengths of the field in different regions of the resonator 1 to be observed. The field is constant with regards to rotation about the length axis 7.

The electric field strength in the air gaps 9,10 is considerably larger than in the dielectric 8 and also much larger proximate to the length axis 7. (The magnitude is illustrated by the number of arrows).

The magnetic field is at right angles to the electric field. Its variation within the cavity is shown in figures 5 and 6, again not drawn to scale. Figure 5 shows the direction of the magnetic field H through the centre of the dielectric plate 8 in the plane normal to the length axis 7. The magnetic field has a rotational symmetry about the length axis 7. The strength is zero in the centre and reaches a peak about two thirds of the distance to the side wall 6 before starting to reduce. Figure 6 illustrates the contours of magnetic field strength in the resonator 1.

For a dielectric resonator 1 with a high Q factor (ie low loss) the loss in the resonator 1 as a whole is dominated by the resistive loss in the surrounding metal. Thus, for the ΕΗ_01Δ mode most of the loss is in the metal of the side wall 6. To reduce resonant frequency with the smallest reduction in Q the air gap 9,10 can be reduced on either one or both sides of the dielectric plate 8. Due to the strength of the E field proximate to the length axis 7 perturbations in the thickness of the air gap 9,10 proximate to the length axis 7 will provide the largest change in frequency. Since the magnetic field H is very weak in this area then the reduction in Q will be minimised.

Shown in figure 7 is an end face 4 of a further embodiment of a resonator 1 according to the invention. Proximate to the end face 4 is a plurality of electrically conducting tuning plates 14. The tuning plates 14 are arranged substantially parallel to the end face 4 and proximate to the length axis 7. Extending from each tuning plate 14 is an electrical conductor 15. Each electrical conductor 15 extends through an associated aperture 15a in the end plate 4 as shown. Connected to the electrical conductor 15 is a switch 16 which can be switched between open and closed configurations. The opposite side of the switch 16 to the tuning plate 14 is connected to an electrical potential, typically an earth 17 or to the end face 4 proximate to the tuning plate 14. When the switch 16 is in the open configuration the potential of the tuning plate 14 floats. When the switch 16 is in the closed configuration the potential of the tuning plate 14 is at earth or the same potential as the end face 4. This effectively reduces the size of the air gap 9 so changing the resonant frequency of the resonator 1 in the ΕΗ₀ι_Δ mode. By switching on various combinations of tuning plates 14 one can change the resonant frequency of the resonator 1 between many different resonant frequencies with minimal Q degradation.

When changing the resonant frequency of the resonator 1 the effective change in capacitive loading will transfer the majority of the stored energy associated with the original resonance to the new resonant frequency quickly and with minimal loss of energy. The microwave resonator 1 according to the invention can therefore be switched between different resonant frequencies far more quickly than known microwave resonators.

To further minimise loss for the resonator the switches 16 are typically MEMS switches. Such switches 16 have a very high on to off impedance change. Alternatively semiconductor switches particularly Gallium Nitride PIN diodes can be used.

Typically a metal input connection pad 18 is connected to the first end face 4. A signal line 19 is connected to the connection pad 18. The microwave signal to be filtered is provided on the signal line 19. An output connection pad 20 is connected to either the first or second end faces 4,5. A further signal line 21 is connected to the output connection pad 20. The filtered microwave signal is received from the output signal line 21. As the ΕΗ₀ι_Δ mode is symmetric about the length axis the angular separation between the input and output pads 18,20 about the length axis 7 can have any value.

An alternative method of providing a microwave signal to be filtered to the microwave resonator 1 is by means of magnetic coupling. The signal line 19 which provides the microwave signal is curved proximate to the dielectric plate 8. The field generated by the curved wire couples the microwave signal to the resonator cavity 2. Returning to figure 3(a) as can be seen when A is in the range about 4.5mm to 5mm the EH_m mode is sufficiently separated in frequency from the other modes to be successfully used for narrowband low loss filters. The Q factor is 50% higher than the ΕΗ₀ι_Δ mode and 50% higher in frequency. This mode has a sinusoidal field variation around the length axis 7. Hence it is a dual mode resonator with each mode orthogonal to each other.

When the resonator 1 is used in the ΕΗ«_Δ mode the output connection pad 20 is typically arranged such that the angular separation between the connection pads 18,20 about the length axis 7 is about 90 degrees. A coupling screw 22 extends through the end face 4 at a point mid-way in angular separation about the length axis 7 between the input and output connection pads 18,20.

The tuneable resonator 1 may further comprise a filter (not shown) connected to the input or output connection pad 18,20. The passband of the filter includes the frequency of the mode of the resonator 1 one wishes to excite (typically the ΕΗ₀ι_Δ mode or EH_m mode) but does not include the resonant frequencies of the other modes.

Shown in figure 8 is a further embodiment of a microwave resonator 1 according to the invention. Connected to an end face 4 of the resonator 1 is a microwave source 23. The microwave source 23 is adapted to provide a microwave signal of a frequency suitable for exciting one of the ΕΗ₀ι_Δ or EH_m modes of the resonator 1.

In the particular example shown, the microwave resonator 1 is being employed as a dual mode resonator 1 (ie ΕΗ_11Δ mode) and so comprises input and output connection pads 18,20 and a coupling screw 22 arranged as previously described.

In an alternative embodiment the source 23 is connected to the end face 4 through a filter as described above. In a further alternative embodiment the filter is connected to the output 21 of the resonator 1. In a further aspect of the invention there is provided a microwave filter 30 comprising a plurality of such resonators 1 electrically coupled together, at least some of which are connected together in cascade. All of the resonators 1 may be single mode resonators. All of the resonators 1 may be dual mode resonators. The resonators 1 may be a mixture of single and dual mode resonators.

Figure 9 shows two of the microwave resonators 1 of the filter 30 coupled together. The two resonators 1 share a common side wall 6. The common side wall 6 has an aperture 31 therein through which the resonators 1 couple together. Preferably the dielectric plates 8 of the two resonators 1 are dimensioned such that they touch as shown.

Claims

A microwave resonator comprising a resonator cavity defined by an electrically conducting cavity wall, the cavity wall comprising first and second spaced apart end faces and a side wall extending therebetween; the cavity having a length axis extending from the centre of the first face to the centre of the second face, the cavity having a length A along the length axis between the first and second end faces; the cavity having a width axis normal to the length axis, the width of the cavity from the side wall through the length axis to the side wall on the opposite side of the length axis being C; a dielectric plate arranged within the cavity normal to the length axis; the dielectric plate being spaced apart from the first and second end faces, so defining first and second gaps therebetween; the width of the dielectric plate normal to the length axis being at least 90% of C, the thickness B of the dielectric plate along the length axis being at least 60% of A; and, wherein C is at least four times A.

A microwave resonant cavity as claimed in claim 1, wherein C is at least five times A, preferably at least six times A.

A microwave resonant cavity as claimed in either of claims 1 or 2 wherein normal to the length axis the shape of the dielectric plate is the same as the shape of the cavity.

A microwave resonant cavity as claimed in any one of claims 1 to 3, wherein the width of the dielectric plate normal to the length axis is at least 95% of C, the dielectric plate being spaced apart from the side wall.

5. A microwave resonator as claimed in claim 3, wherein normal to the length axis the dielectric plate is the same size as the cavity and abuts the side wall.

6. A microwave resonator as claimed in any one of claims 1 to 5, wherein B is at least 70% of A, more preferably at least 80% of A, more preferably at least 90% of A.

7. A microwave resonator as claimed in any one of claims 1 to 6, wherein the relative permittivity of the dielectric plate is in the range 10 to 90, more preferably 30 to 60, more preferably 45.

8. A microwave resonator as claimed in any one of claims 1 to 7, wherein the cavity has a circular cross section normal to the length axis.

9. A microwave resonator as claimed in any one of claims 1 to 8, wherein the cavity has a fourfold rotational symmetry about the length axis, is preferably square.

10. A microwave resonator as claimed in any one of claims 1 to 9, wherein the thickness of the first air gap along the length axis is equal to the thickness of the second air gap along the length axis.

11. A microwave resonator as claimed in any one of claims 1 to 10, wherein the dielectric plate has an aperture extending therethrough, the microwave resonator further comprising a tuning screw extending through the end plate into the aperture.

A microwave resonator as claimed in any one of claims 1 to 11, further comprising at least one electrically conducting tuning plate arranged in one of the first and second gaps and an electrical conductor extending from the tuning plate through an aperture in the end face proximate to the tuning plate.

13. A microwave resonator as claimed in claim 12, further comprising a switch connected in series with the electrical conductor, preferably a MEMS switch or a Gallium Nitride PIN diode.

14. A microwave resonator as claimed in any one of claims 1 to 13, further comprising a microwave filter connected to the resonator cavity.

15. A microwave resonator as claimed in any one of claims 1 to 14, further comprising a microwave source, the microwave source being adapted to provide a microwave signal suitable for exciting either the ΕΗ₀ι_ή or EH_m mode of the resonator.

16. A microwave resonator as claimed in any one of claims 1 to 15, wherein C is in the range 10mm to 60mm, more preferably 20mm to 30mm.

17. A microwave resonator as claimed in any one of claims 1 to 16, wherein B is in the range 2mm to 6mm, more preferably 4 to 5mm.

18. A microwave filter comprising a plurality of resonators as claimed in any one of claims 1 to 17, the microwave resonators being electrically coupled together.

19. A microwave filter as claimed in claim 18, wherein at least some of the resonators are connected together in cascade.

20. A microwave filter as claimed in either of claims 18 or 19 wherein the coupled resonators share a common side wall, the common side wall having an aperture extending therethrough.

21. A method of filtering a microwave signal comprising the steps of providing a microwave resonator as claimed in any one of claims 1 to 17; and, providing a microwave signal to the resonator, the microwave signal having a frequency suitable for exciting either the ΕΗ_01Δ or ΕΗ_11Δ mode of the resonator.

22. A microwave resonator substantially as hereinbefore described.

23. A microwave filter substantially as hereinbefore described.