WO2002054527A2

WO2002054527A2 - A filter including coaxial cavity resonators

Info

Publication number: WO2002054527A2
Application number: PCT/SE2001/002883
Authority: WO
Inventors: Tuomo RÄTY
Original assignee: Allgon Ab
Priority date: 2000-12-29
Filing date: 2001-12-21
Publication date: 2002-07-11
Also published as: AU2002217683A1; WO2002054527A3; SE0004935D0

Abstract

A RF-filter comprising resonator walls (21-26) delimiting a cavity (11) and at least a first and a second resonator (31, 32). A coupling adjuster (611, 621) for adjusting the electromagnetic coupling is arranged between said first and second resonators (31, 32). The coupling adjuster (611, 621) includes an elongated body (611, 624) arranged to be rotatable around an axis located in a plane between the first and the second resonator (31, 32). The coupling adjuster and a supporting wall (612, 622) portion are integral with an internal wall (61, 62) arranged between two resonators.

Description

A filter including coaxial cavity resonators.

Technical field

The present invention relates to a RF-filter including coaxial cavity resonators and a coupling adjuster between the coaxial cavity resonators .

Background of the invention

Coaxial cavity resonators are widely used in telecommunication applications. It has been the aim of research and development to achieve smaller resonators and, at the same time, maintain or increase the Q-value of the resonators (i.e. increase the Q-value per volume) .

A simple type of coaxial cavity resonator comprises a rod, arranged inside a cavity, a so called rod resonator. A first end of the rod is connected to the bottom wall of the cavity and the second end of the rod is in open-circuit relation to the cavity walls. To obtain a reasonable good Q-value, the cross-sectional diameter of the cavity should be approximately 3 times the diameter of the rod.

When coaxial cavity resonators are coupled to each other, coupling arrangements are often physically connected to the resonating body. This results in a decreased unloaded Q-factor when compared with a non-coupled resonator.

A rod resonator is disclosed in WO 99/17394 (Al) . In order to adjust the electromagnetic coupling between two resonators special flaps are arranged on the rods. The flaps are provided on conductor plates, which are separate parts that have to be attached on the resonator bodies . A microwave filter including coaxial cavity resonators is disclosed in US-A-5 225 799. In order to adjust the electromagnetic coupling between the resonators, coupling tabs are arranged in a sidewall of the filter. The adjustment of the coupling is somewhat limited since the tabs are simply adjustable in only one direction.

Another common way to adjust the electromagnetic coupling between resonators is to use coupling screws. However, the use of coupling screws gives a limited coupling adjustment range, being about 5%. At the same time a number of extra components are added to the device. The use of tuning screws and lock nuts thus add component and manufacturing costs as well as mechanical dimensions.

Thus, the coupling devices of prior art devices are suffering of a number of drawbacks, such as the need of many parts to be assembled, a complicated adjustment and/or a small coupling adj ustment range .

Summary of the invention

An object of the present invention is to provide a RF-filter including coaxial resonators with a coupling adjuster in which at least some of the drawbacks described above are reduced.

According to an aspect of the present invention there is provided a RF-filter as specified by claim 1.

An advantage with the present invention is that the filter can be made mechanically strong, due to simple and robust construction of the coupling adjuster. Another advantage with the present invention is that the filter is easy to assemble, since it can be made of relatively few parts.

Another advantage with the present invention is that the tolerance requirements at the coupling area are low or easy to achieve, due to the wide adjustment range. The coupling device and wall aperture are relatively easy to design so that even in small cavities the minimum distance from the coupling device to the resonators is several millimetres. In practice it is possible to repeat the coupling within ±2% accuracy without any measurements .

Another advantage is that a filter according to the present invention is cheap to manufacture.

A further advantage with the present invention is that it provides a wide coupling adjustment range. It is possible to obtain a minimal coupling being less than 50% of the maximal coupling, with a coupling adjuster according to the invention.

Another advantage is that the present invention does not show outside the filter housing and does therefore not add to the mechanical dimensions.

Still another advantage is that while cutting dramatically the number of parts, the tuning of the filter according to the invention is still possible to automate and actually opens new possibilities in automation when compared with screw-tuning.

Brief description of the drawings

Figure 1 shows a filter, according to the invention, in a front view with a partly broken up front wall. Figure 2 shows a first internal wall with a first coupling adjuster, according to the invention, in a view perpendicular to that of figure 1.

Figure 3 shows a second internal wall with a second coupling adjuster, according to the invention, in a view perpendicular to that of figure 1.

Figure 4 shows the filter of figure 1 in a top view.

Figure 5 shows an adjustment tool to be used on a filter according to the invention.

Figure 6 shows an alternative adjustment tool to be used on a filter according to the invention.

Embodiments of the present invention are described below, by way of example only.

Detailed description of the preferred embodiments

Figure 1 shows a filter 1 according to the invention, in a front view with a partly broken up front wall 21. Besides the front wall 21, the filter has a rear wall 22, two side walls 23 and 24, a top wall 25 and a bottom wall 26. The walls 21-26 are conductive resonator walls delimiting a cavity 11, which comprises five resonators 31-35. Each resonator 31-35 includes an inner conductor 41-45 in the form of a conductive rod. Each rod 41-45 is, in this example, connected to a first side of a circular supportive plate 51-55, at the bottom of the respective resonator rod 41-45.

The top end of each rod 41-45 is in open-circuit relation to the cavity walls. A second side, opposite to said first side, of each of said supportive plates 51-55 is conductively connected to the bottom wall 26, e.g. by soldering, welding, conductive glue or in any other suitable way.

Each rod 41-45 and its respective supportive plate 51-55 is preferably coated with a highly conductive material, such as silver, irrespective of if the rods 41-45 and the supportive plates 51-55 are made from one piece or from separate pieces of material .

An alternative to coating is to manufacture the rod and the plate in a solid highly conductive material.

If they are made from separate pieces they have to be conductively connected to each other, e.g. by soldering, preferably before coating.

Between adjacent rods 41-45 there are internal walls 61-64. In this example the internal walls limit the resonators from each other. All internal walls 61-64 include openings for the electromagnetic coupling between the respective adjacent resonators and separate the upper part, the lower part or both parts of the resonators from each other. The internal walls 61-64 run all the way between the front wall 21 and the rear wall 22 or just a portion thereof. When extending up to the front wall 21 and/or the rear wall 22, an internal wall 61-64 is conductively connected thereto. It is also possible that one or more of the internal walls 61-64 is omitted.

It is of course possible to arrange any type of internal wall dependent on the amount of coupling wanted between the resonator bodies, e.g. an internal wall extending from the bottom wall 26 towards the top wall 25 or an internal wall being provided with openings, such as slots or windows, for coupling the resonator bodies with each other. It is even possible to construct a coaxial cavity resonator without any internal -walls or have internal walls separating each resonator body from each other.

The filter 1 thus comprises five coaxial resonators 31-35, each having an inner conductor 41-45 and outer conductor 21-26 and 61-64. The inner and outer conductors are air-insulated from each other (except for the bottom portions) . Further the filter 1 is also provided with an input 12 and an output 13.

As seen in the figure 1 the internal walls 61 and 62 are both provided with adjustable coupling adjusters or coupling devices 611 and 621, for adjusting the electromagnetic coupling between the adjacent resonators 31-32 and 32- 33 , respectively. The coupling adjusters are made of a conductive material and exhibit elongated (horizontal) portions 611 and 624. The coupling adjusters 611 and 621 are, in this example, made in the same piece as the supporting internal wall 61 and 62, respectively. They are for instance formed in a stamping, laser cutting or etching process.

In figure 2, the internal wall 61 with coupling adjuster 611 is shown, in a view perpendicular to that of figure 1. From figure 2 it is clear that the coupling adjuster 611 is supported by the internal wall 61 and wall portion 612. Wall portion 612 is made so that the coupling adjuster 611 can be rotated or twisted around an axis 613 running through wall portion 612 in its direction of extension. Said axis 613 divides the coupling adjuster 611 into a first (left) and a second (right) portion. When adjusting the coupling between resonators 31 and 32, the coupling adjuster is rotated or twisted around the axis 613 and wall portion 612 will be twisted essentially around the same axis. The wall portion 612 is made in such a material and with such dimensions that it remains stable in the adjusted position. The coupling is maximal when the coupling adjuster 611 extends in the plane of the internal wall 61 (unrotated) . The minimal coupling is obtained when the coupling adjuster 611 extends in a plane perpendicular with the internal wall, i.e. when the coupling adjuster is parallel to a line between the inner conductors 41 and 42. By this arrangement it is possible to obtain a minimal coupling being less than 50% of the maximal coupling. The shape of the coupling adjuster in this embodiment is only one example; the shape can vary a lot. The essential is that the coupling adjuster exhibits two open ends. Thus the coupling adjuster could alternatively have a shape being for instance essentially a V, ! or U-shape. Also, the shape of the opening or aperture in the internal wall 61 can vary and have just about any shape .

In figure 3, the internal wall 62 with coupling adjuster 621 is shown, in a view perpendicular to that of figure 1. In figure 3 it is shown that the coupling adjuster 621 is supported by the internal wall 62 and wall portion 622. Coupling adjuster 621 is shaped like a coat-hanger with a horizontal cross bar, or like a stirrup iron. The internal wall 62 then has an opening corresponding to the outer shape of the coupling adjuster 621, and the coupling adjuster 621 has a central opening 625. Wall portion 622 is made so that the coupling adjuster 621 can be rotated or twisted around an axis 623 running through wall portion 612 and essentially perpendicularly to elongated portion 624. Said axis 623 divides the coupling adjuster 621 into a first (left) and a second (right) portion. When adjusting the coupling between resonators 32 and 33, the coupling adjuster is rotated or twisted around the axis 623 and wall portion 622 will be twisted essentially around the same axis. The wall portion 622 is made in such a material and with such dimensions that it remains stable in the adjusted position. The coupling is minimal when the coupling adjuster 621 extends in the plane of the internal wall 62 (unrotated) . The maximal coupling is obtained when the coupling adjuster 621 extends in a plane perpendicular with the internal wall 62, i.e. when the coupling adjuster is parallel to a line between the inner conductors 42 and 43. By this arrangement it is also possible to obtain a minimal coupling being less than 50% of the maximal coupling. The shape of the coupling adjuster in this embodiment is only one example; the shape can vary a lot. The essential is that the coupling adjuster forms a conductive loop. Thus the coupling adjuster could alternatively have a shape being for instance essentially an oval, a circle a trapezium or a trapezoid. Also, the shape of the opening or aperture in the internal wall 62 can vary and have just about any shape .

The internal walls 61 and 62 are preferably connected with the front wall 21 and the rear wall 22.

Figure 4 shows the top wall in a top view. In order to facilitate the adjustment of the coupling, the top wall 25 is provided with openings 251, 251', 252 and 252' through which an adjustment tool 7 can be introduced. The openings 251, 251', 252 and 252' are all substantially parts of a circular curve and arranged in pairs above the respective coupling adjuster. If maximum tuning range is desired they are all substantially semicircularly shaped and arranged to form almost circular slots .

Depending on the location of the coupling adjuster the openings could be placed in the bottom, front or rear wall. The coupling adjuster could namely be attached to any of these walls with the supporting internal wall or without the internal wall 61, 62, i.e. directly on a cavity wall 21-26, preferably with the help of a supporting wall portion 612 or 622.

An adjustment tool 7 is shown in figure 5. This adjustment tool is fork-like and has two pins 71 and 71' and a central gripping portion 72. The pins 71 and 71' are introduced in e.g. openings 251 and 251', respectively, with one pin on the other side of the coupling adjuster 611 (shown with broken lines in figure 4) as the other pin. The gripping portion 72 is then turned (around axis 613) and pins 71 and 71' act to rotate the coupling adjuster 611. The coupling adjuster 611 can thus be rotated in both directions. Since coupling adjuster 621 is coplanar with the internal wall 62 before adjustment, the coupling adjuster 621 has to be rotated before the assembly of the filter so that the adjustment tool 7 can act on the coupling adjuster 621. Alternatively (see figure 6), an adjustment tool ^'8 with extra pins 83 and 83' attached to pins 81 and 81' and having a part being perpendicular to those can be used to put the coupling adjuster 621 into a position where the adjustment tool 7 (or 8) can act on the coupling adjuster 621.

The adjustment procedure can advantageously be automatised when using a fork-like tool.

Although the described filter comprises two adjustable coupling adjusters, it could comprise such coupling adjusters between any number of included resonators. The coupling adjusters could be of any or both of the described types. There could also be more than one coupling adjuster, e.g. two coupling adjusters, of the same or different kind between a pair of resonators. When the coupling device is grounded, as they are in the examples above, the means of adjusting will be inductive. This is, when the coupling is capacitive, it can be adjusted with an inductive portion in the total coupling. However, the means of adjusting the coupling are inductive if resonator hats do not get very close to each other.

It is also possible to arrange coupling adjusters to be not grounded, but instead isolated from ground, for instance soldered on a PCB support. In this case the means of adjusting will be capacitive.

In the filter described, tuning means may also be provided in the coaxial cavity resonators, which may be arranged on the side walls of the cavity, or be arranged on a resonator body.

The rod may have any shape although the shown embodiments only disclose an elongated member having a constant circular cross- section. A square or rectangular cross-section may be used instead, and the shape of the cross-section along the rod may change .

The resonators could be of any suitable type of coaxial cavity resonator. Suitable resonators are disclosed in the international publications WO 00/10220 (Allgon) and WO 01/76004 (Allgon) both included herein by reference. A number of examples are disclosed above, where coaxial cavity resonators have inner and outer conductors, which are air- insulated in relation to each other. Although not specifically shown, it is possible that the resonators are dielectrically loaded, i.e. the inner and outer conductors are only partly air-insulated in relation to each other.

By RF-filter is meant, in this application, a filter operating in frequencies used for radio transmission and radio communication, e.g. for voice and/or data communication purposes. The filter according to the invention is preferably used in frequency range of about 200 MHz to 6 GHz, i.e. in a frequency range in which coaxial resonators are frequently used.

Although the invention has been described in conjunction with a number of preferred embodiments, it is to be understood that various modifications may still be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A RF-filter comprising:

resonator walls delimiting a cavity, at least a first and a second resonator including a first and a second inner conductor, respectively, each conductively connected to a first of said walls, input and output devices, and at least one coupling adjuster for adjusting the electromagnetic coupling between said first and second resonators, said at least one coupling adjuster includes an elongated body having a first and a second portion, said elongated body is arranged to be rotatable around an axis located in a plane between the first and the second resonator, and said axis intersects the elongated body between the first and the second portion

characterised in that

the elongated body is conductively connected to a support allowing the elongated body to be rotated around said axis, the support includes a wall portion arranged between said first and second resonator bodies, and the coupling adjuster and the supporting wall portion are integral with an internal wall arranged between two resonators .

2. The RF-filter according to claim 1, wherein the coupling adjuster has two open ends.

3. The RF-filter according to claim 1 or 2 , wherein the coupling adjuster includes a closed loop.

4. The RF-filter according to any of claims 1-3, wherein the coupling adjuster essentially has a shape selected from the group consisting of a shape of a coat-hanger with a horizontal crossbar, a shape of a stirrup iron, an oval, a circle a trapezium and a trapezoid.

5. The RF-filter according to any of claims 1-4, wherein the axis intersects the elongated body at a central portion of the elongated body.

6. The RF-filter according to any of claims 1-5, wherein said wall portion is located in said plane.

7. The RF-filter according to any of claims 1-6, wherein the coupling adjuster and the wall portion together essentially has a shape selected from the group consisting of V- , Y- , LT- , and T-shapes.

8. The RF-filter according to any of claims 1-7, wherein at least one of said cavity walls is provided with at least one pair of oppositely located slots, each shaped essentially as a part of a circle, and being arranged over an adjustable coupling adjuster for introduction of an adjustment tool.

9. The RF-filter according to any of claims 1-8, wherein two coupling adjusters are arranged between said at least first and second resonators .

10. The RF-filter according to any of claims 1-9, wherein a plurality of resonators are arranged in cascade and at least one coupling adjuster is arranged between every pair of adjacent resonators of said cascade coupled resonators.

11. The RF-filter according to any of claims 1-10, wherein the coupling adjuster is conductively connected to ground.