Dielectric filter and process for producing same

Details Dielectric filter and process for producing same Dielectric filter and process for producing same

2000-09-11

2001-09-25

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S206000, C333S222000

Reexamination Certificate

active

06294968

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to dielectric filters chiefly for use with high-frequency signals of hundreds of megahertz to a few gigahertz.
DESCRIPTION OF THE RELATED ART
Various high-frequency filters are used in communications devices. Compactness and improved electrical characteristics are required of filters with prevalent use of cellular or portable telephones and like mobile communications devices. The conventional filters for use at high frequencies include dielectric filters comprising coaxial dielectric resonators as disclosed, for example, in JP-A-283904/1994. The dielectric filter is of the surface mount type which is mountable directly on the surface of a circuit board so as to be compacted.
FIG. 12
is an exploded perspective view showing the dielectric filter of the surface mount type. The filter comprises a plurality of (two in the illustration) coaxial dielectric resonators
100
arranged side by side in contact with each other on a base
7
.
FIG. 13
is a perspective view showing the coaxial dielectric resonator
100
. The resonator
100
comprises a dielectric body
2
made of a dielectric material and formed with a through-bore
22
extending through opposite end faces
20
,
21
of the body
2
. An outer conductor layer
30
, an inner conductor layer
31
and an end conductor layer
32
arc provided respectively on the outer peripheral surface of the dielectric body
2
, the inner surface of the body defining the bore
22
and one of the end faces,
21
, of the body
2
. As shown in
FIGS. 12 and 13
, the resonator
100
has a bottom surface
23
formed by partly removing the outer conductor layer
30
. The adjoining surfaces of the adjacent resonators
100
,
100
arc locally removed to form interstage coupling windows
10
,
10
. In the case of the surface mount type, the resonator
100
is generally in the form of a rectangular parallelepiped so as to be mounted on the base
7
with case.
As shown in
FIG. 12
, the base
7
has strip lines
81
,
81
for external connection and a grounding pattern
80
which are formed on the upper surface of a dielectric base plate
70
. Each of the strip lines
81
on the upper surface of the base plate
70
extends from an inward region of the plate surface to be in contact with the bottom surface
23
of the resonator
100
to the region of the plate surface to be in contact with the exposed end face
20
of the resonator
100
. The grounding pattern
80
is formed on the upper surface of the base plate
70
except inward regions of the plate surface to be contact with the bottom surfaces
23
,
23
of the resonators
100
,
100
and the regions thereof around the strip lines
81
,
81
.
The resonators
100
,
100
and the base
7
thus constructed are so arranged that the resonators
100
,
100
are adjacent to each other and positioned in place on the base
7
. In this arrangement, the resonators
100
,
100
are electromagnetically coupled to each other through the interstage coupling windows
10
,
10
, and the strip lines
81
,
81
are capacitance-coupled respectively to the inner conductor layers
31
,
31
of the resonators
100
,
100
, whereby a band-pass filter is provided.
While the filter of
FIG. 12
comprises two coaxial dielectric resonators
100
,
100
, at least one coaxial dielectric resonator formed with interstage coupling windows
10
,
10
in opposite side surfaces thereof can be disposed between the two resonators
100
,
100
adjacent thereto. Generally, the greater the number of resonators joined, the greater is the attenuation of output electric power of signals at the boundary between the pass band and the outside thereof.
To ensure matching between the adjoining resonators
100
,
100
of the surface mount dielectric filter thus constructed, the exposed end face
20
of each resonator
100
is adjusted by trimming to obtain a complete product. The completed filter is mounted on a circuit board, with the rear surface of the base in contact with the surface of the circuit board.
To fulfill the requirement that the surface mount dielectric filter be made more compact, it is desired to mount the coaxial resonators only on the circuit board with the base
7
omitted.
In this case, there arises a need to bond the adjacent resonators to each other. However, coaxial dielectric resonators for use with high-frequency signals have a very small axial length of several millimeters and are therefore difficult to bond as positioned properly.
Furthermore, the coaxial dielectric resonators serving respectively as an input stage and an output stage must be provided with external connection electrodes as separated from the outer conductor layers of the resonators, in place of the strip lines
81
,
81
on the base
7
. Generally these electrodes are formed near the exposed end faces of the respective resonators so as to be coupled strongly with their inner conductor layers. The resonators serving as the input stage and the output stage are then substantially shortened in resonance wavelength and become higher in resonance frequency. In the case of the dielectric filter comprising at least three adjacent coaxial dielectric resonators of the same size, the input-stage and output-stage resonators then have a resonance frequency higher than, and failing to match, that of the intermediate-stage resonator, resulting in impaired filter characteristics.
An approximate match in resonance frequency can be obtained by making the intermediate-stage resonator smaller than the other resonators in axial length, whereas the different axial lengths of the resonators result in difficulty in bonding the adjacent resonators in position.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dielectric filter comprising adjacent coaxial dielectric resonators which can be bonded to one another as readily positioned in place, and a process for producing the filter.
Another object of the invention is to provide a dielectric filter which comprises at least three coaxial dielectric resonators having the same axial length and in which the resonance frequency of the input-stage and output-stage resonators can be approximately matched to that of the intermediate-stage resonator, and a process for producing the filter.
To accomplish these objects, the present invention provides a process for producing a dielectric filter comprising a plurality of coaxial dielectric resonators as arranged side-by-side in contact with one another, the process including the steps of preparing the coaxial dielectric resonators each comprising a dielectric body made of a dielectric material and having a through bore extending through opposite end faces thereof, the dielectric body being provided with an outer conductor layer and an inner conductor layer respectively on an outer peripheral surface thereof and an inner surface thereof defining the bore; forming interstage coupling windows by removing the outer conductor layer from the same region of each of adjoining surfaces of the resonators to be adjacent to each other; and bonding the resonators as arranged side-by-side by applying an electrically conductive bonding material to at least one of the outer conductor layers remaining on the adjoining surfaces, placing the resonators in proximity to each other on a smooth-surfaced table and allowing the surface tension of the bonding material to draw the resonators toward each other.
When the coaxial dielectric resonators are arranged side-by-side on the smooth-surfaced table for bonding according to the invention, the surface tension of the bonding material produces between the adjacent resonators a force acting to draw them toward each other. Accordingly, when the outer conductor layers on the respective opposed adjoining surfaces are in the same region, the surface tension of the bonding material
4
exerts a force acting to draw the outer conductor layers
300
,
301
of the adjoining surfaces toward each other as shown in
FIG. 1B
even if the portions to be bonded of the resonators
1
,
1
are somewhat shifte

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