Dielectric filter

Abstract: In a dielectric block (1), resonator holes (2a, 2b) having steps (21) consisting of a portion having larger inner diameter and a portion having smaller inner diameter are provided, and the smaller inner diameter portion of each resonator hole is formed on the side of a short-circuited end surface. By forming the small inner diameter portions of the resonator holes close to each other, the coupling between the two resonators becomes inductive coupling. By contrast, when small inner diameter portions are formed apart from each other, the coupling between the two resonators becomes capacitive coupling. The coupling strength can be changed by adjusting the distance between the small inner diameter portions.

Description: Generally, in a dielectric filter having a plurality of dielectric resonators coupled to each other, when coupling between adjacent resonators is capacitive coupling, an attenuation pole is obtained on the lower frequency range of the pass band, and when the coupling between adjacent resonators is an inductive coupling, an attenuation pole is obtained on the high frequency range of the pass band. Conventionally, in order to obtain capacitive coupling, resonator holes having steps have been formed in a dielectric block, as shown in Fig. 1. In the appended figures, shadowed portions denote portions where the base material of the dielectric block appear, that is, portions which are not provided with a conductor. Referring to Fig. 1, in a conventional dielectric filter having resonator holes with steps, two resonator holes 2a and 2b, for example, are formed piercing through a pair of opposing surfaces 1a and 1b of a dielectric block 1 having approximately rectangular parallellopiped shape. Inner conductors 3, 3 are formed on the inner surfaces of resonator holes 2a and 2b. A pair of input/output electrodes 5, 5 are formed at prescribed portions on the outer surface of dielectric block 1. An outer conductor 4 is formed approximately over the entire outer surface, except the regions where the input/output electrodes 5, 5 are formed. At one apertured surface 1a (hereinafter referred to as an open end surface) of each of the resonator holes 2a and 2b, there is a portion not provided with the inner conductor 3 (hereinafter referred to as a non-conducting portion), so that the inner conductors 3, 3 are isolated (not conducted) from the outer conductor 4. At the other apertured surface 1b (hereinafter referred to as a short-circuited surface), the inner conductors are short-circuited (conducted) with the outer conductor 4. Between the inner conductor 3 of each of the resonator holes 2a, 2b and the input/output electrode 5, an external coupling capacitance is generated, which external coupling capacitance provides an external coupling. In each of the resonator holes 2a and 2b, a step 21 is provided near the center of the open end surface 1a and the short-circuited end surface 1b. The inner diameter of the resonator holes 2a and 2b from the open-end surface 1a to step 21 is made larger than the inner diameter of resonator holes 2a, 2b from short-circuited end surface 1b to step 21. A portion having larger inner diameter on the side of open end surface 1a and a portion having smaller inner diameter on the side of short-circuited end surface 1b are formed coaxially. In the dielectric filter structured as described above, the coupling between two resonators formed in resonator holes 2a and 2b is capacitive coupling, and attenuation pole is formed on the low frequency range of the pass band. By changing the ratio of the length of the portions having larger inner diameter and smaller inner diameter, changing the ratio of inner diameters and so on of the resonator holes 2a and 2b, the degree of capacitive coupling (coupling strength) can be changed. In other words, pass band characteristics such as band width can be adjusted. In order to obtain inductive coupling, a coupling trench 6 is formed on the outer surface of dielectric block 1, such as shown in Fig. 2. More specifically, coupling trenches 6, 6 are formed on both major surfaces of dielectric block 1 between resonator holes 2a and 2b as shown in Fig. 2. Coupling trenches 6, 6 extend parallel to the resonator holes 2a, 2b, from the open end surface 1a and terminate near the center between open end surface 1a and short-circuited end surface 1b. An inner conductor 4 is formed on the surface of each of the coupling trenches 6, 6. Resonator holes 2a and 2b are formed to have constant inner diameter, and the step 21 such as shown in Fig. 1 is not provided. Except these points, the dielectric filter has the similar structure as that shown in Fig. 1, and description thereof is not repeated. In the dielectric filter shown in Fig. 2, the coupling between two resonators formed in the resonator holes 2a and 2b is inductive coupling, and attenuation pole is formed on the high frequency range of the pass band. By changing the length, width, depth, position, cross sectional shape or the like of the coupling trench 6, the coupling strength of the inductive coupling can be changed. In other words, pass band characteristics such as band width can be adjusted. In order to obtain inductive coupling, a step or a slit has been formed on the dielectric block in place of the coupling trenches 6, 6 described above. When attenuation poles are to be obtained on the low frequency range and on the high frequency range of the pass band, three or more resonator holes are formed in the dielectric block, a resonator having a step is formed in order to obtain an attenuation pole on the low frequency range, and a coupling trench or the like is formed on the outer surface of the dielectric block in order to obtain an attenuation pole on the high frequency range, and thus a dielectric filter is formed. However, in the conventional dielectric filter having resonator holes 2a and 2b with steps 21 shown in Fig. 1, the coupling between the resonators is capacitive coupling, and it was difficult to obtain inductive coupling. Further, in order to change the coupling strength, that is, filter characteristics such as bandwidth, troublesome and complicated settings have been necessary, including adjustment of ratio of length of the larger diameter portion and smaller diameter portion, ratio of inner diameters of these portions of the resonator holes 2a and 2b. In the dielectric filter having coupling trench 6 or the like formed on the outer surface of dielectric block 1 such as shown in Fig. 2, outer shape of the dielectric block 1 is complicated, and therefore mounting on a substrate has been troublesome. In order to change the coupling strength, it is necessary to change the dimension, shape or the like of the coupling trench, step or the like, that is, it is necessary to change the outer shape of the dielectric block 1. More specifically, when dielectric filters having different characteristics such as different bandwidths are required, a number of dielectric blocks having different outer shapes corresponding to the required characteristics are necessary, and therefore standardization of dielectric block is difficult. Further, reduction in size of the dielectric filter shown in Fig. 2 is more difficult than the dielectric filter shown in Fig. 1 having a step in the resonator hole of comparable characteristics, because of limitations in shaping the dielectric block. US 5,124,676 describes a dielectric filter having a generally box-like dielectric block, an outer conductor covering side surfaces of the dielectric block, a plurality of inner conductors arranged in the dielectric block in the longitudinal direction so as to extend between two opposite faces of the block, and a short-circuit conductor provided in one of the two faces of the dielectric block to connect the outer and inner conductors. At least one of the inner conductors has a rectangular cross-sectional shape, and the size of this sectional shape in the longitudinal direction of the dielectric block is increased at the face where the short-circuit conductor is provided, and is reduced at the other face. SUMMARY OF THE INVENTION Therefore, the present invention aims to provide a dielectric filter which is compact and capable of readily changing coupling strength between adjacent resonators and changing relation of coupling, that is, capacitive coupling or inductive coupling, without changing outer shape or dimension of the dielectric block. According to the present invention there is provided a dielectric filter, comprising: a dielectric block having a pair of opposing surfaces; a plurality of substantially parallel resonator holes piercing through at least an end surface of said dielectric block, each resonator hole consisting of a larger inner diameter portion along a first axis and a smaller inner diameter portion along a second axis, with the second axis being substantially parallel to the first axis; an outer conductor formed on an outer surface of said dielectric block; and an inner conductor formed on an inner surface of each of said plurality of resonator holes, and wherein, in each of adjacent resonator holes, the second axis is offset from the first axis. Therefore, in embodiments of the invention, by changing the distance between the central axis of small diameter portions of the resonator holes, the coupling strength between the resonators and coupling relation, that is, capacitive coupling or inductive coupling, can be changed. In an embodiment, the distance between central axes of smaller diameter portions of adjacent ones of the plurality of resonator holes is set smaller than the distance between central axes of larger diameter portions, whereby the coupling between the two resonators is made inductive coupling and one attenuation pole can be formed on the high frequency range of the pass band. In another embodiment, the distance between central axes of smaller diameter portions of the resonator holes is made larger than the distance between central axes of larger diameter portions, so that the coupling is made capacitive coupling, the bandwidth is made wider, and one attenuation pole can be formed on the low frequency range of the pass band. Preferably, at least three resonators are formed in the dielectric block, the distance between smaller diameter portions of adjacent resonator holes is made smaller than the distance between the central axes of larger diameter portions to obtain inductive coupling, while the distance between central axes of smaller diameter portions of other adjacent resonators is made larger than the distance between the central axes of larger diameter portions to obtain capacitive coupling, and one attenuation pole can be formed on each of the high frequency range and low frequency range of the pass band. The above and further features of the present invention are set forth with particularity in the appended claims and together with the advantages thereof will become apparent from consideration of the following detailed description of embodiments of the invention which is given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a conventional dielectric filter having resonator holes with steps. Fig. 2 is a perspective view of an appearance of a conventional dielectric filter provided with coupling trenches. Fig. 3A is a perspective view of a dielectric filter in accordance with a first embodiment of the present invention. Fig. 3B is a front view taken from an open end surface of the dielectric filter in accordance with the first embodiment of the present invention. Fig. 4 is a front view taken from the open end surface of the dielectric filter in accordance with a second embodiment of the present invention. Fig. 5 is a graph showing relation between a width d of smaller inner diameter portions of the dielectric filter, coupling coefficient and relation of coupling in accordance with an embodiment of the invention. Fig. 6A is a perspective view of a dielectric filter in accordance with a third embodiment of the present invention. Fig. 6B is a front view taken from the open end surface of the dielectric filter in accordance with the third embodiment of the present invention. Fig. 6C shows frequency attenuation characteristics of the dielectric filter in accordance with the third embodiment of the present invention. Fig. 3A is a perspective view of a dielectric filter of a first embodiment, and Fig. 3B is a front view taken from the open end surface of Fig. 3A. Similar to the dielectric filter of Fig. 1, the dielectric filter shown in Figs. 3A and 3B includes resonator holes 2a and 2b provided with steps 21 approximately at the center between open end surface 1a and short-circuited end surface 1b, and the inner diameter of resonator holes 2a and 2b from open end surface 1a to step 21 is made larger than the inner diameter of the resonator holes 2a and 2b from short-circuited end surface 1b to step 21. As shown in Fig. 3B, the inner diameter portions of resonator holes 2a and 2b on short-circuited end surface 1b are formed closest to each other. More specifically, the distance d (hereinafter referred to as the width of the smaller diameter portion) between the central axes of smaller diameter portions of resonator holes 2a and 2b at the short-circuited end surface 1b is made smaller than the distance (hereinafter referred to as the width of larger diameter portions) between the central axes of larger diameter portions of the resonator holes on the open end surface 1a. Except these points, the dielectric filter has similar structure as the conventional example shown in Fig. 1, and description thereof is not repeated. In the structure shown in Figs. 3A and 3B, the coupling between the two resonators formed at resonator holes 2a and 2b is changed to inductive coupling, and one attenuation pole is formed on the high frequency range of the pass band. Fig. 4 is a front view taken from the open end surface of the dielectric filter of a second embodiment. In the dielectric filter of this embodiment, the smaller diameter portions at the short-circuited end surface 1b of resonator holes 2a and 2b having steps 21 are formed furthermost from each other, as shown in Fig. 4. Namely, the width d of smaller diameter portions on the short-circuited end surface 1b of resonator holes 2a and 2b is made larger than the width of larger diameter portions on the open end surface 1a. Except this point, the dielectric filter has similar structure as the conventional example shown in Fig. 1, and description thereof is not repeated. In the structure of Fig. 4, the coupling between two resonators formed at resonator holes 2a and 2b, which has been originally capacitive coupling, is further enhanced and stronger capacitive coupling is obtained. Therefore, the bandwidth is made wider and one attenuation pole is formed on the low frequency range of the pass band. As described above, by deflecting the central axes of smaller diameter portions of resonator holes having steps from the central axes of larger diameter portions, the distance between the smaller diameter portions of adjacent resonator holes can be changed, whereby coupling strength between adjacent resonators and coupling relation, that is, capacitive coupling or inductive coupling, can be changed. The distance between the coupling strength and the relation of coupling with respect to the width d of smaller diameter portions will be described with reference to the result of experiment. Fig. 5 is a graph showing the coupling coefficient, coupling relation and the width d of smaller diameter portions of the dielectric filter in accordance with an embodiment of the invention. The example of Fig. 5 shows the relation between the width d of smaller diameter portions, coupling coefficient (coupling strength) and coupling relation when two resonator holes are formed in a dielectric block having the thickness 3 mm, width of 6 mm and the length in the direction of the resonator hole of 7 mm, with the diameter of larger diameter portions being 2 mm, the width between larger diameter portions being 3 mm and the inner diameter of smaller diameter portions being 1 mm. Larger diameter portions of the two resonator holes are formed on the side of the open end surface, while smaller diameter portions are formed on the side of the short-circuited end surface. Referring to Fig. 5, when the width d of smaller diameter portions is equal to the width of 3 mm of larger diameter portions, the coupling between the resonators is capacitive coupling, the strength of capacitive coupling becomes weaker as the width d of smaller diameter portions gradually decreases. Coupling ceases when the width d between smaller diameter portions is about 2.5 mm. When the width further decreases, the coupling changes to inductive coupling, and strongest inductive coupling is obtained when the width d of smaller diameter portions is the smallest (2 mm). By contrast, when the width d between smaller diameter portions is increased, the strength of capacitive coupling increases and strongest capacitive coupling is obtained when the width d between smaller diameter portions is the largest (4 mm). The above described phenomenon occurs from the following reason. Namely, the ratio of electric field energy related to the coupling between the resonators hardly changes as the width between larger diameter portions of the resonator holes are fixed on the side of the open end surface, while the ratio of magnetic field energy related to the coupling increases/decreases when the width between smaller diameter portions of the resonator holes is changed on the side of the short-circuited end surface. More specifically, with respect to the coupling between the resonators, when the width between smaller diameter portions is decreased, the ratio of magnetic field energy related to the coupling increases, thus increasing the inductive coupling strength, and when the width between smaller diameter portions is increased, the ratio of magnetic field energy related to coupling decreases, and the capacitive coupling strength increases. Therefore, as in the first embodiment, stable strong inductive coupling can be obtained without the necessity of providing a coupling trench or the like on the outer surface of dielectric block 1. Further, by appropriately setting the width between smaller diameter portions, either capacitive coupling or inductive coupling can be obtained and the coupling strength can also be adjusted. Therefore, desired filter characteristics can be readily obtained. Figs. 6A to 6C are related to the dielectric filter of a third embodiment, in which Fig. 6A is a perspective view, Fig. 6B is a front view taken from the open end surface, and Fig. 6C shows the frequency attenuation characteristics. As shown in Figs. 6A and 6B, the dielectric filter in accordance with this embodiment includes three resonator holes 2a, 2b and 2c having steps 21 in the dielectric block 1. Resonator holes 2a, 2b and 2c are provided with steps 21 approximately at the center between open end surface 1a and short-circuited end surface 1b, and the inner diameter of resonator holes 2a, 2b and 2c from open end surface 1a to the step 21 is made larger than the inner diameter of the holes from short-circuited end surface 1b to step 21. Referring to Fig. 6B, the smaller diameter portions at the side of the short-circuited end surface of resonator hole 2a serving as one input/output stage and of the resonator hole 2c positioned at the center are formed close to each other, while the smaller diameter portions of resonator hole 2b serving as another input/output stage and of resonator hole 2c at the center are formed apart from each other. More specifically, the width between smaller diameter portions of resonator holes 2a and 2c is set smallest, while the width between smaller diameter portions of resonator holes 2b and 2c is made the largest. Except this point, the dielectric filter is similar to the conventional example shown in Fig. 1, and description thereof is not repeated. In this embodiment, the coupling between two resonators formed by resonator holes 2a and 2c is the strongest inductive coupling, while the coupling between two resonators formed by resonator holes 2b and 2c is the strongest capacitive coupling. Therefore, the frequency attenuation characteristic of the filter has maximum bandwidth and two attenuation poles G L and G H formed on the low frequency side and on the high frequency side of the pass band, as shown in Fig. 6C. In the dielectric filter of the present embodiment, a coupling trench such as shown in the conventional example of Fig. 2 may be provided between the resonator holes 2a and 2c to further increase inductive coupling strength between the resonators, and hence to obtain a dielectric filter having wider pass band. Though resonator holes having larger diameter portions on the side of the open end surface and smaller diameter portions on the side of the short-circuited end surface have been described in the embodiments above, the larger diameter portions may be formed on the side of the short-circuited end surface, and the distance between smaller diameter portions on the side of the open end surface may be changed. In that case, the coupling relation between adjacent resonators is reversed to that described above. Namely, when the width of smaller diameter portions is the same as the width of larger diameter portions, the filter indicates inductive coupling, when the width of smaller diameter portions is decreased, inductive coupling becomes weaker and changes to capacitive coupling at a certain width of smaller diameter portions, and when the width of smaller diameter portions is increased, strength of inductive coupling increases. Though a dielectric filter having a pair of input/output electrodes formed at prescribed positions on the outer surface of the dielectric block has been described in the embodiments above, it is not limited thereto. A resin pin may be provided for connection to an outer circuitry, in place of the input/output electrode. Though the inner conductor and the outer conductor are isolated from each other at the side of open end surface, the inner conductor and the outer conductor may be isolated from each other on the open end surface. Further, dielectric filters consisting of two and three resonators have been described above, the filter may consist of four or more resonators. Further, a λ/4 resonator having one end of the inner conductor serving as a short-circuited surface has been described in the embodiments above, the present invention can be similarly applied when a λ/2 resonator having open end surfaces at both ends of the inner conductor serving as the resonator conductor. Further, though the inner conductor is provided on the inner surface of a through hole in the dielectric block, the resonator hole in which the inner conductor is provided may not be a through hole. Although several specific embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same are by way of illustration and example only and are not to be taken by way of limitation, the scope of the present invention being determined by the terms of the appended claims.

Claims: A dielectric filter, comprising: a dielectric block (1) having a pair of opposing surfaces (1a, 1b); a plurality of substantially parallel resonator holes (2a, 2b, 2c) piercing through at least an end surface of said dielectric block, each resonator hole consisting of a larger inner diameter portion along a first axis and a smaller inner diameter portion along a second axis, with the second axis being substantially parallel to the first axis; an outer conductor (4) formed on an outer surface of said dielectric block; and an inner conductor (3) formed on an inner surface of each of said plurality of resonator holes, and characterised in that in each of adjacent resonator holes (2a, 2b, 2c), the second axis is offset from the first axis. The dielectric filter according to claim 1, wherein the distance between the second axes of adjacent resonator holes (2a, 2b, 2c) is smaller than the distance between the first axes of said adjacent resonator holes. The dielectric filter according to claim 1, wherein the distance between the second axes of adjacent resonator holes (2a, 2b, 2c) is larger than the distance between the first axes of said adjacent resonator holes. The dielectric filter according to claim 1, comprising three substantially parallel resonator holes (2a, 2b, 2c); and wherein the distance between the second axes of a first pair (2a, 2c) of the adjacent resonator holes is smaller than the distance between the first axes of the first pair (2a, 2c) of adjacent resonator holes, and the distance between the second axes of a second pair (2b, 2c) of adjacent resonator holes is larger than the distance between the first axes of the second pair (2b, 2c) of adjacent resonator holes. The dielectric filter according to any preceding claim, wherein said plurality of inner conductors (3) have one end electrically isolated from the outer conductor (3) to serve as an open-circuited end, and another end connected to said outer conductor to serve as a short-circuited end. The dielectric filter according to claim 5, wherein a ring-shaped non-conducting portion, at which the inner conductor (3) is not provided, formed near one end of said plurality of inner conductors (3), which electrically insulates said outer conductor (4) from said inner conductor (3). The dielectric filter according to any preceding claim, wherein said outer conductor (4) is formed on an outer peripheral surface of the dielectric block except at one end surface (1a) of said pair of end surfaces (1a, 1b) of the dielectric block, said one end surface (1a) serving as an open end.

Inventors: TADA HITOSHI, TSUJIGUCHI TATSUYA, TADA, HITOSHI, TSUJIGUCHI, TATSUYA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H01P1
IPC codes: H01P7, H01P1