Abstract:
The present invention relates to a coaxial cavity resonator having walls delimiting cavity, and one resonator body having a conductive rod with a first end being in short circuit relation to a first wall of the cavity walls. the first end a cross-sectional area. The resonator further includes a first conductive supportive plate with a first side connected to the first end of each rod. The first side has a larger area than the cross-sectional area of the first end of the rod and a second side, opposite the first side, fo the supportive plate is electrically connected to the first wall. A retainer is provided in the first cavity wall to guide the supportive plate, and an attachment is provided to secure the first supportive plate to the retainer. The invention also relates to a filter and a use of a resonator component in a filter.

Description:
TECHNICAL FIELD 
     The present invention relates to a coaxial cavity resonator. The invention also relates to a filter and a use of a resonator component in a filter. 
     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 simplest type of coaxial cavity resonator comprises a rod, having a length of a quarter wavelength (λ/4), arranged inside a cavity, 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. 
     The most common way of attaching said rod to the bottom wall is by soldering. This manufacturing technique has the drawback of introducing a damaged surface at the joint, and thereby decreasing the Q-value of the resonator. Another drawback is difficulties in aligning the rod during assembly of the resonator. 
     Additional problem is coating of interior walls of cavity necessary to receive a high Q-value. 
     In European patent application EP 0 964 473, a filter is disclosed, see FIG. 1, which provides a cavity made from a material having a good electrical conductivity with integrated resonator bodies. By making the resonators from the same piece of material as the cavity walls, undesired effects at the joint between each resonator and the cavity wall can be avoided since there is no interfaces between them. 
     A drawback with the filter is that it is very expensive to manufacture. Another drawback is that it is not very flexible, since a new mold is required when manufacturing new filter having less or more resonators. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention there is provided a coaxial cavity resonator as specified by the coaxial cavity resonator including walls delimiting a cavity, and at least one resonator body including a conductive rod having a first end being in short circuit relation to a first wall of the cavity walls. The first end has a cross-sectional area. A first conductive supportive plate has a first side connected to the first end of each rod. The first side has a greater area than the cross-sectional area of the first end of the rod. A second side, opposite the first side of the supportive plate, is electrically connected to the first wall. A retainer is provided in the first cavity wall to guide the supportive plate. An attachment is provided to secure the first supportive plate to the retainer, and the supportive plate and at least a portion of the rod closest to the first end have a continuous conductive surface with high conductivity. 
     The present invention is also directed to a filter in which at least one of the described coaxial cavity resonator is mounted as specified in the filter and comprises conductive outer walls, an input and an output, and at least one resonator. 
     The invention is also directed to the use of a resonator component in a filter where the component forms a part of the inside of at least one cavity wall. The component comprises a conductive rod having a first end. The first end has a cross-sectional area. A first conductive supportive plate has a first side connected to the first end of the rod. The first side has a larger area than the cross-sectional area of the first end of the rod. A second side, opposite the first side of the supportive plate, is electrically connectable to a cavity wall of the filter, and the supportive plate and at least a portion of the rod closest to the first end have a continuous conductive surface having high conductivity. 
     An advantage with the present invention is that the resonator body is easier to align during manufacture. 
     Another advantage with the present invention is that the resonators are easy to assembly, since they can be made of relatively few parts. 
     Another advantage is that the present invention is cheap to manufacture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art quarter wavelength coaxial cavity resonator. 
     FIG. 2 a  shows a perspective view in cross section of a preferred embodiment of a quarter wavelength coaxial cavity resonator according to the present invention. 
     FIG. 2 b  shows a top view of the preferred embodiment in FIG. 2 a  with the top wall removed. 
     FIGS. 3 a  and  3   b  shows a second embodiment of a quarter wavelength coaxial cavity resonator. 
     FIGS. 4 a  and  4   b  shows a third embodiment of a quarter wavelength coaxial cavity resonator. 
     FIG. 5 shows a perspective view in cross section of a fourth embodiment of a quarter wavelength coaxial cavity resonator. 
     FIG. 6 shows a perspective view in cross section of a fifth embodiment of a quarter wavelength coaxial cavity resonator. 
     FIG. 7 a  shows a perspective view of a filter comprising several quarter wavelength coaxial resonators according to the present invention. 
     FIG. 7 b  shows a top view of the resonator in FIG. 7 a , without the lid. 
     FIG. 8 shows a perspective view in cross section of a double quarter wavelength resonator body according to the present invention. 
     FIG. 9 shows an exploded side view of a way to assemble the double resonator bodies to the bottom wall in the filter in FIG. 7 a.    
     FIG. 10 shows a perspective view of an alternative way of assemble the double resonator to the bottom wall in FIG. 7 a.    
     FIG. 11 shows a perspective view in cross section of a first embodiment of a half wavelength coaxial cavity resonator according to the present invention. 
     FIG. 12 shows a perspective view in cross section of a second embodiment of a half wavelength coaxial cavity resonator according to the present invention. 
     FIG. 13 shows a perspective view in cross section of a third embodiment of a half wavelength coaxial cavity resonator according to the present invention. 
    
    
     Embodiments of the present invention are described below, by way of example only. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic cross-section of the parts of a prior art resonator  10 , the parts being a frame  11  and a lid part  12 . The frame part comprises an inner conductor  13 , i.e. resonator body, a bottom wall  14  and side walls  15 . The lid part comprises a lid  16  and edges  17 . The parts are dimensioned so that when the lid part is attached to the frame part there is formed a tight, closed outer conductor, which encloses the inner conductor, as shown in the figure. 
     Each part is made from a metallic or metal containing material having good electrical properties in one piece, by extrusion or molding. 
     FIG. 2 a  shows a perspective view in cross section, and FIG. 2 b  shows a top view, of a first embodiment of a half wave coaxial resonator  20 , according to the invention. The resonator body, in this example, comprises of a rod  21  connected to a first side  22   a  of a circular supportive plate  22 , at a first end  23  of the resonator rod  21 . The axis of the supportive plate and the axis of the rod is preferably attached so that they coincide, as shown in FIG. 2 b.    
     A second end  24 , opposite said first end  23 , of the rod  21  is in open-circuit relation to cavity walls delimiting a cavity  25 . The cavity walls comprises a bottom wall  26 , side walls  27  and a top wall  28 . A second side  22   b , opposite to said first side  22   a , of said supportive plate  22  is conductively attached to the bottom wall  26 , e.g. by soldering or conductive glue. 
     The rod  21  and the supportive plate  22  is preferably coated with a highly conductive material, such as silver, irrespective of if the rod  21  and the supportive plate  22  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 attached to each other, e.g. by soldering, preferably before coating. On the other hand if they are made from the same piece, it is easy to manufacture the rod and the supportive plate by machine tooling, e.g. turning, which is relatively cheap. The advantage with attaching the supportive plate  22  to the bottom wall  26  is that the Q-factor of the resonator increases due to a better conductivity across the bottom wall of the cavity. Another advantage is that the positioning of the rod  21  in the cavity  25 , relative to the cavity walls  26 - 28 , is easier during manufacturing. 
     FIG. 3 a  shows an exploded cross-section, and FIG. 3 b  shows a view of the bottom wall along line A—A in FIG. 3 a , of a second embodiment of a quarter wavelength coaxial cavity resonator  30 , according to the invention. The resonator body comprises a rod  21  attached to a supportive plate  22  as described in connection to FIGS. 2 a  and  2   b . In this embodiment, the supportive plate  22  is to be placed in a recess  31  in a bottom wall  32  of the cavity  33 , where the shape of the recess essentially corresponds to the shape of the supportive plate  22 . The size of the recess  31  is approximately the same as the size of the supportive plate  22 . The thickness of the supportive plate  22  is preferably approximately the same as the depth of the plate shaped recess  31 , i.e. The upper surface  22   a  of the supportive plate  22  is in flush with the upper part  32   a  of the bottom wall  32 . 
     Normally the plate is a little smaller compared to the recess for mounting purposes and to obtain a good electric connection to the bottom wall, soldering or conductive glue may be used to fill out the space between them. 
     FIG. 4 a  shows an exploded cross section, and FIG. 4 b  shows a view of the bottom wall along line A—A in FIG. 4 a,  of a third embodiment of a half wavelength coaxial cavity resonator  40 , according to the invention. The resonator body comprises a rod  21  attached to a supportive plate  22 , as described above. The resonator  40  further comprises a guide member  41 , where a first side  41   a  of said guide member  41  is attached to said second side  22   b  of said supportive plate  22 . 
     The resonator  40  is also provided with a bottom wall  42  having an opening  43 , adapted to hold said guide member  41 . The element comprising the rod  21 , the plate  22  and the guide member  41  is arranged in a desired position by sliding the guide member  41  in the elongated opening  43  in a direction marked by the arrow  44 . The plate  22  rests in a recess  45 , having an elongated, half rounded, shape. The position of the element inside the cavity is determined either by the half rounded shape of the recess  45 , corresponding to the shape of the plate  22 , and/or by the depth d and width w of the opening  43  holding the guide member  41 . The guide member  41  may have any shape, but for manufacturing purposes a guide member having a circular cross section is preferred. 
     The element is preferably secured to the bottom wall  42  by soldering or conductive glue arranged on the outside of the cavity around the guide member  41 . 
     The element comprising the rod  21 , the supportive plate  22  and the guide member  41  is preferably coated with a highly conductive material, such as silver, irrespective of if the rod  21 , the supportive plate  22  and the guide member  41  are made from one piece or from separate pieces of material. 
     An alternative to coating is to manufacture the rod, the plate and the guide member in a solid highly conductive material. 
     FIG. 5 shows an exploded view in cross-section of a fourth embodiment of a half wavelength coaxial resonator  50 , according to the present invention. The element making up the rod  21 , the supportive plate  22  and the guide member  41  is the same as described in connection with FIGS. 4 a and  4   b . The resonator  50  only has an opening  51 , without any recess to hold the supportive plate, provided in the bottom wall  52 . The opening may only be an opening corresponding to the shape and size of the guide member  41  or be an elongated opening as described in connection with FIG. 4 b.    
     FIG. 6 shows a cross-section of a fifth embodiment of a half wavelength coaxial cavity resonator  60 , according to the present invention, where the resonator is provided with a second supportive plate  61  in addition to the resonator in FIG. 5. A first side  61   a  of the second supportive plate  61  is attached to a second side  62   b  of a guide member  62 . The length of the guide member  62  is preferably approximately the same as the thickness of the bottom wall  52 . 
     The second supportive plate  61  is preferably made from the same piece of material as the rod  21 , the supportive plate  22  (hereafter referred to as the first supportive plate) and the guide member  62 . All the parts is preferably coated by, or made from, a highly conductive material. 
     The opening  63  in the bottom wall  52  corresponds to the opening described in connection with FIG. 4 b . There may also be provided a recess (not shown), as described in FIG. 4 b , on the inside of the bottom wall and/or on the outside of the bottom wall to further improve the performance of the resonator. 
     The recesses described in the above embodiments are easily manufactured by machine processing, e.g. by etching. 
     FIG. 7 a  shows a perspective view, and FIG. 7 b  shows a top view of a filter device  70  comprising two filters  71  and  72 , where an upper housing portion, lid,  70   a , of the device  70  is partly shown in FIG. 7 a . A lower housing portion of said device  70  has outer walls constituting a top wall  70   b  and side walls  70   c , for each of the included cavities in the filters  71 ,  72 . The filters  71 ,  72  are separated by a common internal wall  73 , which constitutes a bottom wall for each of the included cavities in the filters  71 ,  72 . Each filter comprises a number of resonators, for instance five resonator bodies, separated by internal walls  75 . In this example the internal walls separates the upper part or the lower part of the resonator bodies from each other. 
     In this example each resonator body comprises a rod  74 , having a first end  74   a connected to the bottom wall  73  via a supportive plate  78 , and a hat  76  attached to a second end  74   b , opposite said first end  74   a , of said rod  74 . Further more a ceramic plate  77  is arranged between the hat  76  and the top wall  70   b  to further improve the properties of each filter. This type of resonator is described in more detail in the Swedish patent application SE9904411-7 by the same applicant with the title “A coaxial cavity resonator and a method for manufacturing a coaxial cavity resonator”, which is hereby incorporated by reference. 
     In this example, two adjacent resonators sharing the same bottom wall  73  include an element  80 , which comprises a first rod  74 , a first supportive plate  78 , a guide member  79 , a second supportive plate  78 ′ and a second rod  74 ′. The element  80  is preferably coated by a highly conductive material and preferably made from the same piece of material, as shown in FIG.  8 . The element  80  is similar to the element described in connection with FIG. 6 with the addition of the second rod  74 ′. 
     Each element  80  is held in a desired position by inserting the guide member  79  in an opening  90 , having an alternative shape compared to the opening described in connection with FIG. 6, in the common internal wall (bottom wall)  73 , as shown in FIG. 9, which is an exploded view. Each element  80  is preferably held in position by friction when inserted, which is obtained by adapting the length of the guide member  79  to the thickness of the bottom wall  73 . 
     The bottom wall  73  carrying the elements  80  is then placed in the lower housing portion making up the top walls  70   b  and the side walls  70   c  as indicated by the arrow  91 . The side wall  70   c  is preferably provided with protrusions  92 , having the same thickness, or thinner, as the bottom wall  73 , and having a shape corresponding to the opening not containing the guide member  79 . This way the bottom wall  73  does not have any openings after assembling allowing undesired coupling between resonators sharing the same bottom wall  73 . 
     The protrusions  92  may be made by folding up a part of, or by attaching separate plates to, the side wall  70   b  on which the bottom wall  73  is to be attached to. Alternative ways of providing protrusions is by using die casting, extrusion, machining or other similar techniques. 
     An input  81  and an output  82  is also provided to each filter  71 ,  72 . 
     Although the invention is described together with quarter wavelength resonators, which is preferred, the invention may naturally be implemented in other types of resonators, such as half wavelength resonators. A few different embodiment relating to half wavelength resonators is described hereinafter. 
     FIG. 10 shows a perspective view of an alternative way of assembling the double resonator element  80  to a bottom wall  101 . The difference to the way described in connection with FIG. 9 is that the protrusions  102  are separately provided and have an overlapping shape to prevent any openings in the bottom wall  101  when assembled. By providing an overlapping structure both in each opening  103  and each protrusion  104  the leakage of electromagnetic field between the cavities sharing bottom wall  101  may be reduced and performance increased. 
     Although the invention is described together with quarter wavelength resonators, which is preferred, the invention may naturally be implemented in other types of resonators, such as half wavelength resonators. A few different embodiment relating to half wavelength resonators is described hereinafter. 
     FIGS. 11-13 shows three embodiments of a half wavelength resonator comprising supportive plates according to the invention. 
     FIG. 11 shows a half wavelength coaxial cavity resonator  110  comprising a supportive plate  111  at each end of a resonator rod  112  connected in a way as described in connection with FIGS. 2 a  and  2   b . The volume of the resonator is larger than for a quarter wavelength resonator, as described above, for the same frequency. The mechanical stability of the resonator is higher than for a quarter wavelength resonator as described in FIGS. 2-6. 
     FIGS. 12 and 13 shows half wavelength coaxial cavity resonators  120  and  130 , having a reduced cavity volume compared to the resonator in FIG. 11 working in the same frequency. 
     In FIG. 12, by adding a conductive disc  121  to the rod  112 , approximately half way between the supportive plates  111 , and at the same time adding plates  122  to the cavity wall close to the conductive disc  121 , the length of the cavity may be reduced. 
     In FIG. 13, by adding two conductive discs  131  symmetrically to the rod  112  essentially half way between the supportive plates  111 , the length of the cavity may be reduced. 
     FIGS. 11-13 illustrates that the inventive concept may be applied to a half wavelength resonator as well as a quarter wavelength resonator. Other combinations of conductive discs and added plates may be used in the half wavelength resonator to further shorten the length of the resonator, such is disclosed in the International publication WO 00/10220 by the same applicant, which is hereby incorporated as reference. 
     FIGS. 2-6 only discloses a quarter wavelength coaxial cavity resonator having a single rod as a resonator body, but it is apparent to a person skilled in the arts that any type of resonator body may benefit from the inventive concept of the present invention, as indicated in connection with FIGS. 7-13. 
     Normally, the inside of a cavity have to be coated with some conductive material to obtain a good performance of the resonator. By adding these supportive plates the need for coating the inside of the cavity is reduced, since the supportive plate preferably is coated with a conductive material and the strongest current in the cavity is concentrated around the end of the rod closest to the conductive plate. The surface of the supportive plate, facing inside the cavity, is preferably as large as the size of the bottom wall, or at least as large as possible. A non-circular, e.g. square, rectangular or elliptic, shape of the supportive plate is also possible, but the preferred shape is circular due to manufacturing requirements.