Patent Publication Number: US-6335668-B1

Title: Cavity filter

Description:
FIELD OF INVENTION 
     The present invention relates to an improved cavity filter that can be readily manufactured and worked to a high degree of precision. More particularly, the invention relates to a particular design of the cavity bottom of such a filter. 
     BACKGROUND OF THE INVENTION 
     When constructing and manufacturing high-frequency filters for radio base stations, the filters are built-up to form so-called cavity filters that consist of a plurality of cavities, either with a separate centre conductor in each cavity or with more than one centre conductor per cavity. These filters are used, for instance, in base stations for GSM-based mobile telephony at the frequencies of 900 MHz and 1800/1900 MHz. 
     Each cavity and its centre conductor/conductors functions as an electric oscillating circuit that can be represented by a parallel oscillation circuit having an inductive part L and a capacitive part C when the filter is turned to a quarter wavelength of the received signal. The inductive part is determined essentially by the length of the centre conductor, while the capacitive part is determined essentially by the diameter of the centre conductor and its distance from the cavity side walls and a triming plate provided on the cavity. The filter cavities are disposed in juxtaposed relationship, so that the electromagnetic field generated by the oscillations in one cavity will induce an electric current in an adjacent cavity and therewith also generate oscillations therein. Adaptation of the filter properties is most often achieved through the medium of openings in the walls between two adjacent cavities, so as to enable the coupling factor to be trimmed. The width of the openings between adjacent cavities or the height of the walls therebetween may be variable so as to enable a correct coupling factor to be obtained. However, a cavity filter may also be constructed without walls between mutually sequential centre conductors. In such a filter construction, trimming of the coupling factor between centre conductors can be effected by changing the distance therebetween. 
     It will be evident from the aforegoing that one important aspect of the function of the cavity filter is that the centre conductor is disposed on a very flat cavity bottom. Since the depth of the cavity influences the function of the cavity, it is essential that the filter body can be manufactured to precise measurements with respect to the distance between the cavity bottom and the upper edge of respective walls, so as to achieve good parallelity between said bottom and said upper edges. 
     An example of an earlier known cavity bottom construction is shown in FIG. 1, which also shows a known centre conductor. The Figure illustrates in cross-section a centre conductor  13  mounted in a cavity that comprises a cavity-defining body  11  and a cavity bottom  12 . In this known construction, the bottom  12  is formed in the same working step as the manufacture of the cavity in general. This method of manufacture, however, means that the cavity bottom will not obtain the surface finish, planarity or flatness, that is required in order of the centre conductor to fulfil its function in the best way. 
     According to this known solution, the centre conductor  13  is hollow so that it can be secured to the cavity bottom  12  by means of a screw  14  passed through openings in the bottom surface of the centre conductor and the bottom of said cavity. The electric junction between the centre conductor and the cavity bottom is comprised of the bottom surface that lies around the screw, which bottom surface may be deficient with respect to surface finish and flatness in the case of this known construction. 
     It is thus desirable to obtain a cavity filter that has the aforedescribed advantageous properties and that can be manufactured readily despite the high precision requirements involved. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved cavity filter construction. 
     A first object of the invention is to provide a cavity filter that can be readily manufactured despite high precision requirements. 
     Another object of the present invention is to provide a cavity filter where only certain parts of the cavity and the upper edge of the side walls need be precision worked so as to obtain a very flat surface on which the centre conductors are secured while achieving, at the same time, precise measurements and parallelity between the bottom surface and the upper edge of said cavity. 
     These objects are achieved with an inventive cavity filter that includes a plurality of centre conductors which are disposed on a flat groove or a flat elevation that is preferably elevated slightly above the bottom of the cavity or on a cylindrical portion that forms an integral part of the cavity filter. The cavity filter need not include any intermediate walls. 
     A first advantage afforded by the inventive cavity filter is that it enables manufacture to be simplified. Instead of accurately working, e.g. machining or treating, the whole of the cavity bottom in order to obtain a flat bottom surface, it is only necessary to machine the aforesaid groove or elevation or the upper surface of said cylindrical portion in order to obtain the requisite degree of flatness. This working can be effected in one single manufacturing stage, by excluding the walls between the cavities. The obtained flat surface provides a reference surface in the continued working of the cavity body, so as to obtain an exact measurement and good parallelity between the centre conductor contact surface and the upper edge of the cavity body. 
     Another advantage afforded by the inventive cavity filter is that trimming of the coupling factor between mutually adjacent centre conductors is simplified. The walls between two centre conductors are not required for trimming purposes, and all that is necessary is to change their distance from the machined groove or elevation. 
    
    
     The invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an earlier known arrangement of a centre conductor in a cavity filter. 
     FIG. 2 is a cross-sectional view and a view taken from above of a first embodiment of an inventive cavity filter. 
     FIG. 3 illustrates a part of an inventive cavity filter having means for securing and adjusting a centre conductor. 
     FIG. 4 is a cross-sectional view and a view taken from above of a second preferred embodiment of an inventive cavity filter. 
     FIG. 5 illustrates a centre conductor in an inventive cavity filter according to the first embodiment of the present invention fastened to the bottom of the cavity. 
     FIG. 6 illustrates a centre conductor in an inventive cavity filter according to the second embodiment of the present invention fastened to a cylindrical portion that is an integral part of the cavity. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 is a cross-sectional view and a view taken from above of a first embodiment of an inventive cavity filter. The filter  20  includes a body  21  and a cavity bottom  23  in which there is provided an elevation  22  for securing a centre conductor. In the illustrated embodiment, the elevation is raised about 1-2 mm above the bottom  23  of the cavity and extends through said cavity or through several cavities that are delimited by side walls  21 ,  25 . 
     FIG. 5 is a cross-sectional view showing a part of an inventive cavity filter according to the first embodiment of the present invention and a centre conductor  53 . The cavity filter includes a body  51  and a cavity that is covered by a trimming plate  55 . In the embodiment shown in FIG. 5, the centre conductor  53  is fastened to an elevation  52  raised above the cavity bottom, this elevation corresponding to the elevation  22  in FIG.  2 . The elevation  52  has been worked to a very flat surface and is slightly wider than the diameter of the centre conductor. It will be seen from the figure that an inventive cavity filter which enables a flat surface to be readily created can be used advantageously in arranging the centre conductors when good contact is required between the centre conductor and the cavity bottom. 
     One advantage afforded by the inventive cavity filter is that no particular precision requirements need be made when manufacturing the moulded or cast body  21 . In the preferred embodiment shown in FIG. 2, that part intended for the elevation  22  is raised slightly above the surface of the cavity bottom  23 . This preliminarily formed surface need not fulfil high flatness requirements at this stage. On the other hand, the raised surface must be higher than the level of the desired flat surface, since the flatness of this desired surface is preferably achieved by a milling process or by some flat-polishing process. Manufacture can thus be effected both rationally and inexpensively and with only a small amount of waste. 
     In the case of the inventive cavity filter it is only necessary to machine two parts of the filter to a high degree of precision in order to achieve the requirement of a flat surface for attachment of the centre conductor on the one hand and to obtain precise measurements between the cavity bottom and the upper edge of the cavity walls on the other hand, these two parts corresponding the elevation  22  and the side wall  25  and body  21 . The flat elevation  22  is obtained by milling the pre-formed raised surface on the cavity bottom  23 , or by some other general flat-polishing process. Because the cavity filter according to this preferred embodiment of the invention lacks intermediate walls between the various centre conductors, the cavity bottom can be worked in one single working stage instead of working each individual cavity. The resultant flat bottom surface can be used as a reference surface for working the upper side of the cavity walls. It shall be possible to adjust the distance between the elevation on the bottom of the cavity and the upper edge of the cavity side walls to a very precise measurement. The upper edge and the elevation shall extend parallel with one another so that the same distance can be retained therebetween along the full extent of the cavity filter. 
     A cavity filter according to the first embodiment has an elevation  22 ,  52  which is elevated slightly above the bottom of the cavity and which is worked to obtain a high degree of flatness. When working a raised elevation no burrs will occur, meaning that the edges of the elevation need not be subsequently worked. The height of the elevation, however, can be varied to provide a suitable working measurement. In one conceivable modification of the inventive cavity filter, the flat surface may be provided directly on the bottom of the cavity in the absence of a raised surface, or may even be milled as a groove to a level slightly beneath the level of the bottom of the cavity. On the other hand, it is essential that is groove is at least equally as wide as the diameter of the bottom surface of the centre conductors disposed thereon, so that said conductors will rest on the bottom of the groove with the whole of the bottom surface and particularly with the outer edge. This is a decisive feature, since the electric current in the centre conductors is conducted essentially nearest the surface. In the case of the preferred embodiment, however, the width of the elevation or groove is at least twice as large as the diameter of the bottom surface of the centre conductors. 
     A filter according to the preferred embodiment does not require the presence of intermediate walls between the various centre conductors, and the elevation or groove can thus extend along one or more cavities. As before mentioned, this enables an elevation or groove to be formed more easily on the bottom of the cavity. Mechanical working of the filter is simplified because walls between the cavities need not be worked mechanically in order to trim the coupling factor of the centre conductors. Trimming is normally effected by changing the opening between two mutually adjacent cavities. The field strength of the electromagnetic fields between two centre conductors can be changed through the medium of these openings to obtain different coupling factors. In the case of the inventive cavity filter, trimming is conveniently effected by altering the distance between the juxtaposed centre conductors on the elevation or groove. FIG. 3 shows part of an inventive cavity filter according to the first embodiment of the present invention that includes side walls  31  and an elevation  32  which extends through one or more cavities. The position of a centre conductor  33  mounted on the elevation  32  can be adjusted in relation to an adjacent centre conductor  35 , by moving the centre conductor  33  along a contemplated elevation  36  until the correct distance between the centre conductors  33  and  35  has been reached. The centre conductor  33  can be moved either solely in the longitudinal direction of the elevation or in a composite direction of movement, i.e. simultaneously in the longitudinal direction of the elevation and orthogonally thereto. The centre conductor  33  is only then secured in the elevation  32 . 
     Because the aforesaid requirements relating to flatness, precise measurements and parallelity between the elevation or groove and the upper surface of the cavity walls are fulfilled along the whole of the elevation or groove, the centre conductors can be arranged readily in the desired positions in the elevation or groove for trimming purposes. However, this does not exclude a cavity filter that includes an elevation or a groove such as that described above in which trimming is effected by changing the height or the opening width of intermediate walls  24  between mutually adjacent cavities, provided that the openings are wider than the elevation or groove so that said elevation or groove can be produced in one single working step. Intermediate walls  24  may, however, also be required to enhance the stability of the body in the casting or moulding process when manufacture takes place under very high pressures. 
     FIG. 4 is a cross-sectional view and a view taken from above of a second preferred embodiment of the present invention. The filter  40  includes a body  41  and a cavity bottom  42  in which there are provided cylindrical portions  44  that form an integral part of the cavity filter. According to this embodiment, from the pre-formed upper surfaces  45  of said cylindrical portions  44  a flat surface is obtained by milling said pre-formed surfaces  45 , or by some other general flat-polishing process. At least some of said cylindrical portions  44  are designed with a hollowness  46  through their entire length along with the centre axis and through the cavity bottom. Said hollowness is intended for mounting a centre conductor. Thus, it is only necessary to machine said upper surface  45  of the filter to a high degree of precision in order to achieve the requirement of a flat surface for attachment of the centre conductor on the one hand and to obtain precise measurements between the cavity bottom and the upper edge of the cavity walls on the other hand. Trimming is done by means of pegs  47  that are arranged between said cylindrical portions. The trimming can be influenced by the position of said pegs  47  and their height. 
     FIG. 6 is a cross-sectional view showing a part of an inventive cavity filter according to the second embodiment of the present invention and a centre conductor  63 . The cavity filter includes a body  61  and a cavity that is covered by a trimming plate  65 . In this embodiment, the centre conductor  63  is fastened to a cylindrical portion  62  that is an integral part of the cavity filter. Said cylindrical portion  62  has been worked to a very flat surface  64 . The hollowness  66  along with the centre axis of the cylindrical portion  62  is preferably equipped with threads such that the centre conductor  63 , possibly equipped with an additional trimming element  67  can be screwed in. 
     The inventive cavity filter is preferably produced by a casting or moulding process in which a body  21  is obtained as a one-piece structure. The material from which the filter  20  is comprised must fulfil certain criteria. An advantageous material shall be readily workable so that the elevation or groove in the cavity bottom  23  and the measurement of the side walls  25  and possible intermediate walls  24  can be provided in a simple manner. The material must also be light in weight, so as not to increase the weight of the filter more than necessary and shall have good current conducting capacity. These requirements are fulfilled essentially by aluminium and magnesium, for instance. 
     It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the following claims.