1. Field of the Invention
The present invention relates to a shield and a dielectric filter, and more specifically, to a shield that can prevent the thickness of a dielectric filter from increasing without increasing the manufacturing cost of the dielectric filter and a dielectric filter equipped with the shield.
2. 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 to a dielectric filter. The through holes formed on the dielectric block work as resonators for the high frequency signal. A filter circuit such as a band pass filter circuit is formed by adding capacitance and so forth to the resonators.
When the dielectric filter is mounted on the printed circuit board, the metallization formed on the surfaces of the dielectric block is grounded. However, since the metallization formed on the top surface of the dielectric block (top metallization) is far from a ground electrode formed on the printed circuit board, the potential of the top metallization easily fluctuates. Because such a fluctuation may deteriorate the filter characteristic, grand potential is conventionally bypassed and given to the top metallization using a shield so as to reduce the fluctuation.
FIG. 1 is a schematic perspective view showing a conventional shield 1. FIG. 2 is a schematic sectional view showing the dielectric filter 5 equipped with the shield 1.
As shown in FIGS. 1 and 2, the conventional shield 1 is a metal plate shaped like a capital L having a first plate 2 and a second plate 3 perpendicular to the first plate 2. The shield 1 is fixed to the dielectric filter 5 by adhering the first plate 2 on the top metallization of the dielectric filter 5. When the dielectric filter 5 equipped with the shield 1 is mounted on the printed circuit board, the end of the second plate 3 and the ground electrode formed on the printed circuit board are electrically and mechanically connected so that the fluctuation of the potential on the top metallization of the dielectric filter 5 is restrained.
As described above, because the end of the second plate 3 of the conventional shield 1 is connected to the ground electrode formed on the printed circuit board, it is necessary that the end of the second plate 3 and the bottom surface of the dielectric filter 5 are coplanar when the shield 1 is attached to the dielectric filter 5. However, since the size and the shape of a dielectric block which constitutes the dielectric filter 5 depend on the manufacture conditions, it is extremely difficult to form the shield 1 so that the end of the second plate 3 and the bottom surface of the dielectric filter 5 are coplanar.
Further, because the shield 1 is grounded, the filter characteristics of the dielectric filter 5 changes with the gap between the second plate 3 and the dielectric filter 5. However, because the conventional shield 1 is attached to the top metallization of the dielectric filter 5, it is difficult to fix the gap between the second plate 3 and the dielectric filter 5 to a desired distance.
In order to solve the problems, a technique of using a shield shaped like a capital U is proposed by U.S. Pat. No. 5,745,018.
In recent years, not only small area but also thin shape is strongly required for the various components to be mounted on the printed circuit board. However, according to the technique disclosed on U.S. Pat. No. 5,745,018, because the shield is attached on the top metallization of the dielectric filter similar to another conventional technique shown in FIGS. 1 and 2, total thickness of the dielectric filter mounted on the printed circuit board should be increased.
Also, another technique that forming a cavity for storing a part of the shield on the dielectric block is proposed by U.S. Pat. No. 5,218,329. However, according to this technique, an additional process for forming the cavity is needed so that the manufacture cost should be increased.
One of features of the present invention is to provide an improved shield for a dielectric filter that can prevent the thickness of the dielectric filter from increasing without increasing the manufacturing cost of the dielectric filter.
Another object of the present invention is to provide a dielectric filter equipped with such a shield.
The above and other objects of the present invention can be accomplished by a shield attachable to a dielectric filter, comprising:
a first metallic plate;
a second metallic plate elongated from a first end of the first metallic plate in a predetermined direction;
a third metallic plate elongated from a second end of the first metallic plate opposite to the first end in the predetermined direction; and
a metallic projecting part projecting from the first metallic plate at a portion between the first and second ends of the first metallic plate.
Because the shield according to the present invention can be attached to the dielectric filter such that the second and third metallic plates pinch the dielectric filter from the side surfaces thereof and that the metallic projecting part is in contact with the metallization of the dielectric filter, the shield does not increase total thickness of the dielectric filter equipped therewith. For this reason, it is enabled to satisfy the demand to thin the total thickness of the dielectric filter. Further, according to the present invention, the distance between the shield and the dielectric filter is fixed by the length of the metallic projecting part. Therefore, the distance does not vary so that the fluctuation of the filter characteristics can be avoided. Moreover, since the shield after attached to the dielectric filter has sufficient mechanical strength, the shield can be made of a thin metal plate.
In a preferred aspect of the present invention, a length of the metallic projecting part according to the predetermined direction is shorter than either lengths of the second and third metallic plates according to the predetermined direction.
In a further preferred aspect of the present invention, the metallic projecting part is elongated from a third end or its adjacent portion, the third end being perpendicular to the first end of the first metallic plate.
In a further preferred aspect of the present invention, the metallic projecting part is formed on substantially throughout between first and second ends of the first metallic plate.
In a further preferred aspect of the present invention, the metallic projecting part is formed by folding a part of the first metallic plate using slits formed on the first metallic plate.
In another preferred aspect of the present invention, the metallic projecting part is made of projection member attached to the first metallic plate.
In a further preferred aspect of the present invention, the first metallic plate has a removed portion at a fourth end opposite to the third end.
In a further preferred aspect of the present invention, the shield further comprises another metallic projecting part elongated from the fourth end or its adjacent portion.
The above and other objects of the present invention can be also accomplished by a shield attachable to a dielectric filter, comprising:
a first metallic plate;
a second metallic plate elongated from a first end of the first metallic plate in a predetermined direction;
a third metallic plate elongated from a second end of the first metallic plate opposite to the first end in the predetermined direction;
a first metallic projecting part projecting from the second metallic plate toward the third metallic plate; and
a second metallic projecting part projecting from the third metallic plate toward the second metallic plate.
Because the shield according to the present invention can be attached to the dielectric filter such that the second and third metallic plates pinch the dielectric filter from the side surfaces thereof and that the first and second metallic projecting parts are in contact with the metallization of the dielectric filter, the shield does not increase total thickness of the dielectric filter equipped therewith. Further, according to the present invention, because the distance between the shield and the dielectric filter is fixed by the length of the first and second metallic projecting parts, the distance does not vary so that the fluctuation of the filter characteristics can be avoided. Moreover, since the shield after attached to the dielectric filter has sufficient mechanical strength, the shield can be made of a thin metal plate.
In a preferred aspect of the present invention, the first metallic projecting part is formed by folding a part of the second metallic plate using slits formed on the second metallic plate, the second metallic projecting part is formed by folding a part of the third metallic plate using slits formed on the third metallic plate.
The above and other objects of the present invention can be also accomplished by a dielectric filter, comprising:
a dielectric block which comprises:
a top surface;
a first side surface with a first metallization, being perpendicular to the top surface;
a second side surface with a second metallization, being opposite to the first side surface; and
a third side surface with a third metallization, being perpendicular to the top surface and the first side surface; and
a shield attachable to the dielectric block which comprises:
a first metallic plate;
a second metallic plate elongated from a first end of the first metallic plate in a predetermined direction and connecting with the first metallization;
a third metallic plate elongated from a second end of the first metallic plate opposite to the first end in the predetermined direction and connecting with the second metallization; and
a metallic projecting part projecting from the first metallic plate at a portion between the first and second ends of the first metallic plate and connecting with the third metallization.
According to the present invention, because the total thickness of the dielectric filter is not increased by attaching the shield, it is enabled to satisfy the demand to thin the total thickness of the dielectric filter. Further, according to the present invention, because the distance between the shield and the dielectric block is fixed by the length of the metallic projecting part, the distance does not vary so that the fluctuation of the filter characteristics can be avoided. Moreover, since the shield after attached to the dielectric block has sufficient mechanical strength, the shield can be made of a thin metal plate.
In a preferred aspect of the present invention, the dielectric filter further comprises a top metallization formed on the top surface of the dielectric block, the top metallization and the third metallization being electrically connected to each other.
In a further preferred aspect of the present invention, the dielectric block has through holes passing from the third side surface to a fourth side surface opposite to the third surface.
In a further preferred aspect of the present invention, the metallic projecting part is elongated from a third end or its adjacent portion, the third end being perpendicular to the first end of the first metallic plate.
In a further preferred aspect of the present invention, the metallic projecting part is formed on substantially throughout between first and second ends of the first metallic plate.
In a further preferred aspect of the present invention, a fourth end of the first metallic plate opposite to the third end and a bottom surface of the dielectric block opposite to the top surface are substantially coplanar.
In a further preferred aspect of the present invention, the first metallic plate has a removed portion at the fourth end.
In a further preferred aspect of the present invention, the dielectric filter further comprises a fourth metallization formed on the third side surface of the dielectric block, the shield further comprising another metallic projection part being in contact with the fourth metallization elongated from the fourth end or its adjacent portion.
The above and other objects and features of the present invention will become apparent from the following description made with reference to the accompanying drawings.