Piezoelectric resonator and electronic component using the same resonator

Abstract: A piezoelectric resonator 10 includes a base member 12 formed by laminating piezoelectric layers 14 and internal electrodes 16. The adjacent piezoelectric layers 14 are polarized in different directions with an intervening internal electrode 16. A groove 18 is provided on one lateral surface of the base member 12, and every other internal electrode 16 is exposed at both sides of the groove 18. Insulating films 20 and 22 are disposed on unexposed portions of the internal electrodes 16. External electrodes 24 and 26 are formed at both sides of the groove 18 in the base member 12 and are connected to the internal electrodes 16. The piezoelectric resonator can suppress a reduction in the difference ΔF between the resonant frequency and the anti-resonant frequency.

Description: The present invention relates to a piezoelectric resonator vibratable in a longitudinal vibration mode, comprising: a base member having a longitudinal direction and formed by laminating a plurality of piezoelectric layers and a plurality of internal electrodes; a first group of said internal electrodes and a second group of said internal electrodes respectively extended to and exposed at two regions, i.e. a first region and a second region of a surface of said base member; first and second external electrodes respectively disposed on said first and second regions continuously so as to be connected to said first and second group of said internal electrodes; and said piezoelectric layers being polarizable in the longitudinal direction of said base member, and an electric field being appliable to said base member in its longitudinal direction, thereby exciting a longitudinal vibration in said base member. Such a piezoelectric resonator is known e.g. from US 5 438 232. The present invention also relates to an electronic component using the above described resonator. Description of the Related Art Piezoelectric resonators utilizing mechanical resonance of a piezoelectric member include, for example, a piezoelectric resonator in which electrodes are provided on both surfaces of a piezoelectric plate which is polarized in its thickness direction. In this type of piezoelectric resonator, an AC electric field is applied between the electrodes to excite a longitudinal vibration or a lateral vibration. Howe-ver, this type of piezoelectric resonator utilizes an unstiffened mode in which the direction of an electric field and the direction of polarization are different from the direction of vibration, and has a small electromechanical coupling coefficient and a relatively small difference ΔF between the resonant frequency and the anti-resonant frequency. The use of an unstiffened-mode piezoelectric resonator in an oscillator or a filter disadvantageously results in a small bandwidth and a low degree of flexibility in designing the characteristics. To overcome the above disadvantages, there has been proposed a piezoelectric resonator having a laminated structure, such as the one shown in Fig. 15, as a stiffened-mode piezoelectric resonator having a large electromechanical coupling coefficient and a relatively large difference ΔF between the resonant frequency and the anti-resonant frequency. Fig. 15 is an illustration showing an example of a piezoelectric resonator utilizing the stiffened mode. A piezoelectric resonator 1 includes a rectangular-prism-shaped base member 2. The base member 2 is formed by alternately laminating a plurality of piezoelectric layers 3 and a plurality of internal electrodes 4. Two piezoelectric layers 3 that are adjacent to each other with one internal electrode 4 therebetween are polarized in opposite directions in the longitudinal direction of the base member 2. A groove 5 is formed on one lateral surface of the base member 2, and the internal electrodes 4 are exposed every other layer on both lateral surfaces of the groove 5. Further, on both sides of the grooves 5, external electrodes 6a and 6b are provided on the lateral surface of the base member 2. Accordingly, the adjacent internal electrodes 4 are connected to the different external electrodes 6a and 6b, respectively. In the piezoelectric resonator 1, a signal is input into the external electrodes 6a and 6b, thereby applying an AC electric field between the adjacent internal electrodes 4. Since the adjacent piezoelectric layers 3 are polarized in opposite directions, upon applying an AC electric field to them, the individual piezoelectric layers 3 expand or contract in the longitudinal direction of the base member 2. As a result, a longitudinal fundamental vibration is excited in the overall piezoelectric resonator 1. In this manner, the piezoelectric resonator 1, which utilizes the stiffened mode in which the direction of an electric field and the direction of polarization coincide with the direction of vibration, can increase the electromechanical coupling coefficient and ΔF in comparison with an unstiffened-mode piezoelectric resonator. Thus, the use of a stiffened-mode piezoelectric resonator in an oscillator or a filter makes it possible to increase the bandwidth and elevate the degree of flexibility in designing the characteristics. In the above type of piezoelectric resonator, however, a capacitance is generated between the unexposed portion of each internal electrode and the external electrode which is not connected to the unexposed internal electrode. This capacitance is unfavorably connected in parallel to the capacitance formed between the internal electrodes, which causes vibration, thereby lowering the capacitance ratio of the resonator and further decreasing the difference ΔF between the resonant frequency and the anti-resonant frequency. This results in a small bandwidth when the piezoelectric resonator is used in an oscillator or a filter. Accordingly, it is a main object of the present invention to provide a piezoelectric resonator in which a decrease in the difference ΔF between the resonant frequency and the anti-resonant frequency can be suppressed. It is another object of the present invention to provide an electronic component having a high degree of flexibility in designing the bandwidth by using the above type of piezoelectric resonator. The present invention provides a piezoelectric resonator of above mentioned kind, which is characterized in that an insulating film covers an unexposed portion of each of said first and second group of said internal electrode; and said insulating film is disposed on each of said first and second regions and has a dielectric constant smaller than a dielectric constant of said piezoelectric layers. The present invention further provides a piezoelectric resonator of above mentioned kind, which is characterized in that first and second conductive resin films are respectively disposed on said first and second regions continuously so as to be connected to said external electrodes. In the above piezoelectric resonators, the first and second regions may be respectively located at one side and at the other side in the widthwise direction of the base member on the same lateral surface of the base member. Alternatively, the first and second regions may be respectively located on different lateral surfaces of the base member. The present invention further provides an electronic component including the above described piezoelectric resonator, wherein said electronic component includes a substrate provided with a pattern electrode to which said conductive resin films are electrically connected, and said substrate and said piezoelectric resonator are covered with a cap. In the above described electronic component, a plurality of said pattern electrodes may be provided on said substrate, and said conductive resin films of a plurality of said piezoelectric resonators may be electrically connected to said pattern electrodes so that a ladder filter is provided. When the four piezoelectric resonators are utilized, first, second, third and fourth base members may be provided as said base members; first, second, third and fourth pattern electrodes may be provided as said electrode on said substrate; said first base member may be mounted on said first and second pattern electrodes; said second base member may be mounted on said second and third pattern electrodes; said third base member may be mounted on said second and fourth pattern electrodes; and said fourth base member may mounted on said third and fourth pattern electrodes. According to the above described piezoelectric resonator, an insulating film, which is made from, for example, a resin having a dielectric constant smaller than the piezoelectric layers, is disposed on an unexposed portion of each internal electrode. Accordingly, the insulating film having a small dielectric constant intervenes between the unexposed electrodes and the external electrode which are not connected to the unexposed electrodes, thereby decreasing the capacitance generated therebetween. Further, in the conductive resin, metal particles are uniformly distributed in the insulating resin having a dielectric constant smaller than the piezoelectric layers, as in the above-described insulating film. Thus, the metal particles in the conductive resin films formed on the lateral surface of the base member are almost in point-contact with the base member, thereby significantly decreasing the area of contact in comparison with a piezoelectric resonator in which external electrodes are formed on the lateral surface of the base member. Accordingly, even if the conductive resin films are deposited on the unexposed portions of the internal electrodes of the lateral surface of the base member, the capacitance generated between the conductive resin films and the unexposed portions of the internal electrodes is reduced. Because of the formation of the external electrodes connected to the exposed internal electrodes on the lateral surface of the base member, the internal electrodes can be connected to the conductive resin films with the external electrodes therebetween, thereby enhancing an electrical connection between the conductive resin films and the exposed internal electrodes. The first and second regions where the first and second group of the internal electrodes are exposed may be located on different lateral surfaces or the same surface of the base member according to the intended purpose of use of the piezoelectric resonator. An electronic component, such as an oscillator, a discriminator, or a ladder filter, is fabricated by the use of the foregoing piezoelectric resonator. This electronic component exhibits a less decrease in the difference ΔF between the resonant frequency and the anti-resonant frequency of the piezoelectric resonator and has a high flexibility in designing the bandwidth. Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view illustrating an preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 2 is an illustration showing the piezoelectric resonator shown in Fig. 1. Fig. 2A is an illustration showing a modification of the piezoelectric resonator shown in Fig. 1. Fig. 3 is an illustration showing the relationship of the internal electrodes to a laminated structure for use in the piezoelectric resonator shown in Fig. 2. Fig. 4 is a perspective view illustrating the state in which mother substrates are laminated to produce the piezoelectric resonator shown in Fig. 1. Fig. 5 is an illustration showing the state in which insulating materials and electrodes are formed on a laminated structure which is obtained by laminating the mother substrates shown in Fig. 4. Fig. 6 is an illustration showing another preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 7 is an illustration showing the relationship of the internal electrodes to a laminated structure for use in the piezoelectric resonator shown in Fig. 6. Fig. 8 is an illustration showing still another preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 9 is an illustration showing a further preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 10 is an illustration showing a yet further preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 11 is an illustration showing a further embodiment of a piezoelectric resonator according to the present invention. Fig. 12 is an exploded perspective view illustrating an preferred embodiment of an electronic component formed by using the piezoelectric resonator of the present invention. Fig. 13 is an exploded perspective view illustrating the essential portion of an preferred embodiment of a ladder filter formed by using the piezoelectric resonator of the present invention. Fig. 14 is a circuit diagram illustrating the ladder filter shown in Fig. 13. Fig. 15 is an illustration showing an example of a piezoelectric resonator used as a background of the present invention. PREFERRED EMBODIMENTS OF THE PRESENT INVENTION Fig. 1 is a perspective view illustrating a preferred embodiment of a piezoelectric resonator according to the present invention. Fig. 2 is an illustration showing the embodiment. A piezoelectric resonator 10 includes a columnar base member 12. The base member 12 is formed by alternately laminating a plurality of piezoelectric layers 14 and internal electrodes 16. One piezoelectric layer 14 interposed between internal electrodes 16 and the adjacent piezoelectric layer 14 with the internal electrode 16 therebetween are polarized, as indicated by the arrows shown in Fig. 2, in opposite directions in the longitudinal direction of the base member 12. However, the piezoelectric layers 14 which are positioned at both ends of the base member 12 in its longitudinal direction are not polarized. The piezoelectric layers 14 positioned at both ends of the base member 12 may be, however, polarized as required. Additionally, unpolarized piezoelectric layers may be formed at any positions of the base member. On one lateral surface of the base member 12, a groove 18 extending in the longitudinal direction of the base member 12 is formed at the central portion of the width of the base member 12. On one lateral surface of the groove 18, a first region is provided in which a first group of the internal electrodes 16 are exposed every other layer. On the other lateral surface of the groove 18, a second region is provided in which a second group of the internal electrodes 16 which are not exposed in the first region are exposed. Namely, in the above piezoelectric resonator 10, the first and second group of the internal electrodes 16 exposed at the respective sides of the groove, as illustrated in Figs. 3(a) and 3(b), are alternately laminated. In the first region, a first insulating film 20 is disposed at a position corresponding to the unexposed portion of each internal electrode 16. Similarly, in the second region, a second insulating film 22 is disposed at a position corresponding to the unexposed portion of each internal electrode 16. In this embodiment, as the piezoelectric layer 14, a PZT ceramic having a relative dielectric constant of approximately 1000 is used. As the first and second insulating films 20 and 22, an epoxy resin having a relative dielectric constant of about from 2 to 5 is employed. As the first and second insulating films 20 and 22, a material other than an epoxy resin may be used as long as it has a dielectric constant lower than the piezoelectric layer 14. Moreover, as the piezoelectric layer 14, a piezoelectric material other than a PZT ceramic may be used. Further, on one side of the groove 18, a first external electrode 24 is provided on the first region of the base member 12 on which the first insulating film 20 is disposed. Accordingly, the exposed internal electrodes 16 in the first region are electrically connected to the first external electrode 24. On the other side of the groove 18, a second external electrode 26 is provided on the second region of the base member 12 on which the second insulating film 22 is disposed. Thus, the exposed internal electrodes in the second region are electrically connected to the second external electrode 26. It should be noted that the first and second external electrodes 24 and 26 are provided in the form of a thin film produced by means such as sputtering. Further, it is not essential that the internal electrodes 16 formed within the base member 12 are required to be alternately connected to the different external electrodes. For example, the adjacent internal electrodes 16 may be connected to the same external electrode, or some internal electrodes may be connected to neither of the external electrodes. In the piezoelectric resonator shown in Figs. 1 and 2, the groove 18 is provided. However, it is noted that the groove 18 is not an essential feature for the present invention. As shown in Fig. 2A, the piezoelectric resonator having the features of Fig. 1 without the groove 18 is also applicable. For manufacturing the piezoelectric resonator 10, as shown in Fig. 4, piezoelectric-ceramic mother substrates 40, which will form a large number of piezoelectric layers 14, are first prepared. Formed on each mother substrate 40 are mother electrodes 42 and 44, which will form the internal electrodes 16. The mother electrodes 42 and 44 have electrode-free portions 42a and 44a, respectively, and the mother substrates 40 are alternately laminated in such a manner that the electrode-free portions 42a and the electrode-free portions 44a are displaced from each other. A laminated structure obtained according to the above procedure is then cut at portions indicated by the one-dot-chain lines extending in the transverse direction shown in Fig. 4. An insulating material 46, which will form the first insulating film 20 and the second insulating film 22, is applied, as shown in Fig. 5, to the cross-sectional surface of the cut laminated structure. For the convenience of explanation, the number of layers in this laminated structure illustrated in Fig. 5 does not coincide with the number of piezoelectric layers of the base member 12 shown in Fig. 2. The insulating material 46 is formed on the unexposed portions of the mother electrodes 42 and 44. Further, a plurality of electrodes 48, which will form the external electrodes 24 and 26, are disposed in the direction orthogonal to the surfaces of the mother electrodes 42 and 44. Then, a high DC voltage is applied between the adjacent electrodes 48, thereby polarizing the individual mother substrates 40. Subsequently, the laminated structure is cut at the portions represented by the one-dot-chain lines shown in Fig. 5, and a groove is formed at the center of each cut piece. Thus, the piezoelectric resonator 10 is produced. The provision of the groove 18 is not essential, and it is sufficient if the external electrodes 24 and 26 on both sides of the resonator 10 are reliably separated. In this piezoelectric resonator 10, a signal is supplied between the first external electrode 24 and the second external electrode 26, which makes it possible to apply voltages, which are reverse to each other, to the respective piezoelectric layers 14 of the base member 12 polarized in opposite directions, thereby causing the overall piezoelectric layers 14 to expand and contract in the same direction. Therefore, a longitudinal fundamental vibration using the center of the base member 12 as a node is excited in the overall piezoelectric resonator 10. In this piezoelectric resonator 10, the direction of polarization of the piezoelectric layers 14 and the direction of an electric field generated by the signal coincide with the direction of vibration. Namely, the piezoelectric resonator 10 is a type being a stiffened-mode piezoelectric resonator. The stiffened-mode piezoelectric resonator 10 has a larger electromechanical coupling coefficient and a greater difference ΔF between the resonant frequency and the anti-resonant frequency over an unstiffened-mode piezoelectric resonator in which the direction of polarization and the direction of an electric field are different from the direction of vibration. Moreover, the first insulating film 20 and the second insulating film 22, both of which are made from a resin having a dielectric constant smaller than the piezoelectric layers 14, are provided at positions corresponding to the unexposed portions of the first and second group of the internal electrodes 16. Accordingly, the first insulating film 20 intervenes between the second group of the internal electrodes 16 and the first external electrode 24, and the second insulating film 22 intervenes between the first group of the internal electrodes 16 and the second external electrode 26. This decreases the capacitance generated between the internal electrode 16 and each of the first and second external electrodes 24 and 26. Hence, a reduction in the difference Δf between the resonant frequency and the anti-resonant frequency of the resonator can be suppressed over a piezoelectric resonator without an insulating film. Further, the area of the first insulating film 20 and the second insulating film 22 can be changed to vary the capacitance formed between the second group of the internal electrode 16 and the first external electrode 24, and the capacitance formed between the first group of the internal electrode 16 and the second external electrode 26. This makes it possible to form a piezoelectric resonator having various values of Δf, thereby increasing the designing flexibility. Alternatively, as illustrated in Fig. 6, a first region and a second region may be formed on the opposing surfaces of the base member 12, and the first and second group of the internal electrodes 16 may be alternately extended to the opposing lateral surfaces of the base member 12. In this case, the first and second group of the internal electrodes 16 extended to the opposing portions of the piezoelectric layers 14, as illustrated in Figs. 7(a) and 7(b), may be alternately laminated, thereby forming a piezoelectric resonator 10. Then, first and second insulating films 20 and 22 may be formed at positions corresponding to unexposed portions of the first and second group of the internal electrodes 16, and first and second external electrodes 24 and 26 may be provided on the opposing surfaces of the base member 12 on which the insulating films 20 and 22 are disposed. In this piezoelectric resonator 10, as well as in the counterpart shown in Fig. 2, the formation of the first insulating film 20 decreases the capacitance generated between the second group of the internal electrode 16 and first external electrode 24, and the formation of the second insulating film 22 decreases the capacitance generated between the first group of the internal electrode 16 and second external electrode 26. Alternatively, as illustrated in Fig. 8, a first external electrode 24 may be provided only on each exposed portion of the first group of the internal electrode 16 in the first region of the base member 12, while a second external electrode 26 may be provided only on each exposed portion of the second internal electrode 16 in the second region. Then, a first conductive resin film 28 and a second conductive resin film 30 may be disposed on the first and second external electrodes 24 and 26, respectively. Accordingly, the first and second conductive resin films 28 and 30 are electrically connected to the first and second group of the internal electrodes 16 which are connected to the first and second electrodes 24 and 26, respectively. In the above piezoelectric resonator 10, as well as in the previous resonators, a signal is input between the first conductive resin film 28 and the second conductive resin film 30, which makes it possible to apply an electric field to the piezoelectric layers 14, thereby exciting a longitudinal vibration in the base member 12. In the conductive resin, metal particles are uniformly distributed in the insulating resin which has a dielectric constant smaller than the piezoelectric layer 14, as in the foregoing insulating films. Thus, the metal particles in the conductive resin films 28 and 30 on the base member 12 are almost in point-contact with the base member 12, thereby significantly decreasing the area of contact in comparison with a piezoelectric resonator in which external electrodes are provided on the overall surfaces of the first region and the second region. Consequently, the capacitance generated between the unexposed portion of the second group of the internal electrode 16 and the first conductive resin film 28 and the capacitance generated between the unexposed portion of the first group of the internal electrode 16 and the second conductive resin film 30 become smaller than that generated in a piezoelectric resonator having external electrodes on the overall surface. Additionally, the first and second external electrodes 24 and 26 are provided in such a manner that they are respectively connected to the exposed portions of the first and second group of the internal electrodes 16, thereby increasing the area of contact between the first and second external electrodes 24, 26 and the first and second conductive resin films 28, 30, respectively. Hence, an electrical connection between the first group of the internal electrodes 16 and the first conductive resin film 28, and an electrical connection between the second group of the internal electrodes 16 and the second conductive resin film 30 can be enhanced. It is not necessary to provide the first and second conductive resin films 28 and 30 on the overall surfaces of the first region and the second region of the base member 12. The conductive resin films 28 and 30 may be provided, as illustrated in Fig. 9, only on part of the surface of the base member 12. In the piezoelectric resonator 10 shown in Fig. 9, as well as in the previous resonators, a signal is supplied between the first and second conductive resin films 28 and 30, thereby making it possible to excite a longitudinal vibration in the base member 12. Further, since the conductive resin has the function of damping the vibration of the piezoelectric resonator, Qm of the piezoelectric resonator 10 is reduced with increased areas of the conductive resin film 28 or 30. Accordingly, the area or the configuration of the conductive resin film 28 or 30 can be changed to adjust Qm of the piezoelectric resonator 10. A change in the area of the conductive resin film 28 or 30 also varies the capacitance generated between the second group of the internal electrode 16 and the first conductive resin film 28 and the capacitance generated between the first group of the internal electrode 16 and the second conductive resin film 30, thereby obtaining a piezoelectric resonator 10 exhibiting various values of Δf. Furthermore, as shown in Fig. 10, first and second regions may be located on the opposing surfaces of the base member 12, and the first and second external electrodes 24 and 26 and the first and second conductive resin films 28 and 30 may be provided on the opposing surfaces of the base member 12. In the piezoelectric resonator illustrated in Fig. 10, as well as in the previous resonators, the first and second conductive resin films 28 and 30 may certainly be provided only on part of the surface of the base member 12. The first and second conductive resin films 28 and 30 may be disposed, as illustrated in Fig. 11, on the first and second external electrodes 24 and 26, respectively, of the piezoelectric resonator shown in Fig. 2. In this case, even if the external electrode 24 or 26 is broken caused by expansion and contraction of the first insulating film 20 or the second insulating film 22 due to a temperature change by, for example, a heat shock, the continuity of the external electrode 24 or 26 across the broken portion can be ensured by the provision of the conductive resin film 28 or 30. Thus, the function of the piezoelectric resonator 10 can be preserved. Moreover, the area or the configuration of the conductive resin film 28 or 30 can be changed to adjust Qm of the piezoelectric resonator 10. In a piezoelectric resonator in which the external electrodes 24 and 26 are provided on the overall opposing surfaces of the base member 12, such as the one shown in Fig. 6, the conductive resin films 28 and 30 may be provided on the respective external electrodes 24 and 26. In this case, advantages similar to those offered by the previous resonators can be obtained. By using the foregoing piezoelectric resonator 10, electronic components, such as oscillators and discriminators, are fabricated. Fig. 12 is a perspective view illustrating an embodiment of an electronic component 50. The piezoelectric resonator 10 shown in Fig. 2 is employed in this embodiment. The electronic component 50 has an insulating substrate 52. Four indentations, two for opposing ends of the insulating substrate 52, are provided. Two pattern electrodes 56 and 58 are provided on one surface of the insulating substrate 52. One pattern electrode 56 is provided between the opposing indentations 54 and is extended in the shape of an L from one end of the electrode 56 to the central portion of the insulating substrate 52. The other pattern electrode 58 is provided between the other opposing indentations 54 and is extended in the shape of an L from one end of the electrode 58, which is opposite to the above-described end of the electrode 56, to the central portion of the insulating substrate 52. Then, the two pattern electrodes 56 and 58 face each other with a gap therebetween in the vicinity of the central portion of the insulating substrate 52. As the insulating substrate 52, a known substrate, such as a glass epoxy substrate or an alumina substrate, may be used. Alternatively, a dielectric substrate may be employed. Provided on the first and second external electrodes 24, 26 of the piezoelectric resonator 10 and substantially at the central portion of the base member 12 in the longitudinal direction is a mounting member 60 made of, for example, a conductive adhesive. The mounting member 60 is then connected and fixed on the pattern electrodes 56 and 58 with a conductive adhesive (not shown) provided on the ends of the pattern electrodes 56 and 58 which are placed at the central portion of the insulating substrate 52. Accordingly, the external electrodes 24 and 26 of the piezoelectric resonator 10 can be fixed on the insulating substrate 52 and electrically connected to the pattern electrodes 56 and 58. Further, a metallic cap 62 is placed to cover the insulating substrate 52. At this time, an insulating resin is applied to the insulating substrate 52 and each of the pattern electrodes 56 and 58 in order to avoid an electrical connection between the metallic cap 62 and each of the pattern electrodes 56 and 58. The metallic cap 62 covers the insulating substrate 52, thereby producing the electronic component 50. In the above-described electronic component 50, by the provision of the mounting member 60 formed on the external electrodes 24 and 26 of the piezoelectric resonator 10, the edges of the piezoelectric resonator 10 can be positioned away from the insulating substrate 52, and thus, the vibration is not hampered. Moreover, the central portion of the piezoelectric resonator 10, which serves as a node, is reliably supported by the mounting member 60, thereby preventing a leakage of vibration of the piezoelectric resonator 10. Further, by using a plurality of piezoelectric resonators 10, a ladder filter can be manufactured. Fig. 13 is a perspective view illustrating the essential portion of an electronic component used as a ladder filter having a ladder circuit. In the electronic component 50 illustrated in Fig. 13, four pattern electrodes 64, 66, 68, and 70 are formed on an insulating substrate 52. Provided for the pattern electrodes 64 through 70 are five lands, which are aligned with each other with a spacing therebetween. In this case, the first land as viewed from one end of the insulating substrate 52 is provided for the pattern electrode 64, the second and the fifth lands are provided for the pattern electrode 66, the third land is provided for the pattern electrode 68, and the fourth land is provided for the pattern electrode 70. The piezoelectric resonator 10 provided with the mounting member 60 on the external electrodes 24 and 26 is fixed at the mounting member 60 on the respective lands of the substrate 52 with a conductive adhesive. In this case, piezoelectric resonators 10a through 10d are mounted on the substrate 52 in order to obtain the ladder circuit shown in Fig. 14. Then, a metallic cap (not shown) is placed to cover the insulating substrate 52. In the electronic components shown in Figs. 12 and 13, piezoelectric resonators having a small capacitance between the second group of the internal electrode 16 and the first external electrodes 24 and between the first group of the internal electrode 16 and the second external electrodes 26 can be used, thereby suppressing a reduction in Δf. Accordingly, a decrease in the bandwidth can be suppressed in, in particular, the electronic component 50, such as a ladder filter. As the piezoelectric resonators 10 for use in the electronic components 50, not only the resonator 10 shown in Fig. 2, but also the resonators illustrated in Figs. 6, 8 through 11 may certainly be employed. In particular, if the piezoelectric resonator 10 shown in Fig. 8 or 9 is used in the electronic component 50, the first and second conductive resin films 28 and 30 are attached to the mounting member 60, in which case, Qm of the electronic component 50 can be adjusted. In the ladder filter illustrated in Fig. 13, the capacitance between the internal electrodes of each of the parallel resonators 10b and 10c is set to be sufficiently greater than that of the series resonators 10a and 10d. Consequently, in using piezoelectric resonators having a laminated structure, the number of laminated layers of the series resonators 10a and 10d is small, resulting in a relatively large spacing between the internal electrodes 16. On the other hand, the number of layers of the parallel resonators 10b and 10c is large, resulting in a small spacing between the internal electrodes 16. A reduction in Δf is noticeably presented by the series resonators 10a and 10d having a small capacitance between the internal electrodes and is not significantly observed by the parallel resonators 10b and 10c having a large capacitance. Further, it is easy to partially form the insulating films 20 and 22, as shown in Fig. 2, and to partially form the external electrodes 24 and 26, as illustrated in Fig. 8, in the series resonators 10a and 10d having a large spacing between the internal electrodes 16. It is, however, rather difficult to partially form the above components in the parallel resonators 10b and 10c having a small spacing between the internal electrodes 16. Considering the above correlation, the piezoelectric resonator 10 of the present invention may be used only for the series resonators 10a and 10d in the ladder filter 50 shown in Fig. 12, thereby implementing an easy-to-form ladder filter free from a decrease in the bandwidth. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled men in the art that the forgoing and other changes in form and details may be made therein without departing from the scope of the invention as defined in the appended claims.

Claims: A piezoelectric resonato (10) vibratable in a longitudinal vibration mode, comprising: a base member (12) having a longitudinal direction and formed by laminating a plurality of piezoelectric layers (14) and a plurality of internal electrodes (16); a first group of said internal electrodes and a second group of said internal electrodes respectively extended to and exposed at a first region and a second region of a surface of said base member; first and second external electrodes (24, 26) respectively disposed on said first and second regions continuously so as to be connected to said first and second group of said internal electrodes; and said piezoelectric layers (14) being polarizable in the longitudinal direction of said base member, and an electric field being appliable to said base member in its longitudinal direction, thereby exciting a longitudinal vibration in said base member; characterized in that an insulating film (20 ,22) covers an unexposed portion of each of said first and second group of said internal electrod (16); and said insulating film is disposed on each of said first and second regions and has a dielectric constant smaller than a dielectric constant of said piezoelectric layers. A piezoelectric resonator (10) vibratable in a longitudinal vibration mode. comprising: a base member (12) having a longitudinal direction and formed by laminating a plurality of piezoelectric layers (14) and a plurality of internal electrode (16); a first group of said internal electrodes and a second group of said internal electrodes respectively extended to and exposed at a first region and a second region of a surface of said base member; first and second external electrodes (24, 26) respectively disposed on said first and second regions continuously so as to be connected to said first and second group of said internal electrodes; and said piezoelectric layers (14) being polarizable in the longitudinal direction of said base member, and an electric field being appliable to said base member in its longitudinal direction, thereby exciting a longitudinal vibration in said base member; characterized in that first and second conductive resin films (28,30) are respectively disposed on said first and second regions continuously so as to be connected to said external electrodes. The piezoelectric resonator according to claim 1 or 2, wherein said first and second regions are respectively located at one side and at the other side in the widthwise direction of said base member on the same lateral surface of said base member. The piezoelectric resonator according to claim 1 or 2, wherein said first and second regions are respectively located on different lateral surfaces of said base member. An electronic component including said piezoelectric resonator (20) of one of claim 2 or claims 3 and 4 when dependent from claim 2, wherein said electronic component (50) includes a substrate (52) provided with a pattern electrode (56,58), to which said conductive resin film (28,30) are electrically connected, and said substrate and said piezoelectric resonator are covered with a cap (62). The electronic component according to Claim 5, wherein a plurality of said pattern electrodes are provided on said substrate and said conductive resin films of a plurality of said piezoelectric resonators (10a, ...10d) are electrically connected to said pattern electrodes so that a ladder filter is provided. The electronic component according to claim 6, wherein: first, second, third and fourth base members are provided as said base members; first, second, third and fourth pattern electrodes (64, 66, 68, 70) are provided as said electrode on said substrate; said first base member is mounted on said first and second pattern electrodes; said second base member is mounted on said second and third pattern electrodes; said third base member is mounted on said second and fourth pattern electrodes; and said fourth base member is mounted on said third and fourth pattern electrodes.

Inventors: BABA TOSHIYUKI, KAKUDA TATSUNORI, NISHIMURA TOSHIO, UNAMI TOSHIHIKO, BABA, TOSHIYUKI, KAKUDA, TATSUNORI, NISHIMURA, TOSHIO, UNAMI, TOSHIHIKO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: H03H9
IPC codes: H03H9