DESCRIPTION
FILTER DEVICE AND METHOD FOR MANUFACTURING THE SAME
TECHNICAL FIELD
The present invention relates to a filter device to be used in a
micro wave or a sub-micro wave communication apparatus, and a
method for manufacturing the same filter device.
BACKGROUND ART
Fig. 12 shows a sectional view of a conventional filter device,
which is manufactured through the steps of: machining aluminum
die-cast, then providing the machined die-cast with silver plating to
produce frame 1, and then screwing resonant element 2 into frame 1,
and finally putting lid 3 onto frame 1.
Patent document 1 is known as related art to the present
invention.
Screwing of the resonant element to the frame produces
dispersion in electric resistance at the connected section depending on
the tightening force. The dispersion will lower a Q factor of the
resonator formed of the inside of the frame and the resonant element
mounted in the frame. This phenomenon resultantly degrades the
characteristics of the filter device, such as incurring a greater
insertion loss.
Patent Document 1: Unexamined Japanese Patent Application
Publication No. H08 - 195607

DISCLOSURE OF INVENTION
The present invention addresses the problem discussed above,
and aims to provide a filter device excellent in characteristics of, e.g.
insertion loss. To achieve the foregoing objective, the filter device of
the present invention comprises the following elements:
a filter housing formed of a frame opening at least its upside
and a lid covering the opening of the frame and mounted to the frame,
and the housing being provided with a face plated at least on its inside!
and
a resonant elemer t placed in the filter housing.
The resonant element employs steel sheet whose both faces are plated,
and the plated steel sheet is bent and shaped into a cylindrical form.
A gap formed on a lateral lace of the resonant element is brazed with a
bonding member, and the outer plated face of the resonant element and
the inner plated face of the frame are brazed with a bonding member.
The resonant element is thus brazed with conductive bonding
material, thereby reducing a connection resistance between the
resonant element and the frame. As a result, the Q factor of the
resonator can be increased, so that a filter device having a smaller
insertion loss is obtainable.
Use of the plated steel sheet allows a thickness of the filter
device to be thinner, thereby reducing a weight thereof. On top of that,
the plated steel sheet can be shaped by press-working, which assures a
high productivity, and the filter device thus can be produced at an
inexpensive cost.
BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a sectional view of a filter device in accordance with
a first embodiment of the present invention.
Fig. 2 shows a development view of a frame of the filter device
shown in Fig. 1.
Fig. 3A shows a development view of a resonant element to be
used in the filter device shown in Fig. 1.
Fig. 3B shows a top view of the resonant element shown in Fig.
3A.
Fig. 3C shows a lateral view of the resonant element shown in
Fig. 3A.
Fig. 4A shows an enlarged sectional view of a connected section
bonded only with one side oi plated faces.
Fig. 4B shows an enlarged sectional view of the connected
section bonded with both sides of plated faces.
Fig. 5 shows a sectional view of a filter device in accordance with
a second embodiment of the present invention.
Fig. 6 shows a development view of a frame of the filter device
shown in Fig. 5.
Fig. 7A shows a top view of a resonant element to be used in the
filter device shown in Fig. 5.
Fig. 7B shows a lateral view of the resonant element shown in
Fig. 7A.
Fig. 7C shows a bottom view of the resonant element shown in
Fig. 7A.
Fig. 8 shows a sectional view of a filter device in accordance with
a third embodiment of the prosent invention.
Fig. 9A shows a development view of a resonant element to be

used in the filter device shewn in Fig. 8.
Fig. 9B shows a lateral view of the resonant element shown in
Fig. 9A.
Fig. 10A shows a cross section viewed from the top of the filter
device shown in Fig. 8.
Fig. 10B shows an enlarged sectional view of a tip of a partition
of the filter device shown in Fig. 8.
Fig. 11 shows a cross section viewed from the top of a filter
device using a partition which is described in a second example of the
third embodiment.
Fig. 12 shows a sectional view of a conventional filter device.
DESCRIPTION OF REFERENCE MARKS
11 filter housing
11a frame
l1b lid
l1e bottom
l1d side plate
11e partition
12 resonant element
12b, 13a, 13b, 13c, 13d connected section
14 cream solder (solder)
PREFERRED EMBODIMENTS FOR CARRYING OUT THE
INVENTION
Exemplary Embodiment 1
The first embodiment is demonstrated hereinafter with

reference to the accompanying drawings. Fig. 1 shows a sectional
view of a filter device in accordance with the first embodiment, and Fig.
2 shows a development view of frame 11a of the filter device shown in
Fig. 1. In Fig. 1 and Fig. 2, frame 11a is made of steel sheet which has
been plated with copper and then shaped into a given form by cutting
and bending. Filter housing 11 used in this first embodiment is
formed of frame 11a and lid 11b. Frame 11a is cut into a shape as
shown in Fig. 2 and bent along the dotted lines. Frame 11a thus forms
a box-like shape with bottom 11c and four side plates lid bent along
the four edges of bottom 11c, rising from the edges and crossing with
each other at approx. right angles.
Lid lib is mounted to frame 11a such that it covers the opening
of frame 11a. In this embodiment, frame 11a is brazed to lid lib with
solder 14 (used as an example of the bonding material). Lid lib
includes screw holes at the places above resonant elements 12.
Frequency adjusting screws 15 are put into these screw holes. In this
first embodiment, lid 11b and frame 11a employ the same plated steel
sheet, whose thickness is approx. 1mm.
Side plates 11d bent along the dotted lines shown in Fig. 2 are
joined together, and the jointed section is referred to as connected
section 13a, where side plates 11d adjacent to each other are connected
and fixed together with solder 14. In this embodiment, the steel sheet
is copper-plated in a thickness of approx. 10µm.
Fig. 3A shows a development plan view of resonant element 12 to
be used in the filter device discussed above. Fig. 3B shows a top view
of resonant element 12, ar.d Fig. 3C shows a lateral view of resonant
element 12. In these drawings, resonant element 12 is formed by

press-working the copper-pleted steel sheet as frame 11a is formed, to
be more specific, punched-out flat plate 12a is bent into a cylindrical
form, and shaped into resonant element 12, which is then connected
and fixed to bottom 11c of frame 11a with solder 14.
5 Filter housing 11 of this embodiment is equipped with four
resonant elements 12, which are separated individually with partitions
11e. Gaps between partitions 11e and side plates 11d are brazed with
solder 14, thereby connecting partitions 11e to side plates 11d. Gaps
between partitions 11e and filter housing 11 (respective gaps between
10 partitions 11e and bottom 11c, side plates 11d, 11d 11b) are also brazed
with solder 14 to form connected sections 13b, thereby connecting each
other. Gaps between side plates 11d and lid 11b are brazed With
solder 14 to form connected section 13c, thereby connecting each other.
Partitions 11e cross with each other to form a cross shape at
15
approx. center in frame 11. Connected section 13d (not shown in Fig.
1 but shown in Fig. 10A) of partitions 11e is also brazed with solder 14.
Resonant elements 12 are individually placed at the approx. center of
each cavity separated by partitions 11e.
The foregoing; structure allows resonant element 12 to be hollow
inside, which makes; the weight less than a pole-type resonant element.
Resonant element 12 can be formed by bending a punched-out flat plate
12a, so that gap 12c is produced at the joint, so that gap 12 c is also
connected and fixed to each other with solder 14. This structure
allows reducing an insertion loss of the filter.
In general, electric charges tend to gather at connected sections
13a, 13b, 13c, 13d, and connected sections 12b between resonant
elements 12 and filler housing 11, so that the electric potential at these

connected sections become ligher. Therefore, it is essential to reduce
the resistance at connected sections 12b, 13a, 13b, 13c, and 13d, and it
is desirable to use a metal having the smallest possible resistance as
the bonding material.
In this embodiment, solder 14 is employed as brazing material;
however, the brazing material can be any metal inasmuch as it has a
small resistance, good soliability with a counterpart metal, and is
resistive to metallic erosion.
A cut surface resulting from the press-working done to the steel
sheet exposes basis metal of the steel sheet, i.e. iron is exposed, so that
the basis metal is subject to oxidization or rust with ease, and the
resistance on the cut surface grows great. On top of that, since the
iron is magnetic material, the resistance becomes greater in a high
frequency region. To overcome the foregoing drawbacks, the plated
faces are brazed and connected to each other with solder 14 (as the
bonding material).
To be more specific, at connected sections 13a, plated faces
inside the side plates 11d are connected to each other with solder 14.
At connected sections 13b, plated faces on both sides of partition 11e
are connected to the plated face inside of filter hosing 11 with solder 14.
At connected sections 13c, side plates 11d are connected to lid 11b, and
at connected sections 13d, plated faces on the sides of partitions 11e
are connected to each other. At connected sections 12b, plated faces
inside bottom 11e are connected to the plated faces outside the
resonant elements 12 with solder 14. These connections allow
reducing the resistances at connected sections 12b, 13a, 13b, 13c, and
13d, so that the Q factor of the resonator can be raised, which reduces a

signal loss, and a filter device having a smaller insertion loss is thus
achievable.
On top of that, the structure discussed above diminishes the
concentration of the electric charges on connected sections 12b, 13a,
13b, 13c, and 13d. It is generally known that the electric charges
gather at an angular section, such as connected sections 12b, 13a, 13b,
13c, and 13d. A magnitude of the concentration becomes greater as an
angle of the angular section becomes acuter, and a tip of the angular
section becomes sharper.
The connection between the plated faces with the bonding
member allows the tips of the angular sections of connected sections
12b, 13a, 13b, 13c, and 13d to be round. The bent sections between
bottom 11e and side plates 11d are processed to be round. These
preparations allow diminishing the concentration of the electric
charges on the connected sections 12b, 13a, 13b, 13c, and 13d, which
thus do not so much contribute to the problem discussed previously.
The loss in signals can be thus smaller, and the filter device having a
smaller insertion loss is achievable.
On top of that, cut surfaces of tips 12d of resonant elements 12
are covered with solder 14, so that the cut surfaces are hardly exposed
at tips 12d where electric charges concentrate among others. Electric
resistance at tips 12d can be thus reduced. As a result, use of the
plated steel sheet allows improving the insertion loss of the filter
device.
Frame 11a is connected to 11d 11b with solder 14; however, they
can be connected and fiked to each other with screws. In this case, lid
11b is detachable, and a repair work becomes simpler. Resonant

elements 12 are mounted to bottom 11c; however, they can be mounted
to side plates 11d or 11d 11b instead. It is yet desirable to align the
center axis of adjusting screw 15 and the center axis of resonant
element 12 generally on a straight line.
A method of manufacturing the filter device discussed above is
demonstrated hereinafter. In the press-working step, copper-plated
steel sheet is punched out, then the resultant sheet is bent to form
frame 11a, 11d lib, partitions 11e, and resonant elements 12. After
the press-working step, the brazing step brazes resonant elements 12,
partitions 11e, and 11d 11b to frame 11a.
In this brazing step, soldering and assembling are done firstly,
namely, after the press-working step, resonant elements 12 and
partitions 11e are firstly mounted to bottom 11e of frame 11a, and
cream solder 14 is applied to their connected sections 12b, 13a, 13b, 13c,
and 13d. Then 11d 11b is mounted to frame 11a.
In this first embodiment, cream solder 14 is applied to the
objects through a dispenser; however, when an object is flat plate like
lid 11b, solder 14 can be applied through a screen printing method. In
this case, the cream solder 14 can be applied in a stable amount.
Stick solder can be used instead of cream solder 14, for a more stable
amount of solder can be applied.
In the brazing step, solder 14 is melted by heating after the step
of applying solder 14 and assembling, so that resonant elements 12 and
lid 11b are connected and fixed to frame 11a. Connected sections 13a,
13b, 13c, and 13d of frame 11a are also connected and fixed to the
objects with solder 14.
Paste of cream solder 14 is used for brazing; however, stick

solder or silver solder can be used for brazing. In the case of using the
silver solder, the bonding can be preferably carried out at approx.
900°C in a reducing furnace. As discussed above, the joint of side
plates 11d with each other, the joint of bottom 11e with resonant
elements 12, and covering the gaps 12c of resonant elements 12 with
solder 14 can be done during the one step, i.e. the brazing step, so that
the productivity can be improved.
In an adjustment step following the brazing step, frequency
adjusting screw 15 is mounted to 11d 11b, and a distance between screw
15 and resonant element 12 is adjusted, thereby adjusting the
frequency characteristics of the filter device, which is thus completed.
Fig. 4A shows an enlarged sectional view of the connected
section bonded only with one side of plated faces. Fig. 4B shows an
enlarged sectional view of the connected section bonded with both sides
of plated faces. Fig. 4A shows connected sections 13a, 13c, and Fig. 4B
shows connected sections 12b, 13b. In Fig. 4A and Fig. 4B, elements
similar to those in Fig. 1 - Fig. 3C have the same reference marks, and
the descriptions thereof are simplified here.
In Fig. 4A and Fig 4B, when frame 11a (or resonant element 12)
is press-cut, a clearance of a tooling die for this press-cutting is
adjusted for forming regions 17 at connected sections 13a - 13d for
introducing the plating material onto the cut surface. This
preparation allows simply connecting the objects to the respective
connected sections with solder 14, such as between each side plate 11d,
between partition 11e and 11d 11b, between partition 11e and housing
11, and between housing 11 and resonant element 12.
In this first embodiment, since the plated steel sheet having a

cut surface is used, and the cut surfaces of connected sections 12b, 13a,
13b, 13c, and 13d are placed confronting the plated surfaces. Since
the cut surface has poor sole ability, solder 14 is prevented from flowing,
and thus solder 14 will not spread unnecessarily. A stable and
appropriate shape can be thus formed at each one of the connected
sections 12b, 13a, 13b, 13c, and 13d, so that a dispersion of the
insertion loss can be minimized.
On top of that, connected sections 12b, 13a, 13b, 13c, and 13d are
provided with V-shaped grooves 19 for preventing solder 14 from
flowing and spreading. V-shaped groove 19 prevents melted solder 14
from traversing grooves 19 and spreading, so that a stable and an
appropriate size of round shape can be formed at the respective
connected sections. Thus a smaller insertion loss and a smaller
dispersion thereof can be expected. Instead of V-shaped groove 19,
protrusions or resist film can be used for preventing solder 14 from
spreading. In the case of using the protrusions, no pointed sections
are preferably formed in order to avoid concentration of electric
charges thereon.
Regions 17 are also provided to connected sections 12b and an
outer wall of tip 12d of resonant elements 12 for introducing the plated
material, because cream solder 14 is applied to tip 12d during the
soldering and assembling step in this embodiment. This preparation
shortens the distance between the inner plated face and the outer
plated face of resonant eiement 12 (distance between the cut surfaces
exposed), so that the entire cut surface can be simply covered with
melted solder 14. Tip 12d, where electric charges tend to concentrate,
is covered with solder 14, so that the resistance of tip 12d can be

reduced. As a result, a filter device having a smaller insertion loss is
obtainable.
Partitions 11e in accordance with the first embodiment are
provided with communicat ng windows 18 (shown in Fig. 10A) for
communicating a cavity with an adjacent cavity. Partitions 11e are
also provided with the plated material at edges 18a (shown in Fig. 10A)
confronting the windows, so that the distance between the plated faces
is shortened and the resistance can be reduced.
On top of that, cream solder 14 is applied to the cut surfaces of
edges 18a during the soldering and assembling step, so that the edges,
where an electric potential tends to be higher, of partitions 11e have a
lower resistance. As a result, the filter device having a further
smaller insertion loss is obtainable. Region 17, which introduces the
plated material onto the cut surface, desirably has a wider area, and
specifically, it is preferable for region 17 to have at least 30% area of
the cut surface, more preferably, it has 50% or more than 50% area of
the cut surface. This structure allows the entire cut surface to be
covered steadily with sclder 14. A greater thickness of the plated
surface is desirable in order to introduce the plated material onto the
cut surface, and specifically, the thickness of the plated surface is
preferably at least 0.5% of a thickness of the plated steel sheet, so that
the plated material can be steadily introduced on at least 30% area of
the cut surface.
It is also preferable to introduce the plated material onto the cut
surfaces formed on both sides of gap 12c of resonant element 12. In
this case, the plated material should be introduced on the outer side of
resonant element 12. This preparation allows solder 14 to rise with

ease along gap 12c toward the top of resonant element 12 due to the
capillarity while solder 14 covers the entire cut surfaces, so that gap
12c can be brazed with ease. On top of that, the brazing can be done
at once, so that the productivity can be greatly improved.
The filter device in accordance with this embodiment generates
resonance in the interior space between resonant element 12 and frame
11a, thereby forming a resonator, and a combination of these structures
produces filter characteristics. In this structure, the inner plated
surfaces of filter housing 11 are connected to each other by soldering,
and the outer plated surface of resonant element 12 is connected to the
inner plated surface of housing 11, thereby reducing an electric
resistance in parts of a loop including resonant element 12. The filter
having a higher Q factor of the resonator and a smaller insertion loss is
thus obtainable.
The plating material and the brazing material preferably have a
lower electric resistance from the viewpoint of characteristics of a filter
device, and also these two materials preferably have a greater
difference in the melting points. Because a brazing temperature
should be set between these melting points, and if the difference
between these melting points is small, a viscosity of the brazing
material cannot be small enough to spread. Considering this factor,
use of copper (melting point is approx. 1050°C) as the plating material,
and use of silver solder (melting point is approx. 800°C) or solder 14
(melting point is approk. 180 - 240°C) will make the viscosity of the
brazing material small enough, so that the entire cut surface can be
covered steadily with the brazing material.
In this first embodiment, resonant elements 12 are brazed to the

bottom of the frame; however resonant elements 12 can be brazed to lid
11b or side plates 11d for obtaining the same resonant device as
discussed above. Frequt ncy adjusting screw 15 is mounted to lid 11b;
however, it can be mounted to side plate 11d or bottom 11c. A more
accurate frequency adjustment requires screw 15 to be mounted to a
face confronting the face where resonant element 12 is mounted. The
center of resonant element 12 is preferably aligned with the center of
screw 15 on a substantial straight line.
The brazing material can be attached to the entire sections
before they are put into the reducing furnace, thereby melting the
material in order to spread the brazing material over the entire
sections. Another way to spread the material over the entire sections
is to link connected sections 12b, 13a, 13b, 13c, and 13s to the
non-connected sections, ..e. the cut sections, with narrow grooves, and
then the entire sections are put into the reducing furnace for melting
the brazing material. The melted brazing material travels to the
non-connected sections through the narrow grooves due to the
capillarity. This structure allows the brazing material to cover the
entire cut surfaces with ease. Since those grooves can be formed at
the same time as the press-working step of frame 11 or resonant
elements 12, no additional labor or time is required.
Exemplary Embodiment 2
The second embodiment is demonstrated hereinafter with
reference to the accompanying drawings. Fig. 5 shows a sectional
view of a filter device in accordance with the second embodiment of the
present invention. Fig 6 shows a development view of a frame of the

filter device shown in Fig. 5. In Figs. 5 and 6, elements similar to
those in Fig. 1 have the same reference marks, and the descriptions
thereof are simplified here.
In the first embodiment discussed previously, frame 11a is
formed of bottom 11e and side plates 11d bent from bottom 11c. In
this second embodiment, side plates 11d, which includes top plate 11f,
are separated from bottom 11c, and four side plates 11d are bent at the
edges of top plate llf and depend therefrom, so that they open
downward. 11d 11b is screwed and fixed to top plate llf. Bottom 11c
is connected to side plates 11d with solder 14, thereby forming
connected sections 22.
Resonant elements 21 are brazed to bottom 11e with solder 14,
similar to those in the first embodiment. Fig. 7A shows a top view of
resonant element 21 to be used in the filter device in accordance with
the second embodiment. Fig. 7B shows a lateral view of resonant
element 21, and Fig. 7C shows a bottom view of resonant element 21.
In Figs. 7A - 7C, resonant element 21 is shaped by bending steel sheet
through press-working. Resonant element 21 comprises the following
sections:
mounting plate 21a;
linking section 21b bent from mounting plate 21a; and
cylindrical section 21c linked with linking section 21b.
Cylindrical section 21c is formed of two semicircles which are formed
by bending the steel sheet Resonant element 21 discussed above is
mounted on bottom 11c with its mounting plate 21a placed on bottom
11c and its opening faced to lid 11b. Frame 11a, lid 11b, and resonant
elements 21 are made of stesl sheet plated with copper, so that an outer

plated face of mounting plate 21a and an inner plated face of bottom
11c are brazed together by solder 14. Inner plated faces of the tips at
the opening side of side plates 11d and the inner plated face of bottom
11c are also brazed together by solder 14.
Resonant element 21 is obliged to have gap 21d between two
semicircles of cylindrical sections 21c, and gap 21 can be closed with
solder 14. As a result, use of plated steel sheet allows achieving a
filter device having a smaller insertion loss. Region 17, similar to
that in the first embodiment, is formed at the tip of outer wall of
cylindrical section 21c, so that the plated material can be introduced
and solder 14 can cover the cut surfaces.
The top face of top plate 11f and the underside of 11d 11b confront
each other and are connected together with cream solder. Hole 16a
available on top plate llf produces a step, and the cut surfaces of hole
16a is preferably covered with solder 14.
The cut surface of hole 16a is processed such that the plated
material can be introduced thereon, so that solder 14 can spread
around hole 16a with ease, and electric charges will not so much
concentrate on the step. As a result, the filter device having smaller
insertion loss is obtainable. The plated face is preferably introduced
on the side confronting lid 11b, because the connected section can be
brazed with more ease.
During the step of soldering and assembling in this second
embodiment, cream solder 14 is applied firstly to bottom 11e and lid
11b. To be more specific, cream solder 14 is applied to mounting face
21a of resonant element 21, connected section 22 between bottom 11c
and side plates 11d, and lid 11b at a section confronting top plate 11f.

Since bottom 11e and lid 11b are flat plates, solder 14 can be
applied thereto with ease by a screen printing method, so that an
excellent productivity can be expected. Solder 14 is applied to lid 11b;
however, it can be applied to top plate 11f at the section confronting lid
11b. In this case, since the top face of top plate 11f is flat, solder 14
can be applied thereto with ease by the screen printing method.
Then resonant elements 21, partitions (not shown), and side
plates 11d are mounted to bottom 11c, and then cream solder 14 is
applied to connected sections 13a, 13b, 13c, 13d between each side
plate 11d.
Exemplary Embodiment 3
The third embodiment is demonstrated hereinafter with
reference to the accompanying drawings. Fig. 8 shows a sectional
view of a filter device in accordance with the third embodiment. The
filter device shown in Fig. 8 differs from that of the first embodiment in
the following points: Resonant elements 31 are mounted to lid 11b,
frequency adjusting screws 15 are mounted to bottom lie, and tip 18a
of partition 11e has another shape.
Fig. 9A shows a development view of resonant element 31 in
accordance with this third embodiment, and Fig. 9B shows a lateral
view of resonant element 31. As shown in Figs. 9A and 9B, the tip of
resonant element 31 is bent inside, so that the plated face becomes tip
31a of resonant element 31, and no basis metal is exposed at tip 31a.
Tip 31a thus has a smaller resistance, so that an insertion loss of this
filter device becomes smaller. The bent length of the tip is approx.
3mm.

As shown in Fig. 9A, the corners of the bent section are cut so
that interference in material when the tip is bent can be reduced, and
thus resonant element 31 with accurate dimensions is obtainable.
Fig. 10A shows a cross section viewed from the top of a filter
device in accordance with the third embodiment, and Fig. 10B shows an
enlarged sectional view cf the tip of the partition of the same filter
device. In Figs. 10A and 10B, elements similar to those shown in Fig.
1 have the same reference marks, and the descriptions thereof are
simplified here.
Communicating windows 18 are provided between the edge of
partition 11e and side p\ate 11d for communicating a cavity with an
adjacent cavity, separated by partition 11e. Edge 18a of partition 11e
tends to have a higher electric potential. To overcome this drawback,
edge 18a is pressed from both sides to form V-shaped press-face 32 in
the step of press-working so that the plated material can be introduced
onto the cut surface. Face 32 is cut around its apex for forming a
plated face on press-face 32, so that a smaller area of cut surface can be
exposed at edge 18a of partition 11e.
A smaller resistance is achievable at the place where an electric
potential tends to be higher, so that the filter device having a smaller
insertion loss is obtainable. In this case, edge 18a is preferably
covered with solder 14 as discussed previously.
Fig. 11 shows across section viewed from the top of the filter
device employing the partition, described in the second example, in
accordance with the third embodiment. In Fig. 11, partition 41 is
folded over at its edge, so that a plated face becomes the edge, whose
resistance thus becomes smaller. As a result, the filter device having

a further smaller resistances is obtainable.
INDUSTRIAL APPLICABILITY
The filter device of the present invention has a smaller insertion
loss even when a plated metal sheet is used for forming a frame of the
filter device, so that an excellent productivity can be expected. This
filter device is useful in micro wave or semi-micro wave communication
apparatuses.

CLAIMS
1. A filter device comprising-
a filter housing including a frame opening at least its
upside and a 11d covering the opening of the frame and mounted to the
frame; and
a resonant 3lement placed inside the filter housing;
wherein at le ast an inside of the filter housing is provided
with a plated face, and
wherein the resonant element employs a steel sheet whose
both sides are plated;
wherein the resonant element is shaped into a cylinder by
bending the plated steel sheet, and
wherein a gap formed on a lateral face of the resonant
element is brazed with bonding material, and an outer plated face of
the resonant element is brazed to an inner plated face of the frame
with the bonding material.
2. The filter device of claim 1, wherein a tip of the resonant
element is bent inside.
3. The filter device of claim 1, wherein the frame includes a
bottom, a first side plate and a second side plate rising from the bottom
and crossing with each ather substantially at right angles,
wherein the frame is formed by cutting the steel sheet,
whose both sides are plated, through press-working, and
wherein an inner plated face of the first side plate is

brazed with the bonding material to an inner plated face of the second
side plate.
4. The filter device of claim 3, wherein the plated face is
introduced onto a cut surface of the first side plate, and the plated face
is introduced onto a cut surface of the second side plate.
5. The filter device of claim 3, wherein the frame includes a
third side plate rising from the bottom and placed forming
substantially right angles with respect to the first side plate, and a
communicating window for communicating a cavity with another cavity,
separated by the third side plate,
wherein the third side plate employs a steel sheet whose
both sides are plated, and the plated steel sheet is shaped into the
third side plate by cutting the plated steel sheet through press-working,
and the third side plate is brazed with the bonding material to the
frame at its both sides.
6. The filter device of claim 1 comprising:
the plurality of resonant elements which are separated by
a partition having a communicating window.
7. The filter device of claim 6, wherein at least a part of an
edge of the partition on a side of the communicating window is
provided with a plated face.
8. The filter device of claim 7, wherein the plated face on the

edge of the partition on the side of the communicating window is
formed by introducing a plated face of a side face of the partition
through press working.
9. The filter device of claim 7, wherein the plated face on the
edge of the partition on the side of the communicating window is
formed by folding over the partition.
10. A method of manufacturing the filter device as defined in
claim 1, the method comprising:
cutting and bending a steel sheet, whose both sides are
plated, for obtaining a frame;
brazing sids plates of the frame to each other with
bonding material;
mounting a 11d to the frame; and
brazing the resonant element inside the frame before
mounting the 11d to the frame.
11. The manufacturing method of claim 10, wherein in the step
of mounting the resonant element inside the frame, an outer lateral
face of the resonant element is brazed with bonding material to an
inner plated face of the frame.
12. The manufacturing method of claim 10 further comprising:
obtaining the resonant element before the step of
mounting the resonant element inside the frame,
wherein in the step of obtaining the resonant element,

a plated steel sheet is bent and shaped into a cylindrical form, and then
a gap formed on a lateral face of the resonant element is brazed with
bonding material.

A filter device having a frame made of plated steel sheet
generates a smaller insertion loss and is excellent in productivity.
Resonant elements are shaped into a cylindrical form by bending the
5 steel sheet, whose both sides are plated, before they are placed in a
filter housing. A gap formed on a lateral face of the resonant element
is brazed with solder, and an outer plated face of the resonant element
is brazed with solder to an inner plated face of the frame.