GB2505161B

GB2505161B - A microwave resonator and a tuneable filter including such a resonator

Info

Publication number: GB2505161B
Application number: GB1212285.9A
Authority: GB
Inventors: Ian Mobbs Christopher
Original assignee: Filtronic Wireless Ltd
Current assignee: Filtronic Wireless Ltd
Priority date: 2012-07-10
Filing date: 2012-07-10
Publication date: 2019-09-04
Anticipated expiration: 2032-07-10
Also published as: GB2505161A; GB201212285D0

Description

A microwave resonator and a tuneable filter including such a resonator

The present invention relates to a microwave resonator and a tuneable filter including such a resonator. More particularly, but not exclusively, the present invention relates to a microwave resonator comprising a ceramic resonator body having an aperture extending therethrough and a tuning arm comprising a plunger and a disk, the tuning arm being adapted to be displaced to a position with at least a portion of the plunger in the aperture. More particularly, but not exclusively, the present invention relates to a tuneable filter comprising at least one microwave resonator according to the invention.

The use of tuneable ceramic resonators in filter structures is known. In order to tune the resonator a tuning plunger is typically inserted into the ceramic resonator body. This can often give a reasonably linear tuning of the frequency of the resonator as a function of plunger position. However, coupling of a microwave signal to the resonator can vary significantly as a function of plunger position. This complicates the design of filters including such resonators.

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

Accordingly, in a first aspect of the invention, there is provided a microwave resonator comprising a cavity defined by an electrically conducting cavity wall; a non-metallic resonator body within the cavity spaced from the cavity wall, the resonator body comprising a face having a recess therein; a tuning arm extending through the cavity wall along a displacement axis; a non-metallic plunger connected to the tuning arm; the tuning arm being adapted to be displaced along the displacement axis from a first position towards the resonator body to a second position, at least a portion of the plunger being received in the recess in the second position; and, a non-metallic tuning member connected to the tuning arm on the opposite side of the plunger to the resonator; wherein the tuning member is spaced apart from the plunger.

The microwave resonator according to the invention has a resonant frequency which varies as a linear function of tuning arm position. Significantly, in addition to this the coupling of the microwave signal to the resonator does not vary with tuning arm position.

Preferably, the thickness of the plunger along the displacement axis is less than its width normal to the displacement axis.

Preferably, the plunger is a cylindrical rod.

Preferably, the thickness of the tuning member along the displacement axis is less than its width normal to the displacement axis.

Preferably, the tuning member is a disk

Preferably, the diameter of the disk is larger than the width of the recess.

The recess in the resonator face can extend through the resonator body to form an aperture extending therethrough.

The tuning member can be a dielectric.

The tuning member can be a ceramic.

The plunger can be a dielectric.

The plunger can be a ceramic.

The resonator body can be a dielectric.

The resonator body can be a ceramic.

In a further aspect of the invention there is provided a tuneable filter comprising at least one microwave resonator as claimed in any one of claims 1 to 13.

Preferably, the tuneable filter comprises a plurality of resonators as claimed in any one of claims 1 to 13.

Preferably, the filter comprises first and second input ports and an output port, the filter being a bandstop filter between the first input port and the output port and a bandpass filter between the second input port and the output port.

The filter can comprise a plurality of impedance inverters connected together in series along a signal path, the filter further comprising a microwave resonator according to any one of claims 1 to 13 connected between two adjacent impedance inverters in the signal path.

The filter can comprise a microwave resonator as claimed in any one of claims 1 to 13 between each pair of adjacent impedance inverters.

The filter can be a bandpass filter.

The filter can be a bandstop filter.

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 known ceramic resonator;

Figure 2 shows the resonant frequency of the resonator of figure 1 as a function of tuning arm position;

Figure 3 shows a first embodiment of a resonator according to the invention;

Figure 4 shows the resonator of figure 3 with the plunger just entering the resonator body;

Figure 5 shows the resonator of figure 3 with the plunger further into the resonator body;

Figure 6 shows a further embodiment of a resonator according to invention.

Figure 7 shows a first embodiment of a tuneable filter according to the invention;

Figure 8 shows a further embodiment of a tuneable filter according to the invention;

Figure 9 shows a further embodiment of a tuneable filter according to the invention; and,

Figure 10 shows a further embodiment of a tuneable filter according to the invention.

Shown in figure 1 is a known ceramic resonator 1. The resonator 1 comprises a cavity 2 defined by an electrically conducting cavity wall 3. Arranged within the cavity 2 is a ceramic resonator body 4 having an aperture 5 extending therethrough. The resonator body 4 is arranged on a non-metallic support 6 so spacing it apart from the cavity wall 3. The cavity wall 3 comprises input and output apertures 7,8 for entry of a microwave signal into the cavity 2 and its subsequent exit from the cavity 2. A tuning arm 9 extends through an aperture 10 in the cavity wall 3 and extends along a displacement axis (not shown). Arranged on the end of the tuning arm 9 is a ceramic plunger 11. The plunger 11 has a thickness in the direction of the displacement axis greater than its width. The plunger 11 is dimensioned so as to fit within the aperture 5 of the resonator body 4.

In use the tuning arm 9 is displaced along the displacement axis. As the plunger 11 approaches and then enters into the aperture 5 the resonant frequency of the resonator 1 changes.

Figure 2 shows the resonant frequency of the resonator 1 of figure 1 as a function of position of the tuning arm 9. As can be seen over some range of positions of the tuning arm 9 the resonant frequency varies as a linear function of position of the tuning arm 9. This is the desired behaviour. When the plunger 11 is deep within the aperture 5 however this linear behaviour is lost. At the point where the plunger 11 is arranged close to the centre of the aperture 5 then the resonant frequency becomes substantially insensitive to the position of the plunger 11 as shown. A more significant drawback of such a known resonator 1 is that the coupling of the microwave signal to the resonator 1 varies significantly with position of the tuning arm 9. This considerably complicates the design of circuits including such resonators 1.

Shown in cross section in figure 3 is a microwave resonator 20 according to the invention. The resonator 20 comprises a cavity 21 defined by an electrically conducting cavity wall 22. The cavity wall 22 comprises input and output ports 23,24 for entry of a microwave signal into the cavity 21 and subsequent exit of the signal from the cavity 21.

Arranged within the cavity 21 is a non-metallic resonator body 25. In this embodiment the resonator body 25 is a dielectric, more particularly a ceramic. Extending through the resonator body 25 is an aperture 26.

Extending through the cavity wall is a tuning arm 27. The tuning arm 27 extends along a displacement axis. The tuning arm 27 is a plastics material.

Connected to the tuning arm 27 is a non-metallic plunger 28. In this embodiment the plunger 28 is a dielectric, more particularly a ceramic. The thickness of the plunger 28 along the direction of the displacement axis is greater than the width of the plunger 28 normal to the displacement axis. In this embodiment the plunger 28 is circular in cross section so forming a cylindrical rod and is dimensioned to fit within the circular aperture 26 extending through the resonator body 25.

Also attached to the tuning arm 27 on the opposite side of the plunger 28 to the resonator body 25 is a non-metallic tuning member. The tuning member 29 is a disk arranged normal to the displacement axis. The width of the disk 29 normal to the displacement axis is greater than its thickness along the displacement axis.

The tuning member 29 is spaced apart from the plunger 28 as shown. Typically this is achieved by means of a non-metallic spacer 30.

In use the tuning arm 27 is displaced along the displacement axis. In this embodiment this is achieved by a motor (not shown). Starting from a first position with the plunger 28 remote from the resonator body 25 then as the plunger 28 approaches the resonator body 25 and enters the aperture 26 the resonant frequency of the resonator 20 drops. When the plunger 28 is just entering into the aperture 26 as shown in figure 4 the tuning member 29 has little effect on the resonant frequency. The resonant frequency therefore drops substantially linearly with position of the plunger 28 as with the resonator of the prior art.

As the plunger 28 is inserted further into the aperture 26 to a second position (figure 5) then the effect of displacement of the plunger 28 on the resonant frequency becomes less (as shown in figure 2). At this point however the tuning member 29 is approaching the resonator body 25. This also causes the resonant frequency of the resonator 20 according to the invention to drop. If the plunger 28 and tuning member 29 are dimensioned correctly and separated correctly then the combined effect of the plunger 28 and tuning member 29 is to produce a resonant frequency which continues to drop linearly with position of the tuning arm 27.

The resonator 20 according to the invention has a further important technical advantage over the resonator 20 of the prior art. The coupling of the microwave signal to the resonator 20 does not vary significantly with position of the tuning arm 27.

In an alternative embodiment of the invention (not shown) the tuning arm 27 has a thread on its outer surface. This engages with a corresponding thread around the aperture through which the tuning arm 27 passes into the cavity 21. By rotating the tuning arm 27 about the displacement axis the tuning arm 27 can be displaced into and out of the cavity 21.

In an alternative embodiment the tuning arm 27 can be a ceramic. In a further alternative embodiment the tuning arm 27 can be a metal.

Shown in figure 6 is a further embodiment of a resonator 20 according to the invention. In this embodiment the resonator body 25 has a recess 31 in a face 32. The plunger 28 is received in the recess 31. In this embodiment the recess 31 is square in cross section as are the plunger 28 and tuning member 29. In alternative embodiments other shapes of tuning member 29 and plunger 28 are possible.

Shown in figure 7 is a first embodiment of a tuneable filter 40 according to the invention. The tuneable filter 40 comprises first and second input ports 41,42 and first and second output ports 43,44. Connected between the first input port 41 and first output port 43 is a first signal line 44. Arranged in the first signal line is an impedance inverter 45. Connected between the second input port 42 and second output port 44 is a second signal line 46. Arranged in the second signal line 46 is a further impedance inverter 47.

Connected in parallel between the first and second signal lines 44,46 are third and fourth signal lines 48,49. The third and fourth signal lines 48,49 are connected to the first and second signal lines 44,46 on opposite sides of the impedance inverters 45,47 in the first and second signal lines 44,46 as shown. Each of the third and fourth signal lines 48,49 comprises a plurality of impedance inverters 50 connected together in series. The impedance inverters 50 are connected together at nodes 51.

Connected between each node 51 of the third signal line 48 and the corresponding node 51 of the fourth signal line 49 is an impedance inverter 52. Connected to each node 51 in the third and fourth signal lines 48,49 is a resonator 53 according to the invention.

Typically an internal load (not shown) is connected to the second output port 44.

Between the first input port 41 and the first output port 43 the filter 40 acts as a bandstop filter. Between the second input port 42 and first output port 44 the filter 40 acts as a bandpass filter. The bandstop/bandpass frequency range can be adjusted by tuning the resonant frequencies of the resonators 53.

Shown in figure 8 is a further embodiment of a filter 40 according to the invention. This embodiment is similar to that of figure 7 except the third and fourth signal lines 48,49 each only comprise two impedance inverters 50 connected together in series. The operation of the filter 40 however is substantially identical to that of figure 7. The embodiment of figure 7 is a two stage filter whilst that of figure 8 is a single stage filter.

The output from the first output port 43 of the filter 40 according to the invention could be connected to further filter stages (not shown). In particular the output could be connected to an input port of a further filter 40 according to the invention.

The filter 40 shown in figures 7 and 8 are balanced filters. Shown in figure 9 is an alternative embodiment of a tuneable filter 60 according to the invention. This filter 60 is a conventional band pass filter comprising a plurality of impedance inverters 61 connected together in series along a signal path 62. Connected in the signal path 62 between the impedance inverters 61 are resonators according to the invention modelled as a capacitor 63 and inductor 64 in parallel.

Shown in figure 10 is a further embodiment of a tuneable filter 60 according to the invention. This filter 60 is a conventional band stop filter. The band stop filter comprises a plurality of impedance inverters 61 connected together in series along a signal path 62. Connected between the impedance inverters 61 are resonators according to the invention each modelled as a capacitor 63 and inductor 64 in series.

Claims

1. A microwave resonator comprising A cavity defined by an electrically conducting cavity wall; a non-metallic resonator body within the cavity spaced from the cavity wall, the resonator body comprising a face having a recess therein; a tuning arm extending through the cavity wall along a displacement axis; a non-metallic plunger connected to the tuning arm; the tuning arm being adapted to be displaced along the displacement axis from a first position towards the resonator body to a second position, at least a portion of the plunger being received in the recess in the second position; and, a non-metallic tuning member connected to the tuning arm on the opposite side of the plunger to the resonator; wherein the tuning member is spaced apart from the plunger.

2. A microwave resonator as claimed in claim 1, wherein the thickness of the plunger along the displacement axis is less than its width normal to the displacement axis.

3. A microwave resonator as claimed in claim 2, wherein the plunger is a cylindrical rod.

4. A microwave resonator as claimed in any one of claims 1 to 3, wherein the thickness of the tuning member along the displacement axis is less than its width normal to the displacement axis.

5. A microwave resonator as claimed in any one of claims 1 to 4, wherein the tuning member is a disk

6. A microwave resonator as claimed in claim 5, wherein the diameter of the disk is larger than the width of the recess.

7. A microwave resonator as claimed in any one of claims 1 to 6, wherein the recess in the resonator face extends through the resonator body to form an aperture extending therethrough.

8. A microwave resonator as claimed in any one of claims 1 to 7, wherein the tuning member is a dielectric.

9. A microwave resonator as claimed in claim 8, wherein the tuning member is a ceramic.

10. A microwave resonator as claimed in any one of claims 1 to 9 wherein the plunger is a dielectric.

11. A microwave resonator as claimed in claim 10, wherein the plunger is a ceramic.

12. A microwave resonator as claimed in any one of claims 1 to 9, wherein the resonator body is a dielectric.

13 A microwave resonator as claimed in claim 12, wherein the resonator body is a ceramic.

14. A tuneable filter comprising at least one microwave resonator as claimed in any one of claims 1 to 13. A tuneable filter as claimed in claim 14 comprising a plurality of resonators as claimed in any one of claims 1 to 13. A tuneable filter as claimed in either of claims 14 or 15, the filter comprising first and second input ports and an output port, the filter being a bandstop filter between the first input port and the output port and a bandpass filter between the second input port and the output port. A tuneable filter as claimed in either of claims 14 or 15, the filter comprising a plurality of impedance inverters connected together in series along a signal path; the filter further comprising a microwave resonator according to any one of claims 1 to 13 connected between two adjacent impedance inverters in the signal path. A tuneable filter as claimed in claim 17, comprising a microwave resonator as claimed in any one of claims 1 to 13 between each pair of adjacent impedance inverters. A tuneable filter as claimed in any one of claims 14 to 18, wherein the filter is a bandpass filter. A tuneable filter as claimed in any one of claims 14 to 18, wherein the filter is a bandstop filter.