WO2010033057A1

WO2010033057A1 - Method and arrangement for filtering in a wireless radio communication network

Info

Publication number: WO2010033057A1
Application number: PCT/SE2008/051049
Authority: WO
Inventors: Hannes Medelius; Lennart Schön; Esa Myllyvainio
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-09-19
Filing date: 2008-09-19
Publication date: 2010-03-25

Abstract

The present invention relates to tunable filters and methods for tuning filters adapted to filter radio frequency signals in a wireless communications network. According to the invention filter tuning means are used in order to adjust a position of at least one rod inside a housing of a filter and thereby affect coupling between resonators in the filter.

Description

Method and Arrangement for filtering in a wireless radio communication network

FIELD OF THE INVENTION The present invention generally concerns filters and methods for filtering relating to wireless radio communication network. Specifically, the present invention relates to tunable filters and methods for tuning filters adapted to filter radio frequency signals in a wireless communications network.

BACKGROUND OF THE INVENTION

Dielectric resonator or coaxially loaded cavity filters are used in mobile communication system to suppress unwanted signals (sent and received) and to let wanted signals pass. These two filter types predominate the market.

These filters each consists of a number of resonators, coupled together to provide this filtering, i.e. to get proper transfer of wanted signals and dampening of unwanted signals .

Today's design of filters involves highly sophisticated 3- dimensional simulation techniques. Production is commonly high volume techniques in order to achieve a product with well defined tolerances and cutting down expenses.

Despite this, each produced filter still need adjustment after assembly to achieve proper filter response. The adjustment procedure is called tuning and is due to that one must compensate for variations in mechanical dimensions, mounting tolerances, materials properties etc.

It is still far too expensive to make a tuning free filter for these types of applications. To achieve a desired filter function coupling between resonators is commonly controlled by adding irises in the housings of the filters and/or by adding tuning elements, for example screws .

WO-2004004063 Al, (ERICSSON TELEFON AB L M) , 28 June 2002, relates to a filter having tuning screws to adjust coupling factor between resonators .

This adds complexity and cost as well as insertion loss, i.e. loss of energy of the transferred signal.

It is also difficult to achieve sufficient coupling for certain types of filters. For such filters the resonators have to be closer to each others minimizing spatial separation and thus there will be a minimum of space between resonators . This is especially true for filters with ceramic resonators.

Accordingly, it would be highly desirable in a wireless radio communication network, to have faster and easy tuning of a filter, which result in a less expensive filter.

SUMMARY OF THE INVENTION

The problem dealt with by the present invention is enabling improved tuning of a filter that is fast and easy and less expensive .

An advantage of some exemplary embodiments of the invention is movable dielectric elements for changing the space between them in the housing of the filter, giving a less expensive, faster and easy tuning of a filter.

According to a first aspect, the invention discloses a filter adapted for use in a wireless communications network. The filter includes at least a housing enclosing at least one rod arranged to form part of a resonator adapted to filter radio frequency signals and filter tuning means which are adapted to be used for tuning the filter. The filter tuning means are used to adjust a position of the at least one rod inside the housing and thereby affect coupling between resonators in the filter.

According to a second aspect, the invention discloses a method for tuning a filter which is adapted for use in a wireless communications network. The filter comprises at least a housing enclosing at least one rod arranged to form part of a resonator adapted to filter radio frequency signals. The method comprises the step of tuning the filter, by aid of tuning means, by adjusting a position of at least one of the at least one rod, in order to affect coupling properties in the filter.

Another advantage of some exemplary embodiments of the invention is not to have to add material for tuning, such as screws for example, which would lead to energy loss.

Yet another advantage of some exemplary embodiments of the invention is to not to have to change the structure of the dielectric elements for tuning the filter, which is complicated, takes time and is costly.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims .

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, reference is made to the following drawings and preferred embodiments of the invention.

Fig. 1 is a view of a prior art filter which can incorporate an exemplary embodiment of the tuning of the filter according to the present invention. Fig. 2 and 3 illustrate different shapes of rods making parts of a resonator and/or a filter which can incorporate an exemplary embodiment of the tuning of the filter according to the present invention.

Figures 3a and 3b illustrates rods placed in different angles which can incorporate an exemplary embodiment of the tuning of the filter according to the present invention.

Figures 4a and 4b illustrates resonators and/or filters comprising gaps, antennas and asymmetries which can incorporate an exemplary embodiment of the tuning of the filter according to the present invention.

Figure 5 illustrates a method for tuning a filter according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The present invention is particularly suited but is not limited to use in second generation digital systems, such as e.g. the European Global System for Mobile communications (GSM) , and the advanced versions of GSM, the so called Enhanced Data Rates for Global Evolution (EDGE) radio access networks (GERAN) . Further suited for third generation Public Land Mobile Networks (PLMNs) such as e.g. Wideband Code Division Multiple Access (WCDMA) and CDMA-2000, Time Division-Synchronous Code Division Multiple Access (TD- SCDMA) . The present invention is also applicable to wireless communication systems beyond 3G systems, such as the Long Term Evolution (LTE) systems. Broadly speaking, the present invention is suited for any cellular radio communication network.

According to an exemplary embodiment of the invention, tuning of a filter is done by controlling coupling between individual resonators by simple adjustment of a position of one or more rods, thus minimizing complexity, cost and loss of energy.

The filter provided to be tuned according to the invention comprising at least one resonator.

A resonator is a device or system that exhibits resonance or resonant behaviour. A cavity resonator is a space surrounded by a conductive surface/housing that uses resonance to select a specific band of frequencies.

Fig. 1 shows, from a point of view of the invention, a general concept of a filter where the method for tuning according to prior art can be performed. The filter comprising two or more individually separated rods/posts sharing one cavity, metallic and/or ceramic, inside a housing forming part of a dual mode resonator, such as a TM dual mode resonator. An appropriate number of such or other resonators are combined into one or several filters, microwave filters. In a non-limiting embodiment two rods 3a, 3b, 4a, 4b, 5a and 5b are arranged essentially orthogonally in at least one cavity 2a, 2b and 2c inside a housing Ia, Ib and Ic forming part of a resonator. The cavity 2a, 2b and 2c can be formed out of conductive material or non conductive material comprising a conductive layer. Several such cavities, inside one or more housing Ia, Ib and Ic, forms part of a filter 1. A cavity 2b may have a number of apertures 6 and 7 for electrical or magnetic coupling to adjacent cavities 2a and 2c. The other cavities can also house two rods 4a, 4b, 5a, 5b, but may also house single dielectric rods, metallic rods or other forms of resonators or components. Antennas 8, 9, 10 and 11 may enter any cavity for electrical or magnetic coupling to any other cavity, connectors or components 12a and 12b adjacent to the filter 1. The apertures and antennas are used to achieve single and/or multiple paths through a filter, generate transmission zeros and/or improve filter performance. A preferred placing/positioning of a rod or a pair of rods in a cavity is close to or exactly centred, giving a highest possible Q value, but some offset positioning are also possible if required due to mechanical constraints or other reasons .

A radio based station (RBS) 100 typically comprises one or more such filters as mention above. However, filters are also used wherein they are separated from the RBS.

The order of vertical and horizontal rods, that are essentially orthogonal related to each other in said housing, is typically determined in purpose to achieve high isolation between non adjacent modes that should not couple to each other. An example of such order is to have one cavity 2a inside housing Ia with vertical-horizontal rods and a next cavity Ib in housing 2b with horizontal-vertical order. Alternatively, the order of the rods is varied from one cavity to another in purpose to achieve desired couplings between non adjacent modes. Typically, as is shown in Fig. 1, the rods in one cavity are arranged to mirror the rods in a neighbour cavity in order to achieve the desired couplings and/or isolation between non-adjacent modes, 2b mirrors 2a and 2c mirrors 2b.

The rods in the filter in one or more cavities can be placed slightly un-orthogonal in order to provide acceptable and/or desirable coupling.

In figure 2a, a general embodiment of the present invention is presented. According to this embodiment a dual mode filter is used wherein the tuning means 120a and 120b are placed in connection with the rods. As a first example the tuning means 120a is rotating means, for example a steering wheel, which is used to rotate a non-circular rod, around its one axis, and thereby cause a change in the characteristics of the filter e.g. the coupling factor will vary. The tuning means can be used on one or more non- circular rod and in combination with any other mentioned tuning means. In this example the filter is a dual mode filter, but the tuning mean 120a can be used on any kind of filter rod. As an alternative step the wheel is not placed in the centre of the upper surface of the rod, figure 120b. In another example tuning 120c of a filter is achieved by moving one or more rods along small cavities made in the housing permitting movement of the rods . Thus according to these examples, changing the position of a rod, rotating a non-circular rod or rotating a rod around a non-central axis compared to the axis of the rod.

Figures 2b shows a general example of a single mode filter where the method for tuning according to an exemplary embodiment of the present invention can be performed. The filter includes a rod 4a that is rotatable around its one axis by the aid of a tuning means 40a. A second non- cylindrical, in the middle, is rotatable around its one axis and/or movable along a protruding surface and/or small cavities 40b in the housing. The rod 5a is only movable along a different pattern, by aid of protruding surfaces and/or small cavities (40c) , resulting in a varying coupling factor. Different patterns can be used. In a further embodiment of the present invention there are both horizontal and vertical rods and wherein both horizontally and vertically placed rods are rotatable and or movable according to predetermined patterns .

In Fig. 3a two different non-circular resonators are shown, for example with at least one rod that has elliptic cross- section. According to the exemplary embodiment of the invention tuning is done by rotating the position of the rod, the space between the two dielectric elements will change and resulting in the coupling factor will vary.

In Fig. 3b two different circular rods are shown. According to the exemplary embodiment of the invention a centre point can be defined wherein the rod can be rotated around. If the centre point is eccentrically located on a circular rod and the rod is rotated, the space between that rotated rod and for example a fixed circular rod will change and resulting in the coupling factor will vary.

According to the exemplary embodiment of the invention a combination of Fig. 3a and 3b result in a resonator with a centre point eccentrically located, and another resonator with centre point located in the centre, wherein the shape of the rods are elliptical or non-elliptical or any other shape as shown in Fig. 4a and 4b, different combination of how to vary the coupling factor are accomplished.

According to another exemplary embodiment of the invention a rod may have a protruding surface on one side of the rod. If the wall of the filter has a track that the protruding surface can slide in, the track can become a path that the rod can slide in. This can speed up the fine tuning by having a rough tuning at a predetermined track in which the rod can be moved.

This allows for precise control of coupling between resonators without have to add material for tuning, such as screws for example, which would lead to energy loss. This is of special importance when resonators are closely positioned, for example when utilizing dual mode resonators. Another advantage is that there is no need in changing the structure of the dielectric elements for tuning the filter, which is complicated, takes time and is costly. Fig. 4a and 4b illustrate different shapes of rods making parts of a resonator and/or a filter. The rods can have equal or different shapes belonging to one or more of the group of cylindrical, square, parallelepiped (rectangular), pipe, partially hollowed, curved, or any other similar shape. Elliptical and cylindrical rods are used in figures 2a, 2b, 3a and 3b when describing exemplary embodiments of the tuning according to the invention. However, these are non-limiting examples which are used to explain the invention in a simple manner. The rods used in a filter tuned according to the invention are typically made of ceramic or metal but they can be made of other material, dielectric or conductive. The rods can also have regions of other dielectric constant within them such as holes or inserts 17, 17a, 17b, and 17c in one end, in both ends or in any place of the rod. Alternatively the insert holes are used for moving a rod along a pattern during the tuning process mentioned above.

The rods are normally dielectric, i.e. ceramic. Alternatively, at least one rod is conductive, i.e. made of copper, aluminium or other metal.

At least one rod is dielectric and/or at least one rod is metallic. A single rod can also be made of a part which is dielectric and a part which is metallic. The dielectric material is typically ceramic. One or several conductive rods will in most cases improve spurious performance of a filter significantly but can increase insertion loss slightly. Such resonators with dielectric rods typically have their first higher resonant modes at 1.5 to 2 times a fundamental resonance. Metallic rods on the other hand, typically have their first higher mode at 3-4 times the fundamental resonance. A main merit with dielectric rods is higher unloaded Q. Thus, each inserted metallic rod will normally increase insertion loss a little bit, but also improve filter attenuation in the region 1.5 to 4 times a pass band frequency.

The tuning of the filter according to the exemplary embodiment of the invention is especially useful for filters based on TM mode ceramic resonators but it is viable for many filter types.

Yet another advantage of the embodiments of the present invention is that the rods are not exposed to pressure and vibrations which is the normal case when tuning prior art resonators . Tuning in the solder may case fractures in the ceramic rod which lead to an unusable filter. This is often the case when at the end fine tuning the filter. Thus the whole filter is wasted ending up in extra expenses.

It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims .

Claims

1. A filter (1) adapted for use in a wireless communications network, comprising:

- at least a housing (Ia; Ib; Ic) enclosing at least one rod (3a; 4a; 5a) arranged to form part of a resonator adapted to filter radio frequency signals;

- filter tuning means (40a; 40b; 40c) ;

characterized in that the filter tuning means (40a; 40b; 40c) , which are adapted to be used for tuning the filter, are used to adjust a position of the at least one rod (3a, 4a; 5a) inside the housing and thereby affect coupling between resonators in the filter.

2. The filter (1) of claim 1, further comprising at least one protruding surface and/or at least one small cavity (40c) in the housing, adapted for moving at least one of said rods following along at least a part of a track on a wall of said housing.

3. The filter (1) of claim 1, wherein at least one rod is adapted to be rotated and/or moved inside the housing, by aid of filter tuning means (40a; 40b; 40c) , for tuning the filter.

4. The filter (1) according to any preceding claims wherein at least one rod is movable around its axis for tuning.

5. The filter (1) according to any preceding claims wherein at least one of said dielectric elements having its axis displaced from its centre.

6. The filter (1) according to any preceding claim wherein at least one rod has a cross section shape of the following: elliptical; cylindrical; square; rectangular; pipe; partially hollowed; and curved.

7. The filter (1) according to any preceding claim wherein at least one rod is made of ceramic or metal or any combination of those two.

8. A method for tuning a filter (1), wherein the filter is adapted for use in a wireless communications network, comprising at least one housing (Ia; Ib; Ic) enclosing at least one rod (3a; 4a; 5a) , arranged to form part of a resonator adapted to filter radio frequency signals, characterized in that the method comprising the step: tuning said filter (1) , by aid of tuning means (40a; 40b; 40c) , by adjusting a position of at least one of the at least one rod (3a; 4a; 5a) , in order to affect coupling properties in the filter (1) .

9. The method of claim 8, wherein the step of adjusting the position is to rotate the at least one rod, by aid of rotating means (40a; 40b) , and or move the at least one rod by aid of moving means (40c) along at least a part of a track on a wall of said housing.