A filter for use in a wireless communications network

Technical field

The present invention relates to a filter and a radio base station in a wireless communications network. More particularly, the invention relates to a filter and a radio base station comprising filtering means adapted to filter radio frequency signals in a wireless communications network.

Background

The present invention relates to a wireless communications network. Such a telecommunications network typically includes a plurality of radio base stations in connection with a core network. The radio base stations are adapted to provide access for mobile subscribers to the wireless communications network. Each radio base station typically includes filters, power amplifiers, radio transceivers, connections to antennas (feeders) etc. Typically filters are used for processing radio frequency (RF) signals. Such filters are known as microwave cavity filters which include resonators formed in cavities in order to provide a desired frequency response when signals are input to the filter.

Until the date of this application two main technologies predominate radio base station (RBS) front end filters. The first technology relates to classical coaxial resonators with center conductor(s) for normal performance and low manufacturing cost. These types of resonators are fairly scaleable in size. The second technology relates to ceramic TEOId single mode resonators which are used for high performance. These resonators require however rather large cavities.

Furthermore, a few manufacturers use or have used TM single mode resonators which have the benefit of size reduction without loss of performance relative to a certain coaxial resonator. A typical TM single mode resonator saves 20-50 % volume compared to a coaxial resonator with a same unloaded Q. Unloaded Q refers to capacitor or a tuned circuit itself with no other factors considered. Q is the ratio between reactance and resistance and more general, Q is a measure of efficiency and is frequency-dependent.

Thus, air cavities and/or big resonators are often used in band filters in order to provide a high Q-value. This leads to bulky space demanding filters, which often are heavy and expensive. As an alternative smaller filters can be constructed by means of resonators having another dielectric than air in the cavities.

Also, very complexly shaped TM dual mode and triple mode resonators have recently been developed and used. These resonators have two or three resonances in each resonator making further size saving possible. In some cases the size of a filter is reduced between 30-60%.

Publication WO2006/086414, describes a dual mode ceramic filter that has an enclosure with two cavities separated by a wall, and two TM dual-mode resonators wherein each TM dual-mode resonator is positioned in a corresponding cavity. Each TM dual-mode resonator has first and second modes, and a body having a central portion with a plurality of arms extending outwardly from the central portion. The filter also has two input conductive members, wherein each input conductive member is positioned in a corresponding cavity. Each input conductive member is disposed proximate to a corresponding TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator.

Publication WO02103901 relates to an active filter for suppressing noise and inter- modulation. The filter has at least one impedance circuitry, such as at least one resonator and/or at least one reactive component.

Publication US5764115 relates to a dielectric resonator apparatus that includes a plurality of TM double-mode dielectric resonators. Each dielectric resonator has a dielectric rod-complex, a casing provided with electrically conductive film at the outside surfaces, and metal panels covering the upper and lower openings of the casing. In adjacent TM double-mode dielectric resonators, at portions of the planes of the two casings opposing each other, apertures are provided in the direction of the magnetic field generated by two dielectric rods which have the same axial direction. A coupling

member is also provided so as to form an electrically conductive loop which goes across the magnetic field.

Publication US2004174234 relates to a resonator that includes a cylindrical dielectric and a conductor film covering the surface of the dielectric in close contact therewith. The conductor film is constructed of a cylindrical portion and two flat portions, and is formed by subjecting the surface of the dielectric to metallization or the like. With the conductor film formed in close contact with the dielectric, deterioration of the Q value and the like caused by instability of connection at the corners can be suppressed even when a radio frequency induced current flows from the cylindrical portion over the two flat portions.

In a coaxial resonator, unloaded Q and often also power handling are degraded with size. Thus, given a certain performance, which is often the case, there is very limited possibility to shrink filter size from one product generation to the next. TEOId resonators typically allow performance improvement, not size reduction at constant performance. Filter size at 2 GHz is typically equal to filters with coaxial resonators, but filter size at 1 GHz is significantly larger than with coaxial resonators. Dual and triple mode resonators as the ones mentioned above are geometrically rather complex and in many cases very difficult to manufacture. The complexity of new developed resonators and the costs are still existing problems.

Summary

An object of an aspect of the present invention is to provide a filter which is less expensive, less complex to manufacture and which permits a smaller size with almost maintained high performance.

One advantage with the present invention is that filter size can be reduced. Also signals can much more effectively be processed.

In an embodiment of the present invention there is described a filter which is adapted for use in a wireless communications network, the filter comprising a housing enclosing two

or more filter rods. At least two separate rods inside the housing are arranged essentially orthogonally, relative to each other, forming part of a resonator adapted to filter radio frequency signals.

In another embodiment of the present invention there is provided a radio base station in a communications network, serving one or more mobile equipments, the radio base station comprising at least one filter according to any of claims 1 -13.

Brief description of the drawings

Figure 1 is a view of a filter according to one embodiment of the present invention.

Figures 2 and 3 illustrate different shapes of rods making parts of a resonator and/or a filter according to embodiments of the present invention.

Figure 4 illustrates resonators and/or filters comprising gaps, antennas and asymmetries according to another embodiment of the present invention.

Figure 5 illustrates rods placed in different angles according to additional embodiments of the present invention.

Figure 6 is a view of a filter according to yet another embodiment of the present invention.

Figure 7 is a view of antennas that are used for coupling between cavities according to further embodiments of the present invention.

Detailed description

One purpose of the present invention is to provide a filter comprising at least one resonator to be used in the filter permitting smaller filter size, enabling weight reduction and simplified manufacturing due to less parts and easier mounting of rods in said filter.

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

One general concept according to a first embodiment of the present invention describes a 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. Reference is made to figure 1 which is an illustration of an embodiment of the present invention. 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 1a, 1 b and 1c 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 1 a, 1 b and 1c, 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 centered, giving a highest possible Q value, but some offset positioning are also possible if required due to mechanical constraints or other reasons.

In an embodiment of the present invention a radio based station 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 1 a with vertical-horizontal rods and a next cavity 1 b 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, the rods in one cavity are arranged to mirror the rods in a neighbor cavity in order to achieve the desired couplings and/or isolation between non- adjacent modes, 2b mirrors 2a and 2c mirrors 2b (fig. 1 )

In a further embodiment of the present invention the rods in one or more cavities are placed slightly un-orthogonal in order to provide acceptable and/or desirable coupling. A typical angle α, as seen in figure 5, wherein a rod is located in relation to another is between 70 to 110 degrees, but the angle α may be less like 10 to 70 degrees as for example in a wide pass band design, or higher like 110 to 170 degrees. The location of the rods can vary both in y-axis as well as in x-axis. A rod can also be essentially rotatable, placed angle μ, see figure 5 which illustrates rods placed in different angles related to each other. Both the horizontally and the vertically placed rods are slightly rotatable.

Figures 2 and 3 illustrate different shapes of rods making parts of a resonator and/or a filter according to embodiments of the present invention. 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. Cylindrical rods are used in the figures when describing embodiments of the invention. However, these are non-limiting examples which are used to explain the invention in a more simple manner. The rods 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 in one end, in both ends or in any place of the rod.

In an embodiment of the invention the rods 3a, 3b, 4a, 4b, 5a and 5b are normally dielectric, i.e. ceramic. Alternatively, one or both rods can be conductive, i.e. made of copper, aluminum or other metal.

In yet another embodiment of the present invention 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.

Figure 4 illustrates resonators and/or filters comprising gaps, antennas and asymmetries according to another embodiment of the present invention. Other antennas 13 and 14, dielectric and/or metallic, penetrate the cavity 2b in order to achieve coupling between different modes in the cavity, resonator or filter. Typically, rods are directly attached to a cavity wall, inside of the housing 1 b, in both ends. Alternatively or additionally, one or more asymmetries 19a and 19b in the cavity itself are used, between one or both ends of a rod and the wall, to provide coupling. The asymmetries are made of the same material as the wall or made of other dielectric material. Also, the asymmetries can be made of the same material as the rod. Alternatively, one or more gaps 18 are placed between the rod and the cavity wall. The gap or other material may occur at one or both walls. The tuning means (antennas, gaps asymmetries) are used in purpose to achieve one or more of the following: multiple paths through the filter; possibility to generate transmission zero; flexible filter performance tuning; and, coupling between cavities, connectors and/or components adjacent to the filter. Placing two rods of a cavity slightly un-orthogonally can also be done to provide coupling, as mentioned above.

Figure 6 is a view of a filter according to yet another embodiment of the present invention, in which each cavity only comprises one rod. Several such cavities form a resonator. A filter typically comprises one or more resonators. The placing of a rod in a cavity can vary depending on required coupling characteristics.

Figure 7 gives some examples of antenna constellations that are used for coupling between cavities according to further embodiments of the present invention. These non limiting examples show: Simple loops 20, 21 and twisted loops 22, 23. A twisted loop has opposite sign on coupling coefficient compared with a simple loop. A twisted loop/antenna between two resonators will induce fields with opposite direction compared to an untwisted loop/antenna, in other words, 180 degrees phase shift, changed sign, of a coupling. This is especially important when having multiple paths through a filter generating transmission zeros. As an example: When creating two transmission zeros out of a group of four single mode TM resonators, a filter having resonators 1 to 4, often called a quad section, resonators 2 and 3 should be bypassed by a coupling from resonator 1 to 4 having opposite sign relative to other couplings involved (1 to 2, 2 to 3, 3 to 4). If these other couplings are achieved by having apertures, then resonator 1 to 4 may be a twisted loop or similar.

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