Dielectric filter

Abstract: In a dielectric filter having plural resonator cavities and at least one resonator cavity comprising a large inner-diameter portion, a small inner-diameter portion and a step portion formed between the large and small portions, an inner conductor being formed on the inner surface of each of the resonator cavities and an outer conductor being formed on the outer surface of the dielectric block. The large portion and the small portion of the at least one resonator cavity having the step portion are designed to have different sectional shapes, and may be either coaxial or non-coaxial.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric filter, and particularly to a dielectric filter in which a plurality step of which contain resonator holes having step portions are formed in a single dielectric block. 2. Description of Related Art Conventionally, there has been known a dielectric filter in which a step portion is formed in a resonator cavity, forming two resonant portions having different line impedance, with the step portion at the boundary thereof, thereby obtaining a desired filter characteristic. Figs. 1A and 1B show the construction of a conventional dielectric filter in which resonator cavities having step portions as described above are formed. Fig. 1A is a perspective view of the dielectric filter, taken from the open-circuited end surface of the dielectric filter, and Fig. 1B is a plan view showing the dielectric filter, also taken from the open-circuited end surface. The dielectric filter shown in Figs. 1A and 1B comprises a dielectric block 1 having a substantially rectangular parallelepiped shape, and resonator cavities 2a and 2b which are formed in the dielectric block 1, so as to penetrate through a pair of confronting surfaces of the dielectric block 1. Each resonator cavity has an inner conductor 3 formed on the inner surface thereof. Further, input/output electrodes 5 are formed on the outer surface of the dielectric block 1, and an outer conductor 4 is formed substantially over the whole surface of the dielectric block 1 except for the areas at which the input/output electrodes 5 are formed. At one end surface 1a of the dielectric block 1 (hereinafter referred to as the "open-circuited end surface"), a portion at which no inner conductor 3 is formed (hereinafter referred to as the "non-conductor portion"), is provided at one end portion of each of the resonator cavities 2a, 2b which is in the neighborhood of the end surface 1a as shown in Fig. 1A. Thus the inner conductor 3 formed in each resonator cavity 2a, 2b is separated from the outer conductor 4 by this non-conductor portion. On the other hand, at the opposite end surface 1b of the dielectric block 1 (hereinafter referred to as the "short-circuited end surface), each inner conductor 3 is short-circuited to the outer conductor 4. In the dielectric filter thus constructed, a step portion 21 is provided substantially at the center portion between the open-circuited end surface 1a and the short-circuited end surface 1b in each resonator 2a, 2b, and these resonators 2a and 2b are designed so that the inner diameter thereof at the end surface 1a is larger than that at the end surface 1b. Hereinafter, the portion of each resonator cavity 2a, 2b which has the larger inner diameter is referred to as the "large inner-diameter portion", and the other portion of the resonator cavity which has the smaller inner diameter is referred to as the "small inner-diameter portion". In this structure, the large inner-diameter portion is formed at the side of the open-circuited end surface 1a, and the coupling between both resonators is ordinarily strong capacitive coupling, so that a filter characteristic having a broad pass band and an attenuation pole at a low side of the pass band is obtained. Further, the resonant frequency of each resonator which is formed in each resonator cavity 2a, 2b, and the coupling degree of the resonators, can be varied by changing the ratio of the length of the large inner-diameter portion and the length of the small inner-diameter portion of the resonator cavity 2a or 2b and the ratio of the inner diameters of the resonator cavities 2a and 2b, thereby obtaining a desired filter characteristic. However, in the conventional dielectric filter as described above, the large inner-diameter portions and the small inner-diameter portion of the resonator cavities 2a, 2b are designed to have a circular cross-sectional shape, and the center axes thereof are disposed coaxially. This places restrictions on the self-capacitance which is formed between the inner conductor 3 and the outer conductor 4, and the mutual capacitance which is formed between the neighboring inner conductors 3, and thus the degree of freedom in the design of a desired filter characteristic is low. That is, it is difficult to obtain various filter characteristics in a dielectric block 1 having a required body size. In other words, it is difficult to design the dielectric block 1 so that it has a desired body size and also to obtain required filter characteristics. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a dielectric filter which can overcome the problem of the conventional dielectric filter as described above, and which can enhance the degree of freedom in design of the resonance frequency and the coupling degree between resonators to thereby easily obtain a desired filter characteristic. A filter according to the preamble of claims 1 and 2 is known from patent document EP-A-0641 035 (of figures 9a and 9b). The invention is defined in claims 1 and 2. Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1A is a perspective view of a conventional dielectric filter: Fig. 1B is a plan view of the dielectric filter of Fig. 1A, which is taken from the side of an open-circuited end surface; Fig. 2 is a plan view showing a dielectric filter; Fig. 3 is a plan view showing a dielectric filter; Fig. 4 is a plan view showing a dielectric filter; Fig. 5 is a plan view showing a dielectric filter; Fig. 6 is a plan view showing a dielectric filter according to the first embodiment of the present invention; Fig. 7 is a plan view showing a dielectric filter according to a second embodiment of the present invention; Fig. 8 is a plan view showing a dielectric filter according to a modification of the second embodiment of the present invention; and Fig. 9 is a plan view showing a dielectric filter according to another modification of the second embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Preferred embodiments according to the present invention will be described hereunder with reference to the accompanying drawings in which like reference numerals denote like elements and parts. In these embodiments, the large inner-diameter portion and the small inner-diameter portion of the resonator having the step portion are designed to have different sectional shapes. The structures other than the sectional shapes of the resonator cavities in the following embodiments are substantially identical to those of the conventional resonator cavities shown in Fig. 1A, and the description thereof is omitted. Fig. 2 is a plan view of a dielectric filter , which is taken from the side of the open-circuited end surface thereof. As shown in Fig. 2, the large inner-diameter portion of each through-hole 2a or 2b, having the step portion 21, is designed to have an elongated cross-section having two parallel sides and two arcuate sides, that is, it is designed approximately in an elliptical sectional shape. On the other hand, the small portion of each through-hole 2a, 2b is designed with a circular sectional shape, Further, the large portion is designed so that its longer diameter is parallel to the thickness (height) direction of the dielectric block 1, and the center axes of the large portion and the small portion are collinear with each other. In this structure, since the large portion is designed approximately in an elliptical sectional shape, the distance between the outer conductor and the large portion of each through-hole 2a or 2b is shorter than in the conventional structure. Therefore, as compared with the conventional dielectric filter shown in Figs. 1A and 1B, the self-capacitance at the open-circuited end can be made larger, and the mutual capacitance can be made larger because the confronting surfaces of the large portions of the neighboring through-holes 2a and 2b are large in area. That is, the self-capacitance at the side of the open-circuited end surface can be increased. Thus, the line impedance at the resonance portion of the open-circuited end surface can be reduced, so that the resonance frequency can be lowered. Conversely, in order to obtain a desired resonance frequency, the length (axial length) of the dielectric block can be shortened, and thus miniaturization of the dielectric filter can be promoted. Further, by increasing the mutual capacitance at the open-circuited end, the degree of capacitive coupling between the resonators can be further enhanced. Therefore, it is unnecessary to extremely shorten the distance between the large portions in order to obtain a desired coupling degree, so that a filter characteristic which is stable and small in characteristic fluctuation can be obtained without reducing Q-value. Fig. 3 is a plan view showing a dielectric filter which is taken from the side of the open-circuited end surface. In the dielectric filter of this embodiment, the small inner-diameter portion of each through-hole 2a, 2b, having the step portion 21, is designed approximately in an elliptical sectional shape, and the large inner-diameter portion of each through-hole 2a, 2b is designed in a circular sectional shape. The longer diameter of the inner-diameter small portion is parallel to the thickness (height) direction of the dielectric block 1, and the center axes of the inner-diameter large portion and the small portion are collinear with each other. In this structure, the small portion formed at the side of the short-circuited end surface is designed approximately in an elliptical sectional shape, and thus the distance between the outer conductor and the small portion of each resonator cavity 2a, 2b is shortened. Therefore, as compared with the conventional dielectric filter shown in Fig. 1A, the self-capacitance at the short-circuited end can be made larger. Further, the mutual capacitance can also be made larger because the confronting surfaces of the small portions of the neighboring resonator cavities 2a and 2b are large in area. That is, contrary to the first embodiment, the self-capacitance and the mutual capacitance at the side of the short-circuited end surface can be increased. Therefore, the resonance frequency can be heightened, and the dielectric coupling degree is enhanced, so that the degree of the capacitive coupling can be lowered as a whole. In the filters according to figures 2 and 3, the longer diameter of the elliptical portion of each resonator cavity is parallel to the thickness direction of the dielectric block 1. However, the long-diameter direction may also be parallel to the width direction of the dielectric block 1 as shown in Figs. 4 and 5 or may be oblique with respect to the thickness direction and the width direction of the dielectric block 1 as shown in Fig. 6. Although the small portions are arranged obliquely in the embodiment of Fig. 6, the large portions could be arranged obliquely as well. In the structure shown in Fig. 4, the long-diameter direction of the large inner-diameter portion formed at the side of the open-circuited end surface is set to be parallel to the width direction of the dielectric block 1, and thus the self-capacitance and the mutual capacitance at the open-circuited end can be increased. In the structure shown in Fig. 5, the long-diameter direction of the small inner-diameter portion formed at the side of the short-circuited end surface is set to be parallel to the width direction of the dielectric block 1, and thus the self-capacitance and the mutual capacitance at the short-circuited end can be increased. In the structure shown in Fig. 6, the self-capacitance and the mutual capacitance at the small inner-diameter portion can be varied to various values by changing an oblique (intersectional) angle of the long-diameter direction with respect to the thickness (height) direction of the dielectric block 1. Fig. 7 is a plan view showing a dielectric filter according to a second embodiment of the present invention, which is taken from the side of the open-circuited end surface of the dielectric filter. In the dielectric filter of this embodiment, the long-diameter direction of the elliptical large portions of the resonator cavities 2a and 2b is set parallel to the thickness direction of the dielectric block 1, and the circular small portions of the throug-holes 2a and 2b are formed so as to be spaced away from each other in the thickness direction of the dielectric block 1 as shown in Fig. 7. That is, the center axis of the small portion of each of the through-holes 2a, 2b is eccentrically displaced from the center axis of the corresponding large portion of the through-hole, whereby the small portion of the through-hole 2a is eccentrically formed at the upper side of the dielectric block 1, while the small portion of the through-hole 2b is eccentrically formed at the lower side of the dielectric block 1 as shown in Fig. 7. In this structure, the distance between the outer conductor and the small portion is shorter, so that the self-capacitance at the short-circuited end surface can be made larger, and thus the resonance frequency can be heightened. Further, the distance between the small portions is larger, so that the mutual capacitance at the short-circuited end can be made smaller. Therefore, the degree of inductive coupling is reduced, and the degree of capacitive coupling can be set to a larger value than that of the filter of figure 2. The eccentric orientation of the small portions is not limited to that shown in Fig. 7. As shown in Figs. 8 and 9, the long-diameter direction of the large portions may be set parallel to the width direction of the dielectric block with the circular small portions beeing arranged eccentrically in the width direction as shown in Figs. 8 and 9. In the structure shown in Fig. 8, the distance between the respective axes of the small inner-diameter portions formed at the side of the short-circuited end is as short as possible, so the mutual capacitance at the short-circuited end can be set to a large value. Therefore, the degree of inductive coupling energy can be made higher than the capacitive coupling energy as a whole. That is, the coupling between the resonators can be made inductive coupling, and thus an attenuation pole can be formed at a high side of the pass band. In the structure shown in Fig. 9, the distance between the respective axes of the small inner-diameter portions formed at the side of the short-circuited end is as long as possible, and thus strong capacitive coupling can be obtained. In the dielectric filter of each embodiment as described above, the large inner-diameter portion of the resonator cavity is at the side of the open-circuited end surface. However, the large inner-diameter portion may also be formed at the side of the short-circuited end surface. In this case, variations of the resonance frequency and the coupling type (capacitive coupling or inductive coupling) are substantially converse to those as described above. Furthermore, in the embodiments as described above, either the large portion or the small portion is designed to have an approximately elliptical shape in section, and the other portion is designed to have a circular shape in section. However, the sectional shapes of the through-holes are not limited to the above shapes, and any shape may be adopted insofar as the shapes of the large portion and the small portion are different. Still furthermore, in the embodiments as described above, the dielectric filter has two through-holes each having a step portion, which are formed in the dielectric block. However, the dielectric filter may have three or more through-holes. More generally, this invention is applicable to any dielectric filter in which a plurality of through-holes are formed in a single dielectric block, in which at least one through-hole has a step portion. As described above, according to the dielectric filter of the present invention, the large portion and the small portion of the through-hole having the step portion are designed to have different sectional shapes, whereby the self-capacitance and the mutual capacitance at the large portion and/or the small portion can be set to various values. Therefore, the degree of freedom in design of filter characteristics, such as the resonance frequency, the coupling degree between resonators, the coupling type, etc. can be improved, and various and excellent filter characteristics can be obtained by using a dielectric filter having a desired body size. Furthermore, the adjustable range of the self-capacitance and the mutual capacitance can be broadened because the center axis of the small portion can be shifted from the center axis of the large portion in an up-and-down, a right-and-left or an oblique directiion, so the degree of freedom of design in filter characteristics can be further improved.

Claims: A dielectric filter, comprising a dielectric block (1), said dielectric block extending in a thickness direction, a width direction, and a third direction, said directions being perpendicular to each other; a plurality of throug-holes formed in said dielectric body (1) and extending in the third direction; an inner conductor (3) formed on the inner surface of each of said throug-holes; and an outer conductor (4) formed on the outer surface of said dielectric block (1); wherein at least two through hole (2a, 2b) comprise an large inner-diameter portion, an small inner-diameter portion and a step portion (21) formed between said inner-diameter large and small portions, wherein said large inner-diameter portion and said small inner-diameter (8) portion of said at least two through hole (2a, 2b) having said step portion (21) have different sectional shapes and are substantially coaxial, characterized in that one of said large and small portions has an elongated cross-sectional shape and wherein said portion with elongated cross-sectional shape has a longest diameter extending obliquely with respect to said thickness direction and said width direction of said dielectric block (1). A dielectric filter, comprising a dielectric block (1), said dielectric block extending in a thickness direction, a width direction, and a third direction, said directions being perpendicular to each other; a plurality of through hole formed in said dielectric body (1) and extending in the third direction; an inner conductor (3) formed on the inner surface of each of said through hole; and an outer conductor (4) formed on the outer surface of said dielectric block (1); wherein at least two through hole (2a, 2b) comprise an large inner-diameter portion, an small inner-diameter portion and a step portion (21) formed between said inner-diameter large and small portions, wherein said large inner-diameter portion and said small inner-diameter (8) portion of said at least two through hole (2a, 2b) having said step portion (21) have different sectional shapes, characterized in that said large and small portions of said stepped through hole are non-coaxial, wherein said small portion of said stepped through holeis displaced eccentrically from said large portion along said thickness direction of said dielectric block (1). The dielectric filter of claim 2, wherein said large portion has an elongated cross-sectional shape with a longest diameter extending along said width direction of said dielectric block (1). The dielectric filter of claim 1, wherein the other of said large and small portions has a substantially circular cross-sectional shape. The dielectric filter of claim 1, wherein said portion with elongated cross-sectional shape has a longest diameter extending along the thickness direction of said dielectric block (1). The dielectric filter of any of the claims 1 to 5, wherein said large portion is at an open-circuited end of said through hole (2a, 2b) at which said inner conductor (3) is isolated from said outer conductor (4).

Inventors: MATSUMOTO HARUO, TODA JUN, MATSUMOTO, HARUO, TODA, JUN
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H01P1
IPC codes: H01P1