MICROWAVE STRIP LINE FILTER

Abstract: A microwave strip line filter in which a pair of dielectric bases are superposed one upon another and joined. There is a possibility that the variation of the electrical characteristics of the filter is caused by the misalignment between the resonance electrodes provided on the respective dielectric bases when they are joined. To avoid it, the resonance electrodes (11, 12, 13) on one dielectric base (10) are formed into a pattern of predetermined dimensions, and the resonance electrodes (21, 22, 23) on the other dielectric base (20) are formed into a pattern of smaller dimensions than the resonance electrodes (11, 12, 13). Also, to prevent the bonding agent used for joining the bases from having a bad influence upon the electrical characteristics, the bonding agent (26) is applied in the form of spots to the resonance electrodes (21, 22, 23). Further, to achieve various desired resonance frequency characteristics by the resonance electrodes of the same pattern easily with a high accuracy, the electric bases are made of ceramic materials of different dielectric constants from each other respectively.

Description: TECHNICAL FIELD The present invention relates to a microwave stripline filter which may be used as a band-pass filter. BACKGROUND ART There is known a variety of microwave stripline filters can be used as bandpass filters for a microwave range. Fig. 1 of the accompanying drawings illustrates an example of such microwave stripline filters of a known type in which it comprises two superimposed dielectric substrates 1a and 1b made of a BaO-TiO 2 or BaO-TiO 2 -rare type dielectric ceramic material having a high dielectric constant and a low loss factor. The dielectric substrates 1a and 1b are provided with ground conductors 2a and 2b on the outer surface and peripheral portion thereof, respectively. On the inner surface of one 1a of the dielectric substrates 1a and 1b are disposed a plurality of strip-shaped resonator conductors 3a which operate as a filter element. Each resonator conductor has one end connected to the ground conductor 2a to form a short-circuit terminal, while the other end of each resonator conductor is not connected to any ground conductor to form an open-circuit terminal. The open-circuit terminals of the strip-shaped resonator conductors 3a are alternately arranged to form an interdigitated-type arrangement. A stripline filter of the above described type is disclosed in Japanese Patent Kokai No. 54-87480 (US-A-4 157 517). With such an arrangement in which the resonator conducors 3a having a desired pattern are formed on the inner surface of one of two dielectric substrates 1a and 1b, and sandwiched between said inner surface and the inner surface of the other dielectric substrate, a gap may often be formed partially between the resonator conductors 3a and the inner surface of the other dielectric substrate 1b when the latter is stacked on the former. Therefore, there is disadvantage that electric characteristics between the respective resonator conductors 3a are varied so that the response frequency of the filter may deviated. In order to improve such disadvantage, there has been proposed another microwave stripline filter in which as illustrated in Fig. 2 two dielectric substrates 4a and 4b are provided with respective resonator conductors 5a and 5b having a substantially same pattern and same dimensions on the respective inner surfaces. The resonator conductors 5a and 5b are bonded together as the dielectric substrates 4a and 4b are stacked to each other (see Japanese Patent Kokai 3-41802 and EP-A-0 434 847). In the conventional microwave stripline filter in which two dielectric substrates are provided with respective resonator conductors on the respective inner surfaces, and are stacked and bonded to each other, since respective resonator conductors on the respective inner surfaces have same pattern and same dimensions, the stacking of the dielectric substrates should be performed so that the resonator conductors on one dielectric substrate and the resonator conductors on the other dielectric substrate are overlapped without getting out of position. Consequently, the fact is that this assembling process depends upon the manual procedure of person skilled in the art, and thus there are problems in the manufacturing cost and productivity. Further, it is necessary to provide the resonator conductors on the inner surface of each substrate with very restricted tolerance because the resonator conductors should be formed on the inner surface of each dielectric substrate without involving any positional deviation. Even if the resonator conductors are accurately formed on the inner surface of each dielectric substrate with a predetermined position and dimension, and the dielectric substrates are stacked to each other by those skilled in the art, it is substantially difficult to ascertain whether or not the resonator conductors on the both substrates are deviated from each other at stacking them. As a result, the stacked and bonded resonator conductors on the both substrates may be deviated from each other. As shown in Fig. 3, for example, if any deviation occurs in the axial direction of the resonator conductors, there may occur disadvantages that the distance between the adjacent resonator conductors is deviated from a set value and the resonant frequency is varied. In the field of transmission lines it is known from US-A-2,926,317 to provide conductors on the inner surfaces of two confronting dielectric substrates and to bring the conductors into contact with each other. By making the conductor on one of the substrates wider than that on the other is is possible to permit limited transverse motion between the two. Returning to a consideration of stripline filters it is conventionally proposed that when being assembled two dielectric substrates are stacked and bonded to each other by applying adhesive such as cream solder or the like to the whole surface of the resonator conductors on either one or both of the dielectric substrates. As a result, the adhesive 6 can be partly squeezed by and flows out of the resonator conductors. Then, the squeezed bonding agent can adversely affect the electric characteristic of the resonator conductors so that the filter may have deviated resonant frequency, that cannot be corrected in the subsequent stage of fine adjustment of the resonant frequency of the filter, making the yield of manufacture of filters undesireably low. The use of a relatively large quantity of cream solder and other bonding agents can also deteriorate the Q value of the produced filters because of the low electric conductivity of such agents relative to that of precious material such as silver used for the resonant conductors. One attempt to overcome the problems associated with the variability of the gap in the centre of the sandwiched assembly as a result of the soldering process is described in US-A-4,609,892. In the arrangement described a plurality of leads are each provided with a centrally located cavity or pocket hole which are connected respectively to the filter's input, output and ground connection points. In this way any excess solder accruing during the molten solder process used to permanently attach the filter halves together is accommodated in the lead pocket holes and not under the leads themselves. The result is a filter assembly in which the gap between the base and cover filter halves is carefully controlled. Nevertheless it is an object of the present invention to provide a stripline filter for microwaves capable of overcoming the disadvantages in the conventional filter, having no dispersion in a filtering characteristic and being assembled with a relative large tolerance. A second object of the present invention is to provide a stripline filter for microwaves wherein it has no dispersion in a filtering characteristic, a good yield can be attained, and any reduction of the Q value can be suppressed. DISCLOSURE OF THE INVENTION In order to attain these objects described above, according to the present invention, there is provided a microwave stripline filter having a pair of dielectric substrates each provided with a ground conductor on an outer surface and resonant electrodes on an inner surface, said dielectric substrates being stacked and bonded so that the resonant electrodes on the dielectric substrates are superimposed to each other, the resonant electrodes formed on the inner surface of one of the dielectric substrates having a predetermined pattern and predetermined dimensions, characterized in that the resonant electrodes formed on the inner surface of the other dielectric substrate have a pattern similar to said predetermined pattern and dimensions smaller than said predetermined dimensions and said dielectric substrates are bonded to each other by applying spots of a bonding agent to the resonant electrodes on either one of the dielectric substrates prior to putting the electrodes together. The scaling down of the resonant electrodes on the inner surface of the other dielectric substrate may be arbitrarily set with respect to the predetermined pattern and dimensions on the inner surface of said one dielectric substrate. In the filter according to the first aspect of the present invention, since the resonant electrodes formed on the inner surface of one of the respective dielectric substrates have the predetermined pattern and dimensions, and the resonant electrodes on the other dielectric substrate have a pattern and dimensions smaller than the predetermined pattern and dimensions, the smaller resonant electrodes are not jutted out beyond the bounds of said the resonant electrodes having the predetermined pattern and dimensions even if the both dielectric substrates are superimposed to each other so that the positioning of the resonant electrodes on the both substrates is made with a little shear. It is, therefore, possible to maintain an electric characteristic to a set level which is determined by the resonant electrodes of predetermined pattern and dimensions. A microwave stripline filter according to a second aspect of the present invention a pair of dielectric substrates are bonded to each other by applying a bonding agent such as cream solder to the surfaces of the resonant electrodes on either one of the dielectric substrates to form spots of bonding agent and subsequently putting the electrodes together. Furthermore, in the filter of the present invention, since the bonding agent such as cream solder is applied to the resonant electrodes on either one of the dielectric substrates to form spots of bonding agent, it can hardly be squeezed by and flow out of the resonant electrodes when the dielectric substrates are put together so that the filter may realize intended electric characteristics that are determined by the resonant electrodes having the predetermined pattern and dimensions. Besides, since the amount of bonding agent consumed for producing a required level of adhesive power for the resonant electrodes is minimized, it is possible to suppress any deterioration of the Q value. BRIEF DESCRIPTION OF DRAWINGS In the accompanied drawings: Fig. 1 is a partially cut-out perspective view showing an example of conventional microwave stripline filters; Fig. 2 is an exploded perspective view showing another conventional stripline filter; Fig. 3 is an enlarged partial sectional view of the microwave stripline filter of Fig. 2 showing how the resonant conductors are bonded together. Fig. 4 is an exploded perspective view showing a microwave stripline filter according to one embodiment of the present invention; Fig. 5 is a plan view of one of dielectric substrates which construct the microwave stripline filter of Fig. 4; and Fig. 6 is a plan view of the other dielectric substrate which constructs the microwave strip line filter of Fig. 4. PREFERRED FORMS BY WHICH THE INVENTION IS TO BE EXECUTED The present invention will now be in more detail described by way of embodiments as illustrated in Figs. 4 through 6 of the accompaning drawings. Figs. 4 through 6 illustrate a stripline filter for microwaves according to an embodiment of the present invention. The illustrated filter comprises a pair of dielectric substrates 10 and 20 made of a BaO-TiO 2 or BaO-TiO 2 -rare type dielectric ceramic material having a high dielectric constant and a low loss factor. A ground conductor (not shown) is formed on the outer surface of one of the substrates 10 which is exposed to the atmosphere when the substrates are assembled, while a plurality of (three in the illustrated arrangement) strip-shaped resonant conductors 11, 12 and 13 that operate as a filter element are formed on the inner surface of one substrate 10 which is closed or covered when the substrates are assembled. As will be seen, a plurality of notches 14 are formed along the peripheral portion of the substrate 10, and each of the notches 14 is provided with a short-circuiting conductor 15 extending from the corresponding edge of the inner surface to the corresponding edge of the outer surface except the notches 14a and 14b. An end of each of the strip-shaped resonant conductors 11, 12 and 13 is connected to the ground conductor on the outer surface by way of a corresponding one of the short-circuiting conductors 15 to form a short-circuiting terminal, while the other ends of the strip-shaped resonant conductors 11, 12 and 13 are spaced apart from the corresponding respective short-circuiting conductors 15 to form open-circuit terminals. These open-circuit terminals of the strip-shaped resonant conductors 11, 12 and 13 are arranged in an alternating manner to form an interdigitated type arrangement. The two outer ones 11 and 13 of the three strip-shaped resonant conductors 11, 12 and 13 are provided with respective connecting terminals 16 and 17 which are laterally extended to reach the respective notches 14a and 14b as shown and connected to respective input terminals (not shown). The other dielectric substrate 20 is also provided with a ground conductor short-circuiting conductors and strip-shaped resonant conductors. More specifically, a ground conductor (not shown) is formed on the outer surface of the other dielectric substrate 20, while three strip-shaped resonant conductors 21, 22 and 23 are arranged to corresponding portions on the inner surface of the dielectric substrate 20. These strip-shaped resonant conductors 21, 22 and 23 are arranged to show mirror images of the corresponding respective strip-shaped resonant conductors 11, 12 and 13 but have widths and lengths slightly smaller than those of the strip-shaped resonant conductors 11, 12 and 13. Further, the dielectric substrate 20 is provided with a plurality of notches 24 which are arranged along the peripheral portion of the substrate 20 at positions where correspond to those of the respective notches 14 of the dielectric substrate 10. As in the case of the notches 14 of the dielectric substrate 10, each of the notches 24 is provided with a short-circuiting conductor 25 extending from the corresponding edge of the inner surface to the corresponding edge of the outer surface expect the notches 24a and 24b. In this connection, the width of each of the short-circuiting conductors 25 is determined so that it is slightly smaller than that of the corresponding short-circuiting conductor 15. An end of each of the strip-shaped resonant conductors 21, 22 and 23 is connected to the ground conductor on the outer surface by way of a corresponding one of the short-circuiting conductors 25 to form a short-circuiting terminal, while the other ends of the strip-shaped resonant conductors 21, 22 and 23 are spaced apart from the corresponding respective short-circuiting conductors 25 to form open-circuit terminals. These open-circuiting terminals of the strip-shaped resonant conductors 21, 22 and 23 are arranged in an alternating manner to form an interdigitated type arrangement. The two dielectric substrates 10 and 20 provided with the respective resonant conductors, ground conductors and short-circuiting conductors are then put together with a bonding agent such as cream solder applied to the smaller resonant conductors and short-circuiting conductors. Since the strip-shaped resonant conductors 21, 22 and 23 arranged on the inner surface of the dielectric substrate 20 are smaller than the corresponding respective resonant conductors 11, 12 and 13 having predetermined dimensions, the strip-shaped resonant conductors 21, 22 and 23 can not be displaced out of the effective area of the respective resonant conductors 11, 12 and 13 of the dielectric substrate 10. In particular, the two dielectric substrates 10 and 20 are bonded to each other by applying a bonding agent such as cream solder to the smaller resonant conductors and short-circuiting conductors provided on the dielectric substrate 10 so as to form spots of the bonding agent as shown by 26 in Fig. 6. Since the strip-shaped resonant conductors 21, 22 and 23 arranged on the inner surface of the dielectric substrate 20 are smaller than the corresponding respective resonant conductors 11, 12 and 13 having predetermined dimensions and the bonding agent is spottily applied, the bonding agent 26 can hardly be squeezed out of the effective area of the respective resonant conductors 11, 12 and 13 of the dielectric substrate 10 nor can the strip-shaped resonant conductors 21, 22 and 23 can be displaced out of the effective area of the respective resonant conductors 11, 12 and 13 of the dielectric substrate 10 when the both dielectric substrates 10 and 20 are superimposed to each other. Consequently, the produced filter is not adversely affected by the bonding agent 26 and shows intended and desired electric characteristics even if the resonant conductors of each dielectric substrate are positioned with a slight displacement from tl.eir proper positions at the time of forming them on the inner surface of each dielectric substrate. With the illustrated embodiment, the resonant conductors on the dielectric substrate 10 have a desired pattern of predetermined dimensions while the resonant conductors on the dielectric substrate 20 have dimensions slightly smaller than the predetermined ones. Also, while the resonant conductors are arranged to form an interdigitated type filter, they may be alternatively be so arranged to form a filter of any other type such as a comb-type. INDUSTRIAL APPLICABILITY As described above, according to the present invention since the resonant electrodes on one of the paired dielectric substrates are formed to have a predetermined pattern and the resonant electrodes on the other dielectric substrate are formed to have a pattern similar to said predetermined pattern but smaller dimensions, the smaller resonant electrodes may not be displaced from the area of the resonant electrodes of the predetermined pattern having desired dimensions even if two dielectric substrates are not exactly aligned with each other at assembling of them, and thus the dielectric substrates can be superimposed and assemblied without adversely affecting the intended and desired electric characteristics which depend upon the resonant electrodes having the predetermined pattern and dimensions. As a result, the positioning at bonding of the paired dielectric substrates can be carried out with a sufficient tolerance so that the assembling can be easily performed without resorting to the skilled persons. Also, since the electric characteristics are not substantially varied by any dispersion in the positioning of the resonant electrodes when being assembled, it is possible to provide stripline filters having desired electric characteristics with a high yield. According to the present invention, also, since a bonding agent is applied to the resonant electrodes arranged on one of the paired dielectric substrates to form spots of the bonding agent before the dielectric substrates are put together, the bonding agent can hardly be squeezed out of the effective area of the resonant electrodes and the filter can be assembled without affecting the intended and desired electric characteristics of the resonant electrodes. Moreover, since the amount of the bonding agent to be used is held minimal, any deterioration of the Q value can be effectively prevented and thus strip line filters having stable characteristics can be produced at a high yield.

Claims: A microwave stripline filter having a pair of dielectric substrates (10,20) each provided with a ground conductor on an outer surface and resonant electrodes (11,12,13,21,22,23) on an inner surface, said dielectric substrates (10,20) being stacked and bonded so that the resonant electrodes (11,12,13,21,22,23) on the dielectric substrates (10,20) are superimposed to each other, the resonant electrodes (11,12,13) formed on the inner surface of one of the dielectric substrates (10) having a predetermined pattern and predetermined dimensions, characterized in that the resonant electrodes (21,22,23) formed on the inner surface of the other dielectric substrate (20) have a pattern similar to said predetermined pattern and dimensions smaller than said predetermined dimensions and said dielectric substrates (10,20) are bonded to each other by applying spots of a bonding agent (26) to the resonant electrodes (11,12,13,21,22,23) on either one of the dielectric substrates (10,20) prior to putting the electrodes (11,12,13,21,22,13) together. A microwave stripline filter in accordance with claim 1 wherein said spots of bonding agent (26) are applied to the smaller resonant electrodes (21,22,23).

Inventors: HINO SEIGO, ITO KENJI, SHIMIZU HIROYUKI, HINO, SEIGO, ITO, KENJI, SHIMIZU, HIROYUKI, HINO, SEIGO, NGK SPARK PLUG CO., LTD., ITO, KENJI, NGK SPARK PLUG CO., LTD., SHIMIZU, HIROYUKI, NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO. LTD.
Application year: 1992
Main classification: H01P1
IPC codes: H01P1