Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-09-06
2002-03-19
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S321000, C310S345000, C310S348000, C310S366000
Reexamination Certificate
active
06359532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an energy trapping type piezoelectric filter comprising a substrate having a plurality of oscillating sections.
2. Prior Art
Various energy trapping type piezoelectric filters have been proposed. For example, Japanese Patent Application Laid-Open No. 7-263999 discloses one of such piezoelectric filters. The disclosed filter comprises a pair of oscillating sections laterally arranged and a capacitor electrode centrally arranged on a surface of a single piezoelectric substrate. Such a conventional arrangement, however, has disadvantages that the piezoelectric substrate inevitably is forced to have an oblong profile and the piezoelectric filter has to have a large surface area for carrying the components thereon. Additionally, such a piezoelectric filter is subjected to various restrictions in terms of the profile, the thickness and the regions assigned to the oscillating sections to significantly limit the variable capacitance range, although the piezoelectric filter has only a single coupling capacitor.
In order to overcome such disadvantage there has been proposed another energy trapping type piezoelectric filter comprising two piezoelectric substrates one of which is provided with one of the two oscillating sections and the other is provided with the other oscillating section. That is, such a piezoelectric filter comprises a first and second piezoelectric substrates adapted to produce a single oscillating section, a pair of input/output connection terminals and a grounding terminal when laid one on the other, and a pair of insulating cover plates having a pair of input/output electrodes and a grounding electrode (see, for example, EP 0897217A2). This filter arrangement comprises the first and second piezoelectric substrates are stacked to each other. There are, therefore, advantages that the overall size of the filter can be reduced substantially to half that of the conventional arrangement disclosed in Japanese Patent Application Laid-Open No. 7-263999, the area of the respective piezoelectric substrate can be reduced , and a mounting area of the filter can be reduced. Furthermore, the first and second piezoelectric substrates are provided with respective coupling electrodes that are connected in parallel. This results in other advantages that a coupling capacitance thereof can be suitably set, and a circuit design can be easily made compared with the conventional arrangement having a single coupling capacitor. An equivalent circuit of the piezoelectric filter of this kind is illustrated in
FIG. 8
of the accompanying drawings.
Thus, the first and second piezoelectric substrates, each having a single oscillating section, are laid one on the other in an energy trapping type piezoelectric filter having the above described configuration so that they have only a limited surface area for carrying the related components of the piezoelectric filter. Additionally, the first and second piezoelectric substrates carry thereon respective capacitor electrodes that are connected to each other. Therefore, the coupling capacitance of the device can be selected freely to allow an enhanced degree of freedom for the design of the device if compared with conventional piezoelectric filters of the type under consideration having a single coupling capacitor.
With such an arrangement the first piezoelectric substrate will be laid on the second piezoelectric substrate or vice versa. The distance between the input/output connection terminals of the first and second piezoelectric substrates will be reduced significantly if compared with the conventional device as described in the above. Also, the distance between the input electrode and the output electrode disposed on one of the insulating cover plates of a piezoelectric filter will be reduced. Therefore, it will be apt to easily become electromagnetically coupled with each other to damage the attenuation characteristic of the device.
In the previously proposed energy trapping type piezoelectric filter having a single oscillating section provided on a piezoelectric substrate, when an attempt is made for more reducing the total area of the filter, an output electrode and an input electrode provided on an insulating base plate become closer to each other. Therefore, a mutual electromagnetic coupling may be easily induced. This results in a deterioration of an attenuation characteristic of the filter. In order to avoid any deterioration of the attenuation characteristic of the filter it has been proposed that shielding electrode section is formed on the bottom surface of the insulating base substrate at a region between the output electrode and the input electrode, thereby improving the attenuation characteristic. However, it has been found that an influence of a coupling of the output electrode and the input electrode outside of the bottom surface of the insulating base substrate is not negligible. This is certainly due to the fact that the base plate or substrate has a very thin configuration or a thickness of about 0.5 mm in comparison with a thickness of the piezoelectric filter that is typically about 1.5 mm.
It is, therefore, an object of the invention to provide an energy trapping type piezoelectric filter in which a coupling of an input and output electrodes is more reduced to improve an attenuation characteristic of the filter.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided an energy trapping type piezoelectric filter comprising:
a first piezoelectric substrate having an oscillating section, an output connecting terminal and grounding connector terminals;
a second piezoelectric substrate having an oscillating section, an input connecting terminal and grounding connector terminals;
the first and second piezoelectric substrates being stacked to each other so that the output connector terminal on the first piezoelectric substrate is positioned with respect to the input connector terminal on the second piezoelectric substrate with a certain discrepancy between them in a perpendicular direction;
an insulating base plate having a bottom surface provided with an input electrode which is connected to the input connecting terminal, two output electrodes which are connected to the output connecting terminal, and ground electrodes which are connected to the grounding connector terminals, the insulating base plate being arranged as a most bottom layer; and
a shielding conductive member provided on an upper surface of the insulating base plate for preventing an electromagnetic coupling between the input electrode and the output electrodes, the shielding conductive member being connected to the ground electrodes.
With the provision of the shielding conductive member on an upper surface of the insulating base plate between the input/output electrodes, any coupling of the input/output electrodes can be effectively prevented, thereby improving an attenuation characteristic of the filter.
According to a second aspect of the invention there is provided an energy trapping type piezoelectric filter comprising:
first piezoelectric substrate having a first main surface and a second main surface opposite to the first main surface, the first main surface including a first pair of oscillating electrodes arranged vis-a-vis relative to each other and a first capacitor electrode, one of the oscillating electrodes being electrically connected to the first capacitor electrode, and the other oscillating electrode being electrically connected to an output connection terminal which is arranged on a lateral edge portion of the first piezoelectric substrate to form a first electrode pattern on the first surface, the second main surface including a common electrode arranged oppositely relative to the first pair of oscillating electrodes and a second capacitor electrode arranged opposite to the first capacitor electrode, the second capacitor electrode and the common electrode being electrically connected to each other to form a second electrode pattern
Sugiyama Takeshi
Wakita Manabu
Bettendorf Justin P.
Larson & Taylor PLC
NGK Spark Plug Co. Ltd.
Summons Barbara
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