Piezoelectric Vibrator With Spurious Mode Suppression

The present invention relates to a piezoelectric vibrator. More specifically, the present invention relates to a piezoelectric vibrator utilizing an expansion mode vibration or a breath mode vibration.

2. Description of the Prior Art

FIGS. 1 to 3 are views showing different examples of a square plate type vibrator utilizing a conventional expansion mode vibration in which the present invention can be advantageously employed. Such a piezoelectric vibrator 1 comprises a piezoelectric plate 11 of such as PZT, an electrode 12 formed on one main surface thereof, and an electrode 13 formed on the other main surface thereof. The vibrator shown in FIG. 2 is an example wherein the electrode 14 formed on the other main surface of the piezoelectric plate 11 is formed as a partial electrode i.e. an electrode formed only on a portion of the surface of the plate. The example shown in FIG. 3 is a so-called three-terminal type vibrator wherein the electrode 15 formed on the other main surface of the piezoelectric plate 11 is formed as a dot electrode and the electrode 16 formed on the same other main surface of the piezoelectric plate 11 is formed as a ring electrode. Since the principle of an operation of these piezoelectric vibrators is well-known to those skilled in the art, a more detailed description thereof will be omitted.

FIG. 4 is a graph showing a frequency characteristic of the FIG. 3 vibrator used in an intermediate frequency circuit of an AM band radio receiver, for example, as an example of a square plate type piezoelectric vibrator utilizing a conventional expansion mode vibration. As seen from the graph, the conventional example exhibits a large spurious vibration such as the third harmonic or the fifth harmonic of the fundamental vibration frequency. This spurious is a problem. For example, the third harmonic occurs as shown in FIG. 5. More specifically, such harmonic occurs, because a standing wave of the frequency three times the fundamental vibration frequency is superposed in the expansion direction of the piezoelectric plate 11. Therefore, as an approach for suppressing such spurious vibrations, an improvement for precluding such a standing wave by providing the electrode as partial electrodes 12a and 13a, as shown in FIG. 6, has been known. More specifically, by using the partial electrodes 12a and 13a, as shown in FIG. 6, the surface electric charge at the hatched portion in the surface electric charge distribution in FIG. 6 is canceled, whereby the vibration of the third harmonic is suppressed. By properly changing the geometry of the partial electrodes, the fifth harmonic and seventh harmonic or the edge mode vibration spurious vibration or the like can be suppressed. However, this approach merely suppresses only one intended spurious vibration (such as only the third harmonic) and cannot suppress several various spurious vibrations at the same time. Furthermore, in view of other requirements in the characteristic, it could happen that an electrode arrangement cannot be determined in consideration of only such spurious suppression.

For the purpose of eliminating the above described problems and shortcomings, the present invention employs a piezoelectric plate having a configuration defined by four sides, so that each pair of two adjacent sides defines a corner angle, wherein the dimensions thereof are selected to suppress a spurious vibration.

According to the present invention, spurious vibrations in the third harmonic and the fifth harmonic and other modes such as a edge mode vibration and a bending mode vibration can be fully removed, without any effect upon the fundamental wave, by properly selecting the geometry of the piezoelectric plate.

In a preferred embodiment of the present invention, the piezoelectric plate is configured in a rhombus, wherein the lengths of the four sides are selected to be the same and two opposite angles are equal obtuse angles while the remaining two opposite angles are equal acute angles. The above described two opposite acute angles are selected to be 75.degree. to 89.degree.. Accordingly, in fabricating a piezoelectric vibrator, a piezoelectric plate of a configuration of a rhombus can be readily obtained by only changing the corner angles from the right angle of a conventional piezoelectric plate of a configuration of a square having four sides of the same length and four corner angles of the same right angle to the above described obtuse and acute angles, with a slight change of the existing facilities for fabricating conventional piezoelectric plates having a configuration of a square. Since it is not necessary to change the sides of the piezoelectric plate and it is sufficient to change only the configuration, a packaging case for covering the inventive piezoelectric vibrator may be the same as that for a conventional piezoelectric vibrator of a piezoelectric plate having a square configuration. In this connection, it has been observed that the lengths of the sides of the inventive piezoelectric plate need not be necessarily selected to be the same length and alternatively a piezoelectric plate may be configured as a parallelogram having the two opposite longer sides and the two opposite shorter sides. In such a case, by selecting the side ratio of the shorter side length to the longer side length to be 0.7 to 0.98, a spurious vibration can be effectively suppressed without any effect upon a vibration of the fundamental wave. Furthermore, it has been observed that the inventive piezoelectric vibrator may be configured as a rectangle by selecting the four corner angles defined by the two adjacent longer and shorter sides of the above described parallelogram piezoelectric plate to be the same.

Accordingly, a principal object of the present invention is to provide an improved piezoelectric vibrator which is capable of fully suppressing a spurious vibration without any effect upon a vibration of the fundamental wave.

Another object of the present invention is to provide a piezoelectric vibrator having a configuration of a piezoelectric plate selected to the optimum geometry for fully removing a spurious vibration.

A further object of the present invention is to provide a piezoelectric vibrator capable of fully suppressing a spurious vibration without an influence being exerted upon a vibration of the fundamental wave that can be fabricated using the existing facilities for fabricating conventional piezoelectric vibrators having a configuration of a square, only with a slight modification.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIGS. 1 to 3 are view showing different examples of a square plate type piezoelectric vibrator utilizing a conventional expansion mode vibration wherein the present invention can be advantageously employed;

FIG. 4 is a graph showing one example of a frequency characteristic in the case where the three-terminal piezoelectric vibrator shown in FIG. 3 is utilized in an intermediate frequency circuit of an AM band radio receiver;

FIG. 5 is a view for explaining generation of the third harmonic;

FIG. 6 is a graph showing one example of an electrode structure for suppressing the third harmonic;

FIGS. 7 to 9 are plan views showing different examples of the first embodiment of the present invention;

FIG. 10 is a graph showing a characteristic of an antiresonant impedance/resonant impedance with respect to the angle .theta.1 of the piezoelectric vibrators shown in FIG. 7;

FIG. 11 is a graph showing a frequency characteristic in the case where the FIG. 9 embodiment is used in an intermediate frequency circuit of an AM band radio receiver;

FIGS. 12 to 14 are plan views showing different examples of the second embodiment of the present invention;

FIG. 15 is a graph showing a characteristic of an antiresonant impedance/resonant impedance with respect to the side ratio b/a of the piezoelectric vibrator shown in FIG. 12;

FIG. 16 is a graph showing a frequency characteristic in the case where the FIG. 14 embodiment is used in an intermediate frequency circuit of an AM band radio receiver;

FIGS. 17 to 20 are plan views showing different examples of the third embodiment of the present invention;

FIG. 21 is a graph showing a characteristic of an antiresonant impedance/resonant impedance with respect to the side ratio d/c of the piezoelectric vibrators shown in FIG. 17;

FIG. 22 is a graph showing a characteristic of an antiresonant impedance/resonant impedance with respect to the angle .theta.2 of the piezoelectric vibrator shown in FIG. 17; and

FIG. 23 is a graph showing a frequency characteristic in the case where the embodiment shown in FIG. 20 is employed in an intermediate frequency circuit of an AM band radio receiver.

FIGS. 7 to 9 are plan views showing different examples of the first embodiment of the present invention, wherein various configurations of the piezoelectric plate are shown. FIGS. 7, 8 and 9 correspond to FIGS. 1, 2 and 3, respectively. According to the first embodiment, the main surface of the piezoelectric plate is of a configuration of a rhombus rather than a conventional configuration of a square. The angle .theta.1 of one corner between two sides of the rhombus is selected to be 75.degree. to 89.degree..

FIG. 10 is a graph showing the characteristic of an antiresonant impedance/resonant impedance with respect to the angle, wherein the abscissa indicates the angle (.theta.1) and the ordinate indicates the ratio (dB) of the antiresonant impedance/resonant impedance. Referring to FIG. 10, the curve A shows the frequency of the fundamental vibration, say 455 kHz, the curve B shows the third harmonic, say 1.2 MHz, the curve C shows the fifth harmonic, say 2.0 MHz, the curve D shows the edge mode vibration, say 650 kHz, and the curve E shows the bending mode vibration, say 300 kHz. As seen from FIG. 10, by forming the piezoelectric plate in a configuration of a rhombus and by selecting the angle .theta.1 between the two sides of the rhombus to be approximately 75.degree. to 89.degree., almost all spurious vibration can be effectively suppressed.

FIG. 11 is a graph showing the frequency characteristic in the case where the FIG. 9 three-terminal piezoelectric vibrator is used in an intermediate frequency circuit of an AM band radio receiver, wherein the angle .theta.1 is selected to be 87.5.degree., for example. As seen from FIG. 11, it would be appreciated that any and all of the harmonics can be effectively suppressed as compared with the conventional example (FIG. 4). The same applies to the edge mode vibration (FIG. 10) and the similar effect for suppressing the spurious vibration is performed, whatever electrode structure (for example FIG. 7, 8 or 9) is employed.

As seen also from FIG. 10, although an effect for suppressing the spurious vibration is also attained in the case where the angle .theta.1 is smaller than 75.degree., too small an angle .theta.1 decreases workability and makes complicated a structure for assembling in a casing, while giving rise to a resonance splitting phenomenon. In consideration of these, the angle .theta.1 was selected to be 75.degree. to 89.degree. in the first embodiment.

FIGS. 12, 13 and 14 are plan views of different examples of the second embodiment of the present invention, wherein various configurations of the piezoelectric plate is shown. FIGS. 12, 13 and 14 correspond to FIGS. 1, 2 and 3, respectively. The second embodiment has employed a configuration of the main surface of the piezoelectric plate in the form of a rectangle rather than a conventional configuration of a square. The side ratio b/a of the length a of the longer side and the length b of the shorter side of the rectangle is selected to be 0.7 to 0.98.

FIG. 15 is a graph showing the characteristic of an antiresonant impedance/resonant impedance with respect to the side ratio b/a, wherein the abscissa indicates the side ratio b/a and the ordinate indicates the ratio (dB) of the antiresonant impedance/resonant impedance. Referring to FIG. 15, the curve A shows the frequency of a fundamental vibration, say 455 kHz, the curve B shows the third harmonic, say 1.2 MHz, the curve C shows the fifth harmonic, say 2.0 MHz, the curve D shows the edge mode vibration, say 650 kHz, and the curve E shows a bending mode vibration, say 300 kHz. As seen from FIG. 15, it would be appreciated that almost all spurious vibration can be effectively suppressed by selecting the side ratio b/a to be approximately 0.7 to 0.98.

FIG. 16 is a graph showing the frequency characteristic in the case where the FIG. 14 three-terminal piezoelectric vibrator is used in an intermediate frequency circuit of an AM band radio receiver, wherein the side ratio b/a has been selected to be 0.98, for example. As seen from FIG. 16, it would be appreciated that all the harmonics can be effectively suppressed by the embodiment shown, as compared with the conventional example (FIG. 4). The same applies also to the edge mode vibration (FIG. 15) and a similar effect for suppressing the spurious vibration is also attained, whatever electrode configuration (such as FIG. 12, 13 or 14) is employed.

As seen from FIG. 15, although an effect for suppressing the spurious vibration is performed even when the side ratio b/a is decreased to be smaller than 0.7, too small an a side ratio b/a reduces the workability and makes complicated a structure for assembling the same in a casing, while giving rise to a resonance splitting phenomenon. In consideration of these, the side ratio b/a of the second embodiment was selected to be 0.7 to 0.98.

FIGS. 17 to 20 are plan views of several examples of the third embodiment of the present invention, wherein various configurations of the piezoelectric plate are shown. FIGS. 17, 18 and 19, 20 correspond to FIGS. 1, 2 and 3, respectively. The embodiment shown employs a configuration of the main surface of the piezoelectric plate in the form of parallelogram rather than a conventional configuration of a square. The side ratio d/c of the long side c and the short side d of the parallelogram is selected to be 0.7 to 0.98. The angle .theta.2 between the two sides of the parallelogram is selected to be 75.degree. to 89.degree..

FIG. 21 is a graph showing the characteristic of an antiresonant impedance/resonant impedance with respect to the side ratio d/c, wherein the abscissa indicates the side ratio d/c and the ordinate indicates the ratio (dB) of the antiresonant impedance/resonant impedance. Referring to FIG. 21, the curve A shows a frequency of the fundamental vibration, say 455 kHz, the curve B shows the third harmonic, say 1.2 MHz, the curve C shows the fifth harmonic, say 2.0 MHz, the curve D shows the edge mode vibration, say 650 kHz, and the curve E shows the bending mode vibration, say 300 kHz.

FIG. 22 is a graph showing the characteristic of an antiresonant impedance/resonant impedance with respect to the angle .theta.2 in FIG. 17, wherein the abscissa indicates the angle .theta.2 and the ordinate indicates the ratio (dB) of the resonant impedance and the antiresonant impedance. Referring to FIG. 22, the curve A' shows the frequency of the fundamental vibration, say 455 kHz, the curve B' shows the third harmonic, say 1.2 MHz, the curve C' shows the fifth harmonic, say 2.0 MHz, the curve D' shows the edge mode vibration, say 650 kHz, and the curve E' shows the bending mode vibration, say 300 kHz.

As seen from FIGS. 21 and 22, almost all spurious vibration can be effectively suppressed by forming a configuration of the major surface of the piezoelectric plate in a form of a parallelogram and by selecting the side ratio d/c of the longer side c and the shorter side d of the parallelogram to be 0.7 to 0.98, and by selecting the angle .theta.2 between the longer side and the shorter side to be 75.degree. to 89.degree..

FIG. 23 is a graph showing the frequency characteristic in the case where the three-terminal piezoelectric vibrator shown in FIGS. 19 and 20 is used in an intermediate frequency circuit of an AM band radio receiver, wherein the side ratio d/c is selected to be say 0.96 and the angle .theta.2 is selected to be say 87.5.degree.. As seen from FIG. 23, it would be appreciated that all of the harmonics can be effectively suppressed according to the embodiment shown, as compared with the conventional example (FIG. 4). The same applies to the edge mode vibration (FIGS. 21 and 22), and a similar effect for suppressing the spurious vibration is performed, whatever electrode structure (such as FIG. 17, 18, 19 or 20) is employed.

As seen from FIGS. 21 and 22, although an effect for suppressing the spurious vibration can be achieved even when the side ratio d/c is smaller than 0.7 and the angle .theta.2 is smaller than 75.degree., too small a side ratio d/c reduces the workability and makes complicated a structure for assembling the same in a casing, while giving rise to a resonance splitting phenomenon. In consideration of these, the side ratio d/c was selected to be 0.7 to 0.98 and the angle .theta.2 was selected to be 75.degree. to 89.degree..

Meanwhile, the present invention can be utilized in a resonator, three-terminal filter, a ladder type filter and the like.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.