Patent Publication Number: US-6985048-B2

Title: Surface acoustic wave apparatus and communication apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to surface acoustic wave apparatuses including a surface acoustic wave filter having a balanced-to-unbalanced conversion function. 
   2. Description of the Related Art 
   Remarkable technical progress has been made on portable telephones (communication apparatuses) in terms of their compactness and light weight. To achieve this, the number of components has been reduced, the size of components has been reduced, and components having a plurality of functions have been developed. 
   Surface acoustic wave apparatuses having a balanced-to-unbalanced conversion function, a so-called balun function, used in RF stages of portable telephones have been actively researched, and have been primarily used in global systems for mobile communications (GSMs). 
   Several patent applications relating to surface acoustic wave apparatuses having a balanced-to-unbalanced conversion function have been filed.  FIG. 3  shows a surface acoustic wave apparatus having a balanced-to-unbalanced conversion function with the impedance of an unbalanced signal terminal side set to 50 Ω and the impedance of a balanced signal terminal side set to 200Ω, which is disclosed in Japanese Unexamined Patent Publication No. JP 11-97966. 
   In the structure of  FIG. 3 , in a longitudinally-coupled-resonator-type surface acoustic wave filter  301  in which three interdigital transducers (hereinafter called IDTs) are arranged in direction in which surface acoustic waves propagate, an IDT  303  disposed at the center is divided into two portions substantially symmetrically in the propagation direction of the surface acoustic waves, and the two portions are connected to balanced signal terminals  308  and  309 , and left and right IDTs  302  and  304  having inverted polarities are connected to an unbalanced signal terminal  307 . With this structure, a balanced-to-unbalanced conversion function is provided by the inverted polarities, and the impedance at the balanced signal terminal side is about four times as high as that of the unbalanced signal terminal side by the division of the IDT  303  into the two portions. 
   Filters having a balanced-to-unbalanced conversion function must have equal amplitude characteristics and phases that are inverted by 180 degrees in the transfer characteristics at a pass band between an unbalanced signal terminal and balanced signal terminals. They are called an amplitude-balance degree and a phase-balance degree. 
   The amplitude-balance degree and the phase-balance degree are defined in the following manner assuming that filter apparatuses having a balanced-to-unbalanced conversion function are three-port devices. The amplitude-balance degree=|A|, A=|20 log(S21)|−|20 log(S31)|, the phase-balance degree=|B−180|, B=|∠S21−∠S31|, where an unbalanced input terminal is called port 1, and balanced output terminals are called port 2 and port 3. Ideally, the amplitude-balance degree should be 0 dB and the phase balance degree should be 0 degrees in the pass band of surface acoustic wave filters. 
   However, the balance degrees of the conventional structure shown in  FIG. 3  are insufficient. This is because the electrode fingers ( 310  and  317  in  FIG. 3 ) of the IDT  302  and the IDT  304 , adjacent to the IDT  303  have different polarities from each other, and therefore, parasitic capacitances and bridging capacitances differ at the balanced signal terminals  308  and  309 . 
   SUMMARY OF THE INVENTION 
   To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave apparatus which has a balanced-to-unbalanced conversion function with improved balance degrees and in which the impedance of balanced signal terminals are about four times greater than that of an unbalanced signal terminal. 
   One preferred embodiment of the present invention provides a surface acoustic wave apparatus including a longitudinally-coupled-resonator-type surface acoustic wave filter in which an odd number of three or more interdigital transducers are provided on a piezoelectric substrate in the direction in which surface acoustic waves propagate, one interdigital electrode of an interdigital transducer disposed at the center among the odd number of interdigital transducers is divided into two parts along the propagation direction of the surface acoustic waves and the two parts are connected to balanced signal terminals, respectively, and two interdigital transducers adjacent to the interdigital transducer disposed at the center are inverted with respect to each other and are connected to an unbalanced signal terminal to provide a balanced-to-unbalanced conversion function, wherein an outermost electrode finger of the interdigital transducer disposed at the center is a floating electrode or a grounded electrode, and wiring is provided asymmetrically such that a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is grounded of the two interdigital transducers adjacent to the interdigital transducer disposed at the center has a larger capacitance. 
   According to the above-described structure, since the wiring is provided asymmetrically such that a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is grounded, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center has a larger capacitance, balance degrees between the balanced signal terminals, especially the phase balance degree, is greatly improved. 
   Another preferred embodiment of the present invention provides a surface acoustic wave apparatus including a longitudinally-coupled-resonator-type surface acoustic wave filter in which an odd number of three or more of interdigital transducers are provided on a piezoelectric substrate in the direction in which surface acoustic waves propagate, one interdigital electrode of an interdigital transducer disposed at the center among the odd number of interdigital transducers is divided into two parts in the propagation direction of the surface acoustic waves arid the two parts are connected to balanced signal terminals, respectively, and two interdigital transducers adjacent to the interdigital transducer disposed at the center are inverted with respect to each other and are connected to an unbalanced signal terminal to provide a balanced-to-unbalanced conversion function, wherein an outermost electrode finger of the interdigital transducer disposed at the center is a signal electrode, and wiring is provided asymmetrically such that a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is a signal electrode, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center has a larger capacitance. 
   According to the above-described structure, since the wiring is provided asymmetrically such that a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is a signal electrode, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center has a larger capacitance, balance degrees between the balanced signal terminals, especially the phase balance degree, is greatly improved. 
   The above-described surface acoustic wave apparatus is preferably configured such that the piezoelectric substrate is mounted on a package by flip-chip bonding, and the asymmetrical wiring is provided on the package. 
   In the above-described surface acoustic wave apparatus, wiring on the piezoelectric substrate and on the package is preferably substantially symmetrical about a virtual axis that is substantially perpendicular to the propagation direction of the surface acoustic waves at the center of the interdigital transducer located at the center, except the asymmetrical wiring. 
   Still another preferred embodiment of the present invention provides a surface acoustic wave apparatus including a longitudinally-coupled-resonator-type surface acoustic wave filter in which an odd number of three or more of interdigital transducers are provided on a piezoelectric substrate in the direction in which surface acoustic waves propagate, one interdigital electrode of an interdigital transducer disposed at the center among the odd number of interdigital transducers is divided into two parts in the propagation direction of the surface acoustic waves and the two parts are connected to balanced signal terminals, respectively, and two interdigital transducers adjacent to the interdigital transducer disposed at the center are inverted with respect to each other and are connected to an unbalanced signal terminal to provide a balanced-to-unbalanced conversion function, wherein an outermost electrode finger of the interdigital transducer disposed at the center is a floating electrode or a grounded electrode, and a reactance component or a delay line is added to a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is grounded, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center. 
   According to the above-described structure, since the reactance component or the delay line is added to a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is grounded, balance degrees between the balanced signal terminals, especially the phase balance degree, is greatly improved. 
   Yet another preferred embodiment of the present invention provides a surface acoustic wave apparatus including a longitudinally-coupled-resonator-type surface acoustic wave filter in which an odd number of three or more of interdigital transducers are provided on a piezoelectric substrate in the direction in which surface acoustic waves propagate, one interdigital electrode of an interdigital transducer disposed at the center among the odd number of interdigital transducers is divided into two parts in the propagation direction of the surface acoustic waves and the two parts are connected to balanced signal terminals, respectively, and two interdigital transducers adjacent to the interdigital transducer disposed at the center are inverted with respect to each other and are connected to an unbalanced signal terminal to provide a balanced-to-unbalanced conversion function, wherein an outermost electrode finger of the interdigital transducer disposed at the center is a signal electrode, and a reactance component or a delay line is added to a balanced signal terminal closer to an interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is a signal electrode, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center. 
   According to the above-described structure, since an outermost electrode finger of the interdigital transducer disposed at the center is a signal electrode, and the reactance component or the delay line is added to a balanced signal terminal closer to the interdigital transducer, of which an outermost electrode finger adjacent to the interdigital transducer disposed at the center is a signal electrode, of the two interdigital transducers adjacent to the interdigital transducer disposed at the center, balance degrees between the balanced signal terminals, especially the phase balance degree, is greatly improved. 
   The above-described surface acoustic wave apparatus is preferably configured such that the piezoelectric substrate is mounted on a package by flip-chip bonding, and the reactance component or the delay line is provided on the package. 
   In the above-described surface acoustic wave apparatus, wiring on the piezoelectric substrate and on the package is preferably substantially symmetrical about a virtual axis that is substantially perpendicular to the propagation direction of the surface-acoustic waves at the center of the interdigital transducer disposed at the center, except for the reactance component or the delay line. 
   The above-described surface acoustic wave apparatus may be configured such that the reactance component is a capacitance component, and is connected in parallel between the balanced signal terminal and a ground potential. Alternatively, the above-described surface acoustic wave apparatus may be configured such that the reactance component is an inductance component, and is connected in series to the balanced signal terminal. 
   In the above-described surface acoustic wave apparatus, a surface acoustic wave resonator may be added in series and/or in parallel to the surface acoustic wave filter. In the above-described surface acoustic wave apparatus, a plurality of the surface acoustic wave filters may be connected in cascade to each other. In the above-described surface acoustic wave apparatus, the total number of the electrode fingers of the surface acoustic wave filters connected in cascade is preferably an even number. 
   In the above-described surface acoustic wave apparatus, the interdigital transducers that are arranged at both ends of each of the surface acoustic wave filters are preferably connected in cascade to each other and connected to interdigital transducers arranged at both ends by signal lines, and the phases of signals passing through the signal lines are preferably different by about 180 degrees. 
   In the above-described surface acoustic wave apparatus, an electrode finger of at least one interdigital transducer of adjacent interdigital transducers, close to the boundary of the interdigital transducers is preferably weighted in the surface acoustic wave filter. In the above-described surface acoustic wave apparatus, the weighting is preferably performed by series weighting. 
   The above-described surface acoustic wave apparatus is preferably configured such that the piezoelectric substrate is mounted on a package by flip-chip bonding, the package includes six external terminals, one unbalanced signal terminal, two balanced signal terminals, and three ground terminals, and the six terminals are arranged substantially symmetrically about a virtual axis that is substantially perpendicular to the propagation direction of the surface acoustic waves at the center of the interdigital transducer positioned at the center of the surface acoustic wave filter. 
   The above-described surface acoustic wave apparatus is preferably configured such that the piezoelectric substrate is mounted on a package by flip-chip bonding, the package has five external terminals, one unbalanced signal terminal, two balanced signal terminals, and two ground terminals, and the five terminals are arranged substantially symmetrically about a virtual axis that is substantially perpendicular to the propagation direction of the surface acoustic waves at the center of the interdigital transducer positioned at the center of the surface acoustic wave filter. 
   Still yet another preferred embodiment of the present invention provides a communication apparatus including the surface acoustic wave apparatus according to one of the preferred embodiments described above. Since the communication apparatus includes the surface acoustic wave apparatus according to the above-described preferred embodiments, which have superior balance degrees, the communication apparatus has greatly improved communication characteristics. 
   As described above, a surface acoustic wave apparatus according to various preferred embodiments of the present invention is a longitudinally-coupled-resonator-type surface acoustic wave filter in which three IDTs are provided on a piezoelectric substrate in the direction in which surface acoustic waves propagate. In the surface acoustic wave apparatus, an IDT disposed at the center among the three IDTs is divided into two parts substantially symmetrically in the propagation direction of the surface acoustic waves, the two parts are connected to balanced signal terminals, and left and right IDTs of which the polarities are inverted to each other are connected to unbalanced signal terminals to provide a balanced-to-unbalanced conversion function. A reactance component is connected to either of the balanced signal terminals by at least one of being on the piezoelectric substrate, in the package, and through an external connection to the package. 
   Therefore, the above-described structure provides greatly improved balance degrees between the balanced signal terminals when a reactance component is connected to either of the balanced signal terminals. 
   The above and other elements, characteristics, features, and advantages of the present invention will become clear from the following description of preferred embodiments taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a structural view of a surface acoustic wave apparatus according to a preferred embodiment of the present invention; 
       FIG. 2  is a structural view of a modification (cascade connection) of the surface acoustic wave apparatus; 
       FIG. 3  is a structural view of a conventional surface acoustic wave apparatus; 
       FIG. 4  is a structural view showing the electrode structure of a surface acoustic wave apparatus according to a first preferred embodiment of the present invention; 
       FIG. 5  is a plan showing a layout on a piezoelectric substrate of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 6  is a plan showing the arrangement of terminals at the rear surface of a package in which the surface acoustic wave apparatus according to the first preferred embodiment is accommodated, in a see-through view viewed from the upper-surface (the surface opposite the rear surface) side of the package; 
       FIG. 7  is a cross-sectional view of the package in which the surface acoustic wave apparatus according to the first preferred embodiment is accommodated; 
       FIG. 8  is a graph indicating the phase balance degrees of the first preferred embodiment and a first comparative example; 
       FIG. 9  is a plan showing a layout in a surface acoustic wave apparatus according to the first comparative example; 
       FIG. 10  is a plan showing a layout in a surface acoustic wave apparatus serving as a second comparative example; 
       FIG. 11  is a graph indicating the phase balance degrees of the second comparative example shown in FIG.  10  and the first comparative example; 
       FIG. 12  is a structural view showing the electrode structure of a surface acoustic wave apparatus according to a modification of the first preferred embodiment of the present invention; 
       FIG. 13  is a graph showing the relationships between the frequency and phase balance degree, of the second comparative example and of a case in which the electrode structure shown in  FIG. 12  is used at the layout on the piezoelectric substrate shown in  FIG. 10 ; 
       FIG. 14  is a graph showing the relationships between the frequency and phase balance degree, of the second comparative example and of a case in which the electrode structure shown in  FIG. 12  is used at the layout on the piezoelectric substrate shown in  FIG. 5 ; 
       FIG. 15  is a structural view showing a surface acoustic wave apparatus according to another modification of the first preferred embodiment of the present invention; 
       FIG. 16  is a structural view showing a surface acoustic wave apparatus according to still another modification of the first preferred embodiment of the present invention; 
       FIG. 17  is a graph showing the relationships between the frequency and phase balance degree, of the structure shown in FIG.  15  and of the second comparative example; 
       FIG. 18  is a graph showing the relationships between the frequency and phase balance degree, of the structure shown in FIG.  16  and of the second comparative example; 
       FIG. 19  is a structural view showing a surface acoustic wave apparatus according to still another modification of the first preferred embodiment of the present invention; 
       FIG. 20  is a plan showing another arrangement of electrode terminals in the package of the first preferred embodiment of the present invention; 
       FIG. 21  is a structural view showing still another modification of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 22  is a cross-sectional view showing a manufacturing process of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 23  is a cross-sectional view showing another manufacturing process of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 24  is a structural view showing still another modification of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 25  is a structural view showing still another modification of the surface acoustic wave apparatus according to the first preferred embodiment of the present invention; 
       FIG. 26  is a plan showing an example layout on the piezoelectric substrate in a case in which the electrode structure shown in  FIG. 2  is mounted to the package having the electrode terminals at the rear surface side, shown in  FIG. 6 ; 
       FIG. 27  is a plan showing another example layout on the piezoelectric substrate in a case in which the electrode structure shown in  FIG. 2  is mounted to the package having the electrode terminals at the rear surface side, shown in  FIG. 6 ; 
       FIG. 28  is a plan showing an example layout on the piezoelectric substrate in a case in which the electrode structure shown in  FIG. 2  is mounted to the package having the electrode terminals at the rear surface side, shown in  FIG. 20 ; 
       FIG. 29  is a plan showing another example layout on the piezoelectric substrate in a case in which the electrode structure shown in  FIG. 2  is mounted to the package having the electrode terminals at the rear surface side, shown in  FIG. 20 ; 
       FIG. 30  is a block diagram showing main sections of a communication apparatus according to a preferred embodiment of the present invention; and 
     FIG.  31 A and  FIG. 31B  show cross-sectional views of packages in which the surface acoustic wave apparatus according to the first preferred embodiment is accommodated, and to which a reactance component or a delay line is externally connected.  FIG. 31A  is a view of a case in which a circuit serving as the reactance component or the delay line is formed between a bottom plate and a side wall section, and  FIG. 31B  is a view of a case in which the reactance component or the delay line is formed as a circuit in a multi-layer substrate in which a lamination plate is formed on the bottom plate. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A surface acoustic wave apparatus according to a preferred embodiment of the present invention will be described below by referring to FIG.  1 . The surface acoustic wave apparatus according to a preferred embodiment of the present invention includes, as shown in  FIG. 1 , a longitudinally-coupled-resonator-type surface acoustic wave filter  101  in which three IDTs are provided on a piezoelectric substrate  501  in the direction in which surface acoustic waves propagate. An IDT  103  which is disposed at the center among the three IDTs of the longitudinally-coupled-resonator-type surface acoustic wave filter  101  is divided into two parts substantially symmetrically in the propagation direction of the surface acoustic waves and the two parts are connected to balanced signal terminals  108  and  109 , respectively. The left and right IDTs  102  and  104  of which the polarities are inverted relative to each other are connected to an unbalanced signal terminal  107  to provide a balanced-to-unbalanced conversion function. The surface acoustic wave apparatus includes a reactance component  120  which is provided on the piezoelectric substrate, provided on a package, or externally connected to the package and which is connected to either of the balanced signal terminals  108  and  109 . 
   With the above-described structure, the surface acoustic wave apparatus has the balanced-to-unbalanced conversion function, and the impedance of the balanced signal terminals is about four times that of the unbalanced signal terminal. In addition, the balance degrees thereof are greatly improved by the reactance component  120 . 
   First Preferred Embodiment 
   A first preferred embodiment according to the present invention will be described with reference to  FIG. 4  to FIG.  7 . In the following preferred embodiment, a DCS receiving filter will be described as an example. An electrode structure in the first preferred embodiment will be described first with reference to FIG.  4 . In the first preferred embodiment, a longitudinally-coupled-resonator-type surface acoustic wave filter  401  and a surface acoustic wave resonator  402  connected in series to the longitudinally-coupled-resonator-type surface acoustic wave filter  401  are defined by aluminum (Al) electrodes provided on a piezoelectric substrate  501  preferably made from 40±5-degree Y-cut X-propagation LiTaO 3 . 
   In the longitudinally-coupled-resonator-type surface acoustic wave filter  401 , IDTs  403  and  405  are arranged so as to sandwich an IDT  404  on both sides in direction in which surface acoustic waves propagate, and reflectors  406  and  407  are arranged so as to sandwich the IDTs  403 ,  404  and  405 . 
   The IDT  403  includes two interdigital electrodes each of which is defined by a strip-shaped base end section (bus bar) and a plurality of parallel electrode fingers extending from one side of the base end section, substantially perpendicular to the base end section. The interdigital electrodes are engaged with each other between their electrode fingers such that the sides of electrode fingers of the interdigital electrodes face each other. 
   The signal conversion characteristics and the pass band of the IDT  403  is specified by setting the length and width of each electrode finger, the distance between adjacent electrode fingers, and an overlap width indicating the length of the portions facing each other when the interdigital electrodes are engaged. The other IDTs have the same basic structure as the IDT  403 . The reflectors reflect propagating surface acoustic waves in opposite direction to those in which the waves have propagated. 
   In the above-described structure, as understood from  FIG. 4 , the pitches of several electrode fingers (portions  414  and  415  in  FIG. 4 ) in vicinities of the boundary between the IDT  403  and the IDT  404  and the boundary between the IDT  404  and the IDT  405  are preferably less than that of the other electrode fingers of the IDTs. 
   One interdigital electrode of the IDT  404 , disposed at the center, is divided into sub-interdigital electrodes  416  and  417  in the propagation direction of surface acoustic waves, and the sub-interdigital electrodes  416  and  417  are connected to balanced signal terminals  412  and  413 , respectively. In the first preferred embodiment, the other interdigital electrode of the IDT  404 , facing the sub-interdigital electrodes  416  and  417 , is a floating electrode. Alternatively, it may be a grounded electrode. The IDT  405  has a phase that is inverted relative to that of the IDT  403 . With this structure, the surface acoustic wave filter has a balanced-to-unbalanced conversion function. 
   In the surface acoustic wave resonator  402 , reflectors  409  and  410  are provided so as to sandwich an IDT  408 . One interdigital electrode of the IDT  408  is connected to an unbalanced signal terminal  411 , and the other interdigital electrode of the IDT  408  is connected to the IDT  403  and the IDT  305 . 
     FIG. 5  shows an actual layout on the piezoelectric substrate  501  according to the first preferred embodiment of the present invention. In  FIG. 5 , the same numbers as those used in  FIG. 4  are assigned to portions corresponding to those shown in FIG.  4 . In the layout, electrode pads  502  to  506  are arranged to be electrically connected to a package. The electrode pad  502  corresponds to the unbalanced signal terminal  411 , the electrode pads  503  and  504  correspond to the balanced signal terminals  412  and  413 , respectively, and the electrode pads  505  and  506  are grounded terminals. Each IDT is shown in a simplified manner. 
     FIG. 6  shows (in a see-through view viewed from the upper-surface side of a surface acoustic wave apparatus) electrode terminals  641  to  645  at the rear surface (rectangle-shaped) of a substantially rectangular package  640  in which the structure according to the first preferred embodiment is accommodated. The electrode terminal  641  is disposed at the approximate center of one end section in the longitudinal direction of the rear surface  640   a . The electrode terminals  642  and  643  are disposed at both corner sections of the other end section in the longitudinal direction of the rear surface  640   a . The electrode terminals  644  and  645  are disposed at the approximate centers of both side sections in the longitudinal direction of the rear surface  640   a.    
   The electrode terminal  641  is the unbalanced signal terminal connected to the electrode pad  502 , the electrode terminals  642  and  643  are the balanced signal terminals connected to the electrode pads  503  and  504 , respectively, the electrode terminals  644  and  645  are the grounded terminals connected to the electrode terminals  505  and  506 , respectively. 
   The surface acoustic wave apparatus according to the first preferred embodiment is produced preferably using a face-down method as shown in  FIG. 7 , where the electrode surface of the piezoelectric substrate  501  and the die-attach surface  653  of the package  640  are electrically connected via bumps  656 . 
   The package  640  has a substantially rectangular plate-shaped bottom plate  651 , side wall sections  652  adjacent to each other and extending upward from the sides of the bottom plate  651 , and a cap  654  for covering and contacting the upper ends of the side wall sections  652  to seal the inside of the package  640 . 
   A feature of the first preferred embodiment is that strip-shaped wiring  508  which connects the sub interdigital electrode  417  and the electrode pad  504  has a larger capacitance to the ground, corresponding to a reactance component  120  shown in  FIG. 1 , than strip-shaped wiring  507  which connects the sub-interdigital electrode  416  and the electrode pad  503 . 
   To make the capacitance to the ground larger, a protrusion  509  is additionally provided for the wiring  508  on the piezoelectric substrate  501  in the outside direction. 
   It is preferred that the protrusion  509  be provided for the wiring  508  at a location close to wiring  511  which connects the ground-side electrode pad  506  and the IDT  405 . 
   It is also preferable that the protrusion  509  be arranged approximately perpendicular to the wiring  508  in its longitudinal direction and approximately parallel to the wiring  511  in its longitudinal direction, and extend separately from the wiring  511 . 
   With the protrusion  509 , the capacitance to the ground of the balanced signal terminal  413 , shown in  FIG. 4 , is greater than that of the balanced signal terminal  412 , for example, by about 0.16 pF. Therefore, the wiring  508  and the wiring  507  are arranged asymmetrically to each other. 
   The electrode fingers of the IDT  404  (the sub-interdigital electrodes  416  and  417 ), adjacent to the IDTs  403  and  405  are signal electrodes. The electrode finger of the IDT  405 , adjacent to the sub-interdigital electrode  417  and connected to the electrode pad  504  with the wiring which makes the capacitance to the ground larger, is also a signal electrode. The electrode finger of the IDT  403 , adjacent to the sub-interdigital electrode  416  and connected to the electrode pad  503  is a ground electrode. 
   Further, in the first preferred embodiment, except for the protrusion  509 , the structure, the layout on the piezoelectric substrate  501 , and the package  640  are all symmetrical about a virtual axis A that is substantially perpendicular to the propagation direction of surface acoustic waves and at the approximate center of the IDT  404  divided into the two electrodes, as shown in  FIG. 4  to FIG.  6 . With this, any unbalanced component is prevented, except for the different polarities of the electrode fingers of the IDT  403  and the IDT  405  adjacent to the IDT  404 . 
   Detailed design parameters for the longitudinally-coupled-resonator-type surface acoustic wave filter  401  are shown below, where Al indicates the wavelength determined by the pitch of the electrode fingers for which the pitch has not been reduced. 
   Overlap width: about 78.9λl 
   Number of electrode fingers in IDTs (in the order of IDT  403 , IDT  404 , and IDT  405 ): 19 (3), (3) 26 (3), (3) 19 (the numbers of electrode fingers for which the pitch has been made smaller are indicated in parentheses) 
   Number of electrode fingers in reflectors: 200 
   Duty: about 0.67 (for both IDTs and reflectors) 
   Electrode film thickness: about 0.095λl 
   Detailed design parameters for the surface acoustic wave resonator  402  are shown below. 
   Overlap width: about 46.5λl 
   Number of electrode fingers in IDTs: 150 
   Number of electrode fingers in reflectors: 100 
   Duty: about 0.67 
   Electrode film thickness: about 0.097λl 
   The operation and advantages of the structure of the first preferred embodiment will now be described.  FIG. 8  shows the phase balance degree of the structure of the first preferred embodiment. A first comparative example for comparison is the same as the first preferred embodiment shown in  FIG. 5  in structure, in design of the surface acoustic wave apparatus, in layout on the piezoelectric substrate  501 , and in package mounting method, except that, in the first comparative example, as shown in  FIG. 9 , the protrusion  509 , which makes the capacitance to the ground larger, is not provided for the wiring  508  in the layout on the piezoelectric substrate  501  and the wiring  508  and the wiring  507  are symmetrical about the virtual axis A.  FIG. 8  also shows the phase comparative degree of the first comparative example, which has no protrusion on the piezoelectric substrate  501 . 
   The pass band of DCS receiving filters ranges from 1805 MHz to 1880 MHz. From  FIG. 8 , it is found that, whereas the first comparative example has a maximum shift of about 22 degrees in the phase balance degree in the pass band, the first preferred embodiment has a maximum shift of about 12 degrees, which is improved by about 10 degrees. This is because the capacitance of the balanced signal terminal  413  to the ground is larger, which compensates for a shift in phase between the balanced signal terminal  412  and the balanced signal terminal  413 . 
   In the first preferred embodiment, the protrusion  509 , which makes the capacitance to the ground larger, is provided for the wiring  508 . Conversely, a protrusion  515 , which makes the capacitance to the ground larger, is provided for the wiring  507 , as shown in  FIG. 10 , and thereby, the capacitance of the balanced signal terminal  412  to the ground is larger by about 0.16 pF. A phase balance degree in this case is examined.  FIG. 11  shows the phase balance degree obtained in the case shown in FIG.  10 .  FIG. 11  also shows the result of the first comparative example shown in  FIG. 9 , for comparison. 
   When the capacitance of the balanced signal terminal  412  to the ground is larger, the phase balance degree is worse than that of the first comparative example. The capacitance of which balanced signal terminal to the ground is larger must be determined by the arrangement of the electrode fingers of the IDTs  403  to  405 , adjacent to each other, that is, whether there is a no-electric-field area, where signal electrodes are disposed adjacent to each other or where ground electrodes are disposed adjacent to each other. 
   In the first preferred embodiment, the electrode fingers of the IDT  404 , adjacent to the IDTs  403  and  405  are the sub-interdigital electrodes  416  and  417 , which are signal electrodes. The electrode finger of the IDT  405  adjacent to the IDT  404 , which is adjacent to the sub-interdigital electrode  417 , and which is connected to the wiring which makes the capacitance to the ground larger and connected to the electrode pad  504 , is a signal electrode, and forms a no-electric-field or weak-electric-field area together with the outermost electrode finger of the opposing sub interdigital electrode  417 , which is a signal electrode. The electrode finger of the IDT  403  adjacent to the IDT  404 , which is adjacent to the sub-interdigital electrode  416 , and that is connected to the electrode pad  503 , is a ground electrode, and defines an electric-field area which is larger in electric-field strength than the no-electric field or weak-electric field-area, together with the most outside electrode finger of the opposing sub-interdigital electrode  416 , which is a signal electrode. 
   When electrode fingers are arranged in this manner, if the protrusion  509 , for example, is arranged such that the capacitance of the balanced signal terminal  413  to the ground, connected to the sub interdigital electrode  417  having a no-electric-field area in a vicinity of its most outside electrode finger (or in contact with the most outside electrode finger) is larger than that of the balanced signal terminal  412  connected to the sub-interdigital electrode  416 , as shown in the first preferred embodiment, the phase balance degree is greatly improved. 
   A case in which the electrode fingers of an IDT  704 , adjacent to the IDTs  703  and  705  are neutral-point electrodes (either floating electrodes or ground electrodes can be used) as shown in  FIG. 12  will now be described.  FIG. 13  shows the phase balance degree obtained with the electrode structure shown in  FIG. 12  on the piezoelectric substrate  501  shown in  FIG. 10  (a modification of the first preferred embodiment).  FIG. 14  shows the phase balance degree obtained with the electrode structure shown in  FIG. 12  on the piezoelectric substrate  501  shown in  FIG. 5  (third comparative example). FIG.  13  and  FIG. 14  also show the phase balance degree obtained with the electrode structure shown in  FIG. 12  on the piezoelectric substrate  501  (without a protrusion) shown in  FIG. 9  as a second comparative example. FIG.  13  and  FIG. 14  show the results obtained when the protrusions  515  and  509  are adjusted so as to correspond to a capacitance to the ground of about 0.02 pF. 
   The phase balance degree is improved when the capacitance to the ground, of the balanced signal terminal  412 , connected to the sub-interdigital electrode  716 , is larger than of the balanced signal terminal  713 , connected to the sub-interdigital electrode  717 , as shown in the layout of  FIG. 10  if electrode fingers are arranged as shown in FIG.  12 . 
   Cases in which a delay line and an inductance component are added in series to a balanced signal terminal in an unbalanced manner at the electrode structure shown in  FIG. 12  will now be described. 
     FIG. 15  shown another modification of the first preferred embodiment in which a delay line  720  defining the reactance component  120  shown in  FIG. 1  is added to the balanced signal terminal  712 , connected to the sub-interdigital electrode  716 .  FIG. 16  shows still another modification of the first preferred embodiment in which an inductance component  722  defining the reactance component  120  shown in  FIG. 1  is added. 
   FIG.  17  and  FIG. 18  show the phase balance degree obtained with the structures shown in FIG.  15  and  FIG. 16 , respectively. FIG.  17  and  FIG. 18  also show the phase balance degree obtained with the electrode structure shown in  FIG. 12  with the layout of  FIG. 9 , where neither a delay line nor an inductance component is added, as the second comparative example. 
   Specific methods for forming the delay line  720  and the inductance component  722  are omitted, the delay line may be formed of long wiring on the piezoelectric substrate or in the package, and the inductance component may be formed of a microstrip line. 
   If possible, the delay line and the inductance component may be provided outside of the package and externally connected, as shown in FIG.  31 A and FIG.  31 B. In  FIG. 31A , a circuit  655  defining the delay line and the inductance component (reactance component) is provided at the boundary of the bottom plate  651  and a side wall section  652 . In  FIG. 31B , a lamination plate  657  is provided on the bottom plate  651 , a via hole  658  is provided for the lamination plate  657  in its thickness direction, and a circuit  659  defining the delay line and the inductance component is connected through the via hole  658  and is provided between the bottom plate  651  and the lamination plate  657 . 
   It is understood from FIG.  17  and  FIG. 18  that the phase balance degree obtained when either of the delay line  720  and the inductance component  722  is inserted is better than that obtained in the second comparative example. In the electrode structure shown in  FIG. 4 , the delay line  720  or the inductance component  722  must be added to the balanced signal terminal  413 . 
   As described above, in the surface acoustic wave apparatus according to the first preferred embodiment having the longitudinally-coupled-resonator-type surface acoustic wave filter in which the three IDTs are provided on the piezoelectric substrate in the direction in which surface acoustic waves propagate, where the IDT disposed at the center among the three IDTs is divided into two parts in the propagation direction of the surface acoustic waves and the polarities of the left and right IDTs are inverted to each other to provide a balanced-to-unbalanced conversion function, when the reactance component defined by at least one of the capacitance to the ground, the inductance component connected in series, and the delay line connected in series is made asymmetrical between the two balanced signal terminals, the phase balance degree of the surface acoustic wave apparatus is greatly improved. 
   In the first preferred embodiment, as shown in  FIG. 5 , a signal electrode is disposed close to a ground electrode on the piezoelectric substrate  501  to make the capacitance to the ground larger. Instead, a capacitor  517  may be formed by interdigital electrodes as shown in FIG.  19 . Alternatively, wiring may be adjusted in the package  640 . 
   In the first preferred embodiment, the layout on the piezoelectric substrate  501  and the package  640  are preferably made in the same manner for the balanced signal terminals in order to avoid an extraneous unbalanced component, except that the capacitor to the ground, the inductance component, or the delay line is asymmetrically provided. To this end, there are five electrode terminals  641  to  645  at the rear surface  640   a  of the package  640  (see FIG.  6 ). The present invention is not limited to such a package. Any package may be used as long as the package is symmetrical about a virtual axis A that is substantially perpendicular to the propagation direction of surface acoustic waves and dividing a center IDT into two parts. 
   For example, a package  800  having six electrode terminals  801  to  806 , as shown in  FIG. 20 , is symmetrical about a virtual axis A when the electrode terminal  801  is used as an unbalanced signal terminal, the electrode terminals  802  and  803  are used as balanced signal terminals, and the electrode terminals  804  to  806  are used as ground terminals. 
   In this case, a layout on the piezoelectric substrate  501  is arranged such that the propagation direction of surface acoustic waves are substantially parallel to the longitudinal direction of the piezoelectric substrate  501 . When an electric pad  901  on the piezoelectric substrate  501  is connected to the electrode terminal  801 , an electrode pad  902  is connected to the electrode terminal  802 , an electrode pad  903  is connected to the electrode terminal  803 , and electrode pads  904  to  906  are connected to the electrode terminals  804  to  806  which define ground terminals. This layout on the piezoelectric substrate  501  is also symmetrical about the virtual axis A. 
   In the first preferred embodiment, the package and the piezoelectric substrate are electrically connected preferably by the face-down method to make the surface acoustic wave apparatus, as shown in FIG.  7 . Alternatively, a wire bonding method may be used. 
   The structure used to make the surface acoustic wave apparatus by the face-down method is not limited to that shown in FIG.  7 . For example, as shown in  FIG. 22 , the structure may be arranged such that piezoelectric substrates  1002  are connected to an assembly board  1001  by a flip-chip method, are then covered and sealed by resin  1003 , and are cut into each package by dicing. Alternatively, the structure may be arranged as shown in  FIG. 23  such that piezoelectric substrates  1002  are connected to an assembly board  1001  by a flip-chip method, are then covered and sealed by a sheet-shaped resin member  1003 , and are cut into each package by dicing. 
   In the first preferred embodiment, the surface acoustic wave resonator is connected in series to the longitudinally-coupled-resonator-type surface acoustic wave filter having the three IDTs. It is obvious that the same advantages are obtained when the surface acoustic wave resonator is not connected, or further when the surface acoustic wave resonator is connected in parallel. Alternatively, as shown in  FIG. 24 , IDTs may be provided at both sides of the three IDTs to define a longitudinally-coupled-resonator-type surface acoustic wave filter having five IDTs. 
   Furthermore, even when a weight is applied to an electrode finger  130  close to the boundary of adjacent IDTs, as shown in  FIG. 25 , the same advantages as in various preferred embodiments of the present invention are obtained. The balance degrees are further improved in the structure shown in  FIG. 25. A  thinning-out weight, an overlap-width weight, or a duty weight may be applied. 
   In the present invention, as shown in  FIG. 2 , a longitudinally-coupled-resonator-type surface acoustic wave filter  101  may be connected in cascade to another longitudinally-coupled-resonator-type surface acoustic wave filter  201 . In this case, it is preferable that an IDT  203  arranged at the approximate center of the longitudinally-coupled-resonator-type surface acoustic wave filter  201  has an even number of electrode fingers. 
   It is also preferred that the direction of IDTs  102 ,  104 ,  202 , and  204  be adjusted such that signals transferring through signal lines  205  and  206  which connect the longitudinally-coupled-resonator-type surface acoustic wave filter  101  and the longitudinally-coupled-resonator-type surface acoustic wave filter  201  have an almost-180-degree phase difference. With this arrangement, a surface acoustic wave apparatus having further better balance degrees is obtained. 
   FIG.  26  and  FIG. 27  show example layouts on the piezoelectric substrate  501 , used with the electrode structure shown in  FIG. 2  when the package having five electrode terminals shown in  FIG. 6  is used. FIG.  28  and  FIG. 29  show example layouts on the piezoelectric substrate  501 , used with the electrode structure shown in  FIG. 2  when the package having six electrode terminals shown in  FIG. 20  is used. 
   Electrode pads  1201 ,  1301 ,  1401 , and  1502  are connected to unbalanced signal terminals, electrode pads  1202 ,  1203 ,  1302 ,  1303 ,  1401 ,  1403 ,  1502 , and  1503  are connected to balanced signal terminals, and the remaining electrode pads are connected to ground terminals. 
   In the first preferred embodiment, a 40±5-degree Y-cut X-propagation LiTaO 3  substrate is preferably used as the piezoelectric substrate  501 . The present invention is not limited to this piezoelectric substrate  501 . With other piezoelectric substrates, such as 64 to 72-degree Y-cut X-propagation LiNbO 3  substrates and a 41-degree Y-cut X-propagation LiNbO 3  substrate, the same advantages are obtained. 
   A communication apparatus in which a surface acoustic wave apparatus according to one of the first preferred embodiment and modifications of the first preferred embodiment of the present invention, or a combination thereof, is provided will now be described with reference to FIG.  30 . 
   As shown in  FIG. 30 , the communication apparatus  600  preferably includes in a receiver side (Rx side) for receiving, an antenna  601 , an antenna duplexer/RF top filter  602 , an amplifier  603 , an inter-Rx-stage filter  604 , a mixer  605 , a first IF filter  606 , a mixer  607 , a second IF filter  608 , a first+second local synthesizer  611 , a temperature compensated crystal oscillator (TCXO)  612 , a divider  613 , and a local filter  614 . It is preferred that balanced signals be transmitted from the inter-Rx-stage filter  604  to the mixer  605  in order to maintain balance, as indicated by a doubled line in FIG.  30 . 
   The communication apparatus  600  also includes in a transceiver side (Tx side) for transmission, the antenna  601  and the antenna duplexer/RF top filter  602 , both of which are shared with, a Tx IF filter  621 , a mixer  622 , an inter-Tx-stage filter  623 , an amplifier  624 , a coupler  625 , an isolator  626 , and an automatic power control (APC)  627 . 
   The surface acoustic wave apparatus according to the present preferred embodiment described above are suitably used for the inter-Rx-stage filter  604 , the first IF filter  606 , the Tx IF filter  621 , the inter-Tx-stage filter  623 , and the antenna duplexer/RF top filter  602 . 
   A surface acoustic wave apparatus according to the preferred embodiments of the present invention has a balanced-to-unbalanced conversion function as well as a filter function, and also has outstanding characteristics in which the amplitude characteristic and the phase characteristic between balanced signals are closer to the ideal characteristics. Therefore, a communication apparatus according to the present invention, having the above-described surface acoustic wave apparatuses included therein has a reduced number of components, a reduced size, and a greatly improved transfer characteristic. 
   The present invention is not limited to each of the above-described preferred embodiments, and various modifications are possible within the range described in the claims. An embodiment obtained by appropriately combining technical means disclosed in each of the different preferred embodiments is included in the technical scope of the present invention.