Dielectric filter and its method of manufacturing

Abstract: The filter (100) includes a block (110) of ceramic material having one or more holes (119) extending from a top surface (111) to a bottom surface (113), each of which is interiorly covered with conductive material so as to form an inner conductive layer (121a, ....121d). The bottom surface and side surfaces (115a, 115b, 117a, 117b) of the block are also covered with bottom and side conductive layers electrically connected to the inner conductive layers at the bottom surface. The inner conductive layer is further connected to spaced apart top conductive layer portions (131a, .... 131d) provided on the top surface of the block surrounding each hole. The top layer portions are spaced from each other and have an oblique edge portion which is capacitively coupled with, and obliquely faces an upper edge portion of the side conductive layers, the oblique edge facilitating adjustment of the resonant frequency by removal of a predetermined amount of conductive material therefrom. In accordance with the method of manufacture of this filter, the filter is initially con­ structed to have a resonant frequency which is greater than that ultimately desired, and after measuring the resonant frequency initially obtained, a predetermined amount of conductive material is removed from the top conductive layer at a location along the oblique edge portion according to the required reduction in resonant frequency in order to reduce the resonant frequency to a desired value.

Description: The present invention relates to a dielectric filter comprised of ceramic material, and more particularly to a dielectric filter according to the preamble of claim 1 and to its method of manufacturing according to the preamble of claim 11, to which radio frequency signals (in the following referred to as RF signals) having a frequency from the ultra high frequency (UHF) bands to the relatively low frequency microwave bands can be coupled, and which is well adapted for a band bus filter coupling to RF signals having either of the frequency ranges from 825 Mhz to 845 Mhz or from 870 Mhz to 890 Mhz, which are used by mobile telephones. The GB 2165098A describes a known single-block dielectric filter wherein the block is covered with conductive material and includes at least two holes each having an elongated cross section. The holes are seperated from each other by unplated capacitive stripes which include sinusoidal-shaped portions so that the coupling between adjacent holes can easily be adjusted by simply removing plating. The document "Patent Abstracts of Japan", vol. 10, No. 251 (E432) (2307), 28th August, 1986; and JP-A-6180901, 24-04-1986 describes a microwave filter using an electric conductor pattern on the end face of the dielectric and adjusting this electric conductor pattern by a mechanical method or an optical method to provide a desired filter characteristic. The known patent application GB 2105909A describes a trimmer capacitor which comprises at least a fixed trimming electrode, a cooperating electrode disposed in a plane parallel to that of the trimming electrode and a dielectric body sandwiched between these two electrodes. The published application DE 35 06 471 A1 describes a known method for adjusting a dielectric resonator, wherein on the top side of the dielectric resonator a metallic layer is deposited and the area covered by the metallic layer is changed for fine adjustment of the dielectric resonator. A dielectric filter must be tuned after the filter is initially constructed and tested. A conventional dielectric filter structure whose frequency response may be finely adjusted is described in detail in U.S. Patent No. 4,431,977 (WO 83 02 853) and Japanese laid-open Patent Publication No. 84-128801. A fine frequency adjustment of the filter described in U.S. Patent No. 4,431,977 is performed by removing an amount of the conductive material from around the conductor-lined holes formed in the dielectric material, the amount of the material removed determining the amount of adjustment. There has been a continuing effort, particularly in the field of mobile telephones, to reduce the size of the filters. A problem arises, however, in reducing the size of a filter which is tunable in the manner of the prior art because the amount of conductive material to be removed for a given adjustment will be necessarily decreased, and thus the removal process is more sensitive and therefore more time consuming and expensive. Another adjustment approach which is described in Japanese laid-open Patent Publication No. 84-128801 is to perform the fine frequency response adjustment of the filter by cutting conductive strip lines which are provided on the top surface, surrounding the holes. This other adjustment approach may be used to finely adjust the frequency response of the filter. However, it has been found that with this approach, portions of the ceramic material provided between the holes and the strip lines reduce the unloaded Qu of the filter. Accordingly, it is an object of the present invention to provide an improved dielectric filter whose frequency response can be finely adjusted without reduction of the unloaded Qu of the filter. It is another object of the present invention to provide an improved dielectric filter which can be easily tuned and is well adapted for automatic tuning. This objects are solved by the dielectric filter according to claim 1 and by the method according to claim 11. The dielectic filter of the present invention includes a block of ceramic material having one or more holes extending from a top surface to a bottom surface, each of which is interiorly covered with conductive material so as to form an inner conductive layer. The bottom surface and side surfaces of the block are similarly covered with bottom and side conductive layers electrically connected to the inner conductive layers at the bottom surface. The inner conductive layer is further connected to spaced apart top conductive layer portions provided on the top surface of the block surrounding each hole. The top layer portions are spaced from each other and have an oblique edge portion which is capacitively coupled with, and obliquely faces an upper edge portion of the side conductive layers. As with the known methods of manufacture of dielectric filters (such as are disclosed in U.S. Patent No. 4,431,977 and Japanese laid-open Patent Publication No. 84-128801), the filter is designed to initially have a resonant frequency which is greater than that ultimately desired, and after measuring the resonant frequency initially obtained, a portion of the top conductive layer is removed in order to reduce the resonant frequency to a desired value. However, the amount by which the resonant frequency is reduced by removing a portion of the top conductive layer depends not only on the amount of material removed, but also on the distance from the removed portion to the opposing upper edge portion of the side layer. Therefore, the resonant frequency of the filter can be, and in accordance with the method of the invention is, reduced by a predetermined amount by selection of a location along the oblique edge portion appropriate to the amount of reduction required for removal of a predetermined amount of conductive material. In accordance with another aspect of the invention, the oblique edge portion of the top conductive layer is straight or uniformly staircase-shaped and the upper edge portion of the side layer is straight, so that the distance between them changes in a linear or uniformly incremental manner. This facilitates the selection of the appropriate location for the removal of conductive material depending on the amount by which the resonant frequency must be reduced. BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the invention will be more completely understood from the following detailed description of the preferred embodiments with reference to the accompanying drawings in which: Fig. 1 is a perspective view of a first embodiment of a dielectric filter in accordance with the present invention; Fig. 2 is a cross section of the dielectric filter shown in Fig. 1, taken along lines A-A; Fig. 3 is a partial plan view from the top of the dielectric filter in Fig. 1; Fig. 4 is a graph illustrating the relation between the reduced resonant frequency and the trimming area according to the selection of the trimming portion from the edge portion of the top conductive layer in Fig. 3; and Figs. 5-8 are partial plan views of other embodiments of the dielectric filter according to the present invention showing one of four identical holes in the filter and surrounding conductive layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is illustrated a dielectric filter 100 embodying the present invention. The filter 100 includes a substantially rectangularly shaped block 110 of ceramic materials, primarily BaO and TiO₂. The block 110 has a top surface 111, a bottom surface 113, a pair of mutually parallel first side surfaces 115a and 115b and a pair of mutually parallel second side surfaces 117a and 117b. The block 110 further has four cylindrical interior surfaces 118 therein which respectively define corresponding holes 119 each extending from the top surface 111 to the bottom surface 113 and arranged in a vertical plane parallel to the first side surfaces 115a and 115b. Each of the interior surfaces in the block 110 is entirely covered with a layer of a conductive material such as a silver or copper so as to form inner conductive layers 121a, 121b, 121c and 121d as shown in Fig. 2, which is a cross section of the dielectric filter 100 in Fig. 1 taken along lines A-A. Referring to Fig. 2, the inner conductive layers 121a-121d are electrically connected with one another by means of a bottom conductive layer 123 which may also be formed, for example, of silver or copper on the bottom surface 113 of the block 110. The bottom conductive layer 123 is electrically connected with similarly formed side conductive layers 125 provided on the side surfaces 115a, 115b, 117a, and 117b. Each of the four inner conductive layers, surrounded by the dielectric material enclosed in the side and bottom conductive layers, acts as a dielectric resonator which is resonant with predetermined RF signals inputted from an input electrode 129a and outputted to an output electrode 129b. The four resonators have respective top conductive layers 131 on the top surface 111, designated layers or layer portions 131a, 131b, 131c and 131d. The top conductive layers 131a-131d respectively form collars covering the portions of the top surface 111 surrounding the four corresponding holes 119 and are respectively connected to the corresponding inner conductive layers 121a-121d. The thickness of each of the conductive layers 121, 123, 125 and 131 is about 2 microns. Referring to Fig. 3, there is illustrated a partial plan view of the filter 100 shown in Fig. 1. The exemplary top layer 131 as shown in Fig. 3 has a rectangular configuration, and has side edge portions 126a and 126b respectively facing the straight upper edge portions 125a and 125b of the side conductive layer 125. The side edge portions 126a and 126b are respectively provided with substantial identical right angled triangle shaped recesses 127a and 127b. According to the first embodiment, the width (a) of the filter 100 is 6.00mm; the width (b) of each top layer 131 is 3.00mm; each of the distances (c1) and (c2) between the side portions 126a, 126b and the upper edge portions 125a, 125b is 0.5mm; the length (d) of the top layer 131 is 5.00mm; the depths (e1) and (e2) of the recesses 127a and 127b are each 1.50mm; the diameter (f) of the inner conductive layer 121 is 2.00mm; the lengths (g1) and (g2) of the sections of each of the conductive layer edge portions 126a and 126b which are parallel to the upper edge portions 125a and 125b is 0.50mm; and the base (h) of each of recesses 127a and 127b is 2.00mm. The frequency response of a resonator having the above-mentioned structure can be adjusted by changing its capacitance which is mainly established between the upper edge portions 125a and 125b and the side edge portions 126a and 126b including the straight oblique edge portions 128a and 128b formed by the recesses 127a and 127b. The capacitance can be reduced by removing in the form of a notch 130 a portion of the conductive from the top conductive layer 131 by means of a sandblast trimmer or a laser trimmer. The amount of reduction in the capacitance is determined by the location or locations of one or more such notches 130 along the oblique edge portions 128a and 128b, defined, for example, by its X-coordinate as measured along the upper edge portions 125a and 125b as shown in Fig. 3. As shown in Fig. 4, in the case of removing conductive material at the location on the oblique edge portion 128b defined by the X-coordinate X1, the resonant frequency of the resonator is sharply reduced because the oblique edge portion 128b at X1 is relatively close to the upper edge portion 125b and, therefore, sets up a relatively large capacitance with the upper edge portion 125b. On the other hand, in the case of removing the conductive material from the oblique edge portion 128b at X3, the resonant frequency of the resonator is only slightly reduced because the oblique edge portion at X3 is relatively far from the upper edge portion 125b and, therefore, creates a relatively small capacitance with the upper edge portion. In the case of removing the conductive material from the oblique edge portion 128b at X2, the resonant frequency of the resonator experiences an intermediate reduction. The resonant frequency of the resonator, therefore, can be adjusted within a large range of values by choosing a trimming location on an oblique edge portion and forming there a notch of a dimension previously selected independently of the location. In the first embodiment shown in Fig. 3, the X-coordinates X₁ and X₂ are respectively distances i₁ and i₂ from the center location X₂ equal to 0.75mm and distances j₁ and j₂ from the respective extremes of the oblique edge portion 128b equal to 0.25mm. The resonant frequency of the resonator in Fig. 3, of which the center frequency is around 880MHz, is reduced by 2.0MHz in the case of removing 1.57mm² of the conductive material from the oblique edge portion 128b at the X-coordinate X1 and is reduced by 0.2MHz in the case of removing 1.57mm² of the conductive material from the oblique edge portion 128b at the X-coordinate X3. There will now be described four additional embodiments of the invention which differ from the first embodiment only in the shape of each of the top surface conductive layers surrounding each of the holes 119. Referring to Fig. 5, there is illustrated a second embodiment according to the present invention. The conductive layer 531 in Fig. 5 has a rectangular configuration, of which the length (a) is 5.00mm, the width (b) is 4.0mm, and side edge portions 532a and 532b, facing each of upper edge portions 525a and 525b, are provided with respective regular trapezoid shaped recesses 526a and 526b. Each of the trapezoid shaped recesses has a short side (c) 2.40mm long and a height (d) of 1.00mm, and also has two staircase-shaped oblique sides, respectively consisting of four steps, each of the treads of which is 0.20mm long and each of the risers of which is 0.25mm high. The other dimensions of the resonator in Fig. 5 are substantially the same as those of the resonator shown in Fig. 3. The staircase-shaped oblique sides facilitate automation of the trimming process by reducing the need for precision in locating the X-coordinates where the notch is to be placed. Referring to Fig. 6, there is illustrated a third embodiment according to the present invention. The top conductive layer 631 in Fig. 6 has staircase-shaped edge portions 632a and 632b respectively facing upper edge portions 625a and 625b, each of four steps thereof defining a right-angle triangle-shaped recess. The tread of each of the steps is 1.0mm long and the riser of each step is 0.40mm high. The other dimensions of the resonator shown in Fig. 6 are substantially the same as those of the resonator shown in Fig. 5. This embodiment has a similar advantage to that of Fig. 5 in reducing the need for precision in locating where the notch is to be placed, particularly in an automated trimming process. Referring to Figs. 7 and 8, there are illustrated two other embodiments according to the present invention. The conductive layers 731 in Fig. 7 has a parallelogram configuration, having a pair of edge portions 732a and 732b obliquely facing respective side conductive layer upper edge portions 725a and 725b. The conductive layer 831 in Fig. 8 has a configuration in which edge portions 832a and 832b, respectively obliquely facing conductive side layer upper edge portions 825a and 825b, curve away from the latter edge portions from left to right and from right to left, respectively. In each of the top conductive layers surrounding holes 119 according to the above-mentioned embodiments, locations along oblique edge portions have varying predetermined distances from the outer conductive layer edge portion. Thus, the resonant frequency of the resonator can be reduced from a relatively large amount to a relatively small amount by removing a predetermined same amount of the conductive material from an appropriately selected location along the oblique edge portion. The top surface of the filter is covered with a regular pattern of the conductive layers surrounding the holes to form with the upper edge portions 125a and 125b a plurality of resonators. Since there are no exposed portions of ceramic material on the top surface between the inner conductive layer and the top conductive layer, little reduction of the unloaded Qu of the filter will occur.

Claims: A dielectric filter comprising: a dielectric block (110) having a top surface (111), a bottom surface (113) and two opposite first side surfaces (115a, 115b), the dielectric block (110) further having a plurality of holes (119), the holes extending from the top surface (111) to the bottom surface (113); a plurality of top conductive layers (131a to 131d) on respective portions of the top surface (111) surrounding the respective holes (119); inner conductive layers (121a to 121d) respectively covering the interior surface (118) of the holes (119) to provide resonator elements; a side conductive layer (125) covering the two first side surfaces (115a, 115b) and a bottom conductive layer (123) covering the bottom surface (113) and electrically connecting the side conductive layer (125), the side conductive layer having an upper edge portion (125a, 125b) adjacent to the top surface (111); characterized in that each top conductive layer (121a to 121d) having a side edge portion (126a, 126b) substantially obliquely opposing the upper edge portion (125a, 125b) of the side conductive layer so as to be capacitively coupled with the upper edge portion of the side conductive layer, whereby the frequency response of the filter (100) can be adjusted by removing a predetermined same amount of at least one of the top conductive layers (121a to 121b) from a selected location (x1, x2, x3) along the side edge portion of the top conductive layer, the location being selected according to the amount of adjustment desired. The dielectric filter according to claim 1, wherein the top conductive layers (121a to 121d) each have a substantially right-angle triangle-shaped recess (127a, 127b) and the side edge portion of the respective top conductive layers is formed by the hypothenuse side (128a, 128b) of the recess. The dielectric filter according to claim 1, wherein the side edge portions (632a, 632b) each have a staircase-shaped configuration. The dielectric filter according to claim 1, wherein the top conductive layers each have a regular trapezoid-shaped recess (526a, 526b) having two oblique sides and the respective side edge portion is formed along the two oblique sides. The dielectric filter according to claim 4, wherein the side edge portions each have staircase-shaped configurations. The dielectric filter according to claim 1, wherein the top conductive layers (731) each have a paralellogram configuration with two sides perpendicular to the upper edge portion and two sides (732a, 732b) oblique to the upper edge portion and the side edge portion is one of the two oblique sides and facing the upper edge portion. The dielectric filter according to claim 6, wherein the side edge portions each have a staircase-shaped configuration. The dielectric filter according to claim 6, wherein the side edge portions each have a curled or curved configuration. A dielectric filter as in claim 1, wherein the block (110) is rectangular-shaped and further has two second side surfaces (117a, 117b) which extend from the bottom surface (113) to the top surface (111), the side conductive layer (125) entirely covering the first and the second side surfaces, the upper edge portion of the side conductive layer (125) surrounding the top surface (111), the plurality of holes (119) being aligned in a line parallel to the first side surfaces (115a, 115b), each hole (119) having the interior surface having a cylindrical shape and being entirely covered by the inner conductive layer (121a to 121d), the top conductive layers (131a to 131d) surrounding the respective holes (119). The dielectric filter according to claim 1, wherein said holes (119) are cylindrically shaped. A method for manufacturing a dielectric filter (100) of selected resonant frequency values, the filter including a dielectric block (110) having a top surface (111), a bottom surface (113), and two opposite first side surfaces (115a, 115b), the dielectric block further having a plurality of holes (119) extending from the top surface to the bottom surface between the side surfaces, the method comprising steps of: covering the side surfaces with a conductive material so as to produce a side conductive layer (125) having an upper edge portion (125a, 125b) adjacent to the top surface (111); covering the bottom surface (113) with conductive material so as to produce a bottom conductive layer (123) electrically connecting the side conductive layer (125); covering respective spaced apart portions of the top surface (111) surrounding the holes (119) with conductive material so as to produce respective spaced apart top conductive layers (131a to 131d) thereon; covering each of the interior surfaces (118) of the holes with conductive material so as to produce a plurality of inner conductive layers (127a to 127d) each of the inner conductive layers being electrically connected to the bottom layer and the respective top layer; measuring the resonant frequencies of the filter; characterized in that each of the produced top conductive layers having a side edge portion substantially obliquely opposing the upper edge portion of the side conductive layer so that the side edge portion of the top conductive layer is capacitively coupled with the upper edge portion so as to produce a dielectric filter (100) having resonant frequencies greater than the selected resonant frequency values; a predetermined amount of the conductive material is removed from respective selected portions of the side edge portion depending on the measured resonant frequencies so as to reduce and thereby adjust the resonant frequencies of the filter to the preselected resonant frequency values.

Inventors: HORII HIROYUKI OKI ELECTRIC IN, MATSUKURA HISAO OKI ELECTRIC I, YAMATO OSAMU C O OKI ELECTRIC, HORII, HIROYUKI    OKI ELECTRIC INDUSTRY CO., LTD., MATSUKURA, HISAO OKI ELECTRIC INDUSTRY CO., LTD., YAMATO, OSAMU C/O OKI ELECTRIC INDUSTRY CO., LTD., HORII, HIROYUKI OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1988
Main classification: H01P1
IPC codes: H01P1, H01P7