Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-12-09
2001-05-08
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000
Reexamination Certificate
active
06229246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric resonators used as various kinds of resonators and oscillators, and piezoelectric resonance components incorporating such resonators. More specifically, the present invention relates to thickness-extensional piezoelectric resonators utilizing harmonics of a thickness-extensional vibration mode and piezoelectric resonance components incorporating such resonators.
2. Description of the Related Art
Piezoelectric resonators have been used in piezoelectric resonance components such as a piezoelectric oscillator and a piezoelectric filter. It is known that such conventional piezoelectric resonators use different piezoelectric resonance modes according to frequencies used.
In Japanese Unexamined Patent Publication No. 1-117409, an energy-trap type piezoelectric resonator utilizing the second-order harmonic of the thickness-extensional vibration mode is disclosed. This piezoelectric resonator will be described below referring to
FIGS. 9 and 10
.
FIG. 9
is an exploded perspective view of the piezoelectric resonator. In
FIG. 9
, the piezoelectric resonator is produced by integrally firing two laminated piezoelectric ceramic green sheets
51
and
52
. A round excitation electrode
53
is provided at the center of the ceramic green sheet
51
and is led out to an edge of the ceramic green sheet
51
via a leading electrode
54
. In addition, a round excitation electrode
55
is provided at the center of the upper surface of the ceramic green sheet
52
and is led out to an edge of the ceramic green sheet
52
via a leading electrode
56
. An excitation electrode
57
is provided on the lower surface of the ceramic green sheet
52
and is led out to an edge of the ceramic green sheet
52
via a leading electrode
58
. This is indicated by a reflection of the configuration below the ceramic green sheet
52
.
Pressure is applied to the laminated ceramic green sheets
51
and
52
in the thickness direction of the laminated structure so as to obtain a fired body. Subsequently, polarization processing is performed on the fired body so as to produce a piezoelectric resonator
60
shown in FIG.
10
.
In the piezoelectric resonator
60
, polarization processing is performed on piezoelectric layers
61
and
62
in a direction indicated by arrows shown in FIG.
10
. That is, polarization processing is performed on the fired body uniformly in the thickness direction of the structure.
In the case of driving, the excitation electrodes
53
and
57
are commonly connected and an AC voltage is applied between the excitation electrodes
53
,
57
, and
55
so as to vibrate the piezoelectric resonator
60
. Excitation energy is trapped in a region where the excitation electrodes
53
,
55
, and
57
overlap, that is, in a resonating portion A.
As described above, the piezoelectric resonator
60
using a harmonic of the thickness-extensional vibration mode defines an energy-trapping-type piezoelectric resonator. Thus, it is necessary to dispose a vibration-attenuating portion for attenuating vibration at an area surrounding the resonating portion A. In other words, a vibration-attenuating portion that is larger than the area of the resonating portion needs to be provided. As a result, it is very difficult to reduce the size of the conventional piezoelectric resonator
60
.
Japanese Unexamined Patent Publication No. 2-235422 discloses another energy-trap type piezoelectric resonator using a strip-type piezoelectric ceramic body. In the piezoelectric resonator, it is not essential to provide an extra piezoelectric-substrate portion around the resonating portion. As shown in
FIG. 11
, an excitation electrode
72
a
is provided on the upper surface of a narrow piezoelectric substrate
71
, and an excitation electrode
72
b
is provided on the lower surface thereof. The excitation electrodes
72
a
and
72
b
are formed such that their widths are equal to the width of the piezoelectric substrate. The excitation electrodes
72
a
and
72
b
oppose each other at the center in the length direction of the piezoelectric substrate
71
to define a resonating portion. In addition, the excitation electrodes
72
a
and
72
b
are arranged to extend to opposite edges
71
a
and
71
b
in the length direction of the piezoelectric substrate
71
.
In a piezoelectric resonator
70
shown in
FIG. 11
, in the case of excitation of the thickness-extensional vibration mode, unnecessary vibrations occur due to the dimensional relationship between the width W and the thickness T of the piezoelectric substrate
71
. Regarding this problem, in Japanese Unexamined Patent Publication No. 2-235422, it is shown that, in order to reduce unnecessary spurious vibrations between a resonant frequency and an anti-resonant frequency while a fundamental wave is used, the value of W/T must be substantially equal to 5.33 in the 16 MHz resonant frequency. In the event the third-order harmonic is used, the value of W/T must be substantially equal to 2.87 in the 16 MHz resonant frequency.
In the energy-trap type piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 2-235422, since it is not necessary to provide a vibration-attenuating portion around the resonating portion, the size of the piezoelectric resonator can be reduced. However, when the harmonic of the thickness-extensional vibration mode is actually used, in addition to spurious vibrations generated between a resonant frequency band and an anti-resonant frequency band, various undesirable spurious vibrations occur, with the result that effective resonance characteristics cannot be obtained. Furthermore, the electric capacity of this piezoelectric resonator is relatively small, the resonator is thereby susceptible to influence from the stray capacitance of a circuit board.
SUMMARY OF THE INVENTION
To overcome the above described problems, preferred embodiments of the present invention provide a thickness-extensional piezoelectric resonator adapted to be vibrated in a harmonic of the thickness-extensional vibration mode and a piezoelectric resonance component incorporating such a resonator, which achieve significant reduction in size, have large electric capacity so as to not be susceptible to influence from the stray capacitance of a circuit board, and have small variations in resonance characteristics.
One preferred embodiment of the present invention provides a thickness-extensional piezoelectric resonator including a resonating portion, a vibration-attenuating portion disposed at each side of the resonating portion and adapted to be vibrated in an N-order harmonic of a thickness-extensional vibration mode, a substantially rectangular-plate piezoelectric body; a first excitation electrode and a second excitation electrode disposed on individual surfaces of the substantially rectangular-plate piezoelectric body in such a way that the electrodes oppose each other with the piezoelectric body therebetween; at least one internal electrode disposed inside of the piezoelectric body and arranged such that the at least one internal electrode at least partially opposes both the first excitation electrode and the second excitation electrode with the piezoelectric body therebetween, the resonating portion being defined by a portion where the first excitation electrode, the second excitation electrode, and the at least one internal electrode overlap in the thickness direction of the piezoelectric body; when a direction connecting the vibration-attenuating portions at both sides of the resonating portion is a first direction, the first excitation electrode and the second excitation electrode being arranged such that the electrodes extend close to or to the edges of the piezoelectric body, in a direction that is substantially perpendicular to the first direction; and a relationship of |Dr−Di|=Di/10 is satisfied, wherein Dr indicates the thickness of the piezoelectric layers between the excitation electrodes and the internal electrode or the thi
Budd Mark O.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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