Band pass filter

BACKGROUND OF THE INVENTION

The present invention relates to a band pass filter, and particularly, to a compact band pass filter that can be fabricated at low cost.

DESCRIPTION OF THE PRIOR ART

In general, a dielectric block having through holes passing from one surface to the opposite surface and all of whose surfaces except said one surface are metallized is used as a band pass filter. The through holes formed on the dielectric block work as resonators for the high frequency signal. The band pass filter is formed by adding capacitance and so forth to the resonators.

Many proposals have been made regarding methods for adding capacitance and so forth to the resonators constituted by the through holes.

According to one such method the dielectric block with the through holes is mounted on a substrate and the capacitors etc. are added to the substrate as separate components to form the band pass filter circuit. This method has the advantage that complex processing of the dielectric block is not required but has the disadvantage that the overall circuit size is enlarged because numerous components are used. The method is therefore not suitable for application to equipment that requires miniaturization, such as mobile phones.

According to another proposed method, conductive patterns that work as capacitors etc. are formed on said one surface of the dielectric block by screen-printing to form the band pass filter circuit. This method has the advantage that overall circuit size can be reduced because no capacitors etc. are added as different components, but has the disadvantage that it is extremely difficult to form the conductive patterns. Specifically, although the dielectric block should be stood and fixed with said one surface facing upward in order to print the conductive patterns on said one surface of the dielectric block, this is a very unstable state because the dimensions of said one surface and the opposite surface are small compared with the other surfaces. Further, high patterning accuracy is required for forming the conductive patterns on said one surface of the dielectric block but it is very difficult to form the conductive patterns accurately by the screen-printing in such an unstable state. Moreover, in certain types of the band pass filters, said opposite surface of the dielectric block is not flat but has a step. In this case it is extremely difficult to make the dielectric block stand with said one surface facing upward. For this reason, it is difficult to form the conductive patterns on said one surface of the dielectric block accurately and the fabricating cost increases. Another problem is that it is difficult to obtain large capacitance etc. by only forming the conductive patterns which work as capacitors on said one surface of the dielectric block.

In still another method, grooves or cavities are formed on said one surface of the dielectric block to form the band pass filter circuit by intentional disruption of the electromagnetic field balance. Like the method explained earlier, this method has the advantage that overall circuit size can be reduced but has the disadvantage that it increases fabricating cost because the conductive pattern must be formed on said one surface having the grooves or cavities by screen-printing with the dielectric block maintained with said one surface facing upward.

In still another method, proposed in Japanese Patent Laid Open No. 11-27006, a multilayered circuit member is added to said one surface of the dielectric block. This method has the advantage that it is easy to fabricate the dielectric block because no special processing for forming conductive patterns, cavities or the like on the dielectric block is required but has the disadvantage of high fabricating cost because the fabrication of the multilayered circuit member is complex, i.e., it has to be fabricated by laminating a number of dielectric layers each having a predetermined conductive pattern and through holes.

As pointed out above, the conventional methods for adding the capacitance and so forth to the resonators formed by the dielectric block having through holes encounter such problems as that overall circuit size is enlarged owing to the formation of numerous components or that the fabricating cost increases because it is difficult to fabricate the dielectric block or the multilayered circuit member to be added thereon.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a compact band pass filter that can be fabricated at low cost.

The above and other objects of the present invention can be accomplished by a band pass filter comprising a dielectric block having through holes formed from one surface thereof to another surface opposite the one surface and a single layered dielectric plate having a front surface and a back surface joined to the dielectric block such that the back surface faces the one surface of the dielectric block, the dielectric block having metallizations formed on all surfaces except the one surface, and the dielectric plate having metallizations of predetermined patterns formed on the front and the back surfaces.

According to the present invention, a compact band pass filter that can be fabricated at low cost can be provided because it is formed by joining the single layered dielectric plate to the dielectric block. Further, since it is not necessary to form metallization on the one surface of the dielectric block, the dielectric block can be formed easily. Moreover, since metallizing on the front and back surfaces of the dielectric plate can be preformed in a stable state, the dielectric plate can be also formed easily.

In a preferred aspect of the present invention, the metallizations formed on the front and the back surfaces of the dielectric plate respectively form the one electrodes and the other electrodes of capacitors.

According to this preferred aspect of the present invention, capacitance can be added easily to the dielectric block because the metallizations formed on the opposite surfaces of the single layered dielectric plate form a capacitor.

In a further preferred aspect of the present invention, the metallizations formed on the back surface of the dielectric plate are in contact with the metallizations formed on inner walls of the through holes.

In a further preferred aspect of the present invention, the dielectric plate has cavities formed on the back surface thereof and the metallizations are formed on inner walls of the cavities.

According to this preferred aspect of the present invention, because the cavities are formed on the back surface of the dielectric plate, various characteristic can be given to the band pass filter by varying the shape of the cavities. Further, in the case where capacitors are formed by the metallizations formed on the inner walls of the cavities and metallizations formed on the front surface of the dielectric plate, large capacitance can be obtained.

In a further preferred aspect of the present invention, the dielectric plate has metallized projections projecting from the cavities, the metallizations formed on the projections being in contact with the metallizations formed on inner walls of the through holes.

In a further preferred aspect of the present invention, the through holes include at least a first through hole and a second through hole, the metallizations formed on the front surface of the dielectric plate include at least a first pattern and a second pattern, and the metallizations formed on the back surface of the dielectric plate include at least a third pattern and a fourth pattern, a first capacitor being formed by the first and third patterns, a second capacitor being formed by the second and fourth patterns, and the third and fourth patterns being in contact with the metallizations formed on inner walls of the first and second through holes, respectively.

In a further preferred aspect of the present invention, the first and second patterns are an input terminal and an output terminal, respectively.

In a further preferred aspect of the present invention, the dielectric block and the dielectric plate are made from the same material.

In a further preferred aspect of the present invention, the dielectric block and the dielectric plate are fixed by solder.

In a further preferred aspect of the present invention, every metallization is made of a conductive paste, and the dielectric block and the dielectric plate are fixed by sintering the conductive paste with the dielectric block and the dielectric plate joined.

According to this preferred aspect of the present invention, because the dielectric block and the dielectric plate are fixed by sintering the conductive paste, no solder is required. Therefore, lead (Pb) free soldering that should be performed at higher temperature than ordinary can be applied when the band pass filter is mounted.

In a further preferred aspect of the present invention, the back surface of the dielectric plate has a different area from that of the one surface of the dielectric block.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.

1

(

a

) is a perspective view showing a band pass filter

1

that is a preferred embodiment of the present invention with its components separated.

FIG.

1

(

b

) is a perspective view showing the band pass filter

1

with its components joined.

FIG. 2

is a circuit diagram of the band pass filter

1

.

FIG. 3

is a graph showing the frequency characteristic curve of the band pass filter

1

.

FIG.

4

(

a

) is a perspective view showing a band pass filter

16

that is another preferred embodiment of the present invention with its components separated.

FIG.

4

(

b

) is a perspective view showing the band pass filter

16

with its components joined.

FIG. 5

is a circuit diagram of the band pass filter

16

.

FIG. 6

is a graph showing the frequency characteristic curve of the band pass filter

16

.

FIG.

7

(

a

) is a perspective view showing a band pass filter

32

that is a further preferred embodiment of the present invention with its components separated.

FIG.

7

(

b

) is a perspective view showing the band pass filter

32

with its components joined.

FIG. 8

is a circuit diagram of the band pass filter

32

.

FIG. 9

is a graph showing the frequency characteristic curve of the band pass filter

32

.

FIG.

10

(

a

) is a perspective view showing a band pass filter

48

that is a further preferred embodiment of the present invention with its components separated.

FIG.

10

(

b

) is a perspective view showing the band pass filter

48

with its components joined.

FIG. 11

is a schematic plan view showing a band pass filter

51

that is a further preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be explained with reference to the drawings.

FIG. 1

is a perspective view showing a band pass filter

1

that is an embodiment of the present invention; FIG.

1

(

a

) shows the components of the band pass filter in a separated state and FIG.

1

(

b

) shows them in a joined state.

As shown in

FIG. 1

, the band pass filter

1

is constituted of a dielectric block

2

and a dielectric plate

3

.

The dielectric block

2

has the shape of an almost rectangular prism measuring 6.0 mm×2.6 mm×8.8 mm and is made of a ceramic (εr=92) composed mainly of barium titanate. The dielectric block

2

has two through holes

5

-

1

and

5

-

2

that are formed in parallel from a surface

4

to the opposite surface (not shown). The through holes

5

-

1

and

5

-

2

are 1.2 mm in diameter. Further, all surfaces of the dielectric block

2

(including the inner walls of the through holes

5

-

1

and

5

-

2

) except the surface

4

are metallized and are grounded during mounting. In

FIG. 1

, the metallized portions are shown in the color of the drawing sheet and the portion without metallization is speckled.

The dielectric plate

3

is a single layered plate measuring 6.0 mm×2.6 mm×0.8 mm made of ceramic (εr=92) composed mainly of barium titanate, the same material as the dielectric block

2

. The dielectric plate

3

has a surface (front surface)

6

that faces outward when the dielectric plate

3

is joined to the dielectric block

2

, a surface (back surface)

7

that faces toward the surface

4

of the dielectric block

2

when the dielectric plate

3

is joined to the dielectric block

2

, and an edge surface

8

. As shown in FIG.

1

(

b

), metallizations

9

-

1

and

9

-

2

are formed on the front surface

6

at the portions to be input/output terminals and a metallization

11

is formed on the entire front surface

6

except at clearance portions

10

located at the peripheries of the metallizations

9

-

1

and

9

-

2

. As shown in FIG.

1

(

a

), metallizations

12

-

1

and

12

-

2

are formed on the back surface

7

at the portions corresponding to the two through holes

5

-

1

and

5

-

2

. When the dielectric plate

3

is joined to the dielectric block

2

, the metallizations

12

-

1

and

12

-

2

are directly in contact with the metallizations formed on the inner walls of the through holes

5

-

1

and

5

-

2

, respectively. The metallizations

12

-

1

and

12

-

2

face the metallizations

9

-

1

and

9

-

2

, respectively, formed on the front surface

6

of the dielectric plate

3

so as to form capacitors C

1

and C

3

(FIG.

2

). The metallizations

12

-

1

and

12

-

2

have portions proximate to each other (proximate portions)

13

so as to form a capacitor C

2

. Further, as shown in FIG.

1

(

a

), metallizations

14

whose widths are 0.8 mm are formed on parts of the periphery of the back surface

7

of the dielectric plate

3

. As shown in FIG.

1

(

b

), metallizations

15

are formed on the whole of the edge surface

8

except at portions in contact with the metallizations

9

-

1

and

9

-

2

and the clearance portions

10

.

The dielectric plate

3

having the above described structure is attached to the dielectric block

2

with its back surface

7

facing the surface

4

of the dielectric block

2

, and in this state, solder (not shown) is applied at the interface between the metallizations formed on the dielectric block

2

and the metallizations formed on the dielectric plate

3

to complete the band pass filter

1

of this embodiment. Thus, the metallizations formed on the dielectric block

2

are connected to the metallization

11

formed on the front surface

6

of the dielectric plate

3

through the metallizations

14

formed on the back surface

7

of the dielectric plate

3

and the metallizations

15

formed on the edge surface

8

of the dielectric plate

3

. As mentioned above, the metallizations

12

-

1

and

12

-

2

are in contact with the metallizations formed on the inner walls of the through holes

5

-

1

and

5

-

2

, respectively.

These metallizations provided on the dielectric block

2

and the dielectric plate

3

are formed of silver paste to a prescribed thickness.

FIG. 2

is a circuit diagram of the band pass filter

1

.

As shown in

FIG. 2

, the band pass filter

1

is constituted of the capacitors C

1

to C

3

connected in series between the metallizations

9

-

1

and

9

-

2

, which are the input/output terminals, and resonators each connected between the nodes of the capacitors C

1

and C

2

and the capacitors C

2

and C

3

and the ground. One of the metallizations

9

-

1

and

9

-

2

is used as the input terminal and the other is used as the output terminal. The two resonators are realized by the metallizations formed on the inner walls of the through holes

5

.

FIG. 3

is a graph showing the frequency characteristic curve of the band pass filter

1

.

As shown in

FIG. 3

, the band pass filter

1

has a characteristic of passing a high frequency signal of approximately 1030 MHz to 1080 MHz and effectively eliminating a high frequency signal of approximately 910 MHz to 940 MHz. Therefore, the band pass filter

1

of this embodiment can be used as a component requiring such a frequency characteristic.

The frequency characteristic of the band pass filter

1

depends on the shape (length in particular) of the dielectric block

2

and the pattern of the metallizations formed on the dielectric plate

3

. Thus, a desired frequency characteristic can be given to the band pass filter

1

by varying the shape of the dielectric block

2

and the pattern of the metallizations formed on the dielectric plate

3

.

A method of fabricating the band pass filter

1

of this embodiment will now be explained.

First, the main bodies of the dielectric block

2

and dielectric plate

3

are formed by press molding and sintering ceramic powder including barium titanate as main component. When the dielectric block

2

is formed, two rods are disposed where the through holes

5

are to be formed during press molding in order to form the block with two through holes and the block is then sintered. By this, the main body of the dielectric block

2

is formed to measure 6.0 mm×2.6 mm×8.8 mm. The dimensions of the dielectric plate

3

formed by press molding and sintering are 6.0 mm×2.6 mm×0.8 mm.

Next, the metallizations are applied to the entire inner walls of the through holes

5

of the main body of the dielectric block

2

by introducing an Ag (silver) paste having low viscosity. Further, metallizations are applied to the surfaces other than the surface

4

of the main body of the dielectric block

2

by screen-printing. Then, these metallizations are sintered to complete the dielectric block

2

. Since the metallizations applied to the surfaces other than the surface

4

of the main body of the dielectric block

2

are formed on the entirety of each surface, high accuracy is not required during the screen-printing. The forming of the metallizations on the main body of the dielectric block

2

is therefore easy to carry out.

Next, the metallizations

9

-

1

,

9

-

2

and

11

are applied to the front surface

6

of the main body of the dielectric plate

3

by screen-printing and the metallizations

12

-

1

,

12

-

2

and

14

are applied to the back surface

7

of the main body of the dielectric plate

3

by screen-printing. When applying the metallizations

9

-

1

,

9

-

2

and

11

to the front surface

6

of the main body of the dielectric plate

3

, the screen-printing can be performed with the back surface

7

of the main body of the dielectric plate

3

mounted on a stage (not shown) so the front surface

6

faces upward. When applying the metallizations

12

-

1

,

12

-

2

and

14

to the back surface

7

of the main body of the dielectric plate

3

, the screen-printing can be performed with the front surface

6

of the main body of the dielectric plate

3

mounted on the stage (not shown) so that the back surface

7

faces upward. As shown in

FIG. 1

, since the front surface

6

and the back surface

7

of the main body of the dielectric plate

3

have large areas compared with the edge surface

8

, the dielectric plate

3

is in a very stable state when mounted on the stage (not shown) with the front surface

6

or the back surface

7

facing upward. The forming of the metallizations on the front surface

6

and the back surface

7

of the main body of the dielectric plate

3

is therefore relatively easy to carry out.

Further, the metallizations

15

are applied to the edge surface

8

of the main body of the dielectric plate

3

by transfer printing. Transfer printing is a method for forming metallization patterns corresponding to grooves on an object by preparing a rubber (not shown) having grooves corresponding to the patterns to be transferred, charging an Ag paste into the grooves and forcing the object onto the rubber. Although it is much easier to form metallizations by transfer printing than by screen-printing, it is impossible to form metallizations having accurate patterns by transfer printing. However, the accuracy of transfer printing is sufficient for forming the metallizations

15

on the edge surface

8

of the dielectric plate

3

because the metallizations

15

are only required to connect the metallizations formed on the dielectric block

2

and metallization

11

formed on the front surface

6

of the dielectric plate

3

while preventing the metallizations formed on the dielectric block

2

from connecting to the metallizations

9

-

1

and

9

-

2

formed on the front surface

6

of the dielectric plate

3

. The forming of the metallizations on the edge surface

8

of the main body of the dielectric plate

3

is therefore also relatively easy to carry out. Then, these metallizations are sintered to complete the dielectric plate

3

.

Next, the dielectric block

2

and the dielectric plate

3

formed by the above described method are joined as shown in FIG.

1

(

b

) and, in this state, solder (not shown) is applied at the interface between the metallizations formed on the dielectric block

2

and the metallizations formed on the dielectric plate

3

to adhere them. Because these metallizations have a prescribed thickness, a gap corresponding to the thickness of the metallizations is formed between the surface

4

of the dielectric block

2

and the back surface

7

of the dielectric plate

3

at the portion where the metallizations

12

-

1

,

12

-

2

and

14

are not formed. However, since the metallizations

14

are formed on a part of peripheral portion of the back surface

7

of the dielectric plate

3

, the dielectric block

2

and the dielectric plate

3

are adhered at these portions. Therefore, the electrical and mechanical connection between the dielectric block

2

and the dielectric plate

3

can be surely established by providing the solder (not shown) to these portions. This completes the band pass filter

1

.

As described above, the band pass filter

1

is made of the combination of the dielectric block

2

, which can be fabricated merely by forming the metallizations on the whole of all surfaces except the surface

4

, and the dielectric plate

3

, which can be fabricated merely by forming the metallizations on both main surfaces (front surface

6

and back surface

7

) and the edge surface of a single layered plate. Since the front surface

6

and the back surface

7

of the dielectric plate

3

have large areas compared with the edge surface

8

, the dielectric plate

3

is in a very stable state when mounted on the stage (not shown) with the front surface

6

or the back surface

7

facing upward. The forming of the metallizations on the front surface

6

and the back surface

7

of the dielectric plate

3

is therefore relatively easy to carry out. Since it is thus relatively easy to fabricate the band pass filter

1

of this embodiment, the fabrication cost thereof can be lowered.

Further, the band pass filter

1

does not require formation of complex metallization patterns on the dielectric block

2

because the capacitance which is to be added to the resonators realized by the through holes

5

of the dielectric block

2

is formed by the metallizations

9

-

1

and

9

-

2

formed on the front surface

6

of the dielectric plate

3

and the metallizations

12

-

1

and

12

-

2

formed on the back surface

7

of the dielectric plate

3

, and by the proximate portions

13

of the metallizations

12

-

1

and

12

-

2

. Therefore, not only is fabrication of the dielectric block

2

very easy but band pass filters

1

having various characteristics can be obtained by joining dielectric plates

3

having different metallization patterns to the dielectric block

2

. Because band pass filters

1

having various characteristics can be obtained by changing the metallization patterns formed on the dielectric plate

3

, band pass filters

1

suitable for mass production can be provided.

Moreover, in the band pass filter

1

, the metallizations formed on the back surface

7

of the dielectric plate

3

are positioned between the dielectric materials forming the dielectric block

2

and the dielectric plate

3

after the dielectric block

2

and the dielectric plate

3

are joined. Thus, the capacitance of the capacitor C

2

formed by the proximate portions

13

of the metallizations

12

-

1

and

12

-

2

is larger than the capacitance obtained by forming the same metallization patterns on the surface

4

of the dielectric block

2

. Therefore, a band pass filter that requires a large capacitance can be provided. On the other hand, in case of forming a band pass filter that does not require a large capacitance, because the distance (gap) between the proximate portions

13

of the metallizations

12

-

1

and

12

-

2

can be widened, design freedom is sufficiently ensured.

Next, a band pass filter according to another embodiment of the present invention will be explained.

FIG. 4

is a perspective view showing a band pass filter

16

of this embodiment; FIG.

4

(

a

) shows the components of the band pass filter in a separated state and FIG.

4

(

b

) shows them in a joined state.

As shown in

FIG. 4

, the band pass filter

16

is constituted of a dielectric block

17

and a dielectric plate

18

.

The dielectric block

16

has the shape of an almost rectangular prism measuring 10.0 mm×4.0 mm×7.3 mm and is made of a ceramic (εr=92) composed mainly of barium titanate. The dielectric block

17

has three through holes

20

-

1

,

20

-

2

and

20

-

3

that are formed in parallel from a surface

19

to the opposite surface (not shown). The through holes

20

-

1

,

20

-

2

and

20

-

3

are 1.4 mm in diameter. Further, all surfaces of the dielectric block

17

(including the inner walls of the through holes

20

-

1

,

20

-

2

and

20

-

3

) except the surface

19

are metallized and are grounded during mounting.

The dielectric plate

18

is a single layered plate measuring 10.0 mm×4.0 mm×0.6 mm made of a ceramic (εr=92) composed mainly of barium titanate, the same material as the dielectric block

17

. The dielectric plate

18

has a surface (front surface)

21

which faces outward when the dielectric plate

18

is joined to the dielectric block

17

, a surface (back surface)

22

which faces toward the surface

19

of the dielectric block

17

when the dielectric plate

18

is joined to the dielectric block

17

, and, an edge surface

23

. As shown in FIG.

4

(

b

), metallizations

24

-

1

and

24

-

2

are formed on the front surface

21

at portions to be input/output terminals and a metallization

26

is formed on the entire front surface

21

except at clearance portions

25

located at the periphery of the metallizations

24

-

1

and

24

-

2

. As shown in FIG.

4

(

a

), metallizations

27

-

1

,

27

-

2

and

27

-

3

are formed on the back surface

22

at portions corresponding to the three through holes

20

-

1

,

20

-

2

and

20

-

3

. When the dielectric plate

18

is joined to the dielectric block

17

, the metallizations

27

-

1

,

27

-

2

and

27

-

3

are directly in contact with the metallizations formed on the inner walls of the through holes

20

-

1

,

20

-

2

and

20

-

3

, respectively. Further, three cavities

28

-

1

,

28

-

2

and

28

-

3

in which metallizations

29

-

1

,

29

-

2

and

29

-

3

are formed on inner walls thereof are formed on the dielectric plate

18

. These metallizations

29

-

1

,

29

-

2

and

29

-

3

are in contact with the metallizations

27

-

1

,

27

-

2

and

27

-

3

, respectively. The metallizations

29

-

1

and

29

-

2

formed on the inner walls of the cavities

28

-

1

and

28

-

2

face the metallizations

24

-

1

and

24

-

2

, respectively, formed on the front surface

21

of the dielectric plate

18

so as to form capacitors C

4

and C

5

. Further, the metallization

29

-

3

formed on the inner wall of the cavity

28

-

3

faces the metallization

26

formed on the front surface

21

of the dielectric plate

18

so as to form a capacitor C

6

. Further, as shown in FIG.

4

(

a

), metallizations

30

whose widths are 0.8 mm are formed on parts of the periphery of the back surface

22

of the dielectric plate

18

. As shown in FIG.

4

(

b

), metallizations

31

are formed on the whole of the edge surface

23

except at the portions in contact with the metallizations

24

-

1

and

24

-

2

and the clearance portions

25

. A capacitor C

7

is formed between the metallization

29

-

1

formed in the cavity

28

-

1

and the metallizations

31

formed on the side surfaces

23

and the metallization

26

formed on the front surfaces

21

. A capacitor C

8

is formed between the metallization

29

-

2

formed in the cavity

28

-

2

and the metallizations

31

formed on the side surfaces

23

and the metallization

26

formed on the front surfaces

21

. As shown in FIG.

4

(

a

), the cavities

28

-

1

and

28

-

2

have portions proximate to the edge surface

23

so as to enhance the capacitance of the capacitors C

7

and C

8

.

The dielectric plate

18

having the above described structure is attached to the dielectric block

17

with its back surface

22

facing the surface

19

of the dielectric block

17

to complete the band pass filter

16

of this embodiment. Thus, the metallizations formed on the dielectric block

17

are connected to the metallization

26

formed on the front surface

21

of the dielectric plate

18

through the metallizations

30

formed on the back surface

22

of the dielectric plate

18

and the metallizations

31

formed on the edge surface

23

of the dielectric plate

18

. As mentioned above, the metallizations

27

-

1

,

27

-

2

and

27

-

3

formed on the back surface

22

of the dielectric plate

18

are in contact with the metallizations formed on the inner walls of the through holes

20

-

1

,

20

-

2

and

20

-

3

, respectively.

FIG. 5

is a circuit diagram of the band pass filter

16

.

As shown in

FIG. 5

, the band pass filter

16

is constituted of the capacitor C

4

having one node connected to the metallization

24

-

1

, which is an input/output terminal; a first resonator and the capacitor C

7

each connected between the other node of the capacitor C

4

and ground; the capacitor C

5

having one node connected to the metallization

24

-

2

, which is an input/output terminal; a second resonator and the capacitor C

8

each connected between the other node of the capacitor C

5

and ground; a third resonator inductively coupled with the first and second resonators; and the capacitor C

6

connected to the third resonator. One of the metallizations

24

-

1

and

24

-

2

is used as the input terminal and the other is used as the output terminal. Each of the first to third resonators is realized by the metallizations formed on the inner walls of the through holes

20

-

1

,

20

-

2

and

20

-

3

.

In the band pass filter

16

of this embodiment, the capacitors C

4

to C

6

formed by the metallizations

29

-

1

,

29

-

2

and

29

-

3

have relatively large capacitance because they are formed on the inner walls of the cavities

28

-

1

,

28

-

2

and

28

-

3

formed on the back surface

22

of the dielectric plate

18

. Thus the capacitance produced between the metallizations

27

-

1

and

27

-

3

and the capacitance produced between the metallizations

27

-

2

and

27

-

3

are substantially negligible so that the coupling between the first and third resonators each realized by the metallizations formed on the inner walls of the through holes

20

-

1

and

20

-

3

and the coupling between the second and third resonators each realized by the metallizations formed on the inner walls of the through holes

20

-

2

and

20

-

3

become not capacitive but inductive.

FIG. 6

is a graph showing the frequency characteristic curve of the band pass filter

16

.

As shown in

FIG. 6

, the band pass filter

16

has a characteristic of passing a high frequency signal of approximately 820 MHz to 850 MHz and effectively eliminating a high frequency signal of approximately 870 MHz to 900 MHz. Therefore, the band pass filter

16

of this embodiment can be used as a component requiring such a frequency characteristic.

The frequency characteristic of the band pass filter

16

strongly depends on the patterns of the metallizations and the shapes of the cavities formed on the dielectric plate

18

. Thus, a desired frequency characteristic can be given to the band pass filter

16

by varying the patterns of the metallizations and/or the shapes of the cavities formed on the dielectric plate

18

.

Almost the same method can be used to fabricate the band pass filter

16

of this embodiment as that used for the band pass filter

1

explained earlier except that the cavities

28

-

1

,

28

-

2

and

28

-

3

should be formed during the press molding of the main body of the dielectric plate

18

.

To form the cavities

28

-

1

,

28

-

2

and

28

-

3

on the main body of the dielectric plate

18

, a die having convex portions corresponding to the cavities

28

-

1

,

28

-

2

and

28

-

3

can be used during the press molding. It is easier to form the cavities on the main body of the dielectric plate

18

by press molding than it is to form cavities on the main body of the dielectric block. It is also easy to manufacture the die used therefor. This is because in case of forming cavities on the main body of the dielectric block, since the cavities should be formed on the surface at the portion where the through holes are to be formed, the die for the press molding would be complicated because a main die, the convex portions (sub die) to form the cavities and the rods disposed at almost center of the convex portions (sub die) to form the through holes are needed. Therefore, much time and expense would be required to manufacture the die and, in addition, the process of the press molding using such a die would be complicated. On the contrary, in case of forming cavities on the main body of the dielectric plate

18

, since forming the convex portions to form the cavities is only required for the die, manufacturing the die is easy and, in addition, the process of the press molding using such a die is easy. Since, similarly to the case of the band pass filter

1

, it is thus relatively easy to fabricate the band pass filter

16

of this embodiment, the fabrication cost thereof can be lowered.

Further, also in the band pass filter

16

, complex metallization patterns do not have to be formed on the dielectric block. Therefore, not only is fabrication of the dielectric block

17

very easy but band pass filters

16

having various characteristics can be obtained by joining dielectric plates

18

having different metallization patterns and/or different cavity shapes to the dielectric block

17

, so that band pass filters

16

suitable for mass production can be provided.

Next, the band pass filter according to a further embodiment of the present invention will be explained.

FIG. 7

is a perspective view showing a band pass filter

32

of this embodiment; FIG.

7

(

a

) shows the components of the band pass filter in a separated state and FIG.

7

(

b

) shows them in a joined state.

As shown in

FIG. 7

, the band pass filter

32

is constituted of a dielectric block

33

and a dielectric plate

34

.

The dielectric block

33

has the shape of an almost rectangular prism measuring 9.6 mm×2.5 mm×3.5 mm and is made of a ceramic (εr=92) composed mainly of barium titanate. The dielectric block

33

has six through holes

36

-

1

to

36

-

6

that are formed in parallel from a surface

35

to the opposite surface (not shown). The through holes

36

-

1

to

36

-

6

are 0.7 mm in diameter. Further, all surfaces of the dielectric block

33

(including the inner walls of the through holes

36

-

1

to

36

-

6

) except the surface

35

are metallized and are grounded during mounting.

The dielectric plate

34

is a single layered plate measuring 9.6 mm×2.5 mm×0.6 mm and made from a ceramic (εr=92) composed mainly of barium titanate, the same material as the dielectric block

33

. The dielectric plate

34

has a surface (front surface)

37

that faces outward when the dielectric plate

34

is joined to the dielectric block

33

, a surface (back surface)

38

that faces the surface

35

of the dielectric block

33

when the dielectric plate

34

is joined to the dielectric block

33

, and an edge surface

39

. As shown in FIG.

7

(

b

), metallizations

40

-

1

,

40

-

3

and

40

-

6

are formed on the front surface

37

at portions to be a transmitter terminal TX (FIG.

8

), an antenna terminal ANT and a receiver terminal RX, respectively, and a metallization

42

is formed on the entire front surface

37

except at clearance portions

41

located at the peripheries of the metallizations

40

-

1

,

40

-

3

and

40

-

6

. As shown in FIG.

7

(

a

), cavities

44

-

1

to

44

-

3

in which metallizations

45

-

1

to

45

-

3

are formed on inner walls thereof are formed on the back surface

38

of the dielectric plate

34

at positions corresponding to the three through holes

36

-

1

to

36

-

3

, respectively. Further, projections are formed at approximately the centers of these cavities

44

-

1

to

44

-

3

. When the dielectric plate

34

is joined to the dielectric block

33

, these projections are interfitted into the through holes

36

-

1

to

36

-

3

so that the metallizations

45

-

1

to

45

-

3

formed on the cavities

44

-

1

to

44

-

3

are directly in contact with the metallizations formed on the inner walls of the through holes

36

-

1

to

36

-

3

, respectively. Metallizations

43

-

4

to

43

-

6

are formed on the back surface

38

at portions corresponding to the three through holes

36

-

4

to

36

-

6

, respectively. When the dielectric plate

34

is joined to the dielectric block

33

, the metallizations

43

-

4

to

43

-

6

are directly in contact with the metallizations formed on the inner walls of the through holes

36

-

4

to

36

-

6

, respectively.

The metallization

45

-

1

formed in the cavity

44

-

1

, the metallization

45

-

3

formed in the cavity

44

-

3

and the metallization

43

-

6

face the metallizations

40

-

1

,

40

-

3

and

40

-

6

, respectively, formed on the front surface

37

of the dielectric plate

34

so as to form capacitors. Further, as shown in FIG.

7

(

a

), metallizations

46

are formed on a part of periphery of the back surface

38

of the dielectric plate

34

. As shown in FIG.

7

(

b

), metallizations

47

are formed on the whole of the edge surface

39

except at the portions in contact with the metallizations

40

-

1

,

40

-

3

and

40

-

6

and the clearance portions

41

.

As shown in FIG.

7

(

a

), the proximate portions between the metallizations

45

-

3

and

43

-

4

, the metallizations

43

-

4

and

43

-

5

and the metallizations

43

-

5

and

43

-

6

are formed so as to enhance the capacitance between these metallizations.

The dielectric plate

34

having the above described structure is attached to the dielectric block

33

with its back surface

38

facing the surface

35

of the dielectric block

33

to complete the band pass filter

32

of this embodiment.

FIG. 8

is a circuit diagram of the band pass filter

32

.

As shown in

FIG. 8

, the band pass filter

32

can be used as a so-called duplexer which works as a band pass filter having the antenna terminal ANT as an input terminal and the receiver terminal RX as an output terminal and works as a band pass filter having the transmitter terminal TX as an input terminal and the antenna terminal ANT as an output terminal.

FIG. 9

is a graph showing the frequency characteristic curve of the band pass filter

32

.

As shown in

FIG. 9

, the band pass filter

32

has a characteristic of passing a high frequency signal of approximately 2090 MHz to 2200 MHz and effectively eliminating a high frequency signal of lower than around 2000 MHz between the antenna terminal ANT and the receiver terminal RX; and a characteristic of passing a high frequency signal of approximately 1850 MHz to 2000 MHz and effectively eliminating a high frequency signal of higher than around 2050 MHz between the antenna terminal ANT and the transmitter terminal TX. Therefore, the band pass filter

32

of this embodiment can be used as a component requiring such a frequency characteristic.

The frequency characteristic of the band pass filter

32

strongly depends on the patterns of the metallizations and the shapes of the cavities formed on the dielectric plate

34

. Thus, a desired frequency characteristic can be given to the band pass filter

32

by varying the patterns of the metallizations and/or the shapes of the cavities formed on the dielectric plate

34

.

Almost the same method can be used to fabricate the band pass filter

32

of this embodiment as that used for the band pass filter

1

explained earlier except that the cavities

44

-

1

,

44

-

2

and

44

-

3

should be formed during the press molding of the main body of the dielectric plate

34

. Since, similarly to the case of the band pass filter

1

, it is relatively easy to fabricate the band pass filter

32

of this embodiment, the fabrication cost thereof can be lowered.

Further, also in the band pass filter

32

, complex metallization patterns do not have to be formed on the dielectric block

33

. Therefore, not only is the fabrication of the dielectric block

33

very easy but band pass filters

32

having a various characteristics can be obtained by joining dielectric plates

34

having different metallization patterns and/or different cavity shapes to the dielectric block

33

, so that band pass filters

32

suitable for mass production can be provided.

Next, the band pass filter according to a further embodiment of the present invention will be explained.

FIG. 10

is a perspective view showing a band pass filter

48

of this embodiment; FIG.

10

(

a

) shows the components of the band pass filter in a separated state and FIG.

10

(

b

) shows them in a joined state.

As shown in

FIG. 10

, the band pass filter

48

is constituted of a dielectric block

49

and a dielectric plate

50

.

Although the band pass filter

48

of this embodiment cannot achieve as wide a pass band as that of the band pass filter

16

, it can provide characteristics similar to those of the band pass filter

16

without forming the cavities

28

-

1

,

28

-

2

and

28

-

3

.

In the band pass filter

48

of this embodiment, since, as in the case of the band pass filter

1

explained earlier, no cavities are formed on the dielectric plate

50

, it can be fabricated by easier processes. Further, in the band pass filter

48

of this embodiment, since metallization is formed on of the whole periphery of the back surface of the dielectric plate

50

, the gap between the dielectric block

49

and the dielectric plate

50

is covered with this metallization when the dielectric plate

50

is joined to the dielectric block

49

. Contamination by foreign matter entering into the gap is therefore prevented.

Next, the band pass filter according to a further embodiment of the present invention will be explained.

FIG. 11

is a schematic plan view showing a band pass filter

51

that is a further preferred embodiment of the present invention.

As shown in

FIG. 11

, the band pass filter

51

of this embodiment is constituted of a dielectric block

52

and a dielectric plate

53

. The dielectric block

52

has the same structure as the dielectric block

2

,

17

,

33

or

49

of the band pass filter

1

,

16

,

32

or

48

. On the other hand, although the dielectric plate

53

has the same structure as the dielectric plate

3

,

18

,

34

or

50

of the band pass filter

1

,

16

,

32

or

48

, its size in plan view is a little larger than that of the dielectric block

52

. Metallizations (not shown) are formed on the periphery of the back surface (the surface facing the dielectric block

52

) of the dielectric plate

53

.

In the band pass filter

51

of this embodiment, the dielectric block

52

and the dielectric plate

53

whose size in plan view is larger than that of the dielectric block

52

are used and are fixed by solder

54

applied to the portion of the back surface of the dielectric plate

53

not covered with the dielectric block

52

and a part of the side surface of the dielectric block

52

.

According to the band pass filter

51

of this embodiment, the dielectric plate

53

and the dielectric block

52

can be strongly fixed because the solder

54

is provided over a wide area of the dielectric block

52

and the dielectric plate

53

.

The present invention has thus been shown and described with reference to specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the described arrangements but changes and modifications may be made without departing from the scope of the appended claims.

For example, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, solder is used to adhere the dielectric plates

3

,

18

,

34

and

50

to the dielectric blocks

2

,

17

,

33

and

49

. However the dielectric plates

3

,

18

,

34

and

50

can be joined to the dielectric blocks

2

,

17

,

33

and

49

prior to the sintering of the Ag paste and sintering then be performed to adhere them to each other. In this case, a soldering process for the dielectric blocks

2

,

17

,

33

and

49

and the dielectric plates

3

,

18

,

34

and

50

is not necessary. According to this method, because solder is not used, lead (Pb) free soldering that should be performed at higher temperature than ordinary can be applied when the band pass filter is mounted.

Further, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, a ceramic composed mainly of barium titanate is used as the material of the dielectric blocks

2

,

17

,

33

and

49

and the dielectric plates

3

,

18

,

34

and

50

. However the present invention is not limited to use of this material and dielectric blocks and dielectric plates can instead be made of any of various other materials such as ceramic of barium oxide type.

Moreover, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, the capacitances are obtained by the metallization patterns formed on the dielectric plates

3

,

18

,

34

and

50

. However, metallization patterns which work as other elements such as inductors or elements controlling the phase of input/output signals can be formed on the dielectric plates

3

,

18

,

34

and

50

according to the desired electrical characteristics.

Further, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, the dielectric plates

3

,

18

,

34

and

50

are joined to the dielectric blocks

2

,

17

,

33

and

49

after the metallizing has been completed. However, insofar as at least the processes for the back surface of the dielectric plates

3

,

18

,

34

and

50

(forming the cavities and metallizations) have been finished, they can be joined at anytime and the metallizing be performed at the other portions thereafter.

Moreover, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, although screen-printing is used to form the metallizations on the surfaces of the dielectric blocks

2

,

17

,

33

and

49

(excluding the inner walls of the through holes), the present invention is not limited to this method and any of various other methods can be used instead to form the metallizations. For example, it is possible to form the metallizations on the surfaces of the dielectric blocks

2

,

17

,

33

and

49

by spraying Ag paste onto the surfaces of the dielectric blocks

2

,

17

,

33

and

49

or pressing a sponge impregnated with Ag paste thereon.

Further, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, although the metallizations are formed on inner walls of the through holes of the dielectric blocks

2

,

17

,

33

and

49

by introducing Ag paste having low viscosity, the present invention is not limited to this method and any of various other methods can be used to form the metallizations. For example, it is possible to form the metallizations on the inner walls of the through holes of the dielectric blocks

2

,

17

,

33

and

49

by introducing the Ag paste from one openings of the through holes while the other openings are vacuumed.

Moreover, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, although screen-printing is used to form the metallizations on the back surfaces of the dielectric plates

3

,

18

,

34

and

50

, the present invention is not limited to this method and any of various other methods can be used instead to form the metallizations so as long as pattern accuracy can be ensured.

Further, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, although transfer printing is used to form the metallizations on the side surfaces of the dielectric plates

3

,

18

,

34

and

50

, the present invention is not limited to this method and any of various other methods can be used instead to form the metallizations.

Moreover, in the band pass filters

1

,

16

,

32

and

48

of the above described embodiments, although Ag paste is used as the material of the metallizations, the present invention is not limited to use of Ag paste and any of various other conductive materials, copper (coppering), for example, can be used instead. In case of using coppering as the material of the metallizations, the copper plating can be performed with a resist formed on the portion where the metallizations should not be formed (certain surfaces of the dielectric blocks

2

,

17

,

33

and

49

) in advance. It is preferable to use an electroless plating.

Further, in the band pass filters

16

and

32

of the above described embodiments, it is preferable for the walls of the cavities

28

-

1

to

28

-

3

and

44

-

1

to

44

-

3

formed on the back surfaces

22

and

38

of the dielectric plates

18

and

34

not to be perpendicular to the back surfaces

22

and

38

of the dielectric plates

18

and

34

but to be tapered at a predetermined angle. In the case where the walls of the cavities are tapered, while the convex portions of the die for the press molding must also be tapered, a die having tapered convex portions has the advantage of enhanced working efficiency because the die and the dielectric plate are easy to separate.

Moreover, in the band pass filter

32

of the above described embodiment, three cavities

44

-

1

to

44

-

3

are formed on the dielectric plate

34

and the metallizations

45

-

1

to

45

-

3

are formed on the inner walls of the cavities

44

-

1

to

44

-

3

. However, it is alternatively possible to form the metallizations in a manner similar to the metallizations on the dielectric plate

50

of the band pass filter

48

as shown in

FIG. 10

, without forming the cavities

44

-

1

to

44

-

3

on the dielectric plate

34

.

Further, in the band pass filter

51

of the above described embodiment, although the area of the dielectric plate

53

is made larger than the area of the dielectric block

52

, the area of the dielectric block

52

can instead be made larger than the area of the dielectric plate

53

. In this case, the metallizations formed on the side surfaces of the dielectric block

52

and the metallizations formed on the edge surface of the dielectric plate

53

should be electrically and mechanically connected by providing Ag paste on the exposed portion of the surface of the dielectric block

52

not covered with the dielectric plate

53

using a syringe or the like.

As described above, according to the present invention, a compact band pass filter that can be fabricated at low cost can be provided.

Number	Name	Date	Kind
4703291	Nishikawa et al.	Oct 1987	A
4987393	Yorita et al.	Jan 1991	A
5150089	Komazaki et al.	Sep 1992	A
5157365	Hoang	Oct 1992	A
5170141	Yamazaki et al.	Dec 1992	A
5208565	Sogo et al.	May 1993	A
5239280	Noguchi et al.	Aug 1993	A
5764118	Saito et al.	Jun 1998	A
6052040	Hino	Apr 2000	A

Number	Date	Country
03173202	Jul 1991	JP
05063406	Mar 1993	JP
11027006	Jan 1999	JP

Band pass filter

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (9)

Foreign Referenced Citations (3)