Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-09-25
2003-05-13
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S189000, C310S321000, C310S330000, C310S357000, C310S359000
Reexamination Certificate
active
06563400
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric resonator bending vibrations and a ladder-type filter including such a piezoelectric resonator.
2. Description of the Related Art
As piezoelectric resonators for use in a kHz band or for use in a frequency band of 100 kHz to 1 MHz in particular, piezoelectric resonators utilizing spreading vibrations are well known. In piezoelectric resonators utilizing spreading vibrations, the product of the length Ls of a side and the resonance frequency fr is generally constant, and it is defined by:
As=Ls×fr
Here, As is a constant (frequency-related constant) and As≅2100 kHz·mm. For example, in order to obtain resonators having a resonance frequency of fr=455 kHz as in AM radios, the length Ls of the side becomes 4.62 mm.
In recent years, the miniaturization of equipment is more frequently required, and smaller-sized and thinner electronic components are being demanded. Under such circumstances, it is difficult to use electronic components having a length of a side that is 5 mm as described above. Furthermore, in ladder-type filters, the attenuation value is determined by the capacitance ratio between the parallel resonator and the series resonator. That is, in order to obtain a higher attenuation value, the capacitance between terminals of the parallel resonator may be increased and the capacitance between terminals of the series resonator may be decreased. However, when the capacitance between terminals of the parallel resonator is increased, it is inevitable to decrease the thickness of its piezoelectric substrate, and accordingly the mechanical strength is deteriorated and the range of the thickness is therefore very limited.
In consideration of such circumstances, the present inventor has proposed a piezoelectric resonator in which four or more electrode layers and three or more piezoelectric layers are alternately laminated, at least two of the piezoelectric layers are polarized in the thickness direction, and the above-described electrode layers are connected to each other such that, in a portion of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction of the piezoelectric layers and that, in a portion of the other piezoelectric layers, an electric field is generated in a direction that is opposite to the polarization direction of the piezoelectric layers (Japanese Patent Application No. 11-294491).
In this piezoelectric resonator, since the piezoelectric layer in which the polarization direction and the direction of electric field are the same contracts in the planar direction and the piezoelectric layer in which the polarization direction and the direction of electric field are opposite to each other expands, bending vibration is generated as an entire piezoelectric resonator. In such a piezoelectric resonator utilizing bending vibrations, the dimensions thereof can be more reduced, compared with a piezoelectric resonator utilizing spreading vibrations, when both have the same resonance frequency. Moreover, since four or more electrode layers are provided, capacitance can be individually generated between electrode layers, and the total capacitance between terminals can be increased. Moreover, the piezoelectric layers are laminated, and accordingly even if each piezoelectric layer is thin, there is an advantage that a sufficient mechanical strength can be achieved.
However, in the piezoelectric resonator having the above-described construction, since an input electrode and an output electrode are separately disposed on both top and bottom main surfaces, when the piezoelectric resonator is mounted on a board, or substrate, after the electrode on one main surface has been connected to an electrode of the board, the electrode on the other main surface must be connected to another electrode on the board by wire bonding or by using terminals, etc., and accordingly it was difficult to establish connections. That is, the above-described piezoelectric resonator was not constructed in such a way that electrical characteristics can be taken out from either of the top and bottom main surfaces, and it was difficult to surface mount the piezoelectric resonator.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a very small-sized piezoelectric resonator in which electric characteristics can be taken out from either of the top and bottom main surfaces and which is appropriate for surface mounting, and to provide a ladder-type filter including such a piezoelectric resonator.
According to a first preferred embodiment of the present invention, a piezoelectric resonator utilizing bending vibrations, includes three or more electrode layers and two or more piezoelectric layers that are alternately laminated, first and second end surface electrodes provided on the end surfaces of the laminated piezoelectric layers, recessed portions for insulating the electrode layer, which is not connected to the first or second end surface electrodes, from the first or second end surface electrodes, the recessed portions provided in the periphery portion of the electrode layer not connected to the first or second end surface electrodes, insulation layers provided on the surface of the outermost electrode layers, and first and second external electrodes, which are connected to the first and second end surface electrodes respectively, provided on the outer surface of the insulation layers. At least a first and second of the piezoelectric layers are polarized in the thickness direction, and the first and second end surface electrodes and the electrode layers are connected such that, in at least the first of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction in at least the first of the piezoelectric layers and that, in at least the second of the piezoelectric layers, an electric field is generated to be opposite to the polarization direction in the piezoelectric layer.
According to another preferred embodiment of the present invention, a piezoelectric resonator utilizing bending vibrations, includes three or more electrode layers and two or more piezoelectric layers alternately laminated, first and second end surface electrodes provided on the end surfaces of the laminated piezoelectric layers, a recessed portion, which is insulated from the first end surface electrode, provided in the periphery portion of one outermost layer of the electrode layers, and a first external electrode, which is connected to the first end surface electrode, provided in the recessed portion, and a recessed portion, which is insulated from the second end surface electrode, provided in the periphery portion of the other outermost electrode layer, and a second external electrode, which is connected to the second end surface electrode, provided in the recessed portion. Then, at least two of the piezoelectric layers are polarized in the thickness direction, and the first and second end surface electrodes and the electrode layers are connected such that, in at least a first one of the piezoelectric layers, an electric field is generated in the same direction as the polarization direction in the first of the piezoelectric layers and that, in at least a second one of the piezoelectric layers, an electric field is generated to be opposite to the polarization direction in the second of the piezoelectric layers.
Regarding the first preferred embodiment of the invention, since a piezoelectric layer in which the polarization direction and the direction of electric field are the same contracts in the planar direction and a piezoelectric layer in which the polarization direction and the direction of electric field are opposite to each other expands, bending vibration is generated as an entire piezoelectric resonator. In such a piezoelectric resonator utilizing bending vibrations, the dimensions thereof are greatly reduced, compared with a
Itasaka Yasuhiro
Yoshino Hirohide
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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