Electronic component and ladder filter

<b>BACKGROUND OF THE INVENTION</b>

The present invention relates to an electronic component comprising an insulating substrate, pattern electrodes provided on said insulating substrate, and a plurality of one electronic component element having an external electrode provided on said insulating substrate. An electronic component may be a ladder filter using a plurality of electronic component elements including a piezoelectric resonator using mechanical vibration of a piezoelectric body.

Fig. 19 is a perspective view of a conventional piezoelectric resonator 1. The piezoelectric resonator 1 includes a piezoelectric substrate 2 in the form of, for example, a flat block having rectangular major surfaces. The piezoelectric substrate 2 is polarized along the direction of the thickness thereof. On both the major surfaces of the piezoelectric substrate 2, electrodes 3 are formed. When a signal is input between the electrodes 3, an electric field is applied to the piezoelectric substrate 2 along the direction of the thickness of the piezoelectric substrate 2. The piezoelectric substrate 2 vibrates thereby in the longitudinal direction. Fig. 20 shows a piezoelectric resonator 1 including a piezoelectric substrate 2 in the form of a flat block having square major surfaces, and electrodes 3 formed on the major surfaces of the piezoelectric substrate 2. Also in this piezoelectric resonator 1, the piezoelectric substrate 2 is polarized along the direction of the thickness thereof. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electric field is applied to the piezoelectric substrate 2 along the direction of the thickness and the piezoelectric substrate 2 vibrates in a square-type vibration mode (in a plane direction). The piezoelectric resonators shown in Figs. 19 and 20 are of an unstiffened type, in which the direction of an electric field and the direction of polarization differ from the direction of vibration. The electromechanical coupling coefficient of such unstiffened piezoelectric resonators is smaller than that of stiffened piezoelectric resonators, in which the direction of an electric field, the direction of polarization and the direction of vibration coincide with each other. Therefore, unstiffened piezoelectric resonators have a relatively small difference ΔF between the resonant frequency and the antiresonant frequency. This leads to a disadvantage of restricting a filter band width if the type of piezoelectric resonator is used in a filter. Therefore, the degree of freedom in characteristics design is low when an electronic component using this type of piezoelectric resonator is designed. The piezoelectric resonator shown in Fig. 19 uses the longitudinal first-order resonance mode. However, it also has, due to its structure, large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in width modes. To suppress these spurious resonances, some measures are considered, such as polishing, adding a mass, and changing the shape of the electrodes. These measures, however, increase the manufacturing cost.

In addition, since the piezoelectric substrate has the shape of a flat block having rectangular major surfaces, the substrate cannot be markedly reduced in thickness due to a restriction relating to strength. Therefore, the distance between the electrodes cannot be reduced and the capacitance between terminals cannot be increased. This is extremely inconvenient for achieving impedance matching with an external circuit. To form a ladder filter by connecting a plurality of piezoelectric resonators in series and in parallel alternately, the capacitance ratio of the parallel resonators and the series resonators needs to be made large in order to increase attenuation. Because of the above-described shape restriction, however, large attenuation cannot be obtained. In the piezoelectric resonator shown in Fig. 20, the plane-direction first-order vibration mode is used but large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated due to the structure. Since this piezoelectric resonator needs a large size as compared with a piezoelectric resonator using longitudinal vibration in order to obtain the same resonant frequency, it is difficult for this piezoelectric resonator to be reduced in size. When a ladder filter is formed by a plurality of piezoelectric resonators, the method of reducing the capacitance of the resonators in the series connection by forming the electrodes only on restricted portions of the piezoelectric substrate as well as increasing the thickness of the series resonators is used in order to increase the capacitance ratio of the parallel resonators to the series resonators. In this case, since the electrodes are made only partially, the difference ΔF between the resonant frequency and the antiresonant frequency as well as the capacitance is reduced. The resonators connected in parallel are required to have ΔF reduced correspondingly. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased. The applicant of the present invention proposed, for example, in the specification of Japanese Patent Application 8-110475, a piezoelectric resonator in which a plurality of piezoelectric layers and a plurality of electrodes forming a base member having a longitudinal shape are alternately laminated, in which the plurality of piezoelectric layers are polarized along the longitudinal direction of the base member, and in which vibration in the longitudinal basic mode is excited. This piezoelectric resonator is of the stiffened type and is therefore free from the drawbacks in using the above-described unstiffened piezoelectric resonators. A ladder filter may be formed by using such a stiffened type piezoelectric resonator, for example, as shown in Fig. 21. Fig. 21 is an exploded perspective view of a ladder filter 4 using a piezoelectric resonator constructed as described in Japanese Patent Application 8-110475 made by the applicant of the present invention, and Fig. 22 is an exploded perspective view of essential elements of the ladder filter 4. In the ladder filter 4 shown in Fig. 21, three pattern electrodes 6a, 6b, and 6c are formed on an insulating substrate 5. Four supporting members 7a, 7b, 7c, and 7d are formed of an electroconductive adhesive on the pattern electrodes 6a to 6c. The supporting member 7a is formed on the pattern electrode 6a, the supporting members 7b and 7d are formed on the pattern electrode 6b, and the supporting member 7c is formed on the pattern electrode 6c. Each of four piezoelectric resonators 1a, 1b, 1c, and 1d has a pair of electrodes 3, one of which is mounted on the corresponding one of the supporting members 7a to 7d. The other electrodes 3 on the three piezoelectric resonators 1a, 1b, and 1c are connected to each other by conductor wires 8. The other electrode 3 of the piezoelectric resonator 1d is connected to the pattern electrode 6c by a conductor wire 8. A metallic cap 9 is placed on the insulating substrate 5. This ladder filter 4 has a ladder type circuit shown in Fig. 23. Because the ladder filter constructed as shown in Fig. 21 requires conductor wires for connection between the resonators and for connection between the resonators and the pattern electrodes, the process of manufacturing the ladder filter is complicated and there is a limit to the reduction in the size of the ladder filter. Therefore, if the pattern electrodes of this ladder filter are formed close to each other on the insulating substrate, a stray capacity is formed between the pattern electrodes, so that the amount of attenuation through the filter is reduced. The reduction in size is also limited for the purpose of avoiding this undesirable effect.

<b>SUMMARY OF THE INVENTION</b>

In view of these circumstances, a main object of the present invention is to provide an electronic component which has a piezoelectric resonator having reduced spurious responses and having an increased range of selection of the difference ΔF between the resonant frequency and the antiresonant frequency, and which can be reduced in size. Another object of the present invention is to provide a ladder filter which has a piezoelectric resonator having reduced spurious vibrations and having an increased range of selection of the difference ΔF between the resonant frequency and the antiresonant frequency, and which can be reduced in size. To achieve these objects, the invention provides an electronic component of the above mentioned kind, which is characterized in that at least one of said plurality of one electronic component element is a piezoelectric resonator, said piezoelectric resonator has a base member having a longitudinal direction in which a plurality of internal electrodes and a plurality of piezoelectric layers are laminated and a pair of said external electrodes provided on at least one side face of said base member are connected to said plurality of internal electrodes, and one of said external electrode of one of said electronic component element and the other of said external electrode of the other of said electronic component element adjacent to each other are placed on and electrically connected to one of said pattern electrodes. In the above described electronic component, major surfaces of said plurality of internal electrodes of said piezoelectric resonator may be perpendicular to the longitudinal direction of said base member, said piezoelectric layers may be polarized along the longitudinal direction of said base member, and said piezoelectric resonator may excite vibration in a longitudinal vibration mode. In the above described electronic component, said plurality of one electronic component element may include two chip capacitors, one of said external electrodes of said piezoelectric resonator and an external electrode of one of said capacitors adjacent to each other are connected to one of said pattern electrodes, and the other of said external electrodes of said piezoelectric resonator and an external electrode of the other of said capacitors adjacent to each other are connected to another of said pattern electrodes, whereby a π-type circuit is formed. In the above described electronic component, said electronic component may be a ladder filter having a plurality of said piezoelectric resonators, and two of said external electrodes of two of said piezoelectric resonators adjacent to each other are placed on and electrically connected to one of said pattern electrodes. In the above described ladder filter, said plurality of piezoelectric resonators may include four piezoelectric resonators which are a first series resonator, a second series resonator, a first parallel resonator and a second parallel resonator. And, said four piezoelectric resonators may be arranged in a row in the order of the first series resonator, the first parallel resonator, the second parallel resonator and the second series resonator. Alternatively, said four piezoelectric resonators may be arranged in a row in the order of the first series resonator, the second series resonator, the second parallel resonator and the first parallel resonator. The piezoelectric resonator used in the electronic component according to the present invention is of the stiffened type in which the direction of vibration coincides with the direction of polarization and the direction of an electric field and can therefore have a larger electromechanical coupling coefficient than unstiffened piezoelectric resonators in which the direction of vibration is different from the direction of polarization and the direction of an electric field, so that the range of selection of ΔF which is the difference between the resonant frequency and the antiresonant frequency can be increased.

The piezoelectric resonator used in the electronic component according to the present invention is arranged as a stiffened piezoelectric resonator to reduce the possibility of occurrence of vibrations in any modes such as width and thickness modes different from the longitudinal basic mode, thus reducing spurious vibrations. In the electronic component according to the present invention, external electrodes of the piezoelectric resonators or other electronic component elements can be connected directly to pattern electrodes on the insulating substrate without using any conductive wire. The electronic component and the ladder filter can therefore be reduced in size. In the electronic component according to the present invention, two external electrodes of an adjacent pair of the piezoelectric resonators or other electronic component elements are placed on and electrically connected to one pattern electrode. Therefore, there is no need for insulation between the external electrodes, and the adjacent electronic component elements can be placed close to each other. This arrangement also enables the electronic component and the ladder filter to be reduced in size. In the electronic component according to the present invention, two external electrodes of an adjacent pair of the piezoelectric resonators or other electronic component elements are placed on and electrically connected to one pattern electrode. Therefore, simpler pattern electrodes for connection of the electronic component elements may be formed on the insulating substrate instead of complicated pattern electrodes to achieve a further reduction in size. According to the present invention, an electronic component can be obtained which has a piezoelectric resonator having reduced spurious vibrations and capable of increasing the range of selection of the difference ΔF between the resonant frequency and the antiresonant frequency, and which can be reduced in size. Also, according to the present invention, a ladder filter can be obtained which has a piezoelectric resonator having reduced spurious vibrations and capable of increasing the range of selection of the difference ΔF between the resonant frequency and the antiresonant frequency, and which can be reduced in size. The above-described and other objects, features and advantages of the present invention will become apparent from the following detailed description of embodiments of the invention in conjunction with the accompanying drawings.

<b>BRIEF DESCRIPTION OF THE DRAWINGS</b>

<dl id="dl0001" compact="compact"><dt>Fig. 1</dt><dd>is a plan view of an example of a capacitor incorporation type oscillator in accordance with the present invention;</dd><dt>Fig. 2</dt><dd>is a front view of the capacitor incorporation type oscillator shown in Fig. 1;</dd><dt>Fig. 3</dt><dd>is an exploded perspective view of the capacitor incorporation type oscillator shown in Fig. 1;</dd><dt>Fig. 4</dt><dd>is a circuit diagram of the capacitor incorporation type oscillator shown in Fig. 1;</dd><dt>Fig. 5</dt><dd>is a perspective view of a piezoelectric resonator used in the capacitor incorporation type oscillator shown in Fig. 1;</dd><dt>Fig. 6</dt><dd>is a diagram for explaining the structure of the piezoelectric resonator shown in Fig. 5;</dd><dt>Fig. 7</dt><dd>is a plan view of an essential portion of the piezoelectric resonator shown in Fig. 5;</dd><dt>Fig. 8</dt><dd>is a perspective view of an unstiffened piezoelectric resonator capable of vibrating in the longitudinal direction and provided as a comparative example;</dd><dt>Fig. 9</dt><dd>is a perspective view of a stiffened piezoelectric resonator capable of vibrating in the longitudinal direction;</dd><dt>Fig. 10</dt><dd>is a perspective view of an unstiffened piezoelectric resonator capable of vibrating in the plane direction and provided as a comparative example;</dd><dt>Fig. 11</dt><dd>is a plan view of an example of a ladder filter in accordance with the present invention;</dd><dt>Fig. 12</dt><dd>is a front view of the ladder filter shown in Fig. 11;</dd><dt>Fig. 13</dt><dd>is an exploded perspective view of the ladder filter shown in Fig. 11;</dd><dt>Fig. 14</dt><dd>is a circuit diagram of the ladder filter shown in Fig. 11;</dd><dt>Fig. 15</dt><dd>is a plan view of another example of the ladder filter in accordance with the present invention.</dd><dt>Fig. 16</dt><dd>is a front view of the ladder filter shown in Fig. 15;</dd><dt>Fig. 17</dt><dd>is an exploded perspective view of the ladder filter shown in Fig. 15;</dd><dt>Fig. 18</dt><dd>is a circuit diagram of the ladder filter shown in Fig. 15;</dd><dt>Fig. 19</dt><dd>is a perspective view of a conventional piezoelectric resonator related to the present invention;</dd><dt>Fig. 20</dt><dd>is a perspective view of another conventional piezoelectric resonator related to the present invention;</dd><dt>Fig. 21</dt><dd>is an exploded perspective view of a ladder filter using the piezoelectric resonator related to the present invention;</dd><dt>Fig. 22</dt><dd>is an exploded perspective view of essential elements of the ladder filter shown in Fig. 23; and</dd><dt>Fig. 23</dt><dd>is a circuit diagram of the ladder filter shown in Fig. 21.</dd></dl>

<b>DESCRIPTION OF THE PREFERRED EMBODIMENTS</b>

A capacitor incorporation type oscillator which represents an embodiment of the present invention will be described below with reference to Figs. 1 to 7. A capacitor incorporation type oscillator 10 shown in Fig. 1 includes an insulating substrate 12 having, for example, the shape of a rectangular plate.

Three pattern electrodes 14a, 14b, and 14c are formed on one major surface of the insulating substrate 12 by being spaced apart from each other. Each of the pattern electrodes 14a and 14b is formed into the shape of I extending from one end to a central portion of the insulating substrate 12. The other pattern electrode 14c is generally U-shaped around the pattern electrodes 14a and 14b and along three sides of the surface of the insulating substrate 12 other than the side from which the pattern electrodes 14a and 14b extend. The pattern electrode 14c has opposite end portions formed at the side of the substrate 12 surface from which the pattern electrodes 14a and 14b extend. Four lands 16a, 16b, 16c and 16d are formed in a row as portions of the pattern electrodes 14a to 14c while being suitably spaced apart from each other. The lands 16a and 16b are formed as end portions of the pattern electrodes 14a and 14b while the lands 16c and 16d are formed as intermediate portions of the pattern electrode 14c. A piezoelectric resonator 20 is connected to the lands 16a and 16b of the pattern electrodes 14a and 14b. The piezoelectric resonator 20 will be described below in detail particularly with respect to its specific structure. The piezoelectric resonator 20 includes a base member 22 having, for example, the shape of a rectangular block. The base member 22 includes a plurality of piezoelectric layers 22a formed of, for example, a piezoelectric ceramic. A plurality of internal electrodes 24 are respectively formed on two major surfaces of the piezoelectric layers 22a perpendicular to the longitudinal direction of the base member 22 in a middle section of the base member 22 defined by sectioning the base member 22 in the longitudinal direction of the base member 22. That is, the internal electrodes 24 are suitably spaced apart from each other with its major surfaces set perpendicular to the longitudinal direction of the base member 22. The piezoelectric layers 22a in the middle section of the base member 22 are polarized along the longitudinal direction of the base member 22 in such a manner that the pair of the piezoelectric layers 22a on the opposite sides of each internal electrode 24 are polarized in opposite directions, as indicated by the arrows in Fig. 6. However, the piezoelectric layers 22a in two end sections of the base member 22 at the opposite ends in the longitudinal direction are not polarized. In one side face of the base member 22, a groove 26 is formed so as to extend in the longitudinal direction of the base member 22. The groove 26 is formed at the center in the widthwise direction of the base member 22 to bisect the side face of the base member 22. As shown in Fig. 6, on the side face divided by the groove 26, first insulating film 28a and second insulating film 28b are formed. Edges of the internal electrodes 24 exposed in one of the two portions of the side face of the base member 22 bisected by the groove 26 are alternately covered and left uncovered with the first insulating film 28a. Edges of the internal electrodes 24 which are exposed in the other of the two portions of the side face of the base member 22 bisected by the groove 26 and which are opposite from those not covered with the first insulating film 28a are covered with the second insulating film 28b. Further, external electrodes 30a and 30b are formed on the portions of the base member 22 on which the first and second insulating films 28a and 28b are formed, i.e., on the opposite sides of the groove 26. Therefore, the internal electrodes 24 not covered with the first insulating film 28a are connected to the external electrode 30a while the internal electrodes 24 not covered with the second insulating film 28b are connected to the external electrode 30b. That is, in each adjacent pair of the internal electrodes 24, one is connected to the external electrode 30a or 30b while the other is connected to the external electrode 30a or 30b. Supporting members 32a and 32b are respectively formed on the external electrodes 30a and 30b defined at the center of the same in the longitudinal direction. These supporting members 32a and 32b are formed of an electroconductive material. In this piezoelectric resonator 20, the external electrodes 30a and 30b are used as input/output electrodes. When the piezoelectric resonator 20 is operated, an electric field is applied to the polarized portion between each adjacent pair of the internal electrodes 24 in the longitudinal direction middle section of the base member 22.

The base member 22 is thereby made piezoelectrically active in the middle section. Since at this time a voltage is applied in opposite directions to the portions of the base member 22 polarized in opposite directions, the base member 20 expands and contracts in the same direction as a whole. As a result, in the entire piezoelectric resonator 20, vibration is excited in the longitudinal basic mode with a node corresponding to the center of the base member 22 in the longitudinal direction. The base member 22 is not polarized in its two end sections at the opposite ends in the longitudinal direction. These sections are not polarized and no electric fields are applied thereto since no electrodes are formed therein. Therefore, these opposite end sections are piezoelectrically inactive. In the piezoelectric resonator 20, the direction of polarization of the base member 22, the direction of each electric field according to an input signal and the direction of vibration of the base member 22 coincide with each other. That is, the piezoelectric resonator 20 is of the stiffened type. The stiffened piezoelectric resonator 20 has a larger electromechanical coupling coefficient than an unstiffened piezoelectric resonator in which the direction of vibration differs from the direction of polarization and the direction of an electric field. Therefore, the piezoelectric resonator 20 is capable of increasing the range of selection of the difference ΔF between the resonant frequency and the antiresonant frequency in comparison with the conventional piezoelectric resonator. This means that the piezoelectric resonator 20 can obtain a characteristic of a larger band width in comparison with the conventional unstiffened piezoelectric resonator. To measure differences between stiffened and unstiffened piezoelectric resonators, piezoelectric resonators shown in Figs. 8, 9, and 10 were made. The piezoelectric resonator shown in Fig. 8 was made by forming electrodes on both surfaces in the thickness direction of a piezoelectric substrate measuring 4.0 mm by 1.0 mm by 0.38 mm. This piezoelectric resonator was polarized in the thickness direction and was capable of vibrating in the longitudinal direction when a signal was applied to the electrodes. The piezoelectric resonator shown in Fig. 9 had the same dimensions as the piezoelectric resonator shown in Fig. 8. Electrodes were formed on both surfaces in the longitudinal direction of a piezoelectric substrate. The piezoelectric resonator was polarized in the longitudinal direction and was capable of vibrating in the longitudinal direction when a signal was applied to the electrodes. The piezoelectric resonator shown in Fig. 10 was made by forming electrodes on both surfaces in the thickness direction of a piezoelectric substrate measuring 4.7 mm by 4.7 mm by 0.38 mm. This piezoelectric resonator was polarized in the thickness direction and was capable of vibrating in the plane direction when a signal was applied to the electrodes. The piezoelectric resonators shown in Figs. 8 and 10 were of the unstiffened type while the piezoelectric resonator shown in Figs. 9 was of the stiffened type. The resonant frequency Fr and the electromechanical coupling coefficient K of each of these piezoelectric resonators were measured. The results of this measurement are shown in Tables 1, 2, and 3. Table 1 shows the measured values of the piezoelectric resonator shown in Fig. 8. Table 2 shows the measured values of the piezoelectric resonator shown in Fig. 9. Table 3 shows the measured values of the piezoelectric resonator shown in Fig. 10. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center">Longitudinal Basic Vibration</entry><entry namest="col3" nameend="col3" align="center">Longitudinal Triple Wave</entry><entry namest="col4" nameend="col4" align="center">Width Mode Vibration</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Resonant Frequency (MHz)</entry><entry namest="col2" nameend="col2" align="left">0.460</entry><entry namest="col3" nameend="col3" align="char" char=".">1.32</entry><entry namest="col4" nameend="col4" align="char" char=".">1.95</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Electromechanical Coupling Coefficient (%)</entry><entry namest="col2" nameend="col2" align="left">18.9</entry><entry namest="col3" nameend="col3" align="char" char=".">3.9</entry><entry namest="col4" nameend="col4" align="char" char=".">25.2</entry></row></tbody></tgroup></table></tables><tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center">Longitudinal Basic Vibration</entry><entry namest="col3" nameend="col3" align="center">Longitudinal Triple Wave</entry><entry namest="col4" nameend="col4" align="center">Width Mode Vibration</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Resonant Frequency (MHz)</entry><entry namest="col2" nameend="col2" align="left">0.455</entry><entry namest="col3" nameend="col3" align="char" char=".">1.44</entry><entry namest="col4" nameend="col4" align="center">1.96</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Electromechanical Coupling Coefficient (%)</entry><entry namest="col2" nameend="col2" align="left">42.9</entry><entry namest="col3" nameend="col3" align="char" char=".">12.2</entry><entry namest="col4" nameend="col4" align="center">4.0</entry></row></tbody></tgroup></table></tables><tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center">Plane Direction Fundamental Vibration</entry><entry namest="col3" nameend="col3" align="center">Plane Direction Triple Wave</entry><entry namest="col4" nameend="col4" align="center">Thickness Mode Vibration</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Resonant Frequency (MHz)</entry><entry namest="col2" nameend="col2" align="left">0.458</entry><entry namest="col3" nameend="col3" align="char" char=".">1.25</entry><entry namest="col4" nameend="col4" align="char" char=".">5.65</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Electromechanical Coupling Coefficient (%)</entry><entry namest="col2" nameend="col2" align="left">35.0</entry><entry namest="col3" nameend="col3" align="char" char=".">11.5</entry><entry namest="col4" nameend="col4" align="char" char=".">23.3</entry></row></tbody></tgroup></table></tables>

As can be read from these tables, the stiffened piezoelectric resonator had a larger electromagnetic coupling coefficient K than the unstiffened piezoelectric resonators and, therefore, could have a larger difference ΔF between the resonant frequency and the antiresonant frequency than the unstiffened piezoelectric resonators. The largest spurious vibration of the stiffened piezoelectric resonator was a longitudinal triple wave and the electromagnetic coupling coefficient K was 12.2 % with respect to this vibration while the electromagnetic coupling coefficient K with respect to width-mode vibration different from the basic vibration was small, 4.0 %. In contrast, the unstiffened longitudinal-vibration piezoelectric resonator had a large electromagnetic coupling coefficient K of 25.2 % in the width mode and the unstiffened plane-direction vibration piezoelectric resonator had a large electromagnetic coupling coefficient K of 23.3 % in the thickness mode. Thus, it is understood that the stiffened piezoelectric resonator had smaller spurious vibrations than the unstiffened piezoelectric resonators. The capacitance of the piezoelectric resonator 20 can be adjusted by, for example, changing the area of the opposed surfaces of internal electrodes 24, the number of piezoelectric layers 22a and the number of internal electrodes 24, and/or the size of the piezoelectric layers 22a in the longitudinal direction of the base member 22. That is, the capacitance of the resonator can be increased by increasing the area of the opposed surfaces of internal electrodes 24, by increasing the number of piezoelectric layers 22a and the number of internal electrodes 24 and/or by reducing the size of the piezoelectric layers 22a in the longitudinal direction of the base member 22. Conversely, the capacitance of the resonator can be reduced by reducing the area of the opposed surfaces of internal electrodes 24, by reducing the number of piezoelectric layers 22a and the number of internal electrodes 24 and/or by increasing the size of the piezoelectric layers 22a in the longitudinal direction of the base member 22. Because of this capacitance adjustment by changing the area of the opposed surfaces of internal electrodes 24, the number of piezoelectric layers 22a and the number of internal electrodes 24, and/or the size of the piezoelectric layers 22a in the longitudinal direction of the base member 22, the degree of freedom in capacitance design of the piezoelectric resonator 20 is high. Therefore, the piezoelectric resonator 20 can easily be impedance matched to an external circuit when used by being mounted on a circuit board or the like. In the capacitor incorporation type oscillator 10, the supporting members 32a and 32b are bonded to the lands 16a and 16b of the pattern electrodes 14a and 14b, respectively, by an electroconductive adhesive at the center of the piezoelectric resonator 20 in the longitudinal direction. The external electrodes 30a and 30b of the piezoelectric resonator 20 are thereby connected electrically to the pattern electrodes 14a and 14b, respectively. In the capacitor incorporation type oscillator 10, two electrodes of an electronic component element, e.g., external electrodes 42a and 42b of a chip capacitor 40 are bonded to the lands 16a and 16c of the pattern electrodes 14a and 14c, respectively, by an electroconductive adhesive. The external electrodes 42a and 42b of the capacitor 40 are thereby connected electrically to the pattern electrodes 14a and 14c, respectively. Further, two electrodes of an electronic component element, e.g., external electrodes 52a and 52b of a chip capacitor 50 are bonded to the lands 16b and 16d of the pattern electrodes 14b and 14c, respectively, by an electroconductive adhesive. The external electrodes 52a and 52b of the capacitor 50 are thereby connected electrically to the pattern electrodes 14b and 14c, respectively. Consequently, the capacitor incorporation type oscillator 10 has a π-type circuit shown in Fig. 4. In the capacitor incorporation type oscillator 10, a metallic cap (not shown) is placed on the insulating substrate 12 so as to cover the piezoelectric resonator 20 and the capacitors 40 and 50. To prevent the metallic cap from being short-circuited to the pattern electrodes 14a to 14c, an insulating resin is applied to the insulating substrate 12 and the pattern electrodes 14a and 14c.

As described above, the capacitor incorporation type oscillator 10 has the piezoelectric resonator 20 having reduced spurious vibrations and having an increased range of selection of the difference ΔF between the resonance frequency and the antiresonant frequency. In the capacitor incorporation type oscillator 10, there is no need for any conductor wire since the external electrodes 30a, 30b, 42a, 42b, 52a, and 52b of the piezoelectric resonator 20 and the capacitors 40 and 50 provided as electronic component elements are directly connected to the pattern electrodes 14a to 14c on the insulating substrate 12 by the supporting members 32a and 32b. Therefore, the overall size of the capacitor incorporation type oscillator 10 can be reduced. In the capacitor incorporation type oscillator 10, adjacent two of the external electrodes of two electronic component elements adjacent to each other are placed on the lands of one pattern electrode to be connected to the pattern electrode electrically and mechanically. Therefore, there is no need for insulation between the two external electrodes, so that the electronic component elements adjacent to each other can be placed close to each other. This connection structure also contributes to the reduction in size. In the capacitor incorporation type oscillator 10, simpler pattern electrodes formed on the insulating substrate may suffice for connection of electronic component elements without complicated pattern electrodes since the connection structure is such that adjacent two of the external electrodes of two electronic component elements adjacent to each other are placed on the lands of one pattern electrode to be connected to the pattern electrode electrically and mechanically. The overall size of the capacitor incorporation type oscillator 10 can also be reduced by simplifying the pattern electrodes. In the capacitor incorporation type oscillator 10, damping of vibration of the piezoelectric resonator 20 due to proximity to the insulating substrate 12 is limited because the piezoelectric resonator 20 is supported and suitably spaced apart from the insulating substrate 12 by the supporting members 32a and 32b formed at its node. Since the supporting members 32a and 32b are formed on the piezoelectric resonator 20 at a node of the same in the capacitor incorporation type oscillator 10, the piezoelectric resonator 20 can easily be supported only by fixing the supporting members 32a and 32b so that its vibration is not strongly damped. Fig. 11 is a plan view of a ladder filter 60 which represents a second embodiment of the present invention, and Figs. 12, 13, and 14 are a front view, an exploded perspective view, and a circuit diagram, respectively, of the ladder filter shown in Fig. 11. The ladder filter 60 shown in Fig. 11 includes an insulating substrate 12 having, for example, the shape of a rectangular plate. Four pattern electrodes 14a, 14b, 14c, and 14d are formed on one major surface of the insulating substrate 12 while being spaced apart from each other. The pattern electrode 14a extends from one side of the substrate 12 surface so as to be generally L-shaped along the contour of the substrate 12 surface at one corner of the same. The pattern electrode 14b has a straight portion extending along the side of the substrate 12 surface opposite from the side from which the pattern electrode 14a extends, a portion extending from an intermediate position on the straight portion to a central portion of the substrate 12, and a portion extending from a position in the vicinity of one end of the straight portion and along a near edge of the substrate 12. The pattern electrode 14c is generally I-shaped and has one end formed at the center of the insulating substrate 12 and the other end formed adjacent to the side of the substrate 12 surface from which the pattern electrode 14a extends. The pattern electrode 14d is generally L-shaped and has one end formed near the center of the insulating substrate 12 and an intermediate portion formed adjacent to the side of the substrate 12 surface from which the pattern electrode 14 extends. Five lands 16a, 16b, 16c, 16d, and 16e are formed in a row as portions of the pattern electrodes 14a to 14d while being suitably spaced apart from each other. The Lands 16a to 16d are respectively formed as end portions of the pattern electrodes 14a to 14d while the land 16e is formed as the other end portion of the pattern electrode 14b. Four piezoelectric resonators 20a, 20b, 20c, and 20d provided as electronic component elements are placed on the lands 16a to 16e of the pattern electrodes 14a to 14d by being arranged in this order. Each of the piezoelectric resonators 20a to 20d has the same structure as the above-described piezoelectric resonator 20. However, two piezoelectric resonators 20a and 20d are used as series resonators while the other two piezoelectric resonators 20b and 20c are used as parallel resonators. Accordingly, the piezoelectric resonators 20a to 20d are designed so that the capacitance of the parallel piezoelectric resonators 20b and 20c is markedly larger than the capacitance of the series piezoelectric resonators 20a and 20d. That is, the supporting members 32a and 32b of the piezoelectric resonator 20a provided as the first series piezoelectric resonator are bonded to the lands 16a and 16b of the pattern electrodes 14a and 14b, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20a are thereby connected to the pattern electrodes 14a and 14b, respectively. Also, the supporting members 32a and 32b of the piezoelectric resonator 20b provided as the first parallel piezoelectric resonator are bonded to the lands 16b and 16c of the pattern electrodes 14b and 14c, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20b are thereby connected to the pattern electrodes 14b and 14c, respectively. Also, the supporting members 32a and 32b of the piezoelectric resonator 20c provided as the second parallel piezoelectric resonator are bonded to the lands 16c and 16d of the pattern electrodes 14c and 14d, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20c are thereby connected to the pattern electrodes 14c and 14d, respectively. Further, the supporting members 32a and 32b of the piezoelectric resonator 20d provided as the second series piezoelectric resonator are bonded to the lands 16d and 16e of the pattern electrodes 14d and 14b, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20d are thereby connected to the pattern electrodes 14d and 14b, respectively. Consequently, the ladder filter 60 has a ladder type circuit shown in Fig. 14. That is, in the ladder filter 60, the pattern electrode 14a is used as an input terminal, the pattern electrode 14d is used as an output terminal, and the pattern electrode 14c is used as a grounding terminal. In the ladder filter 60, a metallic cap (not shown) is placed on the insulating substrate 12 so as to cover the piezoelectric resonators 20a to 20d. To prevent the metallic cap from being short-circuited to the pattern electrodes 14a to 14d, an insulating resin is applied to the insulating substrate 12 and the pattern electrodes 14a to 14d. The ladder filter 60 has the piezoelectric resonators 20a to 20d having reduced spurious vibrations and having an increased range of selection of the difference ΔF between the resonance frequency and the antiresonant frequency, as does the above-described oscillator 10. In the ladder filter 60, there is no need for any conductor wire since the external electrodes 30a and 30b of the piezoelectric resonators 20a to 20d provided as electronic component elements are directly connected to the pattern electrodes 14a to 14d on the insulating substrate 12 by the supporting members 32a and 32b. Therefore, the overall size of the ladder filter 60 can be reduced. In the ladder filter 60, adjacent two of the external electrodes of each adjacent pair of the piezoelectric resonators are placed on the lands of one pattern electrode to be connected to the pattern electrode electrically and mechanically. Therefore, there is no need for insulation between the external electrodes and the electronic component elements adjacent to each other can be placed close to each other. This arrangement also enables the electronic component and the ladder filter to be reduced in size. Since in the ladder filter 60 adjacent two of the external electrodes of each adjacent pair of the piezoelectric resonators are placed on the lands of one pattern electrode to be connected to the pattern electrode electrically and mechanically, simpler pattern electrodes formed on the insulating substrate may suffice for connection of electronic component elements without complicated pattern electrodes. This connection structure also contributes to the reduction in size. In the ladder filter 60, damping of vibration of the piezoelectric resonators 20a to 20d due to proximity to the insulating substrate 12 is limited because the piezoelectric resonators 20a to 20d are supported and suitably spaced apart from the insulating substrate 12 by the supporting members 32a and 32b formed at their nodes. Since the supporting members 32a and 32b are formed on the piezoelectric resonators 20a to 20d at nodes of the same in the ladder filter 60, the piezoelectric resonators 20a to 20d can easily be supported only by fixing the supporting members 32a and 32b so that their vibrations are not strongly damped. The attenuation in the ladder filter is influenced by the capacitance ratio of the parallel resonators and the series resonators. The capacitance of each of the piezoelectric resonators 20a to 20d can be adjusted by, for example, changing the area of the opposed surfaces of internal electrodes 24, the number of piezoelectric layers 22a and the number of internal electrodes 24, and/or the size of the piezoelectric layers 22a in the longitudinal direction of the base member 22. Therefore, if the capacitance of the piezoelectric resonators 20a to 20d is adjusted, the ladder filter 60 can be realized as a filter having a smaller number of resonators and having a larger amount of attenuation in comparison with those using conventional unstiffened piezoelectric resonators. Also, since the range of selection of ΔF of the piezoelectric resonators 20a to 20d can be increased relative to that in the conventional piezoelectric resonators, the ladder filter 60 can have a larger passband width than those using conventional piezoelectric resonators. Fig. 15 is a plan view of a ladder filter 60 which represents a third embodiment of the present invention, and Figs. 16, 17, and 18 are a front view, an exploded perspective view, and a circuit diagram, respectively, of the ladder filter shown in Fig. 15. The ladder filter 60 shown in Figs. 15 to 18 differs from that shown in Figs. 11 to 14 only in the placement of the piezoelectric resonator 20b provided as the first parallel resonator and the piezoelectric resonator 20d provided as the second series resonator. That is, in the ladder filter 60 shown in Figs. 15 to 18, the supporting members 32a and 32b of the piezoelectric resonator 20d provided as the second series resonator are bonded to the lands 16b and 16c of the pattern electrodes 14b and 14c, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20d are thereby connected to the pattern electrodes 14b and 14c, respectively. Also, the supporting members 32a and 32b of the piezoelectric resonator 20d provided as the first parallel resonator are bonded to the lands 16d and 16e of the pattern electrodes 14d and 14b, respectively, by an electroconductive adhesive. The external electrodes 30a and 30b of the piezoelectric resonator 20b are thereby connected to the pattern electrodes 14d and 14b, respectively. Consequently, this ladder filter 60 has a ladder type circuit shown in Fig. 18. That is, in this ladder filter 60, the pattern electrode 14a is used as an input terminal, the pattern electrode 14c is used as an output terminal, and the pattern electrode 14d is used as a grounding terminal. The ladder filter 60 shown in Figs. 15 to 18 are as advantageous as that shown in Figs. 11 to 14. The base member 22 of the piezoelectric resonator 20, and 20a to 20d of the above-described capacitor incorporation type oscillator and the ladder filters 60 has a groove 26 formed in one side face. However, it is not always necessary to form such a groove 26 in the base member. In each of the piezoelectric resonators 20, 20 to 20d of the above-described oscillator and filters, the longitudinal direction end sections of the base member 22 are piezoelectrically inactive. However, any portion other than the longitudinal direction end sections of the base member 22 may be formed as such a piezoelectrically inactive section, or the entire base member 22 may be formed so as to be piezoelectrically active through its overall length. In the electronic component of the present invention, any other type of piezoelectric resonator capable of being mounted in a surface mount manner and using the longitudinal vibration mode may be used. In the electronic component of the present invention, any electronic component element capable of being mounted in a surface mount manner other than the chip capacitor may also be used. In the electronic component of the present invention, the number of electronic component elements used, including piezoelectric resonators, may be selected as desired.