Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1998-04-09
1999-09-14
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
310326, 310348, 310353, H03H 956, H03H 910, H03H 917
Patent
active
059528982
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a piezoelectric filter of the surface mounting type.
BACKGROUND ART
A piezoelectric filter using a thickness sliding oscillation or the like of a piezoelectric substrate is generally arranged such that there is housed, in a container, an element comprising a piezoelectric substrate of crystal or the like provided on one surface thereof with a pair of input/output oscillating electrodes (split electrodes) with a predetermined gap provided, and on the other surface thereof with a common electrode opposite to the pair of input/output oscillating electrodes.
Recently, such a piezoelectric filter is required to be made in a small size particularly for a portable-type communication device or the like. Further, to simplify the mounting process, it is required that the element is not only made in a small size, but also housed in a package of the surface mounting type. In such a structure in which the element in a small size is housed in a package of the surface mounting type, however, a leakage current is readily generated due to an electromagnetic coupling between the input/output oscillating electrodes. This is remarkably disadvantageous in that the assured attenuation amount is lowered.
To overcome the problem above-mentioned, there has been proposed a piezoelectric filter which comprises (i) a surface mounting-type package body made of an insulator in which a layer-like shield electrode uniform in grounding potential is formed on a flat portion and (ii) a piezoelectric filter element incorporated in and supported and by the package body, the piezoelectric filter element having a pair of input/output oscillating electrodes arranged to be opposite to the flat layer-like shield electrode, and in which the pair of input/output oscillating electrodes and the shield electrode are opposite to each other with a small distance provided therebetween (For example, Japanese Patent Laid-Open Publications No. 6-29776 and 6-85599 and U.S. Pat. No. 5,382,929).
According to the arrangement above-mentioned, as seen in each of the publications above-mentioned, the assured attenuation amount is increased by reducing the distance between the input/output oscillating electrodes and the shield electrode disposed at the package body. However, when the distance is reduced down to a certain limit value, the assured attenuation amount reaches the peak. As schematically shown in FIG. 10, when the distance is further reduced, floating capacitances C are generated between each of a pair of input/output oscillating electrodes 71, 72 and a shield electrode 73. Accordingly, a leakage current iL is generated between the pair of input/output oscillating electrodes 71, 72, causing the assured attenuation amount to be gradually reduced.
More specifically, even though a flat shield electrode is disposed opposite to a pair of input/output oscillating electrodes with a predetermined gap provided therebetween, the assured attenuation amount is limited to the peak value determined based on the distance between the shield electrode and the pair of input/output oscillating electrodes, and cannot be increased more than this peak value.
In view of the foregoing, the present invention is proposed with the object of providing a piezoelectric filter of the surface mounting type so arranged as to increase the assured attenuation amount as compared with the conventional piezoelectric filter.
DISCLOSURE OF THE INVENTION
To achieve the object above-mentioned, the present invention provides a piezoelectric filter of the surface mounting type including (i) a multiple-mode piezoelectric filter element in which a piezoelectric substrate is provided on one surface thereof with a pair of in put/output oscillating electrodes with a predetermined gap provided, and on the other surface thereof with a common electrode opposite to the pair of input/output oscillating electrodes, and (ii) a package body of the surface mounting type made of an insulating material, the filter element being supported, at predetermined pos
REFERENCES:
patent: 5382929 (1995-01-01), Inao et al.
Arimura Hiroyuki
Nakata Hozumi
Daishinku Corporation
Pascal Robert
Summons Barbara
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