BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to a method of forming
electrodes of a dielectric filter comprising a plurality of
resonators disposed in parallel.
Description of the Related Art:
There have been proposed various kinds of dielectric
filters in which inner surfaces of through-holes of a
dielectric ceramic block are coated with inner conductors so
as to form a plurality of resonators in parallel, and the
outer surface of the dielectric ceramic block is coated with
an outer conductor except for the open end surface to which
the through-holes are opened.
On the open end surface of such a dielectric filter,
auxiliary conductors extend from the edges of open ends of,
for example, two adjacent resonators such that an isolation
gap is formed between facing edges of the auxiliary
conductors for inter-stage capacitance coupling of the
resonators. Moreover, input/output pads are formed on a side
surface at positions facing the open ends of the resonators
such that the input/output pads are electrically insulated
from the outer conductor. If necessary, extension conductors
are extended from the input/output pads toward the open end
surface, so that isolation gaps are formed between the
extension conductors and the auxiliary conductors. As a
result, the input/output pads are connected to the resonators
through the capacitances of the isolation gaps. In the
dielectric filter, as described above, various kinds of
conductor pattern are formed.
Conventionally, the above-described conductor pattern
is formed by applying an electrode-forming material into a
desired shape or pattern through use of screen pattern
printing. However, use of the screen pattern printing causes
generation of blur at the boundary of the pattern or
patchiness due to variation in the concentration of ink.
Moreover, if positioning of a screen is inadequate, the
position of the conductor pattern relative to the block
changes slightly, resulting in positional deviation. As a
result, the characteristics of resulting filters greatly vary,
requiring a step of adjusting the characteristics, thus
leading to increased production time, difficulty in mass-production,
and a low yield.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present
invention is to provide an improved method of forming
electrodes of a dielectric filter in which the electrodes can
be formed stably in a desired pattern.
To achieve the above object, the present invention
provides an improved method of forming electrodes of a
dielectric filter in which inner surfaces of through-holes of
a dielectric ceramic block are coated with inner conductors
so as to form a plurality of resonators in parallel; the
outer surface of the dielectric ceramic block is coated with
an outer conductor except for the open end surface to which
the through-holes are opened; auxiliary conductors are formed
on the open end surface of the dielectric filter such that
they are connected to the inner conductors of the respective
resonators; and input/output pads are formed on a side
surface at positions facing the open ends of outermost
resonators such that the input/output pads are electrically
insulated from the outer conductor. The method comprises the
steps of: forming a ceramic block having through holes and
concave portions, the positions and planar shapes of the
concave portions corresponding to the positions and planar
shapes of the auxiliary conductors to be formed on the open
end surface of the ceramic block; forming an electrode layer
over the entire open end surface; and grinding the open end
surface to a depth smaller than the depth of the concave
portions so as to leave the electrode layer within only the
concave portions.
In the case where extension conductors are formed to
extend from the input/output pads toward the open end surface,
concave portions having planar shapes corresponding to the
shapes of the respective extension conductors are formed at
positions corresponding to the positions of the respective
extension conductors, in a manner similar to the case of the
auxiliary conductors. Subsequently, the electrode layer
forming step and the grinding step are carried out.
In the method according to the present invention, since
parts of the electrode layer are left in the concave portions
through the step of forming an electrode layer over the
entire open end surface through dipping and the step of
grinding the open end surface, the shapes of the auxiliary
conductors and/or extension conductors can be defined
accurately by the concave portions. Accordingly, unlike the
case of screen printing, neither blur nor patchiness occurs,
thereby enabling stable pattern formation.
The above-described concave portions may be formed
through press forming performed simultaneously with formation
of the peripheral shape of the block. In this case, since
the positions and shapes of the concave portions are defined
by dies, the concave portions can be formed with high
accuracy and can be formed at desired positions with improved
reproducibility.
Alternatively, the above-described concave portions may
be formed by milling or the like after the ceramic block is
sintered.
Accordingly, the input/output coupling capacitances and
the inter-stage coupling capacitance become constant, so that
dielectric filters having stable filter characteristics can
be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and many of the
attendant advantages of the present invention will be readily
appreciated as the same becomes better understood by
reference to the following detailed description of the
preferred embodiment when considered in connection with the
accompanying drawings, in which:
FIG. 1 is a perspective view of a dielectric filter; FIG. 2 is a plan view of the dielectric filter shown in
FIG. 1; FIGS. 3A - 3C are vertically sectioned views of an
upper portion of the dielectric filter, showing the steps of
a method of forming conductors according to an embodiment of
the present invention; and FIG. 4 is an equivalent circuit of the dielectric
filter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be
described with reference to the drawings. Identical portions
are indicated by the same reference symbols throughout the
drawings for the purpose of simplifying the description.
FIGS. 1 and 2 show a two-stage type dielectric filter 1
having two resonators 3. The dielectric ceramic block 2 of
the filter 1 is made of a titanium oxide ceramic and
substantially has the shape of a rectangular prism. The
resonators 3 are formed parallel to each other. The
resonators 3 are formed by coating the through-holes 4 with
inner conductors 5, and the outer surface of the dielectric
ceramic block 2 is coated with an outer conductor 7 except
for an open end surface 8 to which the through-holes 4 are
opened. The outer conductor 7 serves as a shield electrode.
The resonators 3 have a length corresponding to λ/4, where λ
is a resonant frequency.
Next, a description will be given of the main portion
of the present invention.
As shown in FIG. 1, on the open end surface 8 of the
dielectric ceramic block 2, auxiliary conductors 6 for inter-stage
coupling extend from the open ends of the two adjacent
resonators 3 such that auxiliary conductors 6 for inter-stage
coupling are connected to the inner conductors 5 of the
through-holes 4. The resonators 3 are coupled with each
other through the capacitance C1 (see FIG. 4) of an isolation
gap g1 formed between the auxiliary conductors 6 for inter-stage
coupling.
Moreover, input/output pads 9 are formed on a side
surface of the dielectric ceramic block 2 at positions facing
the open ends of the resonators 3 such that the input/output
pads 9 are electrically insulated from the outer conductor 7.
The input/output pads 9 enable electrical connection with
electrical paths on a printed circuit board. Extension
conductors 10 are extended from the input/output pads 9
toward the open end surface so that isolation gaps g2 are
formed between the extension conductors 10 and the auxiliary
conductors 6. As a result, the input/output pads 9 are
connected to the auxiliary conductors 6 through the
capacitances Co of the isolation gaps g2 (see FIG. 4). When
the dielectric filter 1 includes three or more resonators,
the input/output pads 9 are formed such that they face the
outermost resonators.
The auxiliary conductors 6 and the extension conductors
10 are formed through the method illustrated in FIGS. 3A - 3C.
First, powder of a titanium oxide is placed in a die,
and press forming is carried out in order to obtain a green
block 2' having a peripheral shape corresponding to that of
the dielectric ceramic block 2. At this time, as shown in
FIG. 3A, concave portions 20a' and 20b' are formed in the
open end surface 8' of the green block 2' at positions
corresponding to the positions of the respective auxiliary
conductors 6 and the respective extension conductors 10 to be
formed on the open end surface 8 of the dielectric ceramic
block 2. The concave portions 20a' and 20b' have planar
shapes corresponding to the shapes of the respective
auxiliary conductors 6 and the respective extension
conductors 10.
Upon removal of the green block 2' from the die,
through-holes 4' are formed, followed by a sintering process.
As a result, the dielectric ceramic block 2 is obtained.
Subsequently, the dielectric ceramic block 2 is dipped in an
electrode material paste in order to form an electrode layer
21 over the entire surface, as shown in FIG. 3B
Subsequently, as shown in FIG. 3C, the open end surface
8 is subjected to grinding operation. This grinding
operation is carried out such that the grinding depth is less
than the depth of the concave portions 20a and 20b (see line
I-I in FIG. 3B). As a result, the electrode layer 21
covering the open end surface 8 of the dielectric ceramic
block 2 is removed such that the electrode layer 21 in the
concave portions 20a and 20b are left unground. The portions
of the electrode layer 21 remaining in the concave portions
20a and 20b serve as the auxiliary conductors 6 and the
extension conductors 10, respectively. Subsequently, in
order to separate the input/output pads 9, which are integral
with the extension conductors 10, from the outer conductor 7,
square-C-shaped exposed portions 23 are formed through
partial removal of the outer conductor 7.
Since the above-described concave portions 20a' and
20b' are formed through press forming performed
simultaneously with formation of the peripheral shape of the
green block 2', the positions of the concave portions 20a'
and 20b' become always constant with respect to the
peripheral shape of the green block 2', so that the concave
portions 20a' and 20b' can be formed with improved
reproducibility. Accordingly, the auxiliary conductors 6 and
the extension conductors 10 formed in the concave portions
20a and 20b are defined by the concave portions 20a and 20b,
so that the auxiliary conductors 6 and the extension
conductors 10 can be formed with high accuracy and at desired
positions. Moreover, since the auxiliary conductors 6 and
the extension conductors 10 can be formed through dipping,
unlike the case of screen printing, the conductor pattern does
not cause patchiness, and the boundary of the pattern becomes
clearer and does not become blurred. Since none of blur,
patchiness, and positional shift occurs, the pattern can be
formed with high accuracy, thereby enabling the stable
generation of capacitance coupling.
Moreover, since the concave portions 20a' and 20b' are
formed through press forming performed simultaneously with
formation of the peripheral shape of the green block 2', no
additional step is required, so that the manufacture of the
dielectric filter can be facilitated.
The above-described manufacturing method may be
modified such that after the press forming of the green block
2', the concave portions 20a' and 20b' are formed by milling
or the like simultaneously with the formation of the through-holes
4'. In this case as well, the auxiliary conductors 6
for inter-stage coupling can be formed without causing blur
or patchiness.
Moreover, the concave portions 20a' and 20b' can be
formed by milling or the like after the green block 2' is
sintered.
The extension conductors 10 extended from the
input/output pads 9 are not necessarily required, and the
auxiliary conductors 6 may be extended further such that the
auxiliary conductors 6 are coupled directly with the
input/output pads 9 via insulation gaps g2. In this case,
the concave portions are formed at only locations where the
auxiliary conductors are to be formed.
Obviously, numerous modifications and variations of the
present invention are possible in light of the above
teachings. It is therefore to be understood that within the
scope of the appended claims, the present invention may be
practiced otherwise than as specifically described herein.