Patent Publication Number: US-7211925-B2

Title: Surface acoustic wave device and branching filter

Description:
This application claims priority under 35 USC § 120 to U.S. patent application Ser. No. 10/800,649, filed Mar. 16, 2004, the disclosure of which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a surface acoustic wave device and a branching filter. 
   In recent years, rapid development has been made in mobile communication terminals including portable telephones. It is desired that these terminals be particularly lightweight for ease of handling and portability. 
   To realize compact and lightweight mobile communication terminals, it is essential that electronic parts used therein be also compact and lightweight. For this reason, surface acoustic wave devices, i.e., surface acoustic wave filters, which are advantageous in making them compact and lightweight, are widely used. 
   As important characteristics required for the surface acoustic wave filters, it is arisen the insertion loss and the spurious response attenuation. The insertion loss affect power consumption of the apparatus, and the lower the loss, the more the battery life is prolonged, so that the battery capacity can be reduced, contributing to the compact and lightweight apparatus. In addition, if high spurious response attenuation can be obtained by a single surface acoustic wave filter, it contributes to the compact and lightweight apparatus. 
   Conventional surface acoustic wave filters will be described below. 
   As a filter which satisfies low loss and high attenuation characteristics, it is possible to cite a ladder-type filter. 
   In the ladder-type filter, a surface acoustic wave resonator connected in series to input and output signal electrodes and a surface acoustic wave resonator connected in parallel thereto are set as one section, and a plurality of such sections are cascade connected. Since these ladder-type filters excel in the low loss and the attenuation of a vicinity of a passband, they are widely used as high-frequency filters of portable telephones. 
   However, as for the ladder-type filter, if a surface acoustic wave resonator connected in series to input and output signal electrodes and a surface acoustic wave resonator connected in parallel thereto are set as one section, it is difficult to satisfy the required characteristics if only this one section is used. Therefore, it is necessary to use a plurality of sections by cascade connecting them. For example, in a reception-side filter of a branching filter for W-CDMA, it is necessary to use at least seven surface acoustic wave resonators. For that reason, there is a problem in that the chip size increases, and runs counter to the demand for the compact size. 
   In addition, since the surface acoustic wave resonator needs to be disposed at a position where it does not interfere with the other surface acoustic wave resonators, a long connection line is required to effect predetermined electrical connection. The loss attributable to this connection line becomes such a large problem that it cannot be ignored. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the invention is to provide a surface acoustic wave device which makes it possible to obtain low loss and high attenuation characteristics, while reducing the number of surface acoustic wave resonators used. 
   To overcome the above-described problems, the surface acoustic wave device in accordance with the invention is characterized by comprising: an input signal electrode and an output signal electrode to and from which an electric signal is inputted or outputted; a first surface acoustic wave resonator including an input terminal connected to the input signal electrode and having a plurality of comb electrodes, an output terminal connected to the output signal electrode and having a plurality of comb electrodes, a first common terminal which has a plurality of comb electrodes and some comb electrodes of which form an interdigital transducer together with the comb electrodes of the input terminal, and a second common terminal which has a plurality of comb electrodes and some comb electrodes of which form an interdigital transducer together with the comb electrodes of the output terminal; and a second surface acoustic wave resonator connected between a grounding electrode and at least one of the common terminals of the first surface acoustic wave resonator. 
   In addition, to overcome the above-described problems, the surface acoustic wave device in accordance with the invention is characterized by comprising: an input signal electrode-and an output signal electrode to and from which an electric signal is inputted or outputted; a third surface acoustic wave resonator connected between the input signal electrode and the output signal electrode; and a fourth surface acoustic wave resonator including a plurality of signal-side terminals having a plurality of comb electrodes and connected to a midpoint between the input signal electrode and the third surface acoustic wave resonator and a midpoint between the output signal electrode and the third surface acoustic wave resonator, and a ground-side terminal having a plurality of comb electrodes to form interdigital transducers by the comb electrodes and the comb electrodes of the plurality of signal-side terminals and connected to a grounding electrode. 
   Furthermore, to overcome the above-described problems, the surface acoustic wave device in accordance with the invention is characterized by comprising: an input signal electrode and an output signal electrode to and from which an electric signal is inputted or outputted; a plurality of third surface acoustic wave resonators connected in series between the input signal electrode and the output signal electrode; and a fourth surface acoustic wave resonator including a plurality of signal-side terminals having a plurality of comb electrodes and connected to a midpoint between the input signal electrode and the third surface acoustic wave resonator, a midpoint between the output signal electrode and the third surface acoustic wave resonator, and a midpoint between adjacent ones of the third surface acoustic wave resonators, and a ground-side terminal having a plurality of comb electrodes to form interdigital transducers by the comb electrodes and the comb electrodes of the plurality of signal-side terminals and connected to a grounding electrode. 
   According to the above-described invention, it becomes possible to obtain low loss and high attenuation characteristics, while reducing the number of surface acoustic wave resonators used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view illustrating an electronic part in which a surface acoustic wave device in accordance with a first embodiment of the invention has been packaged; 
       FIG. 2  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a first embodiment of the invention; 
       FIG. 3  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 3 ; 
       FIG. 4  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a first object of consideration by the present inventors; 
       FIG. 5  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 4 ; 
       FIG. 6  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a second embodiment of the invention; 
       FIG. 7  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in FIG.  6 ; 
       FIG. 8  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a second object of consideration by the present inventors; 
       FIG. 9  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 8 ; 
       FIG. 10  is a diagram illustrating the element layout of the surface acoustic wave device shown in  FIG. 6 ; 
       FIG. 11  is an explanatory diagram illustrating the element layout of the surface acoustic wave device shown in  FIG. 8 ; 
       FIG. 12  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a modification of the second embodiment of the invention; 
       FIG. 13  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a third object of consideration by the present inventors; 
       FIG. 14  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a third embodiment of the invention; 
       FIG. 15  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 14 ; 
       FIG. 16  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a fourth object of consideration by the present inventors; 
       FIG. 17  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 16 ; 
       FIG. 18  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a first modification of the third embodiment of the invention; 
       FIG. 19  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a fifth object of consideration by the present inventors; 
       FIG. 20  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a second modification of the third embodiment of the invention; and 
       FIG. 21  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a sixth object of consideration by the present inventors. 
       FIGS. 22 and 23  are graphs illustrating the frequency characteristics of a surface acoustic wave device according to embodiments of the invention. 
       FIG. 24  shows the structure of the interdigit traded electrode fingers used to produce/receive surface acoustic waves. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereafter, a description will be given more specifically of the embodiments of the invention with reference to the drawings. In the appended drawings, identical members are denoted by the same reference numerals, and repetition descriptions are omitted. It should be noted that the embodiments of the invention are forms which are particularly useful when the invention is carried out, and the invention is not limited to its embodiments. 
   (First Embodiment) 
     FIG. 1  is a cross-sectional view illustrating an electronic part in which a surface acoustic wave device in accordance with a first embodiment of the invention has been packaged.  FIG. 2  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a first embodiment of the invention.  FIG. 3  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 3 .  FIG. 4  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a first object of consideration by the present inventors.  FIG. 5  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 4 . 
   In an electronic part  10  shown in  FIG. 1 , a, surface acoustic wave device  11 , in which a predetermined conductive pattern has been formed on a piezoelectric substrate (e.g., 39° rotated, Y-cut, X-propagated LiTaO 3  piezoelectric substrate), is mounted on a ceramic-made or resin-made mounting substrate  12  which consists of a single layer or a multiple layer and on which a predetermined wiring pattern or a circuit pattern has been formed. Further, the pattern forming surface in the piezoelectric substrate is disposed in face-to-face relation to the mounting surface of the mounting substrate  12 , and the surface acoustic wave device  11  and the mounting substrate  12  are flip-chip connected via bumps  13 . It should be noted that the two members may be wire-connected by a wire bonding technique. 
   Here, the piezoelectric substrate is formed by a piezoelectric single crystal of such as LiNbO 3 , LiTaO 3 , or crystal, or a piezoelectric ceramic such as a lead zirconate titanate based piezoelectric ceramic. However, a piezoelectric substrate in which a piezoelectric thin film such as a ZnO thin film has been formed on an insulating substrate may be used as the piezoelectric substrate. 
   Further, a cap  14  is bonded to the mounting substrate in such a manner as to surround the surface acoustic wave device  11 , and protects the surface acoustic wave device  11  from such as dust and mechanical shocks. 
   In the surface acoustic wave device  11 , as shown in  FIG. 2 , formed on the piezoelectric substrate are an input signal electrode  15  and an output signal electrode  16 , to and from which an electric signal is inputted or outputted; a pair of grounding electrodes  17  which are set at ground potential; and a first surface acoustic wave resonator  18  and a second surface acoustic wave resonator  19  which resonate with the surface acoustic wave of a predetermined frequency. 
   Here, the first surface acoustic wave resonator  18  consists of an input terminal  18   a  connected to the input signal electrode  15  and having a plurality of comb electrodes, an output terminal  18   b  connected to the output signal electrode  16  and having a plurality of comb electrodes, and a common terminal (first and second common terminals)  18   c  having a plurality of comb electrodes. Further, one interdigital transducer A is formed by some comb electrodes of the common terminal  18   c  and the comb electrodes of the input terminal  18   a,  while one interdigital transducer B is formed by the other comb electrodes of the common terminal  18   c  and the comb electrodes of the output terminal  18   b,  It should be noted that reflectors  20  for reflecting the surface acoustic wave are respectively disposed on both sides of the first surface acoustic wave resonator  18  and the second surface acoustic wave resonator  19  (the reflectors on the second surface acoustic wave resonator  19  side are not shown). 
   The second surface acoustic wave resonator  19  is connected between the common terminal  18   c  of the above-described first surface acoustic wave resonator  18  and the grounding electrodes  17 . 
   Further, a setting is provided such that the antiresonant frequency of the second surface acoustic wave resonator  19  substantially agrees with the resonant frequency of the first surface acoustic wave resonator  18 . 
   It should be noted that the number of electrodes and the aperture width of each interdigital transducer making up the second surface acoustic wave resonator can be set freely, including the description that follows. 
   In addition, although electrode periods and the numbers of pairs of the interdigital transducers of the first surface acoustic wave resonator are set to identical values in this embodiment, different electrode periods and different numbers of pairs may be used. Further, the aperture width need not completely agree, and may be different. 
   For example, in  FIG. 2 , if a electrode period of the interdigital transducer B is made different from a electrode period of the interdigital transducer A, passing band can be widened.  FIG. 22  shows a frequency characteristic of the surface acoustic wave device in that an electrode period of the interdigital transducer A is set to λ and an electrode period of the interdigital transducer B is set to 0.99λ (or 1.01λ). In  FIG. 22 , a broken line indicates a frequency characteristic when both electrode periodes of the transducers A and B are identical each other. As apparent from  FIG. 22 , it is preferable that the passing band is widened at high frequency side when the electrode periodes are differentiated. The difference of electrode periodes is appropriately adjustable in accordance with necessitated characteristic. The ratio of the period is not limited to 0.99 or 1.01. In the case of three or more interdigital transducers are arranged as shown in  FIG. 6  or  12 , it is clearly understandable that the electrode period can be independently set irrespective of the others. 
   Furthermore, the electrode periods may be alternately identical or different in relationship with the interdigital transducers making up the second surface acoustic wave resonator, or any ones of the electrode periods maybe identical. However, to obtain desirable frequency characteristics, the electrode period of the interdigital transducers making up the second surface acoustic wave resonator is preferably set to be greater than the electrode period of the interdigital transducers making up the first surface acoustic wave resonator. 
   The frequency characteristic of the surface acoustic wave device having the above-described configuration is shown in  FIG. 3 . 
   Here, an equivalent circuit of a surface acoustic wave device as a first object of consideration by the present inventors is shown in  FIG. 4 . 
     FIG. 4  is a surface acoustic wave device of a so-called ladder type. Two surface acoustic wave resonators  118  and  119  are connected in series between an input signal electrode  115  and an output signal electrode  116 . Further, one surface acoustic wave resonator  120  is connected between a midpoint between these surface acoustic wave resonators  118  and  119  and a pair of grounding electrodes  117 . Namely, a total of three surface acoustic wave resonators are used in the illustrated case. Further, a setting is provided such that the antiresonant frequency of the second surface acoustic wave resonator  120  substantially agrees with the resonant frequencies of the surface acoustic wave resonators  118  and  119 . 
   The frequency characteristic of the surface acoustic wave device having the configuration of  FIG. 4  is shown in  FIG. 5 . 
   As can be understood through a comparison between the frequency characteristic ( FIG. 3 ) of the surface acoustic wave device of this embodiment and the frequency characteristic ( FIG. 5 ) of the surface acoustic wave device which is the first object of consideration, the insertion losses and the attenuation characteristics of the two devices are substantially equivalent. 
   As described before, only two surface acoustic wave resonators including the first surface acoustic wave resonator  18  and the second surface acoustic wave resonator  19  are used in the surface acoustic wave device of this embodiment. In contrast, three surface acoustic wave resonators are used in the surface acoustic wave device which is the first object of consideration. 
   Thus, in accordance with the invention, in the first surface acoustic wave resonator  18 , the input terminal  18   a  and the output terminal  18   b  having the plurality of comb electrodes are respectively connected to the input signal electrode  15  and the output signal electrode  16 . Furthermore, the surface acoustic wave resonator  19  is connected between the grounding electrodes  17 , on the one hand, and the common terminal  18   c,  on the other hand, which makes up the interdigital transducers together with the comb electrodes of the input terminal  18   a  and the comb electrodes of the output terminal  18   b.  Therefore, the number of surface acoustic wave resonators required can be reduced, and the connection line length necessary for electrical connection can be made short. Hence, it is possible to obtain frequency characteristics of low loss and high attenuation characteristics, while reducing the number of surface acoustic wave resonators used. 
   As a result, it is possible to make the chip size small and make the device compact. 
   (Second Embodiment) 
     FIG. 6  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a second embodiment of the invention.  FIG. 7  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 6 .  FIG. 8  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a second object of consideration by the present inventors.  FIG. 9  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 8 .  FIG. 10  is a diagram illustrating the element layout of the surface acoustic wave device shown in  FIG. 6 .  FIG. 11  is an explanatory diagram illustrating the element layout of the surface acoustic wave device shown in  FIG. 8 .  FIG. 12  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a modification of the second embodiment of the invention.  FIG. 13  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a third object of consideration by the present inventors. 
   It should be noted that the electronic component in which the surface acoustic wave device has been packaged is similar to the one shown in  FIG. 1  in accordance with the first embodiment, including the embodiment described below. 
   As shown in  FIG. 6 , one first surface acoustic wave resonator  18  and two second surface acoustic wave resonators  19   a  and  19   b  are formed in the surface acoustic wave device in accordance with this embodiment. 
   Here, the first surface acoustic wave resonator  18  consists of the input terminal  18   a  connected to the input signal electrode  15  and having the plurality of comb electrodes, the output terminal  18   b  connected to the output signal electrode  16  and having the plurality of comb electrodes, and a first and a second common terminal  18   c   1  and  18   c   2  respectively having the plurality of comb electrodes. Further, an interdigital transducer A is formed by some comb electrodes of the first common terminal  18   c   1  and the comb electrodes of the input terminal  18   a,  and an interdigital transducer B is formed by some comb electrodes of the second common terminal  18   c   2  and the comb electrodes of the output terminal  18   b.  Furthermore, an interdigital transducer C is formed by the other comb electrodes of the first common terminal  18   c   1  and the other comb electrodes of the second common terminal  18   c   2 . 
   One second surface acoustic wave resonator  19   a  is connected between the first common terminal  18   c   1  of the first surface acoustic wave resonator  18  and the grounding electrodes  17 , while the other second surface acoustic wave resonator  19   b  is connected between the second common terminal  18   c   2  of the first surface acoustic wave resonator  18  and the grounding electrodes  17 . It should be noted, however, that it suffices if the second surface acoustic wave resonator is connected either between the first common terminal  18   c   1  and the grounding electrodes  17 , or between the second common terminal  18   c   2  and the grounding electrodes  17 . 
   Further, a setting is provided such that the antiresonant frequencies of the second surface acoustic wave resonators  19   a  and  19   b  substantially agree with the resonant frequency of the first surface acoustic wave resonator  18 . 
   The frequency characteristic of the surface acoustic wave device having the above-described configuration is shown in FIG.  7 . 
   Here, an equivalent circuit of a surface acoustic wave device as a second object of consideration by the present inventors is shown in  FIG. 8 . 
     FIG. 8  is also a surface acoustic wave device of the so-called ladder type. Three surface acoustic wave resonators  121 ,  122 , and  123  are connected in series between the input signal electrode  115  and the output signal electrode  116 . Further, two surface acoustic wave resonators  124  and  125  are connected between the grounding electrodes  117  and respective midpoints between these surface acoustic wave resonators  121 ,  122 , and  123 . Namely, a total of five surface acoustic wave resonators are used in the illustrated case. Further, a setting is provided such that the antiresonant frequencies of the surface acoustic wave resonator  124  and  125  substantially agree with the resonant frequencies of the surface acoustic wave resonators  121 ,  122 , and  123 . 
   The frequency characteristic of the surface acoustic wave device having the configuration of  FIG. 8  is shown in  FIG. 9 . 
   As can be understood through a comparison between the frequency characteristic ( FIG. 7 ) of the surface acoustic wave device of this embodiment and the frequency characteristic ( FIG. 9 ) of the surface acoustic wave device which is the second object of consideration, the insertion losses and the attenuation characteristics of the two devices are substantially equivalent. 
   As described before, only a total of three surface acoustic wave resonators including one first surface acoustic wave resonator  18  and two second surface acoustic wave resonator  19  are used in the surface acoustic wave device of this embodiment. In contrast, five surface acoustic wave resonators are used in the surface acoustic wave device which is the second object of consideration. 
   Accordingly, in accordance with the invention, in the first surface acoustic wave resonator  18 , the input terminal  18   a  and the output terminal  18   b  having the plurality of comb electrodes are respectively connected to the input signal electrode  15  and the output signal electrode  16 . Further, the second surface acoustic wave resonator  19   a  is connected between the grounding electrodes  17 , on the one hand, and the first common terminal  18   c   1 , on the other hand, which makes up the interdigital transducers together with the comb electrodes of the input terminal  18   a.  Furthermore, the second surface acoustic wave resonator  19   b  is connected between the grounding electrodes  17 , on the one hand, and the second common terminal  18   c   2 , on the other hand, which makes up the interdigital transducers together with the comb electrodes of the input terminal  18   b.  Therefore, it is possible to obtain frequency characteristics of low loss and high attenuation characteristics, while reducing the number of surface acoustic wave resonators used. As a result, it is possible to make the chip size small and make the device compact. 
   The element layout of the surface acoustic wave device in accordance with the invention in  FIG. 6  is shown in  FIG. 10 . The element layout of the surface acoustic wave device as the second object of consideration in  FIG. 8  is shown in  FIG. 111 . 
   As is apparent from these drawings, in the surface acoustic wave device in accordance with the invention, as compared with the surface acoustic wave device as the second object of consideration having a frequency characteristic equivalent thereto, the number of surface acoustic wave resonators used is small, so that it can be understood that the chip size is made substantially small. 
   Here, an equivalent circuit of the surface acoustic wave device as a modification of the second embodiment is shown in  FIG. 12 , and an equivalent circuit of a surface acoustic wave device as a third object of consideration by the present inventors is shown in  FIG. 13 . 
   In  FIG. 12 , a total of four surface acoustic wave resonators, including one first surface acoustic wave resonator  18  and three second surface acoustic wave resonators  19   a,    19   b,  and  19   c  are formed. 
   The first surface acoustic wave resonator  18  consists of the input terminal  18   a  connected to the input signal electrode  15  and having the plurality of comb electrodes, the output terminal  18   b  connected to the output signal electrode  16  and having the plurality of comb electrodes, and first, second, and third common terminals  18   c   1 ,  18   c   2 , and  18   c   3  respectively having the plurality of comb electrodes. Further, the interdigital transducer A is formed by some comb electrodes of the first common terminal  18   c   1  and the comb electrodes of the input terminal  18   a,  and the interdigital transducer B is formed by some comb electrodes of the second common terminal  18   c   2  and the comb electrodes of the output terminal  18   b.  Furthermore, the interdigital transducer C is formed by the other comb electrodes of the first common terminal.  18   c   1  and some comb electrodes of the third common terminal  18   c   3 , and an interdigital transducer D is formed by the other comb electrodes of the second common terminal  18   c   2  and the other comb electrodes of the third common terminal  18   c   3 . 
   In addition, the three surface acoustic wave resonators  19   a,    19   b,  and  19   c  are connected between the grounding electrodes  17 , on the one hand, and the respective common terminals  18   c   1 ,  18   c   2 , and  18   c   3  of the first surface acoustic wave resonator  18 , on the other hand. However, it suffices if the second surface acoustic wave resonator is provided between at least one common terminal and the grounding electrodes. 
   In  FIG. 13 , four surface acoustic wave resonators  126 ,  127 ,  128 , and  129  are connected in series between the input signal electrode  115  and the output signal electrode  116 . Further, three surface acoustic wave resonators  130 ,  131 , and  132  are connected between the grounding electrodes  117  and respective midpoints between these surface acoustic wave resonators  126 ,  127 ,  128 , and  129 . Namely, a total of seven surface acoustic wave resonators are used. 
   The surface acoustic wave device in accordance with the invention shown in  FIG. 12  and the surface acoustic wave device which is the third object of consideration shown in  FIG. 13  also have equivalent frequency characteristics. 
   Thus, in the invention, in a case where the number of common terminals making up the first surface acoustic wave resonator  18  has increased, it suffices if the second surface acoustic wave resonator is connected between the common terminal and the grounding electrodes. 
   Incidentally, it is possible to expand the third common terminal. Namely, a plurality of third common terminals can be provided. In this case, an interdigital transducer is formed by some comb electrodes of one third common terminal and some comb electrodes of another third common terminal. Consequently, respective ones of the input terminal  18   a,  the first common terminal  18   c   1 , the plurality of third common terminals  18   c   3 , the second common terminal  18   c   2 , and the output terminal  18   b  are coupled via the interdigital transducers. 
   (Third Embodiment) 
     FIG. 14  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device in accordance with a third embodiment of the invention.  FIG. 15  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 14 .  FIG. 16  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a fourth object of consideration by the present inventors.  FIG. 17  is a graph illustrating the frequency characteristic of the surface acoustic wave device shown in  FIG. 16 .  FIG. 18  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a first modification of the third embodiment of the invention.  FIG. 19  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a fifth object of consideration by the present inventors.  FIG. 20  is a circuit diagram illustrating an equivalent circuit of the surface acoustic wave device as a second modification of the third embodiment of the invention.  FIG. 21  is a circuit diagram illustrating an equivalent circuit of a surface acoustic wave device as a sixth object of consideration by the present inventors. 
   As shown in  FIG. 14 , a third surface acoustic wave resonator and a fourth surface acoustic wave resonator  22  which resonate with the surface acoustic wave of a predetermined frequency are formed in the surface acoustic wave device in accordance with this embodiment. 
   Here, the third surface acoustic wave resonator  21  is connected between the input signal electrode  15  and the output signal electrode  16 . It should be noted that the number of electrodes and the aperture width of each interdigital transducer making up the third surface acoustic wave resonator  21  can be set freely. 
   In addition, the fourth surface acoustic wave resonator  22  consists of two (a plurality of) signal-side terminals  22   a  respectively having the plurality of comb electrodes and connected to a midpoint between the input signal electrode  15  and the third surface acoustic wave resonator  21  and a midpoint between the output signal electrode  16  and the third surface acoustic wave resonator  21 , as well as a ground-side terminal  22   b  having the plurality of comb electrodes to form interdigital transducers by these comb electrodes and the comb electrodes of the respective signal-side terminals  22   a  and connected to the grounding electrodes  17 . Here, one interdigital transducer A is formed by the comb electrodes of one signal-side terminal  22   a  and some comb electrodes of the ground-side terminal  22   b,  while the other interdigital transducer B is formed by the comb electrodes of the other signal-side terminal  22   a  and the other comb electrodes of the ground-side terminal  22   b.  It should be noted that the reflectors  20  for reflecting the surface acoustic wave are respectively disposed on both sides of the fourth surface acoustic wave resonator  22  (the reflectors on the third surface acoustic wave resonator  21  side are not shown). 
   Further, a setting is provided such that the antiresonant frequency of the fourth surface acoustic wave resonator  22  substantially agrees with the resonant frequency of the third surface acoustic wave resonator  21 . 
   In addition, although the electrode periods and the numbers of pairs of the interdigital transducers A and B of the fourth surface acoustic wave resonator  22  are set to identical values in this embodiment, different electrode periods and different numbers of pairs may be used. Further, the aperture width need not completely agree, and may be different. 
   For example, in  FIG. 141  if a electrode period of the interdigital transducer B is made different from a electrodes period of the interdigital transducer A, passing band can be widened.  FIG. 23  shows a frequency characteristic of the surface acoustic wave device in that an electrode period of the interdigital transducer A is set to λ and an electrode period of the interdigital transducer B is set to 0.99λ (or 1.01λ) In  FIG. 23 , a broken line indicates a frequency characteristic when both electrode periodes of the transducers A and B are identical each other. As apparent from  FIG. 23 , it is preferable that the passing band is widened at low frequency side when the electrode periodes are differentiated. The difference of electrode periodes is appropriately adjustable in accordance with necessitated characteristic. The ratio of the period is not limited to 0.99 or 1.01. In the case of three or more interdigital transducers are arranged as shown in FIG.  18  or  20 , it is clearly understandable that the electrode period can be independently set irrespective of the others. 
   Furthermore, the electrode periods may be alternately identical or different in relationship with the interdigital transducers making up the third surface acoustic wave resonator  21 , or any ones of the electrode periods may be identical. However, to obtain desirable frequency characteristics, the electrode period of the interdigital transducers making up the fourth surface acoustic wave resonator  22  is preferably set to be greater than the electrode period of the interdigital transducers making up the third surface acoustic wave resonator  21 . 
   The frequency characteristic of the surface acoustic wave device having the above-described configuration is shown in  FIG. 15 . 
   Here, an equivalent circuit of a surface acoustic wave device as a fourth object of consideration by the present inventors is shown in  FIG. 16 . 
     FIG. 16  is a surface acoustic wave-device of the so-called ladder type. One surface acoustic wave resonator  133  is connected between the input signal electrode  115  and the output signal, electrode  116 . Further, two surface acoustic wave resonators  134  and  135  are respectively connected in parallel between the grounding electrodes  117  and a midpoint between the input signal electrode  115  and the surface acoustic wave resonator  122  and between the grounding electrodes  117  and a midpoint between the output signal electrode  116  and the surface acoustic wave resonator  122 . Namely, a total of three surface acoustic wave resonators are used in the illustrated case. Further, a setting is provided such that the antiresonant frequencies of the surface acoustic wave resonators  134  and  135  substantially agree with the resonant frequency of the surface acoustic wave resonator  133 . 
   The frequency characteristic of the surface acoustic wave device having the configuration of  FIG. 16  is shown in  FIG. 17 . 
   As can be understood through a comparison between the frequency characteristic ( FIG. 15 ) of the surface acoustic wave device of this embodiment and the frequency characteristic ( FIG. 17 ) of the surface acoustic wave device which is the fourth object of consideration, the insertion losses and the attenuation characteristics of the two devices are substantially equivalent. 
   As described before, only two surface acoustic wave resonators including the third surface acoustic wave resonator  21  and the fourth surface acoustic wave resonator  22  are used in the surface acoustic wave device of this embodiment. In contrast, three surface acoustic wave resonators are used in the surface acoustic wave device which is the fourth object of consideration. 
   Thus, in accordance with the invention, in the fourth surface acoustic wave resonator  22 , the signal-side terminals  22   a  having the plurality of comb electrodes are respectively connected to the midpoint between the input signal electrode  15  and the third surface acoustic wave resonator  21 ,and the midpoint between the output signal electrode  16  and the third surface acoustic wave resonator  21 . The interdigital transducers are formed by the comb electrodes of the ground-side terminal  22   b  and the comb electrodes of the respective signal-side terminals  22   a,  and the ground-side terminal  22   b  and the grounding electrodes  17  are connected. Therefore, it is possible to obtain frequency characteristics of low loss and high attenuation characteristics, while reducing the number of surface acoustic wave resonators used. As a result, it is possible to make the chip size small and make the device compact. 
   Here, a description will be given of the form of connection in a case where a plurality of third surface acoustic wave resonators are used, through a comparison with the surface acoustic wave device which is used as an object of consideration. 
   First, in a case where two third surface acoustic wave resonators  21   a  and  21   b  are used, these third surface acoustic wave resonators  21   a  and  21   b  are connected in series-between the input signal electrode  15  and the output signal electrode  16 , as shown in  FIG. 18 . 
   In addition, the fourth surface acoustic wave resonator  22  consists of three (a plurality of) signal-side terminals  22   a  respectively having the plurality of comb electrodes and connected to the midpoint between the input signal electrode  15  and one third surface acoustic wave resonator  21   a,  the midpoint between the output signal electrode  16  and the other third surface acoustic wave resonator  21   b,  and the midpoint between the third surface acoustic wave resonator  21   a  and the third surface acoustic wave resonator  21   b,  as well as the ground-side terminal  22   b  having the plurality of comb electrodes to form interdigital transducers by these comb electrodes and the comb electrodes of the respective signal-side terminals  22   a  and connected to the grounding electrodes  17 . Here, the interdigital transducers A, B, and C are respectively formed by the signal-side terminals  22   a  and portions of the ground-side terminal  22   b.    
   A surface acoustic wave device as a fifth object of consideration having a frequency characteristic equivalent to that of the surface acoustic wave device having the above-described configuration is shown in  FIG. 19 . 
   In  FIG. 19 , two surface acoustic wave resonators  136  and  137  are connected in series between the input signal electrode  115  and the output signal electrode  116 . Further, three surface acoustic wave resonators  138 ,  139 , and  140  are respectively connected in parallel between the grounding electrodes  117  and the midpoint between the input signal electrode  115  and the surface acoustic wave resonator  136 , between the grounding electrodes  117  and the midpoint between the surface acoustic wave resonator  136  and the surface acoustic wave resonator  137 , and between the grounding electrodes  117  and the midpoint between the output signal electrode  116  and the surface acoustic wave resonator  137 . Namely, a total of five surface acoustic wave resonators are used in the illustrated case. 
   As is apparent from a comparison between  FIG. 18  illustrating the surface acoustic wave device in accordance with the invention and  FIG. 19  illustrating the surface acoustic wave device which is the fifth object of consideration, in order to obtain predetermined frequency characteristics, three surface acoustic wave resonators (the third surface acoustic wave resonators  21   a  and  21   b  and the fourth surface acoustic wave resonator  22 ) are required in the former case, whereas five surface acoustic wave resonator  136  to  140  are required in the latter case. 
   Next, in a case where three third surface acoustic wave resonators  21   a,    21   b,  and  21   c  are used, these third surface acoustic wave resonators  21   a,    21   b,  and  21   c  are connected in series between the input signal electrode  15  and the output signal electrode  16 , as shown in  FIG. 20 . 
   In addition, the fourth surface acoustic wave resonator  22  consists of four (a plurality of) signal-side terminals  22   a  respectively having the plurality of comb electrodes and connected to the midpoint between the input signal electrode  15  and the third surface acoustic wave resonator  21   a,  the midpoint between the output signal electrode  16  and the third surface acoustic wave-resonator  21   b,  the midpoint between the third surface acoustic wave resonator  21   a  and the third surface acoustic wave resonator  21   b,  and the midpoint between the third surface acoustic wave resonator  21   b  and the third surface acoustic wave resonator  21   c,  as well as the ground-side terminal  22   b  having the plurality of comb electrodes to form interdigital transducers by these comb electrodes and the comb electrodes of the respective signal-side terminals  22   a  and connected to the grounding electrodes  17 . Here, the interdigital transducers A, B, C, and D are respectively formed by the signal-side terminals  22   a  and portions of the ground-side terminal  22   b.    
   Thus, in the case where the third surface acoustic wave resonators which are connected in series between the input signal electrode  15  and the output signal electrode  16  have increased, it suffices if signal-side terminals which are respectively connected to midpoints between adjacent ones of the third surface acoustic wave resonators are increased in number. 
   A surface acoustic wave device as a sixth object of consideration having a frequency characteristic equivalent to that of the surface acoustic wave device having the above-described configuration is shown in  FIG. 21 . 
   In  FIG. 21 , three surface acoustic wave resonators  141 ,  142 , and  143  are connected in series between the input signal electrode  115  and the output signal electrode  116 . Further, four surface acoustic wave resonators  144 ,  145 ,  146 , and  147  are respectively connected in parallel between the grounding electrodes  117  and the midpoint between the input signal electrode  115  and the surface acoustic wave resonator  141 , between the grounding electrodes  117  and the midpoint between the surface acoustic wave resonator  141  and the surface acoustic wave resonator  142 , between the grounding electrodes  117  and the midpoint between the surface acoustic wave resonator  142  and the surface acoustic wave resonator  143 , and between the grounding electrodes  117  and the midpoint between the output signal electrode  116  and the surface acoustic wave resonator  143 . Namely, a total of seven surface acoustic wave resonators are used in the illustrated case. 
   As is apparent from a comparison between  FIG. 20  illustrating the surface acoustic wave device in accordance with the invention and  FIG. 21  illustrating the surface acoustic wave device which is the sixth object of consideration, in order to obtain predetermined frequency characteristics, four surface acoustic wave resonators (the third surface acoustic wave resonators  21   a,    21   b,  and  21   c  and the fourth surface acoustic wave resonator  22 ) are required in the former case, whereas seven surface acoustic wave resonator  141  to  147  are required in the latter case. 
   The invention devised by the present inventors has been described above through its embodiments, but inductance elements can be respectively connected between the grounding electrodes  17  and the aforementioned surface acoustic wave resonators  19 ,  19   a,    19   b,  and  19   c.  In addition, an inductance element can be connected also between the grounding electrodes  17  and the ground-side terminal  22   b  of the fourth surface acoustic wave resonator  22 . If such an arrangement is provided, it is possible to obtain greater spurious response attenuation, so that the frequency characteristic improves further. 
   In addition, a plurality of surface acoustic wave devices of this application can be connected, or a surface acoustic wave device having another configuration can be connected to the surface acoustic wave device of this application. 
   The surface acoustic wave device of the invention is suitable for a filter, particularly a filter for a branching filter using a plurality of filters. Moreover, the surface acoustic wave device of the invention is also suitable for a composite filter having a plurality of passbands with mutually different band center frequencies. In these cases, it suffices if at one of the surface acoustic wave devices is the surface acoustic wave device in accordance with the invention. 
   However, the scope of application of the invention is not limited to the filter, and may be applied to various surface acoustic wave devices other than the field of filters in which a plurality of surface acoustic wave resonators are mounted. 
   It should be noted that a one-terminal pair surface acoustic wave resonator and a two-terminal pair surface acoustic wave resonator are disclosed on page 217 of “ Danseisoshi Gijutsu Handobukku  (Handbook of Elastic Component Technology)” (Ohmsha, Ltd., published on Nov. 30, 1991). 
   In addition, a surface acoustic wave filter combining a one-terminal pair surface acoustic wave resonator and a two-terminal pair surface acoustic wave resonator is disclosed in JP-A-5-48055, for example. A configuration in which a one-terminal pair surface acoustic wave resonator and a two-terminal pair surface acoustic wave resonator are connected in series is disclosed therein. 
   In contrast, in this application, in the first surface acoustic wave resonator  18 , the input terminal  18   a  and the output terminal  18   b  having the plurality of comb electrodes are respectively connected to the input signal electrode  15  and the output signal-electrode  16 . Furthermore, the second surface acoustic wave resonator  19  is connected between the grounding electrodes  17 , on the one hand, and the common terminal  18   c,  on the other hand, which has the plurality of comb electrodes and makes up the interdigital transducers together with the input terminal  18   a  and the output terminal  18   b.  In addition, in the fourth surface acoustic wave resonator  22 , the signal-side terminal  22  having the plurality of comb electrodes is connected to the midpoint between the input signal electrode  15  and the third surface acoustic wave resonator  21  and the midpoint between the output signal electrode  16  and the third surface acoustic wave resonator  21 . In a case where a plurality of third surface acoustic wave resonators are provided, the signal-side terminal  22  having the plurality of comb electrodes is connected to midpoints between adjacent ones of the third surface acoustic wave resonators, the interdigital transducers are formed by the comb electrodes of the signal-side terminals  22   a  and the ground-side terminal  22   b  having the plurality of comb electrodes, and the ground-side terminal  22   b  is connected to the grounding electrodes  17 . 
   The above-described configurations are not even suggested in the aforementioned documents, and have been disclosed for the first time by this application. 
   As is apparent from the foregoing description, the following advantages are offered in accordance with the invention. 
   The number of surface acoustic wave resonators required can be reduced, and the connection line length necessary for electrical connection can be made short. Therefore, it is possible to obtain frequency characteristics of low loss and high attenuation characteristics, while reducing the number of, surface acoustic wave resonators used. 
   As a result, it becomes possible to make the chip size small and make the device compact.