Dielectric filter

Abstract: A dielectric filter comprising a plurality of resonators formed by boring a plurality of through holes between the oppositely disposed end surfaces of a dielectric block in parallel with each other and arranging internal conductors on the inner peripheral surfaces of the respective through holes and an external conductor formed on a substantial portion of the outer peripheral surface of the dielectric block, wherein a region devoid of short-circuit conductor is formed on the short-circuit end surface and extending in parallel with the direction connecting the resonators, thereby the filter can be controlled satisfactorily for coupling the dielectric coaxial resonators and have a reduced height.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a dielectric filter comprising a plurality of dielectric coaxial resonators arranged in parallel with each other. Prior Art Such dielectric filters have been disclosed in particular in GB- A -2 163 606 and EP-A-0 566 573 and JP-58 204 601. GB-A-2 163 606 discloses a dielectric filter comprising: a plurality of parallel through holes in a dielectric block the inner peripheral surface of each through hole is provided with an internal conductor for forming a plurality of resonators an external conductor on a substantial portion of the outer peripheral surface one of the end surfaces is a short-circuit end surface and the other end surface is an open-circuit end surface at least one region devoid of short-circuiting conductor is formed on the short circuit end surface that region has an extension parallel to the direction connecting the resonators EP-A-0 556 573 discloses the dependency of resonator parameters on location and dimension of non conductive regions in a short-circuit surface of a dielectric resonator. There have also been known various dielectric filters realized by boring a plurality of through holes through a dielectric block in parallel with each other between a pair of oppositely disposed end surfaces of a dielectric block, providing internal conductors on the inner peripheral surfaces of the respective through holes to form a plurality of dielectric coaxial resonators, providing an external conductor on a substantial portion of the outer peripheral surface of the dielectric block and providing a short-circuit conductor on one of the end surfaces of the dielectric block to make it a short-circuit end surface for connecting the external conductor on the outer peripheral surface and the internal conductors on the inner peripheral surfaces of the through holes. A dielectric filter having such a configuration is typically used as filters for high frequency bands. FIG. 1 of the accompanying drawings illustrates such a conventional dielectric filter comprising a dielectric block A which is provided with two resonators B and has one end surface or a short-circuit end surface C provided with a region D having no short-circuit conductor between the resonators B as insterstage coupling means of the resonators of the dielectric filter (Refer to Japanese Patent Kokai No. 3-293802). The applicant of the present application proposed in Japanese Patent Application No. 6-163189 (Japanese Patent Kokai No. 8-8607) a dielectric filter for a high frequency band as shown in FIG. 2 realized by providing a dielectric block 1 with a pair of through holes 2a and 2b extending in parallel with each other therethrough, providing internal conductors 3a and 3b on the inner surfaces of the respective through holes 2a and 2b to produce a pair of dielectric coaxial resonators, providing an external conductor 4 on a given area of the outer peripheral surface and providing a short-circuit conductor on one of the end surfaces of the dielectric block 1 to make it a short-circuit conductor end surface 5 for connecting the external conductor 4 and the internal conductor 3a and 3b, the other end surface being left as open-circuit end surface 6, wherein a slot 7 is formed by removing the external conductor 4 along a direction perpendicular to the through holes 2a and 2b in an area of the outer peripheral surface located close to the short-circuit conductor end surface 5 in order to couple and polarize the dielectric coaxial resonators at a same time in a simple manner. With a conventional dielectric filter for a high frequency band provided with a slot devoid of a short-circuit conductor formed along a direction perpendicular to and between the dielectric coaxial resonators on the short-circuit conductor end surface in order to couple resonators as described above by referring to FIG. 1, the slot is required to have a considerable width and hence a large surface area that occupies about 2/3 of the total area of the short-circuit end surface to ensure a sufficient coupling effect at the cost of a reduced Q value. Additionally, the slot formed on the short-circuit end surface along a direction perpendicular to and between the resonators can reduce the mechanical strength of the dielectric filter (Refer to Japanese UM Kokai No. 62-61504). With such a dielectric filter illustrated in FIG. 2 a shield case is applied thereto typically after directly connecting input/output terminals formed on the lower portion of the outer peripheral surface located opposite to the side of the coupling slot is connected directly with a printed circuit board. The inner wall of the shield case has to be separated from the coupling slot on the top side of the outer peripheral surface by at least 3mm because the coupling effect of the slot changes significantly when the inner wall of the shield case comes too close to or into contact with the coupling slot on the top side of the outer peripheral surface. This involves a problem that a dielectric filter for a high frequency band cannot reduce its height beyond a certain degree. Therefore, it is an object of the present invention to provide a dielectric filter with a reduced height that can solve the above problem without damaging the coupling effect and reducing the Q value. SUMMARY OF THE INVENTION According to the invention, the above object is achieved by providing a dielectric filter having the general structure defined in the preamble of annexed claim 1, and comprising at least one non-conductive region such as defined in the characterizing part of same claim 1. Particular embodiments of the invention are set out in the dependent claims. With a dielectric filter for a high frequency band having the configuration defined in claim 1, the dielectric coaxial resonators formed in the respective through holes are coupled for electric field at the open-circuit end surface and for magnetic field at the short-circuit end surface. Therefore, the magnetic coupling of the resonators are intensified because the magnetic field is made less apt to swerve to the external conductor side by the region devoid of short-circuit conductor formed along an edge of the short-circuit end surface running in parallel with the direction connecting the resonators. Thus they are coupled more intensely for magnetic field than for electric field so that any adjacently located resonators are coupled intensely for magnetic filed to give rise to an interstage coupling. Such a region devoid of short-circuiting conductor may advantageously be formed by producing a transversal slit as defined in claim 1 on an end surface of the dielectric block typically by means of a dicing saw and removing the short-circuit conductor in that region. The slit may be formed in the operation of producing a dielectric block by press machining. If such is the case, the short-circuit conductor surface is produced by applying a conductive material onto that surface except the slit. Alternatively, the region devoid of short-circuiting conductor may be produced by applying a conductive material to the entire surface and thereafter partly removing it to form a slit by means of a laser trimmer or sand blast. Still alternatively, the non-conductive region may be formed by screen printing or patterning at the timing of forming a film of a short-circuit conductor. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of a comparable conventional dielectric filter; FIG. 2 is a schematic perspective view of a conventional dielectric filter previously proposed by the inventors of the present invention; FIG. 3 is a schematic perspective view of an embodiment of two-stage type dielectric filter for a high frequency band; FIG. 4A is a schematic view of the embodiment of dielectric filter of FIG. 3, showing one of its end surfaces with dimensional values; FIG. 4B is a schematic front view of the embodiment of dielectric filter of FIG. 3; FIG. 4C is a schematic cross sectional view taken along line X-X in FIG. 4B. FIG. 5 is a schematic perspective view showing a modification of the embodiment of FIG.3; FIG. 6 is a graph showing the electric characteristic of a two-stage type dielectric filter for a high frequency band; FIG. 7 is a graph showing the electric characteristic of a two-stage type dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 6; FIG. 8 is a graph showing the electric characteristic of a two-stage type dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 6 or 7; FIG. 9 is a graph showing the electric characteristic of a two-stage type dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 6, 7 or 8; FIG. 10 is a graph showing the electric characteristic of a comparable conventional dielectric filter as shown in FIG. 1; FIG. 11 is a graph showing the electric characteristic of a comparable conventional dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 10; FIG. 12 is a graph showing the electric performance of a comparable conventional dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 10 or 11; and FIG. 13 is a graph showing the electric performance of a comparable conventional dielectric filter with a region devoid of short-circuit conductor having dimensions different from those of FIG. 10, 11 or 12. DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION The present invention will now be described by referring to FIGS. 3 and 4 illustrating an embodiment of dielectric filter. In FIGS. 3 and 4, there is shown a two-stage type dielectric filter for a high frequency band comprising a pair of dielectric coaxial resonators formed in a single dielectric block 1. Referring to FIGS. 3 and 4, the dielectric block 1 is a rectangular parallelpiped made of a ceramic dielectric material containing titanium and having an end surface 1a, another end surface 1b and four outer lateral side surfaces 1c, 1d, 1e and 1f. A pair of through holes 2a and 2b are bored through the dielectric block 1 in parallel with each other and extend between one end surface 1a and the other end surface 1b. Internal conductors 3a and 3b are formed respectively on the inner peripheral surfaces of the through holes 2a and 2b by applying a conductive material to produce a pair of dielectric coaxial resonators. An external conductor 4 is formed on the outer lateral side surfaces 1c, 1d, 1e and 1f and is used as a grounding conductor. A short-circuit conductor 5 is formed on the end surface 1a and electrically connected to the external conductor 4 on the outer lateral side surfaces 1c, 1d, 1e and 1f. In this way, the short-circuit conductor 5 electrically connects the internal conductors 3a and 3b of the dielectric coaxial resonators to the external conductor 4. Thus, the end surface 1a may well be referred to as short-circuit end surface. The other end surface 1b of the dielectric block 1 carries no conductor and therefore operates as an open-circuit end surface 6. Input/output conductors 7a and 7b are formed on the outer lateral side surface 1c and electrically insulated from the external conductor 4. The input/output conductor 7a is capacitively connected to the internal conductor 3a via the dielectric block 1, whereas the input/output conductor 7b is capacitively connected to the internal conductor 3b also via the dielectric block 1. One of the input/output conductors 7a and 7b is connected to an input terminal of an electric circuit while the other input/output conductor is connected to an output terminal of the circuit to complete the electric connection of the dielectric filter for a high frequency band. With the dielectric filter having a configuration as described above, a pair of regions 8 and 9 devoid of short-circuit conductor or non-conductive regions are formed along the oppositely disposed respective edges of the short-circuit end surface 5, extending along the direction connecting the resonators as shown in FIGS. 3 and 4. The regions 8 and 9 devoid of short-circuit conductor may be formed by coating the end surface 1a with a short-circuit conductor and subsequently removing it to form the regions typically by means of a laser trimmer or sand blast. Alternatively, the regions 8 and 9 devoid of short-circuit conductor may be formed by producing a pair of transversal shallow slits typically by means of a dicing saw and removing the short-circuit conductor in that regions as shown in FIGS. 4B and 4C. The slits may be formed in the operation of producing a dielectric block by press machining. If such is the case, the short-circuit conductor surface is produced by applying a conductive material onto that surface except the slits, which then becomes regions 8 and 9 devoid of shrort-circuit conductor. Still alternatively, the non-conductive regions may be formed by screen printing or patterning at the timing of forming an external conductor and a film of a short-circuit conductor. FIGS. 4A, 4B and 4C show dimensional values of the dielectric filter for a high frequency band thus constructed. FIG. 5 illustrates a modification of the embodiment of FIG.3, in which a single non-conductive region 8 is formed along one of the edges of the short-circuit end surface 5, extending along the direction connecting the resonators. FIGS. 6 through 9 are graphs illustrating an electric characteristic of such a dielectric filter having such a configuration as shown in FIG. 5 observed when different values were used for the width W1 of the region 8. For the purpose of comparison, FIGS 10 through 13 show graphs illustrating the electric characteristics of the conventional dielectric filter as shown in FIG. 1 also observed when different values were used for the width W of the region D devoid of short-circuit conductor. By comparing the graphs for the two dielectric filters, it will be seen that the region devoid of short-circuit conductor of the dielectric filter according to the invention has a relatively small area of about 1/3 of the short-circuit end surface and the dielectric filter shows a satisfactory coupling effect, whereas the comparable region of the conventional dielectric filter of FIG. 1 has to be made as large as about 2/3 of the short-circuit end surface to achieve a similar coupling effect. Thus, the conventional dielectric filter such as shown in FIG. 1 is accompanied by a problem of regulating difficulty and a reduced Q value. The coupling effect became remarkable when the region devoid of short-circuit conductor is expanded along the X-direction rather than along the Y-direction as shown in FIG. 4A. This means that the dielectric filter shows an excellent controllability if compared with the conventional dielectric filter of FIG. 1 where a region devoid of short-circuit conductor is formed between the resonators. While the above embodiment is a two-stage type dielectric filter for a high frequency band comprising a pair of dielectric coaxial resonators, the present invention is applicable to a three-stage type dielectric filter comprising three dielectric coaxial resonators or a four- or higher stage type dielectric filter. The performance of a dielectric filter according to the invention (in terms of frequency bandwidth, attenuation pole generating frequency, etc.) can be controlled by regulating the position and the surface area of the region devoid of short-circuit conductor. Therefore, the region devoid of short-circuit conductor may be modified transversally (in the Y-direction) and/or longitudinally (in the X-direction) to substantially change its location and/or area in order to regulate the performance of the dielectric filter after forming the region. As described above, with the dielectric filter since a region devoid of short-circuit conductor is formed on the short-circuit end surface along a direction connecting the through holes to realize an interstage coupling, no coupling groove, slit nor spot facing has to be formed after producing a dielectric block for the filter so that such a dielectric filter can be manufactured efficiently in a well controlled manner with a desired coupling performance without reducing the Q value and a reduced mechanical strength. Thus, the present invention provide a remarkable technological advantage for manufacturing a dielectric filter for a high frequency band that operates stably and reliably.

Claims: A dielectric filter comprising: a dielectric block (1) having oppositely disposed rectangular end surfaces (1a, 1b) and an outer peripheral surface (1c, 1d, 1e, 1f) therebetween; a plurality of resonators provided in the dielectric block (1) in parallel with respect to each other, each of the resonators including a through hole (2a, 2b) extending between the oppositely disposed end surfaces (1a, 1b) of the dielectric block (1) and an internal conductor (3a, 3b) provided on the inner peripheral surface of the respective through hole (2a, 2b); and an external conductor (4) provided on a substantial portion of the outer peripheral surface (1a, 1c, 1d, 1e, 1f)on the dielectric block (1), one (1a) of the end surfaces (1a, 1b) being a rectangular short-circuit end surface (5) provided with a short-circuiting conductor connecting the respective internal conductors (3a, 3b) of the resonators with the external conductor (4) on the outer peripheral surfaces (1c, 1d, 1e, 1f), and the other end surface (1b) being a rectangular open-circuit end surface (6), characterized in that at least one non-conductive region (8, 9) devoid of short-circuiting conductor is provided on the short circuit end surface (5) said each non-conductive region extends along a longer edge of the rectangular short-circuit end surface in parallel with the direction connecting the respective resonators at the intersection with the adjacent peripheral surface, and has a width equal to or larger than the distance between the axes of the through holes (2a, 2b) of two adjacently located resonators and smaller than the width of the block. A dielectric filter as claimed in claim 1, characterized in that two non-conductive regions (8, 9) devoid of short-circuiting conductor are formed on said short-circuit end surface (5), which extend along the oppositely disposed respective longer edges of said rectangular short-circuit end surface (5). A dielectric filter as claimed in claim 1, characterized in that said non-conductive region (8; 8, 9) devoid of short-circuiting conductor formed on said short-circuit end surface (5) comprises a slot. A dielectric filter as claimed in claim 1, characterized in that said non-conductive region (8, 9) devoid of short-circuiting conductor formed on said short-circuit end surface (5) comprises a pair of shallow slits which extend along the oppositely disposed respective longer edges of said rectangular short-circuit end surface (5). A dielectric filter as claimed in claim 3 or 4, characterized in that each of said slot (8) or slits (8, 9) is formed by a screen printing or patterning at the time of forming the external and short-circuiting conductors on said dielectric block (1). A dielectric filter as claimed in claim 1, characterized in that said each non-conductive region (8; 8, 9) has an area of about 1/3 of the short-circuit end surface (5).

Inventors: ONO SHOJI, SHIBATA MASAKI, ONO, SHOJI, SHIBATA, MASAKI, Ono, Shoji, c/o NGK Spark Plug Co., Ltd., Shibata, Masaki, c/o NGK Spark Plug Co., Ltd.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD.
Application year: 1997
Main classification: H01P1
IPC codes: H01P1