Ladder type filter

Abstract: A ladder type filter (10) designed to reduce a distortion in filtering characteristics has a supporting substrate (12) made of an insulating material. Supporting members (22,24,26,28,30,32,34,36,38) are attached to lands formed as portions of pattern electrodes (14,16,18,20) on the supporting substrate. A plurality of piezoelectric resonators (50a,50b,50c,50d) are mounted on the supporting members and connected in a ladder form to form a filter. Each piezoelectric resonator has a base member (52) in which piezoelectric layers (54) and internal electrodes (56) are laminated. External electrodes (62,64) formed on one side face of the base member (52) are connected to the internal electrodes (56) in the base member (52). The width w mm and the thickness t mm of the piezoelectric resonators (50a,50b,50c,50d) and the number n of laminated layers each consisting of one of the piezoelectric layers (54) and one of the electrodes (56) adjacent to the piezoelectric layer (54) are selected so as to satisfy a expression of 1 ≤ log 10 (n 2 wt) ≤ 3.

Description: BACKGROUND OF THE INVENTION The present invention relates to ladder type filters and, more particularly, to a ladder type filter comprising a plurality of piezoelectric resonators connected in series and in parallel in a ladder fashion. Fig. 16 is a perspective view of a piezoelectric resonator 1 used in a conventional ladder type filter. 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 the two 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. 17 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). A ladder type filter can be formed by connecting piezoelectric resonators of this kind in series and in parallel in a ladder fashion, as shown in Fig. 18. For example, such a ladder type filter is used as an intermediate frequency filter in a radio set, a communication apparatus or the like. In use of such a ladder type filter, however, there is a possibility of a disturbance of the filtered waveform. For example, this disturbance appears as a considerable reduction in 2 dB band width. SUMMARY OF THE INVENTION In view of the above-described problem, a main object of the present invention is to provide a ladder type filter arranged to reduce a distortion in filtering characteristics. To achieve this object, the present invention provides a ladder type filter of the above mentioned kind, which is characterized in that each of said piezoelectric resonators comprises a base member having a plurality of piezoelectric layers and a plurality of internal electrodes alternately laminated along a longitudinal direction, said piezoelectric resonators vibrate in the longitudinal direction of said base member by application of an electric field to said base member via said internal electrodes, and each of said piezoelectric resonators satisfies an expression of 1 ≤ log 10 (n 2 wt) ≤ 3, in which n is the number of laminated layers each consisting of one of said piezoelectric layers and one of said internal electrodes adjacent to said one of said piezoelectric layers, and w mm and t mm are a width and a thickness of each of said piezoelectric resonators respectively. In the above described ladder type filter, each of said piezoelectric layers may be polarized along the longitudinal direction of said base member. In the above described ladder type filter, the adjacent pair of said piezoelectric layers located on the opposite sides of each of said internal electrodes may be polarized in opposite directions. In the above described ladder type filter, each adjacent pair of said internal electrodes may be respectively connected to two different external electrodes which are provided on at least one side face of said base member. In the above described ladder type filter, a plurality of said piezoelectric resonators may be connected in series and in parallel in a ladder fashion on an supporting substrate having pattern electrodes provided on its surface. The inventors of the present invention proposed, for example, in the specification of Japanese Patent Application 8-110475, etc., a laminated piezoelectric resonator which can be used in place of piezoelectric resonators used in conventional ladder type filters. This piezoelectric resonator is a stiffened piezoelectric resonator in which the direction of application of an electric field and the direction of vibration coincide with each other. Such a device is also disclosed in US-A-3 378 704; a technique to calculate the filter characteristics of a stacked crystal filter is discussed in A. BALLATO ET AL.: 'Systematic Design of Stacked-Crystal Filters by Microwave Network Methods', IEEE TRANS. MICROWAVE THEORY AND TECHNIQUES,, 01. January 1974, vol. 22, no. 1, pages 14 to 25. Accordingly, this piezoelectric resonator has a larger number of factors, such as the width, the thickness and the number of laminated layers, which contribute to filtering characteristics, in comparison with the conventional piezoelectric resonators. A study has therefore been made as to whether an improvement in filtering characteristics can be achieved by controlling such factors in a ladder type filter using such a laminated piezoelectric resonator. The result is that a good filtering characteristic can be obtained if the width of the piezoelectric resonator, represented by w, the thickness of the piezoelectric resonator, represented by t, the number of laminated layers each consisting of one piezoelectric layer and one electrode adjacent to the piezoelectric layer, represented by n, satisfy an expression of 1 ≤ log 10 (n 2 wt) ≤ 3. In a piezoelectric resonator used in such a ladder type filter, piezoelectric layers are polarized along the longitudinal direction of a base member and an electric field is applied to the piezoelectric layer between each adjacent pair of the electrodes. External electrodes to be connected to the electrodes in the base member may be formed on one face of the piezoelectric resonator to enable the piezoelectric resonator to be mounted on a substrate in a surface mount manner to form the ladder type filter. Also, a chip type ladder filter can be constructed in such a manner that pattern electrodes are formed on an insulating substrate, piezoelectric resonators are mounted on the substrate so as to form a ladder type filter circuit, and the piezoelectric resonators are thereafter covered with a cap. According to the present invention, a ladder type filter having an improved filtering characteristic can be obtained. This ladder type filter can be arranged to have a larger 2 dB band width. If piezoelectric resonators can be mounted on a substrate in a surface mount manner, there is no need for using lead wires or like for connection of the piezoelectric resonators, thereby enabling the ladder type filter to be reduced in size. Further, if the ladder type filter is designed as a chip component, it can be mounted easily on a printed circuit board or the like. The above-described and other objects, features and advantages of the present invention will become apparent from the detailed description of a preferred embodiment of the invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded perspective view of a ladder type filter which represents an embodiment of the present invention; Fig. 2 is a cross-sectional view of a portion of the ladder type filter shown in Fig. 1; Fig. 3 is a perspective view of a piezoelectric resonator used in the ladder type filter shown in Fig. 1; Fig. 4 is a diagram showing the structure of the piezoelectric resonator shown in Fig. 3; Fig. 5 is a plan view of a base member which is used in the piezoelectric resonator shown in Fig. 4, and on which strips of insulating film are formed; Fig. 6 is a diagram of another example of the piezoelectric resonator used in the ladder filter shown in Fig. 1; Fig. 7 is a diagram showing two cross sections of a base member used in the piezoelectric resonator shown in Fig. 6; Fig. 8 is an equivalent circuit diagram of the ladder type filter shown in Fig. 1; Fig. 9 is a graph showing a filtering characteristic of a sample 1 of the ladder type filter; Fig. 10 is a graph showing a filtering characteristic of a sample 2 of the ladder type filter; Fig. 11 is a graph showing a filtering characteristic of a sample 3 of the ladder type filter; Fig. 12 is a graph showing a filtering characteristic of a sample 4 of the ladder type filter; Fig. 13 is a graph showing a filtering characteristic of a sample 5 of the ladder type filter; Fig. 14 is a graph showing a filtering characteristic of a sample 6 of the ladder type filter; Fig. 15 is a graph showing a filtering characteristic of a sample 7 of the ladder type filter; Fig. 16 is a perspective view of a piezoelectric resonator used in conventional ladder type filters; Fig. 17 is a perspective view of another piezoelectric resonator used in conventional ladder type filters; and Fig. 18 is an equivalent circuit diagram of a conventional ladder type filter using the piezoelectric resonator shown in Fig. 16 or 17. DESCRIPTION OF THE PREFERRED EMBODIMENT Figs. 1 and 2 show a ladder type filter 10 which represents an embodiment of the present invention. The ladder type filter 10 includes a supporting substrate 12 made of an insulating material such as alumina. Three recesses are formed in each of two opposite side portions of the supporting substrate 12. Four pattern electrodes 14, 16, 18, and 20 are formed on the supporting substrate 12. The pattern electrodes 14, 16, and 18 are formed so as to extend from the three recesses in one of the opposite side portions of the supporting substrate 12 toward a central portion of the supporting substrate 12. The pattern electrode 20 is formed so as to extend between the opposite side portions of the supporting substrate 12 where no recess is formed. Two extensions 20a and 20b extend from the pattern electrode 20 toward the central portion of the supporting substrate 12. The extension 20a is formed so as to extend toward a position between the two pattern electrodes 14 and 16 while the other extension 20b is formed so as to extend toward the pattern electrode 18 along one of the opposite sides of the supporting substrate 12 having no recess. Extended end portions of the pattern electrodes 14, 16, and 18 and the extensions 20a and 20b of the pattern electrode 20 are increased in width to form lands, which are arranged in a row. A supporting member 22 is attached to the land of the pattern electrode 14. Two supporting members 24 and 26 are attached to the land of the extension 20a of the pattern electrode 20; two supporting members 28 and 30 to the land of the pattern electrode 16; and two supporting members 32 and 34 to the land of the pattern electrode 18. A supporting member 36 is attached to the land of the extension 20b of the pattern electrode 20. For example, each of these supporting members 22 to 36 is formed in such a manner that surface electrodes 40 are formed on opposite side surfaces of a core 38 formed of an epoxy resin or the like. The supporting members 22 to 36 are connected to the lands by an electroconductive adhesive 42 or the like. When this connection is made, major surfaces of the supporting members 22 to 36 on which no surface electrode 40 is formed are bonded to the lands by the electroconductive adhesive 42. The electroconductive adhesive 42 contacts the surfaces of the surface electrodes 40 on the opposite surfaces of the supporting members 22 to 36 to electrically connect the lands and the surface electrodes 40. Piezoelectric resonators 50 (50a, 50b, 50c, 50d) are mounted on the supporting members 22 to 36. As shown in Fig. 3, each piezoelectric resonator 50 includes a base member 52 having, for example, the shape of a rectangular block. As shown in Fig. 4, the base member 52 is formed by laminating a plurality of piezoelectric layers 54 made of, for example, a piezoelectric ceramic material and a plurality of internal electrodes 56. Each internal electrode 56 is formed so that its major surfaces are perpendicular to the longitudinal direction of the base member 52. The piezoelectric layers 54 are polarized along the longitudinal direction of the base member 52 so that the adjacent pair of piezoelectric layers 54 on the opposite sides of each internal electrode 56 are polarized in opposite directions, as indicated by the arrows in Fig. 4. However, the base member 52 is not polarized at the opposite ends in the longitudinal direction. As shown in Fig. 5, on one side face of the base member 52, strips of insulating film 58 and 60 are formed adjacent to two longer sides of the side face opposite from each other in the widthwise direction of the base member 52. Edges of the internal electrodes 56 are exposed in the side face of the base member 52. Portions of the exposed edges of the internal electrodes 56 adjacent to one longer side of the side face are alternately covered and left uncovered with insulating film 58. In other portions of the exposed edges of the internal electrodes 56 adjacent to the other longer side of the side face, those opposite from the exposed edge portions not covered with insulating film 58 are covered with insulating film 60. Further, external electrodes 62 and 64 are formed on the side face of the base member 52 opposite from each other in the widthwise direction, i.e., on two portions of the side face of the base member 52 on which insulating films 58 and 60 are formed. The internal electrodes 56 not covered with insulating film 58 are connected to the external electrode 62 while the electrodes 56 not covered with insulating film 60 are connected to the external electrode 64. That is, in each adjacent pair of the internal electrodes 56, one is connected to the external electrode 62 or 64 while the other is connected to the external electrode 64 or 62. Alternatively, the internal electrodes 56 may be formed in such a manner that the portions to be connected to the external electrodes 62 and 64 are exposed as layers in the lamination and the other portions are not exposed, as shown in Figs. 6 and 7. In such a case, it is not necessary to specially use insulating films 58 and 60. In the piezoelectric resonator 50, the external electrodes 62 and 64 are used as input/output electrodes. When the piezoelectric resonator 50 is operated, an electric field is applied to the portion between each adjacent pair of the internal electrodes 56 defining a central section of the base member 52. The base member 52 is thereby made piezoelectrically active in the central section. However, the base member 52 is not made piezoelectrically active in its opposite end sections because it is not polarized in the opposite end sections and because no electric field is applied in the opposite end sections since no electrodes are formed on the opposite end surfaces of the base member 52. Thus, the central section of the base member 52 is formed as an active section 66 which is activated by an input signal while the opposite end sections of the base member 52 are formed as inactive sections 68 which are not activated by an input signal. Each inactive section 68 is defined as a section in which no driving force is generated by an input signal. Accordingly, an electric field may be applied to the portion between each adjacent pair of internal electrodes 56 if the portion between the electrodes is not polarized. Also, a structure for inhibiting application of an electric field to each of the corresponding polarized piezoelectric layers 54 may be used. Such inactive section 68 may be provided in necessary places in the base member 52 according to one's need. Also, it is not always necessary to form inactive sections 68; the entire base member 52 may be formed as an active section. When a signal is input to this piezoelectric resonator 50, a voltage is applied in opposite directions to the piezoelectric layers 54 in the active section 66 polarized in opposite directions. All the piezoelectric layers 54 are thereby caused to expand and contract to excite, as vibration of the entire piezoelectric resonator 50, vibration in the longitudinal basic mode with a node corresponding to the center of the base member 52. In this piezoelectric resonator 50, the direction of polarization of the active section 66, the direction of each electric field according to a signal and the direction of vibration of the active section 66 coincide with each other. That is, this piezoelectric resonator 50 is of the stiffened type. Piezoelectric resonators 50a to 50d thus constructed are disposed on the supporting substrate 12. The piezoelectric resonator 50a is mounted on the supporting members 22 and 24 with its external electrodes 62 and 64 bonded to the supporting members 22 and 24, respectively, by an electroconductive adhesive 70 or the like. When the external electrodes 62 and 64 are attached to the supporting members 22 and 24, the electroconductive adhesive 70 contacts the surface electrodes 40 of the supporting members 22 and 24 to electrically connect the external electrodes 62 and 64 to the surface electrodes 40. Similarly, the piezoelectric resonator 50b is mounted on the supporting members 26 and 28 with its external electrodes 62 and 64 bonded to the supporting members 26 and 28, respectively, by electroconductive adhesive 70 or the like. Also, the piezoelectric resonator 50c is mounted on the supporting members 30 and 32 with its external electrodes 62 and 64 bonded to the supporting members 30 and 32, respectively, by electroconductive adhesive 70 or the like. Further, the piezoelectric resonator 50d is mounted on the supporting members 34 and 36 with its external electrodes 62 and 64 bonded to the supporting members 34 and 36, respectively, by electroconductive adhesive 70 or the like. A metallic'cap (not shown) is placed on the supporting substrate 12. To prevent the metal cap from being short-circuited to the pattern electrodes 14, 16, 18, and 20, an insulating resin is applied to the supporting substrate 12 and the pattern electrodes 14, 16, 18, and 20 in advance. By capping with the metallic cap, the manufacture of the ladder type filter is completed. This ladder type filter 10 has a circuit in a ladder form such as that shown in Fig. 8. For example, two piezoelectric resonators 50a and 50d serve as series resonators while the other two piezoelectric resonators 50b and 50c serve as parallel resonators. The thus-constructed ladder type filter 10 is designed so that the parallel piezoelectric resonators 50b and 50c have substantially larger capacitances than the series piezoelectric resonators 50a and 50d. In the ladder type filter, an attenuation characteristic is determined by an impedance characteristic of the series piezoelectric resonators 50a and 50d and an impedance characteristic of the parallel piezoelectric resonators 50b and 50c. Filters having a center frequency of 455 kHz corresponding to the intermediate frequency of radiobroadcasting and communication systems were made as samples 1 to 7 in accordance with the above-described ladder type filter 10. Filtering characteristics of samples 1 to 7 varied by changing the width of the piezoelectric resonators 50a to 50d, represented by w, the thickness of the piezoelectric resonators 50a to 50d, represented by t, and the number of laminated layers each consisting of one of the piezoelectric layers 54 and one of the internal electrodes 56 adjacent to the piezoelectric layer 54, represented by n, were measured. The relationship between the filtering characteristics and the values w, t and n was examined. Table 1 shows the results of the measurement and examination, and Figs. 9 to 15 shows the filtering characteristics of samples 1 to 7 of the ladder type filter. Sample No. 1 2 3 4 5 6 7 2 dB Passb and Width (kHz) 4.7 6.9 10.1 11.9 10.8 8.9 5.8 Series Resonators Number of Laminated Layers n 20 16 10 5 3 2 2 Width w (mm) 1.0 1.0 1.0 1.2 1.1 1.6 0.8 Thickness t (mm) 1.0 0.8 1.0 1.0 1.1 0.8 0.8 log10 (n 2 wt) 2.6 2.3 2.0 1.5 1.0 0.71 0.6 Parallel Resonators Number of Laminated Layers n 44 32 25 20 10 5 5 Width w (mm) 2.0 1.95 1.6 0.75 1.0 2.0 1.0 Thickness t (mm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 log10 (n 2 wt) 3.6 3.3 3.0 2.5 2.0 1.7 1.6 From these measurement results, it is understood that favorable filtering characteristics can be obtained when w, t and n satisfy an inequality: 1 ≤ log 10 (n 2 wt) ≤ 3. In samples 1 and 2, log 10 (n 2 wt) of the parallel resonators is out of this range. In samples 6 and 7, log 10 (n 2 wt) of the series resonators is out of this range. These ladder type filters have distorted filtering characteristics and smaller 2 dB passband widths. In contrast, log 10 (n 2 wt) of samples 3 to 5 of the ladder type filter is within the range shown above, and these ladder type filters have undistorted filtering characteristics and larger passband widths. In particular, sample 4 of the ladder type filter is excellent in the filtering characteristic as well as in 2 dB passband width. In this filter, log 10 (n 2 wt) of the series resonators is 1.5 while log 10 (n 2 wt) of the parallel resonators is 2.5. The width w, the thickness t and the number n of laminated layers of piezoelectric resonators 50 used in ladder type filter 10 are selected so as to satisfy the inequality shown above, thus obtaining a ladder type filter having a favorable filtering characteristic. Also, a chip type filter can be constructed by mounting piezoelectric resonators on an insulating substrate having pattern electrodes formed on its surface and by covering these parts with a metallic cap. This chip type filter can be designed as an electronic component capable of being mounted in a surface mount manner on a printed circuit board or the like. The construction of piezoelectric resonator 50 may alternatively be such that two external electrodes are formed on opposite side faces of base member 52 instead of being formed on one side face. In such a case, one of the external electrodes is placed on and connected to a pattern electrode while the other external electrode is connected to another pattern electrode by a lead wire or the like. However, in the case where two external electrodes are formed on one side face, piezoelectric resonators 50a to 50d can be mounted on supporting substrate 12 in a surface mount manner, thereby enabling ladder type filter 10 to be reduced in size. While a four-element ladder type filter using four piezoelectric resonators 50a to 50d has been described, the present invention can, of course, be applied to a ladder type filter having any other number of elements, e.g., a six-element ladder type filter.

Claims: A ladder type filter (10), comprising a plurality of piezoelectric resonators (50a,50b,50c,50d) connected in series and in parallel in a ladder fashion, characterized in that each of said piezoelectric resonators (50a,50b,50c,50d) comprises a base member (52) having a plurality of piezoelectric layers (54) and a plurality of internal electrodes (56) alternately laminated along a longitudinal direction, said piezoelectric resonators (50a,50b,50c,50d) vibrate in the longitudinal direction of said base member (52) by application of an electric field to said base member (52) via said internal electrodes (56), and each of said piezoelectric resonators (54) satisfies an expression of 1 ≤ log 10 (n 2 wt) ≤ 3, in which n is the number of laminated layers each consisting of one of said piezoelectric layers (54) and one of said internal electrodes (56) adjacent to said one of said piezoelectric layers (54), and w mm and t mm are a width and a thickness of each of said piezoelectric resonators (50a,50b,50c,50d) respectively. A ladder type filter (10) according to Claim 1, characterized in that each of said piezoelectric layers (50a,50b,50c,50d) is polarized along the longitudinal direction of said base member (52). A ladder type filter (10) according to Claim 2, characterized in that the adjacent pair of said piezoelectric layers (50a,50b,50c,50d) located on the opposite sides of each of said internal electrodes (56) are polarized in opposite directions. A ladder type filter (10) according to one of Claims 1 to 3, characterized in that each adjacent pair of said internal electrodes (56) are respectively connected to two different external electrodes (62,64) which are provided on at least one side face of said base member (52). A ladder type filter (10) according to one of Claims 1 to 4, characterized in that a plurality of said piezoelectric resonators (50a,50b,50c,50d) are connected in series and in parallel in a ladder fashion on an supporting substrate (12) having pattern electrodes (14,16,18,20) provided on its surface.

Inventors: INOUE JIRO, TAKESHIMA TETSUO, UNAMI TOSHIHIKO, INOUE, JIRO, TAKESHIMA, TETSUO, UNAMI, TOSHIHIKO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H03H9
IPC codes: H03H9, H01L41