Abstract:
A surface acoustic wave apparatus includes a piezoelectric substrate mounting a surface acoustic wave filter having a plurality of comb-like electrode sections, and another surface acoustic wave filter also having a plurality of comb-like electrode sections, in a manner such that a balun function is obtained. Further, a wiring pattern is provided on the piezoelectric substrate for a connecting reference terminal side electrode of a comb-like electrode section with a reference terminal side electrode of another comb-like electrode section. Moreover, another wiring pattern is provided on the piezoelectric substrate for connecting a reference terminal side electrode of a further comb-like electrode section with a reference terminal side electrode of a still further comb-like electrode section.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a surface acoustic wave apparatus, and more particularly, to a surface acoustic wave apparatus having a balun function. 
   2. Description of the Related Art 
   Conventionally, there has been extremely remarkable progress made in relation to some techniques for making recent cellular phones compact in size and light in weight. In order to obtain such achievements, research activity has been carried out to reduce various components as well as the size of the required components. In fact, this research activity has also attempted to develop some components each having a plurality of functions. 
   In view of the above, there has been active research conducted in recent years to develop an apparatus having a so-called balun function incorporated in surface acoustic wave filters for use in an RF band of a cellular phone. Particularly, the balun function has been mainly utilized in a GSM system (Global System for Mobile Communication) and other such systems. Further, there is also an extremely high possibility that the balun function can be used in AMPS, PCS, DCS and other components. Here, “balun” means a circuit for matching a balanced line such as a parallel two-wire feeder with an unbalanced line such as a coaxial cable. 
     FIG. 12  is an explanatory view showing a widely used basic constitution of a surface acoustic wave apparatus which has such a balun function and having an input impedance and an output impedance which are substantially equal to each other. An arrangement shown in  FIG. 12  represents a surface acoustic wave apparatus which includes a package  101 , and a piezoelectric substrate  108  included in the package. Mounted on the electric substrate  108  are two stages of longitudinally connected surface acoustic wave filters  109  and  110 , belonging to a longitudinally combined resonator type, each consisting of three comb-like electrode sections (which can also be called rattan blind-like electrodes, hereinafter referred to as an IDT (Interdigital Transducer)). 
   The first surface acoustic wave filter  109  is a longitudinally combined resonator type surface acoustic wave filter, which includes an IDT  112  and an IDT  113  arranged respectively on the right side and the left side of a central IDT  111  (along the propagating direction of a surface acoustic wave), also includes a reflector  114  and a reflector  115  which are arranged in a manner such that the above IDTs are interposed therebetween. 
   The second surface acoustic wave filter  110  is constructed in exactly the same manner as the first surface acoustic wave filter  109 , thereby providing an arrangement in which the two longitudinally combined resonator type surface acoustic wave filters are longitudinally connected with each other. Here, a terminal  133  is an unbalanced signal terminal, while terminals  135  and  136  are balanced signal terminals. These terminals are electrically connected with the package  101  through metal wires. The unbalanced signal terminal  133  of the longitudinally combined resonator type surface acoustic wave filter  109  on an unbalanced side, as well as a ground terminal  134 , are connected with unbalanced side electrodes  103  and  104  of the package  101 . On the other hand, the respective balanced signal terminals  135  and  136  of the longitudinally combined resonator type surface acoustic wave filter  110  on the balanced side, are respectively connected with balanced side electrodes  105  and  107  of the package  101 . 
   Accordingly, a surface acoustic wave filter having a balun function is required to have a propagating characteristic that in its pass bands between the unbalanced signal terminals and the balanced signal terminals, the amplitude characteristics are as equal to one another as possible, and that the phases are reversed by as close to 180 degrees as possible. In practice, these characteristics are respectively referred to as amplitude balance and phase balance. 
   With respect to the amplitude balance and the phase balance, it is accepted to regard each of the above-described surface acoustic wave filters having the balun function as a three-port device. For example, when the unbalanced input terminal is assumed to be a first port, the balanced output terminals are assumed to be second and third ports respectively, while the amplitude balance and the phase balance may be defined as follows:
 
Amplitude balance=| A|, A=| 20 log  S 21|−|20 log  S 3|;
 
Phase balance=| B− 180 |,  B=|∠S 21−∠ S 31|
 
   Here, S 21  and S 31  are matrix elements when the three-port device is expressed in a scattered matrix, respectively representing a propagating characteristic between the first port and the second port, as well as a propagating characteristic between the third port and the first port. As to such balances, it is preferable that within the pass bands of the filter, an amplitude balance is 0 dB and a phase balance is 0 degree. On the other hand, when outside the pass bands, it is preferable that the amplitude balance is 0 dB and the phase balance is 180 degree. 
   However, in an arrangement shown in  FIG. 12 , since there is a deviation between an actual value and the aforementioned ideal value, it has been very difficult to achieve the desired level of characteristics. The reason for this may be explained as follows. The shapes of the balanced signal terminal  135  and the balanced signal terminal  136  are not symmetrical with respect to each other. Moreover, since the metal wires  140  and  142  for electrically connecting the package  101  are not symmetrical with each other either with respect to their shapes and lengths, a parasitic capacitance as well as a parasitic inductance in the respective balanced signal terminals  135  and  136  are different from each other. This means the presence of a problem that a balance, particularly a balance outside the pass bands will be deteriorated, hence undesirably reducing an attenuation outside the pass bands. 
   SUMMARY OF THE INVENTION 
   In order to overcome the problems described above, preferred embodiments of the present invention provide an improved surface acoustic wave apparatus having a balun function and also significantly improving an attenuation outside the pass bands. 
   According to a preferred embodiment of the present invention, a surface acoustic wave apparatus includes a piezoelectric substrate, at least two longitudinally connected surface acoustic wave filters provided on the piezoelectric substrate for providing a balun function, the filters having a plurality of comb-like electrode sections arranged in the propagating direction of a surface acoustic wave, and wiring patterns provided on the piezoelectric substrate to mutually electrically connect reference terminal side electrode fingers other than electric terminal side electrode fingers provided on the comb-like electrode sections for electric signals to flow therethrough. 
   Preferably, in the surface acoustic wave apparatus, the respective comb-like electrode sections for use in establishing connections are electrically separated from one another on the piezoelectric substrate, and provided with electric terminal side electrode fingers and reference terminal side electrode fingers which are connected with the electrodes on the next stage. Further, the respective electric terminal side electrode fingers are electrically connected with one another on the piezoelectric substrate. Moreover, the wiring patterns are provided on the piezoelectric substrate to mutually electrically connect the respective reference terminal side electrode fingers corresponding to the respective electric terminal side electrode fingers which are electrically connected with one another. 
   According to the above-described arrangement, since at least two stages of surface acoustic wave filters each having several groups of IDTs arranged in the propagating direction of a surface acoustic wave are longitudinally connected, it is possible to provide a balun function. 
   Further, according to the above-described arrangement, since wiring patterns are provided for mutually electrically connecting the respective reference terminal side electrode fingers, it is possible to maintain the respective reference terminal side electrode fingers at the standard potential in a floated state, and to ensure a filter function as well as a balun function, without having to connect these reference terminal side electrode fingers in a grounded state. 
   In this way, with the use of the above-described arrangement, it is possible to dispense with several connections such as metal wires which were needed in a conventional apparatus, thereby greatly simplifying the structure of the apparatus. Further, since the provision of the wiring patterns makes it possible to alleviate a non-symmetrical connection usually caused by the above-described wire connection, it has become possible to significantly improve an attenuation outside the pass bands, as compared with the conventional apparatus. 
   Alternatively, in the above-described surface acoustic wave apparatus, a package for receiving the piezoelectric substrate is arranged such that at least one ground electrode is disposed between balanced signal electrodes and along the edge portions of the package. Further, a signal connecting section is provided for electrically connecting the respective balanced signal terminals on the piezoelectric substrate and the respective balanced signal terminals on the package. Moreover, a ground connecting section is provided for connecting a ground terminal of the reference terminal side electrode fingers corresponding to the comb-like electrode sections connected with unbalanced signal terminals, and the ground electrode located between the balanced signal electrodes on the package. 
   With the use of the above-described unique arrangement, since it is possible for the ground connecting portion to be present between the respective balanced signal connecting portions, it is possible for the ground connecting portions to provide a shield effect between the balanced signal terminals. For this reason, it becomes possible to make use of the ground connecting portions to reduce a mutual inductance of each balanced signal connecting portion, which is responsible for a balance deterioration, thereby increasing the balance and greatly improving the attenuation outside the pass bands. 
   Further, in the above-described surface acoustic wave apparatus, the wiring patterns on the piezoelectric substrate for mutually electrically connecting the respective reference terminal side electrode fingers corresponding to the respective electric terminal side electrode fingers electrically connected with one another, are electrically connected with the ground electrodes of the package. With the use of such an arrangement, since the wiring patterns may be electrically connected with the ground electrode of the package, a grounded state can be reinforced, thereby greatly improving an attenuation outside the pass bands. 
   In the above-described surface acoustic wave apparatus, it is preferable that the surface acoustic wave filters be arranged in two longitudinally connected stages. In this way, since the apparatus includes two longitudinally connected stages arranged in a miniaturized configuration, it is possible to avoid the formation of a large scale apparatus. 
   Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a surface acoustic wave apparatus according to a first preferred embodiment of the present invention. 
       FIG. 2  is a schematic view showing a surface acoustic wave apparatus according to a second preferred embodiment of the present invention. 
       FIG. 3  is a schematic view showing a surface acoustic wave apparatus according to a third preferred embodiment of the present invention. 
       FIG. 4  is a schematic view showing a surface acoustic wave apparatus according to a fourth preferred embodiment of the present invention. 
       FIG. 5  is a graph showing a frequency characteristic of an insertion loss in each of the first preferred embodiment and a conventional apparatus. 
       FIG. 6  is a graph showing a frequency characteristic of an amplitude balance in each of the first preferred embodiment, the second preferred embodiment and the conventional apparatus. 
       FIG. 7  is a graph showing a frequency characteristic of a phase balance in each of the first preferred embodiment and the second preferred embodiment of the present invention. 
       FIG. 8  is a graph showing a frequency characteristic of an insertion loss in each of the first preferred embodiment and the second preferred embodiment of the present invention. 
       FIG. 9  is a graph showing a frequency characteristic of an insertion loss in each of the second preferred embodiment and the third preferred embodiment of the present invention. 
       FIG. 10  is a graph showing a frequency characteristic of an insertion loss in each of the third preferred embodiment and the conventional apparatus. 
       FIG. 11  is a schematic view showing a surface acoustic wave apparatus according to a fifth preferred embodiment of the present invention. 
       FIG. 12  is a schematic view showing a conventional surface acoustic wave apparatus. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Several preferred embodiments of the present invention will be described in the following with reference to  FIG. 1  through  FIG. 11 .  FIG. 1  schematically shows the structure of a surface acoustic wave apparatus according to the first preferred embodiment of the present invention. However, each of the following preferred embodiments will be described using an EGSM receiving filter as an example but the present invention is not limited thereto. 
   In the first preferred embodiment, a first surface acoustic wave filter  209  and a second surface acoustic wave filter  210  are disposed on a piezoelectric substrate  208  which is preferably made of LiTaO 3  for 40±5 degree Y-cut X-propagation. In fact, these filters are preferably formed by aluminum (Al) electrodes using a photolithography method. 
   Here, the first surface acoustic wave filter  209  and the second surface acoustic wave filter  210  are longitudinal combination resonator type surface acoustic wave filters which have been described above, and are arranged such that each of them has three IDTs arranged in the propagating direction of a surface acoustic wave. Preferably, the first surface acoustic wave filter  209  and the second surface acoustic wave filter  210  are arranged in a manner such that their surface acoustic wave propagating directions are substantially parallel with each other, and such that the center positions of these surface acoustic wave propagating directions are substantially coincident with each other in a direction that is substantially perpendicular to the propagating direction, thereby making it easy to manufacture apparatus compact in size. 
   An electrode  222  on an electric terminal side in an IDT  212  of the first surface acoustic wave filter  209 , is longitudinally connected with an electrode  225  on an electric terminal side in an IDT  217  of the second surface acoustic wave filter  210 . Meanwhile, an electrode  224  on an electric terminal side in an IDT  213  of the first surface acoustic wave filter  209 , is longitudinally connected with an electrode  227  on an electric terminal side in an IDT  218  of the second surface acoustic wave filter  210 . 
   Here, the IDT  212  and the IDT  217  are not grounded but connected with each other in a floated state. Similarly, the IDT  213  and the IDT  218  are not grounded either, but connected with each other also in a floated state. More specifically, a wiring pattern  251  is provided for electrically connecting, on the piezoelectric substrate  208 , a terminal  229  of an electrode  221  on a reference terminal side of the ITD  212  with a terminal  231  of an electrode  226  on a reference terminal side of the ITD  217 . Further, another wiring pattern  252  is provided for electrically connecting, on the piezoelectric substrate  208  a terminal  230  of an electrode  223  on a reference terminal side of the ITD  213  with a terminal  232  of an electrode  228  on a reference terminal side of the ITD  218 . 
   In practice, the respective wiring patterns  251  and  252  are preferably formed by using the aforementioned photolithography method, and are preferably arranged in symmetrical relation with each other, using a virtual line as a symmetrical axis which is substantially perpendicular to the aforementioned propagating direction in the center position of the propagating direction of the surface acoustic wave, thereby increasing the attenuation outside the pass bands. 
   Here, a terminal  233  of the first surface acoustic wave filter  209  is used as an unbalanced signal terminal, while a terminal  235  and a terminal  236  of the second surface acoustic wave filter  210  are used as balanced signal terminals. These balanced and unbalanced signal terminals are all connected with the electrodes of the package  201 , thereby defining a surface acoustic wave apparatus having a balun function. In detail, an electrode  203  of the package  201  is an unbalanced side electrode, while an electrode  205  and an electrode  207  are balanced side electrodes, with other electrodes being ground electrodes. 
   In fact, a first feature of the first preferred embodiment can be described as follows. More specifically, of the connecting IDTs for longitudinally connecting the first surface acoustic wave filter  209  with the second surface acoustic wave filter  210 , those IDTs (IDT  212  and IDT  213 , IDT  217  and IDT  218 ) for use in connection in an identical surface acoustic wave filter are electrically separated from each other on the piezoelectric substrate  208 . 
   Then, a second feature of the first preferred embodiment can be described as follows. More specifically, in the IDTs which are longitudinally connected, the reference terminals (the electrode  221  and the electrode  226 , the electrode  223  and the electrode  228 ) corresponding to (facing) the longitudinally connected electric terminals (the electrode  222  and the electrode  225 , the electrode  224  and the electrode  227 ) are respectively electrically connected with one another on the piezoelectric substrate  208 , all through the respective wiring patterns  251  and  252 . 
   On the other hand, in the conventional apparatus shown in  FIG. 12 , the reference terminals (the electrode  121  and the electrode  126 , the electrode  123  and the electrode  128 ) corresponding to the longitudinally connected electric terminals (the electrode  122  and the electrode  125 , the electrode  124  and the electrode  127 ) are not electrically connected with one another on the piezoelectric substrate  108 . In fact, only the reference terminals (the electrode  121  and the electrode  123 , the electrode  126  and the electrode  128 ) of the surface acoustic wave filters are electrically connected with one another on the piezoelectric substrate  108 . 
   Further, a third feature of the first preferred embodiment can be described as follows. More specifically, the metal wires  137  and  141  required in the conventional apparatus shown in  FIG. 12  can be omitted in the first preferred embodiment shown in FIG.  1 , providing an advantage of reducing the metal wire. However, in order to simplify  FIG. 1 , the number of IDTs as well as reflectors are omitted to some extent. 
   The details in designing an example of a preferred embodiment of the longitudinally combined resonator type surface acoustic wave filter are as follows. 
   Cross width W: about 52.9 λI 
   The number of IDT pairs (in the order of  212 ,  211 ,  213 ): 11 pairs, 15 pairs, 11 pairs 
   IDT wavelength λI: about 4.16 μm 
   Reflector wavelength λR: about 4.64 μm 
   The number of reflectors: 120 
   An IDT—IDT interval: about 0.31 λI 
   An IDT—Reflector interval: about 0.51 λI 
   IDT Duty: about 0.68 
   Reflector Duty: about 0.50 
   Electrode film thickness: about 0.083 λI 
   The operation and advantages in relation to the first preferred embodiment will be described as follows. In the conventional apparatus shown in  FIG. 12 , the reference terminal (the electrode  121 ) of the IDT  112  of the first surface acoustic wave filter  109  and the reference terminal (the electrode  123 ) of the IDT  113  of the first surface acoustic wave filter  109 , the reference terminal (the electrode  126 ) of the IDT  117  of the second surface acoustic wave filter  110  and the reference terminal (the electrode  128 ) of the IDT  118  of the second surface acoustic wave filter  110 , are electrically connected on the piezoelectric substrate  108 . In such an arrangement, the reference terminals (the electrode  121 , the electrode  123 , the electrode  126  and the electrode  128 ) are required to be electrically connected with the ground electrodes of the package  101  by way of metal wires. 
   The reasons for the above-described unique arrangement can be explained as follows. Here, the first surface acoustic wave filter  109  is used as an input side, while the second surface acoustic wave filter  110  is used as an output side. Accordingly, in order to longitudinally connect together the two surface acoustic wave filters and thus provide a desired surface acoustic wave apparatus, it is necessary to transmit an electric signal, which is generated by a surface acoustic wave excited in the surface acoustic wave filter  109 , to the surface acoustic wave filter  110 . 
   If the metal wire  137  and the metal wire  141  are not included, the reference terminals (the electrode  121  and the electrode  126 , the electrode  123  and the electrode  128 ) corresponding to the electrically connected electric terminals will not be electrically connected. At this time, since the potential of the reference terminals corresponding to the electrically connected electric terminals can not be determined, an electric signal generated in the first surface acoustic wave filter  109  will not be transmitted to the second surface acoustic wave filter  110 . As a result, it is impossible for such an apparatus to function as a surface acoustic wave apparatus. For this reason, in a conventional apparatus shown in  FIG. 12 , it is necessary for the reference terminals to be grounded by way of the metal wire  137  as well as the metal wire  141 . 
   In fact, Japanese Unexamined Patent Application Publication No. 10-163803 has disclosed an apparatus which is identical with the conventional apparatus discussed in the present specification, as shown in  FIG. 10 ,  FIG. 11 ,  FIG. 12  and  FIG. 14  of JP 10-163803. 
   In contrast to the above-described conventional apparatus, in the first preferred embodiment of the present invention shown in  FIG. 1 , the reference terminal (the electrode  221 ) of the IDT  212  of the first surface acoustic wave filter  209  and the reference terminal (the electrode  226 ) of the IDT  217  of the second surface acoustic wave filter  210 , the reference terminal (the electrode  223 ) of the IDT  213  of the first surface acoustic wave filter  209  and the reference terminal (the electrode  228 ) of the IDT  218  of the second surface acoustic wave filter  210 , are electrically connected on the piezoelectric substrate  208 . 
   For this reason, various reference terminals with respect to the longitudinally electrically connected electric terminals have the same potential, so that an electric signal generated in the first surface acoustic wave filter  209  will be transmitted to the second surface acoustic wave filter  210 . As a result, in the first preferred embodiment shown in  FIG. 1 , it is not necessary to use metal wires similar to the metal wire  137  and the metal wire  141  used in the conventional apparatus shown in  FIG. 12 . 
   The transmission characteristic of the first preferred embodiment is shown in  FIG. 5 . The transmission characteristic of the conventional apparatus is also shown in  FIG. 5 . The first preferred embodiment of the present invention differs from the above-described conventional apparatus in that various electrodes on the piezoelectric substrate in the first preferred embodiment are connected with the package through the metal wires in a different manner from that used in the conventional apparatus. However, the surface acoustic wave filters in the first preferred embodiment of the present invention are completely the same as those in the conventional apparatus. 
   In this way, as compared with the conventional apparatus shown in  FIG. 12 , the first preferred embodiment shown in  FIG. 1  provides an arrangement in which two metal wires have been omitted, thereby making it possible to provide an improved surface acoustic wave apparatus having a greatly simplified structure. Further, as compared with the above conventional apparatus, the first preferred embodiment of the present invention makes it possible to obtain an advantage of increasing attenuation in the vicinity of pass bands. 
   Next, description will be provided to explain a surface acoustic wave apparatus according to a second preferred embodiment of the present invention.  FIG. 2  shows the second preferred embodiment of the present invention. In fact, the second preferred embodiment shown in  FIG. 2  is almost the same as the first preferred embodiment shown in  FIG. 1  except that the position of metal wire  339  is different. Therefore, the second preferred embodiment uses reference numerals that are changed, for example “200” used in the first embodiment to “300”, i.e., changing only the first digit of each reference numeral, with the last two digits remaining unchanged. However, an explanation of what each reference numeral represents is omitted. Similarly, in each of the other preferred embodiments which will be described in the following, only the first digit of each reference numeral will be changed, but the last two digits remain unchanged. 
   Therefore, the following description will be provided to explain only the points which are different from those in the first preferred embodiment shown in  FIG. 1 . As described above, in the first preferred embodiment, the ground terminal  234  with respect to the unbalanced signal terminal  233  of the first surface acoustic wave filter  209  is connected with the unbalanced side ground electrode  204  of the package  201  via the metal wire  239 . 
   On the other hand, in the second preferred embodiment shown in  FIG. 2 , the ground terminal  334  with respect to the unbalanced signal terminal  333  of the first surface acoustic wave filter  309  is connected with the balanced side ground electrode  306  of the package  301  via the metal wire  339 . 
   The operation and advantages achieved by the second preferred embodiment will be described in the following. In detail, with regard to the second preferred embodiment of the present invention, its amplitude balance is shown in  FIG. 6 , its phase balance is shown in  FIG. 7 , and its attenuation is shown in  FIG. 8 , all in comparison with the conventional apparatus as well as with the first preferred embodiment of the present invention. As shown in  FIG. 6 , the first preferred embodiment indicates that an amplitude balance in a frequency band from about 1.5 GHz to about 2 GHz has a maximum value which is as high as about 8 dB, while the second preferred embodiment indicates a maximum value which is as low as about 2 dB within the same frequency band, thereby showing the improvement. 
   Similar results can also be seen in  FIG. 7  which shows a phase balance. As compared with the first preferred embodiment, the second preferred embodiment shows that at a frequency of about 1.5 GHz or higher, its phase balance will be increased by about 20 degrees to about 30 degrees, so that it can reach about 180 degrees, thereby providing an improvement. In fact, as compared with the conventional apparatus, a considerable improvement has been achieved in amplitude balance, as well as in phase balance. 
   In this way, the second preferred embodiment shows that when outside the pass bands, its amplitude balance is 0 dB and its phase balance is close to 180 degrees, thereby providing a great improvement in attenuation, as shown in  FIG. 8 . In particular, when compared with the first preferred embodiment, at a frequency of about 1.5 GHz or higher, such an attenuation has been improved by about 10 dB. Even when compared with the conventional apparatus, particularly in the vicinity of a frequency of about 1.5 GHz which is close to a pass band, an attenuation has been improved from about 40 dB to about 60 dB. 
   An advantage obtained from the second preferred embodiment can be described as follows. In the first preferred embodiment shown in  FIG. 1 , the balanced signal terminal  235  and the balanced signal terminal  236  are not symmetrical with each other. In addition, the metal wire  240  and the metal wire  242  are not symmetrical with each other, either. For this reason, it is possible to consider that an incidental capacitance and an incidental inductance between the balanced signal terminals are different, and a mutual inductance is generated between the metal wire  240  and the metal wire  242 , hence causing a deteriorated balance. 
   On the other hand, in the second preferred embodiment shown in  FIG. 2 , a metal wire  339  is provided between the metal wire  340  and the metal wire  342 . Therefore, it is possible to consider that the presence of the metal wire  339  provides a shield effect between the balanced terminals, so that the mutual inductance partially responsible for a deteriorated balance has been minimized and thus the balance has been increased. Accordingly, especially outside the pass bands, an amplitude balance is close to 0 dB and a phase balance is close to 180 degrees, thereby greatly improving an attenuation outside the pass bands. 
   In this way, an insufficient attenuation outside the pass bands in the first preferred embodiment can be greatly improved only by making a change in the arrangement of the metal wires, without having to increase the number of the metal wires. 
   In the following, description will be provided to explain a surface acoustic wave apparatus according to a third preferred embodiment of the present invention.  FIG. 3  shows the third preferred embodiment of the present invention. In fact, the third preferred embodiment shown in  FIG. 3  is constructed as in the above-described second preferred embodiment shown in  FIG. 2 , adding only a metal wire  437  and a metal wire  441 . Therefore, the third preferred embodiment is almost the same as the second preferred embodiment except that the number of metal wires in the third embodiment is different from that in the second preferred embodiment. 
   In fact, the third preferred embodiment differs from the first and second preferred embodiments in the following points. That is, in the third preferred embodiment, a reference terminal (an electrode  421 ) of an IDT  412  of a first surface acoustic wave filter  409 , a reference terminal (an electrode  423 ) of an IDT  413  of the first surface acoustic wave filter, a reference terminal (an electrode  426 ) of an IDT  417  of a second surface acoustic wave filter  410 , a reference terminal (an electrode  428 ) of an IDT  418  of the second surface acoustic wave filter, are grounded through the respective wires  437 ,  441  as well as a package  401 . 
   In this way, the structure shown in  FIG. 3  differs from the first preferred embodiment shown in  FIG. 1  and the second preferred embodiment shown in  FIG. 2 , in that the third preferred embodiment uses two more metal wires than the first and second preferred embodiments. However, as may be made clear in the following description, the number of metal wires used in the third preferred embodiment is just the same as the number of metal wires used in the above described conventional apparatus shown in  FIG. 12 . 
   Further, different from the conventional apparatus shown in  FIG. 12 , the third preferred embodiment shows that a ground terminal  434  with respect to an unbalanced signal terminal  433  of the first surface acoustic wave filter  409  is connected via a metal wire  439  to a balanced side ground electrode  406  of the package  401 . 
   On the other hand, in the conventional apparatus shown in  FIG. 12 , a ground terminal  134  with respect to an unbalanced signal terminal  133  of the first surface acoustic wave filter  109  is connected via a metal wire  139  to an unbalanced side ground electrode  104  of the package  101 . For this reason, the number of wires in the third preferred embodiment is just the same as that in the conventional apparatus. 
   In the following, description will be provided to explain the operation and advantage of the third preferred embodiment.  FIG. 9  is a graph showing a comparison between an attenuation in the third preferred embodiment and that in the second preferred embodiment. Now, with regard to the third preferred embodiment, as may be understood from  FIG. 9 ; in the vicinity of the pass bands and in a frequency band from about 0 MHz to about 900 MHz, as well as in a frequency band from about 1 GHz to about 1.5 GHz, the attenuation has been improved by about 10 dB to about 20 dB as compared with the second preferred embodiment. However, the reason for such an improvement may be considered as follows. More specifically, in the third preferred embodiment, since the reference terminals (the electrode  421 , the electrode  423 , the electrode  426  and the electrode  428 ) corresponding to the longitudinally connected IDT electric terminals are grounded through the metal wires  437  and  441  as well as the package  401 , so that the grounding effect has been reinforced. 
     FIG. 10  is a graph showing an attenuation comparison between the conventional apparatus and the third preferred embodiment. The following facts can be understood from the comparison. More specifically, a ground terminal  434  with respect to an unbalanced signal terminal  433  of the first surface acoustic wave filter  409  is connected via the metal wire  439 , not to the unbalanced side ground electrode  404  of the package  401 , but to the balanced side ground electrode  406  of the package  401 , thereby greatly improving the attenuation in a frequency band not within but close to the pass bands. 
   In the following, description will be provided to explain a surface acoustic wave apparatus according to a fourth preferred embodiment of the present invention. In fact, the fourth preferred embodiment represents a surface acoustic wave apparatus in which the electrodes of the surface acoustic wave filter of the third preferred embodiment are connected with the electrodes of the package not through the metal wires but through a flip-chip manner using metal bumps.  FIG. 4  shows the fourth preferred embodiment, but without showing the package. 
   Actually, in the fourth preferred embodiment, by using the same connection method as used in the third preferred embodiment using metal wires, an unbalanced signal terminal  533  of a first surface acoustic wave filter  509  is connected via a metal bump to a diaphragm contact surface connected with an unbalanced signal electrode of the package. On the other hand, various balanced signal terminals  535  and  536  of a second surface acoustic wave filter  510  are also connected via the metal bumps to a diaphragm contact surface connected with a balanced signal electrode of the package. 
   Further, a reference terminal  529  and a reference terminal  532  are connected via the metal bumps to the diaphragm contact surface connected with the ground electrodes of the package, while the ground terminal  534  of an IDT  511  of the first surface acoustic wave filter  509  is connected via a metal bump to a diaphragm contact surface connected with a balanced side ground electrode of the package. 
   By using the flip-chip method using the metal bumps described above, it is possible for the diaphragm contact surfaces, frets and the like of the package to provide the same effect as obtainable from the above-described metal wires, thereby improving an attenuation outside the pass bands. 
   In the following, description will be provided to explain a surface acoustic wave apparatus according to a fifth preferred embodiment of the present invention. Similar to each of the above-described preferred embodiments, the fifth preferred embodiment, as shown in  FIG. 11 , has a structure capable of performing float connection, such that the total number of electrode fingers of each of the IDTs  611  and  616  in the central area is an even number, thereby improving the characteristic outside the pass bands. 
   Here, the fifth preferred embodiment is described using an AMPS receiving filter as an example. More specifically, in the fifth preferred embodiment, IDTs  612  and  617  on one side with respect to the respective central IDTs  611  and  616  are connected with each other, but not in a grounded connection. Further, IDTs  613  and  618  on the other side of the central IDTs are also connected with each other, but not in a grounded connection. In other words, the IDTs  612  and  617  are connected with each other by a floating connection, while the IDTs  613  and  618  are connected with each other also by a floating connection. However, a piezoelectric substrate is not shown in the drawing. 
   Here, the number of electrode fingers of the central IDTs  611  and  616  is preferably an even number, while the IDTs  617  and  618  are connected in a manner such that their phases are mutually opposite to each other. By virtue of such an arrangement, it is possible to improve an amplitude balance and a phase balance outside the pass bands, as well as to achieve a balun function. 
   Further, in the fifth preferred embodiment, it is preferable that the pitch of the outermost electrode fingers of the central IDTs  611  and  616  is changed to a pitch of other electrode fingers, for example, is set to be large. 
   Although in the above-described first to fifth preferred embodiments, a 40±5 degree Y-cut X-propagation LiTaO 3  substrate is preferably used, the substrate is not limited thereto. In fact, it is also possible to use some other substrates such as a 64–72 degree Y-cut X-propagation LiNbO 3  substrate and a 41 degree Y-cut X-propagation LiNbO 3  substrate, thereby obtaining the same effects. Further, in order to obtain a desired frequency characteristic, it is possible to optionally make some changes in the cross width, the number of IDTs, an IDT-IDT interval, an IDT-reflector interval, and even to add a trap if necessary, thereby obtaining the same effects. 
   Thus, according to a surface acoustic wave apparatus of various preferred embodiments of the present invention, as described above, a piezoelectric substrate has at least two longitudinally connected surface acoustic wave filters arranged thereon to provide a balun function, which filters have several groups of IDTs arranged in the propagating direction of a surface acoustic wave. Further provided on the piezoelectric substrate are wiring patterns for mutually electrically connecting reference terminal side electrode fingers corresponding to the electric terminal side electrode fingers which are mutually electrically connected on the IDTs for electric signals to flow therethrough. 
   In addition, with the use of the above arrangement, if the wiring patterns are provided, it is possible to dispense with some connections such as metal wires which were needed in the conventional apparatuses, thereby simplifying the structure of the apparatus. In addition, it is possible to improve an attenuation outside the pass bands, as compared with the conventional apparatus. 
   While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.