Dielectric filter having obliquely oriented stepped resonators

A dielectric filter having plural resonator cavities in a dielectric block, at least one resonator cavity comprising a large inner-diameter portion, a small inner-diameter portion and a diameter-changing portion formed between the large and small portions, an inner conductor being formed on the inner surface of each of the resonator cavities. The large and small inner-diameter portions of the stepped resonator cavity may have different sectional shapes, and may be either coaxial or non-coaxial. One of the portions may have an elongated cross-sectional shape and/or be arranged obliquely with respect to the dielectric block. One of the portions may moreover be arranged eccentrically with respect to the other portion.

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 of resonator cavities 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. As seen in FIG. 1A, 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 (see FIG. 1A) 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 (see FIG. 1B) 
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 
A feature 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. 
According to a first aspect of the present invention, in order to attain 
the above object, a dielectric filter having plural resonator cavities 
contains 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 is formed on the inner 
surface of each of the resonator cavities and an outer conductor is formed 
on the outer surface of the dielectric block. The large inner-diameter 
portion and the small inner-diameter portion of the resonator cavity 
having the step portion are designed to have different cross-sectional 
shapes. 
Since the large and small portions are designed to have different 
cross-sectional shapes, the self-capacitance and the mutual capacitance at 
the large portion and/or the small portion can be set to various values. 
That is, the degree of freedom in design of the resonance frequency and 
the coupling degree is enhanced, and various filter characteristics can be 
obtained by using a dielectric filter having a desired body size. 
According to a second aspect of the present invention, in the dielectric 
filter of the first aspect of the present invention, the center axis of 
the small portion of the resonator cavity having the step portion is 
eccentrically deviated from the center axis of the large portion. 
The axis of the small portion is eccentrically deviated from the axis of 
the large portion so as to broaden an adjustable range of the 
self-capacitance and the mutual capacitance. 
According to a third aspect of the invention, either the large or small 
inner-diameter portion may be circular in cross-section and the other 
portion non-circular. 
According to a fourth aspect of the invention, the large inner-diameter 
portion may be non-circular in cross-section, and the small inner-diameter 
portion either circular or non-circular. 
The third and fourth aspects of the invention further enhance the freedom 
of design of the resonance frequency, coupling degree, range of 
self-capacitance and mutual capacitance, and other electrical filter 
characteristics. 
Other features and advantages of the present invention will become apparent 
from the following description of the invention which refers to the 
accompanying drawings.

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, which may not be 
described in detail in some of the drawing descriptions. 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 according to a first 
embodiment of the present invention, disclosed in U.S. Pat. No. 5,742,214, 
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 resonator cavity 
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 resonator cavity 2a, 2b is designed with a 
circular sectional shape. Further, the large portion is designed so that 
its longer diameter is parallel tc 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 resonator cavity 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 resonator cavities 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 according to a second 
embodiment of the present invention, disclosed in U.S. Pat. No. 5,742,214, 
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 resonator cavity 2a, 2b, having the step portion 21, is designed 
approximately in an elliptical sectional shape, and the large 
inner-diameter portion of each resonator cavity 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 first and second embodiments, 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 third 
embodiment of the present invention, disclosed in U.S. Pat. No. 5,742,214, 
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 resonator cavities 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 resonator cavities 2a, 2b is eccentrically 
displaced from the center axis of the corresponding large portion of the 
resonator cavity, whereby the small portion of the resonator cavity 2a is 
eccentrically formed at the upper side of the dielectric block 1, while 
the small portion of the resonator cavity 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 first embodiment. 
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 being 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 resonator 
cavities 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 resonator cavities each having a step portion, which are 
formed in the dielectric block. However, the dielectric filter may have 
three or more resonator cavities. More generally, this invention is 
applicable to any dielectric filter in which a plurality of resonator 
cavities are formed in a single dielectric block, in which at least one 
resonator cavity 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 resonator cavity 
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 direction, so the degree of 
freedom of design in filter characteristics can be further improved. 
In the structures shown in FIGS. 10-14, the large inner-diameter portions 
have an elongated cross-sectional shape. The longer-diameter direction of 
the large inner-diameter portions formed adjacent to the open-circuited 
end surface is set at an angle with respect to the upper surface of the 
dielectric resonator 1. The mutual capacitance between the resonator 
cavities 2a and 2b can be adjusted by changing the angle. 
Further, the resonance frequency of the resonator 1 can also be changed by 
adjusting the positional relation between the small-diameter portions 
formed adjacent to the short-circuited end surface, as shown in FIGS. 
10-12. In FIGS. 10-12, the smaller inner-diameter portions are circular in 
cross-section. 
In the structures shown in FIGS. 13 and 14, the small inner-diameter 
portions formed adjacent to the short-circuited end surface have an 
elongated shape and are set either at an angle or perpendicular to the 
upper surface of the dielectric resonator 1. The mutual capacitance 
between the resonator cavities 2a and 2b can also be adjusted by changing 
the angle. Accordingly, the degree of freedom in design of the resonance 
frequency can be further enhanced. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. Therefore, the 
present invention is not limited by the specific disclosure herein, but 
only by the appended claims.