Patent Publication Number: US-7898366-B2

Title: Balanced acoustic wave filter device and composite filter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to balanced acoustic wave filter devices having balance-to-unbalance conversion functions, and particularly relates to an acoustic wave filter device in which arrangement of ground lines and ground terminals on a piezoelectric substrate is improved, and a composite filter including the acoustic wave filter. 
     2. Description of the Related Art 
     In recent years, general cellular phones have been manufactured to include a plurality of communication systems. Therefore, in order to reduce adverse effects among the communication systems, there is a strong demand for increasing an out-of-band attenuation amount. Furthermore, for miniaturization of cellular phones, a small-sized dual filter chip formed by two filter elements on a piezoelectric substrate has been developed. 
     As an example of such a dual filter chip, WO2006/003787 described below discloses a balanced acoustic wave filter device including first and second surface acoustic wave filter elements arranged on a piezoelectric substrate. 
       FIG. 13  is a plan view schematically illustrating a balanced acoustic wave filter device disclosed in WO2006/003787. 
     A balanced acoustic wave filter device  1001  has an electrode configuration on a piezoelectric substrate  1002  as shown in  FIG. 13 . In this configuration, first and second surface acoustic wave filters  1011  and  1011 A are formed. 
     The first surface acoustic wave filters  1011  is a balanced acoustic wave filter which includes an unbalanced terminal  1013  and first and second balanced terminals  1014  and  1015  and which has a balanced-to-unbalanced conversion function. The surface acoustic wave filter  1011  has first and second 3-IDT longitudinally-coupled surface acoustic wave filter elements  1017  and  1018  which are connected to the unbalanced terminal  1013  through a single-port surface acoustic wave resonator  1016 . The surface acoustic wave filter elements  1017  and  1018  are connected to first and second balanced terminals  1014  and  1015 , respectively, through single-port surface acoustic wave resonators  1019  and  1020 , respectively. 
     The second surface acoustic wave filter  1011 A is configured similarly to the first surface acoustic wave filter  1011 . Therefore, a description of the first surface acoustic wave filter  1011  is used for a description of the second surface acoustic wave filter  1011 A by adding a character “A” to reference numerals used for the description of the first surface acoustic wave  1011 . 
     In the acoustic wave filter device having the balanced-unbalanced conversion function, good balancing of signals between first and second balanced terminals  1014 A and  1015 A is required. Therefore, not only IDT electrodes and electrodes serving as reflectors formed on the piezoelectric substrate  1002  but also terminals to which input signals and ground potentials are supplied and connection lines for wiring are symmetrically arranged with respect to a virtual line passing through the center of the piezoelectric substrate  1002  to the greatest extent possible. 
     However, as a filter chip is miniaturized, the margin for wiring on the piezoelectric substrate  1002  is reduced. 
     When the acoustic wave filter device  1001  is manufactured, ground lines and ground terminals are connected to one another at a plurality of portions as much as possible. For example, in  FIG. 13 , a ground terminal is arranged in a portion indicated by an arrow mark A. This ground terminal is electrically connected to ground terminals of IDTs connected to the balanced terminals  1014  and  1015  of the first surface acoustic wave filter  1011  and terminals connected to ground potentials of IDTs connected to the balanced terminals  1014 A and  1015 A of the second surface acoustic wave filter  1010 . That is, in the first and second surface acoustic wave filters  1011  and  1011 A, the terminals to be connected to the ground potentials, the terminals being included in the IDTs connected to the balanced terminals, are connected to one another and further connected to a common ground terminal. In this case, the ground terminal indicated by the arrow mark A is arranged at the center between the first and second surface acoustic wave filters  1011  and  1011 A. With this configuration, a symmetric characteristic is improved, and deterioration of balancing is prevented. 
     However, it becomes apparent that in the conventional acoustic wave filter device  1001  configured as described above, in an attenuation characteristic in the vicinity of pass band in an out-of-band of the first and second surface acoustic wave filters  1011  and  1011 A, the balancing is deteriorated. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, preferred embodiments of the present invention provide a balanced acoustic wave filter device which has a balance-to-unbalance conversion function and prevents deterioration of balancing between out-of-band attenuation characteristics, and also provide a composite filter including the balanced acoustic wave filter device. 
     According to a first preferred embodiment of the present invention, a balanced acoustic wave filter device includes an unbalanced terminal and first and second balanced terminals. The balanced acoustic wave filter device includes a piezoelectric substrate, a first longitudinally coupled acoustic wave filter element which is arranged on the piezoelectric substrate, which is connected between the unbalanced terminal and the first balanced terminal, and which includes second, first, and third IDTs arranged in this order in an acoustic wave propagation direction and first and second reflectors arranged so as to sandwich a region including the first to third IDTs in the acoustic wave propagation direction, and a second longitudinally coupled acoustic wave filter element which is arranged on the piezoelectric substrate so as to be spaced away from the first acoustic wave filter element in the acoustic wave propagation direction, which is connected between the unbalanced terminal and the second balanced terminal, and which includes fifth, fourth, and sixth IDTs arranged in this order in the acoustic wave propagation direction and third and fourth reflectors arranged so as to sandwich a region including the fourth to sixth IDTs in the acoustic wave propagation direction. One terminal of the first IDT and one terminal of the fourth IDT are connected to the unbalanced terminal, the other terminal of the first IDT and the other terminal of the fourth IDT are connected to a ground potential, one terminal of the second IDT and one terminal of the third IDT are connected to the first balanced terminal, the other terminal of the second IDT and the other terminal of the third IDT are connected to the ground potential, one terminal of the fifth IDT and one terminal of the sixth IDT are connected to the second balanced terminal, and the other terminal of the fifth IDT and the other terminal of the sixth IDT are connected to the ground potential. On the piezoelectric substrate, a first ground terminal connected to an external ground potential is arranged nearer to the unbalanced terminal relative to the first and second acoustic wave filter elements and nearer to the first acoustic wave filter element relative to the center between the first and second acoustic wave filter elements. A first ground line which connects the terminal, which is connected to the ground potential, of one of the second and third IDTs which is arranged nearer to the first ground terminal to the first ground terminal is longer than a second ground line which connects the terminal, which is connected to the ground potential, of one of the fifth and sixth IDTs which is arranged nearer to the first ground terminal to the first ground terminal. 
     According to a second preferred embodiment of the present invention, a balanced acoustic wave filter device includes an unbalanced terminal and first and second balanced terminals and a balance-to-unbalance conversion function. The balanced acoustic wave filter device includes a piezoelectric substrate, and first to fourth longitudinally coupled resonator type acoustic wave filter elements arranged on the piezoelectric substrate. Phases of signals output in response to signals input to three of the first to fourth acoustic wave filter elements are the same as one another, and are shifted by 180 degrees with respect to a signal output in response to a signal input to the remaining one of the acoustic wave filter elements. The first longitudinally coupled acoustic wave filter element includes second, first, and third IDTs arranged in this order in an acoustic wave propagation direction and first and second reflectors arranged so as to sandwich a region including the first to third IDTs in the acoustic wave propagation direction. The second longitudinally coupled acoustic wave filter element is arranged on the piezoelectric substrate so as to be spaced away from the first acoustic wave filter element in the acoustic wave propagation direction, and includes fifth, fourth, and sixth IDTs arranged in this order in the acoustic wave propagation direction and third and fourth reflectors arranged so as to sandwich a region including the fourth to sixth IDTs in the acoustic wave propagation direction. One terminal of the first IDT of the first acoustic wave filter element and one terminal of the fourth IDT of the second acoustic wave filter element are connected to each other and further connected to the unbalanced terminal, the first acoustic wave filter element is connected to the first balanced terminal through the third acoustic wave filter element, and the second acoustic wave filter element is connected to the second balanced terminal through the fourth acoustic wave filter element. On the piezoelectric substrate, a first ground terminal connected to an external ground potential is arranged nearer to the unbalanced terminal relative to the first and second acoustic wave filter elements and nearer to the first acoustic wave filter element relative to the center between the first and second acoustic wave filter elements. A first ground line which connects the terminal, which is connected to the ground potential, of one of the second and third IDTs which is arranged nearer to the first ground terminal to the first ground terminal is longer than a second ground line which connects the terminal, which is connected to the ground potential, of one of the fifth and sixth IDTs which is arranged nearer to the first ground terminal to the first ground terminal. 
     According to a third preferred embodiment of the present invention, a longitudinally coupled resonator type balanced acoustic wave filter device includes a piezoelectric substrate, a first IDT arranged on the piezoelectric substrate, second and third IDTs arranged so as to sandwich the first IDT in an acoustic wave propagation direction, fourth and fifth IDTs arranged so as to sandwich a region including the first to third IDTs in the acoustic wave propagation direction, and first and second reflectors arranged so as to sandwich a region including the first to fifth IDTs in the acoustic wave propagation direction, an unbalanced terminal and first and second balanced terminals, one terminal of the second IDT and one terminal of the third IDT being connected to the unbalanced terminal, and a first divided IDT section and a second divided IDT section obtained by dividing the first IDT in the acoustic wave propagation direction. The first divided IDT section and the fourth IDT are connected to the first balanced terminal, and the second divided IDT section and the fifth IDT are connected to the second balanced terminal. On the piezoelectric substrate, a first ground terminal connected to an external ground potential is arranged nearer to the unbalanced terminal relative to the first to fifth IDTs and nearer to the fourth IDT relative to the center of the first IDT. A first ground line which connects the terminal of the fourth IDT connected to the ground potential to the first ground terminal is longer than a second ground line which connects the second divided IDT section to the first ground terminal. 
     According to a fourth preferred embodiment of the present invention, a balanced acoustic wave filter device includes an unbalanced terminal and first and second balanced terminals. The balanced acoustic wave filter device includes a piezoelectric substrate, a first longitudinally coupled acoustic wave filter element which is arranged on the piezoelectric substrate, which is connected between the unbalanced terminal and the first balanced terminal, and which includes fourth, second, first, third, and fifth IDTs arranged in this order in an acoustic wave propagation direction and first and second reflectors arranged so as to sandwich a region including the first to fifth IDTs in the acoustic wave propagation direction, and a second longitudinally coupled acoustic wave filter element which is arranged on the piezoelectric substrate so as to be spaced away from the first acoustic wave filter element in the acoustic wave propagation direction or in a direction opposite to the acoustic wave propagation direction, which is connected between the unbalanced terminal and the second balanced terminal, and which includes ninth, seventh, sixth, eighth, and tenth IDTs arranged in this order in the acoustic wave propagation direction and third and fourth reflectors arranged so as to sandwich a region including the sixth to tenth IDTs in the acoustic wave propagation direction. One terminal of the second IDT, one terminal of the third IDT, one terminal of the seventh IDT, and one terminal of the eighth IDT are connected to one another and further connected to the unbalanced terminal, the other terminal of the second IDT, the other terminal of the third IDT, the other terminal of the seventh IDT, the other terminal of the eighth IDT are connected to a ground potential, one terminal of the fourth IDT, one terminal of the first IDT, and one terminal of the fifth IDT are connected to the ground potential, the other terminal of the fourth IDT, the other terminal of the first IDT, and the other terminal of the fifth IDT are connected to the first balanced terminal, one terminal of the ninth IDT, one terminal of the sixth IDT, and one terminal of the tenth IDT are connected to the ground potential, and the other terminal of the ninth IDT, the other terminal of the sixth IDT, and the other terminal of the tenth IDT are connected to the second balanced terminal. On the piezoelectric substrate, a first ground terminal connected to an external ground potential is arranged nearer to the unbalanced terminal relative to the first and second acoustic wave filter elements and nearer to the first acoustic wave filter element relative to the center between the first and second acoustic wave filter elements. A first ground line which connects the terminal of the fourth IDT connected to the ground potential to the first ground terminal is longer than a second ground line which connects the terminal of the ninth IDT connected to the ground potential to the first ground terminal. 
     That is, the first to fourth preferred embodiments of the present invention are the same in that, in a configuration of a balanced acoustic wave filter device including an unbalanced terminal and first and second balanced terminals arranged on a piezoelectric substrate, a first ground terminal connected to an external ground potential is arranged nearer to the unbalanced terminal relative to IDTs of an acoustic wave filter element and at the center of first and second acoustic wave filter elements or nearer to the first acoustic wave filter element relative to the center among IDTs connected to the first and second balanced terminals or nearer to IDTs connected to the first balanced terminal, a first ground line which connects one of the IDTs which is arranged nearest to the first ground terminal to the first ground terminal, the IDTs being arranged between the unbalanced terminal and the first balanced terminal, is longer than a second ground line which connects a terminal of one of the IDTs which is arranged nearest to the first ground terminal, the terminal of one of the IDTs which are arranged between the unbalanced terminal to the second balanced terminal being connected to the ground potential. Accordingly, grounding is prevented from being intensified in the IDT arranged nearest to the first ground terminal, and a difference between a magnitude of the grounding and a magnitude of grounding at terminals of the IDTs arranged between the unbalanced terminal and the second balanced terminal which are connected to the ground potential is small. Consequently, balancing between out-of-band attenuation characteristics can be improved. 
     According to a preferred embodiment of the present invention, a second ground terminal which is connected to the terminals of the IDTs connected to the unbalanced terminal, the terminals being to be connected to the ground potential, and which is arranged in a region opposite to a region including the unbalanced terminal relative to the IDTs may preferably be provided. 
     The composite filter according to another preferred embodiment of the present invention includes the balanced acoustic wave filter device according to one of the above-described preferred embodiments of the present invention. In the composite filter, even when the first ground terminal of the balanced acoustic wave filter device is also used as a ground terminal of another filter chip, balancing between signals is reliably prevented from being deteriorated in the balanced acoustic wave filter device. 
     According to a preferred embodiment of the present invention, the composite filter may include the balanced acoustic wave filter device and an acoustic wave filter device different from the balanced acoustic wave filter device. The balanced acoustic wave filter device and the other acoustic wave filter device are arranged on the single piezoelectric substrate, and the first ground terminal is shared by the balanced acoustic wave filter device and the other acoustic wave filter device. 
     As described above, in each of the balanced acoustic wave filter devices of the first and second preferred embodiments of the present invention, since the first ground line which connects the terminal, which is connected to the ground potential, of one of the second and third IDTs which is arranged nearer to the first ground terminal to the first ground terminal is longer than the second ground line which connects the terminal, which is connected to the ground potential, of one of the fifth and sixth IDTs which is arranged nearer to the first ground terminal to the first ground terminal, grounding is prevented from being intensified in one of the second and third IDTs which is arranged nearest to the first ground terminal. Accordingly, the balancing between signals of the first and second balanced terminals can be improved. Although a reason why the balancing of signals is improved by arranging the first ground line longer than the second ground line is not necessarily clear, it is believed that this is because an adverse effect of a direct wave transmitted to the first ground terminal from the first IDT connected to the unbalanced terminal is prevented. 
     Similarly, in the third and fourth preferred embodiments of the present invention, since the first ground line is longer than the second ground line, grounding in the IDTs connected to the first ground line weakens. Accordingly, also in the third and fourth preferred embodiments of the present invention, the balancing between signals of the first and second balanced terminals can be enhanced. 
     Accordingly, in a case where the acoustic wave filter device according to a preferred embodiment of the present invention is used, even when the first ground terminal is arranged so as to be shifted from the center of the filter element, the balancing between signals is prevented from being deteriorated. Therefore, the first ground terminal is also used as a ground terminal of another adjacent filter chip without deterioration of the balancing between signals. Consequently, miniaturization of the composite filter device, for example, can be enhanced. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view schematically illustrating a composite filter according to a first preferred embodiment of the present invention. 
         FIG. 2  is a plan view schematically illustrating a composite filter for a comparison with the composite filter according to the first preferred embodiment of the present invention. 
         FIG. 3A  is a diagram illustrating a characteristic of attenuation amount frequencies of first and second balanced terminals, and  FIG. 3B  is a diagram illustrating a difference between characteristics of the first and second balanced terminals. 
         FIG. 4A  illustrates a characteristic of the relationship between an attenuation amount and a frequency of a first balanced terminal and a characteristic of the relationship between an attenuation amount and a frequency of a second balanced terminal in the comparative example shown in  FIG. 2 , and  FIG. 4B  is a diagram illustrating a difference between characteristics of the first and second balanced terminals. 
         FIG. 5  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter device according to a second preferred embodiment of the present invention. 
         FIG. 6  is a plan view schematically illustrating an electrode configuration of a conventional acoustic wave filter device for a comparison of the acoustic wave filter device of the second preferred embodiment of the present invention. 
         FIG. 7  is a plan view schematically illustrating an electrode configuration of a composite filter according to a third preferred embodiment of the present invention. 
         FIG. 8  is a plan view schematically illustrating an electrode configuration of a conventional composite filter for a comparison with the composite filter of the third preferred embodiment of the present invention. 
         FIG. 9  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter element according to a fourth preferred embodiment of the present invention. 
         FIG. 10  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter device for a comparison with the acoustic wave filter element of the fourth preferred embodiment of the present invention. 
         FIG. 11  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter device according to a fifth preferred embodiment of the present invention. 
         FIG. 12  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter element for a comparison with the acoustic wave filter of the fifth preferred embodiment of the present invention. 
         FIG. 13  is a plan view schematically illustrating an example of a conventional acoustic surface wave filter device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described hereinafter with reference to preferred embodiments of the present invention and the accompanying drawings. 
       FIG. 1  is a plan view schematically illustrating a composite filter including an acoustic wave filter device according to a preferred embodiment of the present invention. The composite filter of this preferred embodiment is a surface acoustic wave device configured such that an EGSM reception filter and a PCS reception filter are disposed on a piezoelectric substrate  2 . 
     By forming an electrode configuration on the piezoelectric substrate  2  as shown in  FIG. 1 , an EGSM reception filter  3  and a PCS reception filter  4  each of which preferably has a balance-to-unbalance conversion function are provided. The EGSM reception filter  3  includes an unbalanced terminal  6  and first and second balanced terminals  7  and  8 . Similarly, the PCS reception filter  4  includes an unbalanced terminal  9  and first and second balanced terminals  10  and  11 . The impedance on the unbalanced terminals  6  and  9  side preferably is about 50Ω and the impedance on the first and second balanced terminals  7 ,  8 ,  10 , and  11  side preferably is about 100Ω. 
     Each of the reception filters  3  and  4  preferably is a surface acoustic wave filter having the electrode configuration as shown in  FIG. 1 . That is, each of the reception filters  3  and  4  preferably is a surface acoustic wave filter having a balance-to-unbalance conversion function and an impedance conversion function. 
     In a composite filter  1  including the reception filters  3  and  4  of this preferred embodiment, the PCS reception filter  4  corresponds to a balanced acoustic wave filter device according to a preferred embodiment of the present invention. 
     The PCS reception filter  4  includes a single-port surface acoustic wave resonator  12  connected to the unbalanced terminal  9 . First and second 3-IDT acoustic wave filter elements  13  and  14  are connected to the unbalanced terminal  9  through the single-port surface acoustic wave resonator  12 . The first and second 3-IDT acoustic wave filter elements  13  and  14  are 3-IDT longitudinally-coupled surface acoustic wave filters. 
     The first acoustic wave filter element  13  includes a first IDT  21  and second and third IDTs  22  and  23  which are arranged so as to sandwich the first IDT  21  in a surface acoustic wave propagation direction. That is, the second IDT  22 , the first IDT  21 , and the third IDT  23  are arranged in this order in the surface acoustic wave propagation direction. First and second reflectors  31  and  32  are arranged in a surface wave propagation direction so as to sandwich a region in which the IDTs  21  to  23  are arranged. 
     On the other hand, the second acoustic wave filter element  14  includes a fifth IDT  25 , a fourth IDT  24 , and a sixth IDT  26  arranged in this order in a surface acoustic wave propagation direction. Third and fourth reflectors  33  and  34  are arranged in a surface wave propagation direction so as to sandwich a region in which the IDTs  24  to  26  are arranged. The second acoustic wave filter element  14  is arranged so as to be spaced away from the first acoustic wave filter element  13  in the surface acoustic wave propagation direction. 
     The acoustic wave propagation direction of the first acoustic wave filter element  13  and the acoustic wave propagation direction of the second acoustic wave filter element  14  are the same as each other or opposite to each other. 
     The first IDT  21  of the first acoustic wave filter element  13  has one terminal connected to the unbalanced terminal  9  through the single-port surface acoustic wave resonator  12 , and the other terminal connected to the ground potential. The second and third IDTs  22  and  23  each have terminals connected to the ground potential, and the other terminals connected to each other and further connected to a first balanced terminal  10  through a surface acoustic wave resonator  27 . Similarly, in the second acoustic wave filter element  14 , the fourth center IDT  24  has one terminal connected to the unbalanced terminal  9  through the single-port surface acoustic wave resonator  12 . On the other hand, the fifth and sixth IDT  25  and  26  each have terminals connected to the ground potential, and other terminals connected to each other and further connected to a second balanced terminal  11  through the surface acoustic wave resonator  28 . 
     When these electrodes are formed on the piezoelectric substrate  2 , portions to be connected to the ground potential are connected to one another by ground lines. Here, a first ground wiring section  35  is arranged nearer to the unbalanced terminal  9  relative to a region in which the acoustic wave filter elements  13  and  14  are arranged. Furthermore, a second ground wiring section  36  is arranged nearer to the balanced terminals  10  and  11  relative to the region in which the acoustic wave filter elements  13  and  14  are arranged. The first ground wiring section  35  is connected to a first ground terminal  37 , and the second ground wiring section  36  is connected to a second ground terminal  38 . The first and second ground terminals  37  and  38  are external terminals electrically connected to portions connected to a ground terminal outside of the composite filter  1 . 
     On the piezoelectric substrate  2 , the first ground terminal  37  is arranged nearer to the unbalanced terminal  9  relative to the first and second acoustic wave filter elements  13  and  14  and nearer to the first acoustic wave filter element  13  relative to the center between the first and second acoustic wave filter elements  13  and  14 . This is because the first ground terminal  37  is also connected to the ground potential of the EGSM reception filter  3  adjacent to the PCS reception filter  4  which is the acoustic wave filter device of the present preferred embodiment. That is, the first ground terminal  37  is arranged between the two reception filters  3  and  4 . By arranging the first ground terminal  37  connected to the external ground potential between the two reception filters  3  and  4 , and by sharing the ground terminal by the reception filters  3  and  4 , miniaturization of the entire acoustic wave filter device is attained. 
     On the other hand, in this preferred embodiment, the terminals of the first and fourth IDTs  21  and  24 , the IDTs  21  and  24  being to be connected to the unbalanced terminal  9 , which are to be connected to the ground potential, are connected to each other by the second ground wiring section  36  and further electrically connected to the second ground terminal  38 . 
     As is apparent from  FIG. 1 , the terminals of the second and third IDTs  22  and  23  which are connected to the first IDT  10  are connected to the ground potential through the second ground wiring section  36 . 
     One of the unique features of the present preferred embodiment is that a length of a first ground line which connects the first ground terminal  37  to the second IDT  22  which is arranged nearer to the first ground terminal  37  relative to the third IDT  23  is larger than a length of a second ground line which connects the first ground terminal  37  to the terminal of the fifth IDT  25  which is arranged nearer to the first ground terminal  37  relative to the sixth IDT  26 , the terminal being connected to the ground potential. With this arrangement, the out-of-band attenuation characteristic is improved. The reason for this is described in more detail below. 
     Note that in the following description, the characteristics of the present preferred embodiment will be described by comparing the present preferred embodiment with a comparative example shown in  FIG. 2  in which portions different from the present preferred embodiment are denoted by reference numerals including a character “A”. 
     In a composite filter of the comparative example shown in  FIG. 2 , second IDTs  22 A and  23  are arranged so as to sandwich a first IDT  21 , and one terminal of the second IDT  22 A is connected to a first balanced terminal  10 A, and the other terminal is connected to a ground potential. The terminal of the IDT to be connected to the ground potential is connected to one terminal of the IDT  23  to be connected to the ground potential through a ground line  35 A, and further connected to a ground line A which connects a reflector  32  to a first ground terminal  37 . On the other hand, terminals of fifth and sixth IDTs  25  and  26  which are to be connected to the ground potential are connected to each other through a ground line  35 B, and the fifth IDT  25  is also connected to the ground line A. Accordingly, in the comparative example, although the IDT  22 A is arranged nearest to the first ground terminal  37 , a length of a ground line from the terminal of the first IDT  22 A which is to be connected to the ground potential to the first ground terminal  37  is slightly smaller than a length of a ground line which connects the terminal of the fifth IDT  25  which is to be connected to the ground potential to the first ground terminal  37 . In a case where one end of the ground line A is connected to the center between reflectors  32  and  33 , these ground lines preferably have substantially the same lengths. 
     As described above, in general, the ground line which connects the second IDT  22 A to the first ground terminal  37  and the ground line which connects the fifth IDT  25  to the first ground terminal  37  are conventionally devised to have lengths as small as possible. 
     However, in the configuration shown in  FIG. 2 , it became apparent that excellent balancing between a signal output from the first balanced terminal  10 A which is connected to the first IDT  22 A and a signal output from a second balanced terminal  11 A is not attained. This may be because a direct wave from the first IDT  21 A connected to an unbalanced terminal  9 A is supplied to the first ground terminal  37  connected to the external ground potential. 
     On the other hand, in this preferred embodiment, as shown in  FIG. 1 , the second IDT  22  is arranged nearest to the first ground terminal  37 , and one of the terminals of the second IDT  22  to be connected to the ground potential is connected to the second ground wiring section  36  through the first reflector  31 . Accordingly, the first ground line connects the terminal of the second IDT  22  to be connected to the ground potential to the first ground terminal  37  such that the first ground line extends through the first reflector  31 , the second ground wiring section  36 , the reflector  34 , the first ground wiring section  35 , and the ground line A to the first ground terminal  37 . On the other hand, the second ground line which connects the terminal of the fifth IDT  25  to be connected to the ground potential to the first ground terminal  37  has a length including a length of a portion of a bus bar ranging from the terminal of the fifth IDT  25  to the terminal of the second reflector  32  and the ground line A. Accordingly, since the first ground line is sufficiently longer than the second ground line, the grounding is prevented from being excessively intensified in the IDT  22 . In other words, the grounding of the IDT  22  weakens, and therefore, influence due to the direct wave transmitted from the first IDT  21  to the first ground terminal  37  is reduced in the second IDT  22 . Accordingly, balancing between a signal output from the first IDT  10  and a signal output from the second balanced terminal  11  is improved. 
     Accordingly, in the composite filter  1 , although the first ground terminal  37  is arranged at a portion shifted from the center of the PCS reception filter  4  so as to be shared by the reception filter  4  and the reception filter  3  adjacent to the reception filter  4 , the balancing of signals in the PCS reception filter  4  can be enhanced. Therefore, the balancing between signals is not deteriorated and miniaturization of the composite filter  1  is attained. 
       FIG. 3A  is a diagram illustrating insertion losses of the first and second balanced terminals  10  and  11  of the PCS reception filter  4  of the composite filter  1  according to the preferred embodiment, and  FIG. 3B  is a diagram illustrating a differential characteristic between the insertion losses of the first and second balanced terminals. 
       FIG. 4A  illustrates insertion losses of the first and second balanced terminals  10 A and  11 A of the comparative example shown in  FIG. 2 , and  FIG. 4B  is a diagram illustrating a differential characteristic between the insertion losses. 
     As is apparent from  FIGS. 4A and 4B , a difference between an attenuation amount of the balanced terminal  10 A and an attenuation amount of the balanced terminal  11 A in the vicinity of 5 GHz is approximately 7 dB. Therefore, in the differential characteristic shown in  FIG. 4B , an attenuation characteristic on a high frequency side is deteriorated. 
     On the other hand, as shown in  FIG. 3A , in this preferred embodiment, the attenuation characteristic of the second balanced terminal  11  is improved by approximately 5 dB in the vicinity of 5 GHz. The attenuation characteristic of the first balanced terminal  10  is the same as that of the comparative example. Therefore, a difference between the attenuation amount of the first balanced terminal  10  and the attenuation amount of the second balanced terminal  11  in the vicinity of 5 GHz is reduced to approximately 3 dB. Accordingly, in the differential characteristic shown in  FIG. 3B , a sufficiently high attenuation amount is attained on the high frequency side. 
       FIG. 5  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter device according to a second preferred embodiment of the present invention.  FIG. 6  is a plan view schematically illustrating an electrode configuration of a conventional acoustic wave filter device according to a second comparative example provided for comparison with the second preferred embodiment. 
     In an acoustic wave filter device  101  according to the second preferred embodiment, an electrode configuration as shown in  FIG. 5  is formed on a piezoelectric substrate  102  whereby a PCS reception filter is configured. That is, similarly to the PCS reception filter  4  included in the composite filter  1  of the first preferred embodiment, the acoustic wave filter device  101  is a reception band filter for a cellular phone using a PCS method. 
     The acoustic wave filter device  101  includes an unbalanced terminal  103  and first and second balanced terminals  104  and  105 . Impedance on the unbalanced terminal  103  side preferably is about 50Ω, and impedance on the first and second balanced terminals  104  and  105  side preferably is about 150Ω, for example. 
     Accordingly, the acoustic wave filter device  101  is also an acoustic wave filter device having an impedance conversion function and a balance-to-unbalance conversion function. 
     The acoustic wave filter device  101  of this preferred embodiment is basically the same as the PCS reception filter  4  which is the acoustic wave filter according to the first preferred embodiment. That is, the unbalanced terminal  103  corresponds to the unbalanced terminal  9  of the first preferred embodiment, and the first and second balanced terminals  104  and  105  correspond to the first and second balanced terminals  10  and  11  of the first preferred embodiment, respectively. Then, the electrode configuration the same as that of the PCS reception filter  4  according to the first preferred embodiment is connected between the unbalanced terminal  103  and the first and second balanced terminals  104  and  105 . Therefore, components that are the same as those of the first preferred embodiment are denoted by reference numerals the same as those of the first preferred embodiment, and the description of the first preferred embodiment is quoted. 
     This preferred embodiment also includes a first ground terminal  37  connected to an external ground potential and a second ground terminal  38  connected to the external ground potential. The first ground terminal  37  is arranged nearer to the unbalanced terminal  103  relative to first and second acoustic wave filter elements  13  and  14  and near the first acoustic wave filter element  13 . One terminal of a second IDT  22  to be connected to a ground potential is, similarly to the first preferred embodiment, connected to a ground wiring section  36  through a first reflector  31 . The second ground terminal  38  is connected to the second ground wiring section  36 . The ground wiring section  36  is connected to one terminal of a reflector  34 . The other terminal of the reflector  34  is shared with a bus bar of an IDT  26 , and is connected to one terminal of the IDT  25  connected to the ground potential using a first ground wiring section  35 . Furthermore, one terminal of a third IDT  23 , one terminal of a reflector  32 , one terminal of a reflector  33 , and the terminal of the IDT  25  which are to be connected to the ground potential are connected to one another to be a common ground terminal. One terminal of a ground wiring section A is connected to the common ground terminal, and the other terminal of the ground wiring section A is connected to the first ground terminal  37 . The second ground wiring section  36  is arranged nearer to the first and second balanced terminals  104  and  105  relative to the first and second acoustic wave filter elements  13  and  14 . 
     Also in this preferred embodiment, a first ground line is longer than a second ground line. That is, in this preferred embodiment, the first ground line extends from one terminal of the IDT  22  through the first reflector  31 , the second ground wiring section  36 , the reflector  34 , the bus bar shared by the reflector  34  and the sixth IDT  26 , the first ground wiring section  35 , the fifth IDT  25 , a bus bar shared by the reflectors  32  and  33 , and the ground wiring section A to the first ground terminal  37 . On the other hand, the second ground line extends from the terminal of the IDT  25  through a bus bar shared by terminals of the reflectors  32  and  33  and the ground wiring section A to the first ground terminal  37 . Accordingly, also in this preferred embodiment, the first ground line is longer than the second ground line. Accordingly, as with the first preferred embodiment, balancing can be improved in the second preferred embodiment. 
     Note that the second comparative example shown in  FIG. 6  has a configuration substantially the same as the comparative example shown in  FIG. 2 . Therefore, the same components are denoted by the same reference numerals, and descriptions thereof are omitted. In the second comparative example, one terminal of a second IDT  22 A to be connected to a ground potential is directly connected to a first ground terminal  37  through a ground wiring section B. Other configurations are the same as those of the second preferred embodiment. Accordingly, in the second comparative example, the second IDT  22 A connected to a first balanced terminal  104  and IDTs  24 ,  25 , and  26  connected to a second balanced terminal  105  are connected to one another through first ground wiring sections  35 A and  35 , and further connected to the first ground terminal  37 . 
     The acoustic wave filter device according to the second preferred embodiment of the present invention or the second comparative example is configured on a single piezoelectric substrate. Accordingly, instead of the second ground terminal  38  shown in  FIGS. 1 and 2 , the first ground terminal  37  which is not shared by adjacent elements is preferably used, and the first ground terminal  37  is connected to the ground potential. Other configurations of the acoustic wave filter device  101  according to the second preferred embodiment are the same as those of the PCS reception filter  4  according to the first preferred embodiment. Therefore, as with the first preferred embodiment, balancing between out-of-band attenuation characteristics can be improved. 
     As described above, according to various preferred embodiments of the present invention, only the acoustic wave filter device  101  configured in accordance with the present invention, instead of a composite filter, may be formed on a single piezoelectric substrate. 
       FIG. 7  is a plan view schematically illustrating an electrode configuration of a composite filter  201  according to a third preferred embodiment of the present invention. In the composite filter  201 , the electrode configuration is arranged on a piezoelectric substrate as shown in  FIG. 7 . 
     In the composite filter  201  of the third preferred embodiment, according to the electrode configuration, surface acoustic wave filter devices  202  and  203  are provided on the piezoelectric substrate. 
     The surface acoustic wave filter device  202  is a transmission filter using a CDMA 2100 method and the surface acoustic wave filter device  203  is a transmission filter using a CDMA 800 method. 
     Note that between the surface acoustic wave filter devices  202  and  203 , the surface acoustic wave filter device  202  which is the transmission filter using the CDMA 2100 method is a surface acoustic wave filter device according to the present preferred embodiment of the present invention. 
     As shown in  FIG. 7 , the surface acoustic wave filter device  202  includes an unbalanced terminal  204  and first and second balanced terminals  205  and  206 . The surface acoustic wave filter device  203  which is the transmission filter using the CDMA 800 method includes an unbalanced terminal  207  and first and second balanced terminals  208  and  209 . 
     In the surface acoustic wave filter device  202 , first to fourth filter elements  211  to  214  are connected between the unbalanced terminal  204  and the first and second balanced terminals  205  and  206 . 
     Specifically, one terminal of the first filter element  211  and one terminal of the second filter element  212  are connected to the unbalanced terminal  204 . The first filter element  211  includes a first IDT  221  and second and third IDT  222  and  223  which are arranged so as to sandwich the first IDT  221  in a surface wave propagation direction. That is, the second IDT  222 , the first IDT  221 , and the third IDT  23  are arranged in the surface wave propagation direction in this order. First and second reflectors  231  and  232  are arranged so as to sandwich a region in which the IDTs  221  to  223  are arranged in a surface wave propagation direction. 
     Similarly, in the second filter element  212 , a fifth IDT  225 , a fourth IDT  224 , and a sixth IDT  226  are arranged in this order in a surface wave propagation direction, and third and fourth reflectors  233  and  234  are arranged so as to sandwich a region in which the IDTs  224  to  226  are arranged in a surface wave propagation direction. Here, one terminal of the first IDT  221  of the first filter element  211  and one terminal of the fourth IDT  224  of the second filter element  212  are connected to the unbalanced terminal  204  in common. 
     The other terminal of the first IDT  221  and the other terminal of the fourth IDT  224  are connected to a second ground wiring section  242 . 
     First and second ground terminals  37  and  38  are connected to a ground potential outside of the composite filter  201 . The second IDT  222  is nearer to the first ground terminal  37  than the third IDT  223 . One terminal of the second IDT  222  to be connected to the ground terminal is connected to the second ground wiring section  242  through the first reflector  231 . Note that the second ground terminal  38  is connected to the second ground wiring section  242 . 
     Furthermore, one terminal of the third IDT  223 , one terminal of the fifth IDT  225 , one terminal of the first reflector  231 , and one terminal of the reflector  233  are connected to one another and further connected to a first ground wiring section  241 . One terminal of the sixth IDT  226  is also connected to the first ground wiring section  241 . 
     The terminal of the sixth IDT  226  to be connected to the ground potential is connected to one terminal of the fourth reflector  234 , and the other terminal of the fourth reflector  234  is connected to the second ground wiring section  242 . Therefore, the first ground wiring section  241  is connected to the second ground wiring section  242  through the fourth reflector  234 . 
     Note that the terminal of the third IDT  223 , the terminal of the second reflector  232 , the terminal of the third reflector  233 , and the terminal of the fifth IDT  225  which are to be connected to the ground potential are configured as a common terminal and the common terminal is electrically connected to the first ground terminal  37  through a ground wiring section A. The first ground terminal  37  is arranged nearer to the unbalanced terminal  204  relative to the first and second filter elements  211  and the  212  and near the first filter element  211  relative to the center between the first and second filter elements  211  and the  212 . The second IDT  222  is arranged nearer to the first ground terminal  37  than the IDT  223 . The terminal of the second IDT  222  to be connected to the ground terminal is connected to the first ground terminal  37  through a first ground line. Note that the first ground line extends from the terminal of the second IDT  222  to be connected to the ground potential and includes the first reflector  231 , the second ground wiring section  242 , the fourth reflector  234 , the first ground wiring section  241 , and the ground wiring section A. 
     On the other hand, a second ground line of this preferred embodiment connects the fifth IDT  225  which is arranged nearer to the first ground terminal  37  than the sixth IDT  226  to the first ground terminal  37 . Specifically, the second ground line starting from the terminal of the IDT  225  is connected to the first ground terminal  37  through the ground wiring section A. Accordingly, also in this preferred embodiment, the first ground line is longer than the second ground line. 
     Note that the first ground terminal  37  also corresponds to a ground terminal used to electrically connect the surface acoustic wave filter device  203  to the outside. Therefore, the first ground terminal  37  is arranged nearer to the first filter element  211  relative to the center of the surface acoustic wave filter device  202 . 
     In subsequent stages of the first and second filter elements  211  and  212 , third and fourth filter elements  213  and  214  are connected. 
     Specifically, the other terminal of the first IDT  221  of the first filter element  211  is connected to one terminal of a first IDT  251  of the third acoustic wave filter element  213 , and the other terminal of the first IDT  251  of the third filter element  213  is connected to the first balanced terminal  205 . Furthermore, the other terminal of the second IDT  222  and the other terminal of the third IDT  223  are electrically connected to one terminal of a second IDT  252  and one terminal of a third IDT  253  of the third filter element  213 , respectively. The other terminal of the IDT  252  and the other terminal of the IDT  253  are connected to each other through a ground line  245 , and the terminal of the second IDT  252  is electrically connected to a first reflector  261 . The first reflector  261  is electrically connected to the second ground wiring section  242 . Accordingly, the terminals of the IDTs  252  and  253  to be connected to the ground potential are electrically connected to the second ground wiring section  242  through the first reflector  261 . 
     Note that one terminal of a second reflector  262  is electrically connected to the terminal of the third IDT  253  to be connected to the ground potential. 
     Similarly to the second filter element  212 , the fourth filter element  214  includes fourth to sixth IDTs  254  to  256  and third and fourth reflectors  263  and  264 . One terminal of the fourth IDT  254  is electrically connected to the second ground wiring section  242 . The other terminal of the fourth IDT  254  is connected to the second balanced terminal  206 . One terminal of the fifth IDT  255  and one terminal of the sixth IDT  256  are electrically connected to the other terminal of the fifth IDT  225  and the other terminal of the sixth IDT  226 , respectively. The other terminal of the fifth IDT  255  and the other terminal of the sixth IDT  256  are electrically connected to each other through a fourth ground line  246 . 
     The terminal of the sixth IDT  256  to be connected to the ground potential is electrically connected to the fourth reflector  264 , and the fourth reflector  264  is electrically connected to the second ground wiring section  242 . Accordingly, the terminals of the first and sixth IDTs  255  and  256  which are to be connected to the ground potential are electrically connected to the second ground wiring section  242  through the fourth reflector  264 . 
     Also in this preferred embodiment, in the surface acoustic wave filter device  202  having a balance-to-unbalance conversion function described above, the terminal of the second IDT  222  of the first filter element  211  which is to be connected to the ground potential and which is arranged near the first balanced terminal  205  is connected to the first ground terminal  37  through a unique line which is different from a line corresponding to portions of the third IDT  223  connected to the first balanced terminal  205  and the IDT  225  and  226  connected to the second balanced terminal  206  which are connected to the ground potential. 
     Also in this preferred embodiment, as described above, since the first ground line is longer than the second ground line, a difference between an out-of-band attenuation characteristic of the first balanced terminal  205  and an out-of-band attenuation characteristic of the second balanced terminal  206  is reduced, and accordingly, balancing between the out-of-band attenuation characteristics is enhanced. 
     Note that the surface acoustic wave filter device  203  is configured such that a fifth 3-IDT longitudinally coupled resonator type surface acoustic wave filter device  271  and a sixth longitudinally coupled resonator type surface acoustic wave filter element  272  are connected to each other in series between the unbalanced terminal  207  and the first and second balanced terminals  208  and  209 . 
       FIG. 8  is a plan view schematically illustrating an electrode configuration of a conventional composite filter  301  as an example of comparison with that of the third preferred embodiment. The composite filter  301  includes a surface acoustic wave filter device  302  and a surface acoustic wave filter device  203 . The surface acoustic wave filter device  203  is the same as that of the third preferred embodiment. 
     The surface acoustic wave filter device  302  is the same as that of the third preferred embodiment except that one terminal of a second IDT  222 A to be connected to a ground potential is directly connected to a first ground terminal  37  through a ground wiring section B. Since the second IDT  222 A of a first filter element which is nearest to the first ground terminal  37  is connected to a connection point of a ground wiring section A through the ground wiring section B, a first ground line which connects the IDT  222 A to the first ground terminal  37  is shorter than a second ground line which connects an IDT  225  to the first ground terminal  37 . Therefore, an out-of-band attenuation characteristic of a first balanced terminal  205  is different from an out-of-band attenuation characteristic of a second balanced terminal  206 , and accordingly, balancing may be deteriorated. On the other hand, in the third preferred embodiment, as with the first and second preferred embodiments, the balancing between the out-of-band attenuation characteristics can be improved. 
     According to the first to third preferred embodiments, examples in which 3-IDT longitudinally coupled resonator type surface acoustic wave filter devices are included are described. However, the present invention is applicable to a configuration using a 5-IDT longitudinally coupled acoustic wave filter element. Specifically, an acoustic wave filter device  401  according to a fourth preferred embodiment shown in  FIG. 9  is the same as the reception filter  4  shown in  FIG. 1  except that the acoustic wave filter device  401  includes first and second 5-IDT acoustic wave filter elements  402  and  403 . Accordingly, the same components are denoted by the same reference numerals, and descriptions thereof are omitted. Here, the first acoustic wave filter element  402  includes a fourth IDT  414 , a second IDT  412 , a first IDT  411 , a third IDT  413 , and a fifth IDT  415  in this order in a surface acoustic wave propagation direction. The second acoustic wave filter element  403  includes a ninth IDT  424 , a seventh IDT  422 , a sixth IDT  421 , an eighth IDT  423 , and a tenth IDT  425  in this order in a surface acoustic wave propagation direction. 
     In the first acoustic wave filter element  402 , one terminal of the second IDT  412  and one terminal of the third IDT  413  are connected to each other, and further connected to an unbalanced terminal  9 . The other terminal of the second IDT  412  and the other terminal of the third IDT  413  are electrically connected to a ground wiring section  440 . Furthermore, one terminal of the first IDT  411 , one terminal of the fourth IDT  414 , and one terminal of the fifth IDT  415  are connected to one another and further connected to a first balanced terminal  10  through a surface acoustic wave resonator  27 . The other terminal of the IDT  411  and the other terminal of the fifth IDT  415  are connected to the first ground terminal  37  through the ground wiring section A arranged near the unbalanced terminal  9 . Furthermore, the fifth IDT  415  is electrically connected to a reflector  417 . A reflector  414  and the reflector  417  are electrically connected to the ground wiring section  440 . 
     Among the IDTs connected to the first balanced terminal  10 , the other terminal of the fourth IDT  414  which is arranged nearest to the first ground terminal  37 , the terminal being the one to be connected to the ground potential, is not connected to the ground wiring section A, but connected to a first reflector  416 . 
     On the other hand, in the second acoustic wave filter element  403 , the terminals of the seventh and eighth IDTs  422  and  423  connected to the ground potential, the seventh and eighth IDTs  422  and  423  being connected to the unbalance terminal  9 , are connected to the ground line section  440 . 
     On the other hand, the terminals of the sixth IDT  421 , the ninth IDT  424 , and the tenth IDT  425  which are to be connected to the ground potential are connected to one another through a ground wiring section  35 . The first ground wiring section  35  is also connected to reflectors  426  and  427 . The reflector  426  is connected to the reflector  417 , and the reflector  427  is electrically connected to the ground wiring section  440 . 
     In this preferred embodiment, a first ground line electrically connects the terminal of the fourth IDT  414  which is arranged nearest to the first ground terminal  37 , among the IDTs  411 ,  414 , and  415 , the terminal being to be connected to the ground potential, to the first ground terminal  37 . The first line extends from the terminal of the fourth IDT  414  to be connected to the ground potential to the first ground terminal  37  through the reflector  416 , the ground wiring section  440 , the reflector  403 , the first ground wiring section  35 , and the ground wiring section A. On the other hand, a second ground line connects the IDT  424  which is arranged nearest to the first ground terminal  37 , among the IDTs of the second acoustic wave filter element  403  which are connected to the second balanced terminal  11  to the first ground terminal  37 . The second ground line extends from the terminal of the IDT  424  to the first ground terminal  37  through the reflectors  426  and  417  which are connected to the IDT  424  and the ground line A. Also in this preferred embodiment, the first ground terminal  37  is arranged nearer to the unbalanced terminal  9  relative to the first and second acoustic wave filter elements  402  and  403  and arranged nearer to the first acoustic wave filter element  402  relative to the center between the first and second filter elements  402  and  403 . 
     Also in this preferred embodiment, the first ground line is longer than the second ground line. Accordingly, a difference between a grounding state on the balanced terminal  10  side and a grounding state on the balanced terminal  11  is small whereby balancing between out-of-band attenuation amounts is improved. 
     Note that  FIG. 10  is a plan view schematically illustrating an electrode configuration of a conventional example corresponding to the acoustic wave filter device  401  of the third preferred embodiment for reference. In a composite filter  501 , one terminal of a fourth IDT  514  to be connected to a ground potential, among first to fifth IDTs  511  to  515  of a first acoustic wave filter element  502 , is connected through ground wiring sections A and B to one terminal of the IDT  511  and one terminal of the IDT  515  which are to be connected to the ground potential, the IDTs  511 ,  514 , and  515  being to be connected to a first balanced terminal  10 . Accordingly, in the conventional example, a first ground line is shorter than a second ground line. Therefore, in first and second acoustic wave filters  512  and  513 , a difference between a grounding state on the balanced terminal  10  side and a grounding state on the balanced terminal  11  is comparatively large. Accordingly, balancing between out-of-band attenuation amounts may be deteriorated. Similarly, a second acoustic wave filter  513  includes first to fifth IDTs  521  to  525  and reflectors  526  and  527 . Furthermore, the first acoustic wave filter  512  includes first and second reflectors  516  and  517 . 
       FIG. 11  is a plan view schematically illustrating an electrode configuration of an acoustic wave filter element according to a fifth preferred embodiment of the present invention. In an acoustic wave filter device  601  of the fifth preferred embodiment, a 5-IDT longitudinally coupled resonator type acoustic wave filter element  602  is arranged between an unbalanced terminal  9  and first and second balanced terminals  10  and  11 . 
     The acoustic wave filter element  602  includes a first IDT  611 , second and third IDTs  612  and  613  which are arranged so as to sandwich the first IDT  611  in a surface wave propagation direction, and fourth and fifth IDTs  614  and  615  which are arranged so as to sandwich a region including the first to third IDTs  611  to  613  in the surface wave propagation direction. First and second reflectors  616  and  617  are arranged so as to sandwich a region including the first to fifth IDTs  611  to  615  in a surface wave propagation direction. One terminal of the IDT  614  to be connected to a ground potential is electrically connected to one terminal of the first reflector  616 . Similarly, one terminal of the fifth IDT  615  to be connected to the ground potential is electrically connected to one terminal of the reflector  617 . 
     One terminal of the second IDT  612  and one terminal of the third IDT  613  are connected to the unbalance terminal  9  in common through a single-port surface acoustic wave resonator  12 . The other terminal of the second IDT  612  and the other terminal of the third IDT  613  are electrically connected to a second ground wiring section  36 . 
     On the other hand, the first IDT  611  which is connected to the balanced terminals is divided into a first divided IDT  611   a  and a second divided IDT  611   b  in a surface wave propagation direction. The first divided IDT section  611   a  included in the first IDT  611  and one terminal of the fourth IDT  614  are connected to each other, and are further connected to the second balanced terminal  11  through an one-port surface acoustic wave resonator  27 . The second divided IDT section  611   b  and one terminal of the fifth IDT  615  are connected to each other, and are further connected to the second balanced terminal  11  through an one-port surface acoustic wave resonator  28 . 
     One terminal of the first IDT  611  to be connected to the ground potential and the other terminal of the fifth IDT  615  to be connected to the ground potential are electrically connected to a ground wiring section A. In addition to the ground wiring section A arranged near the unbalanced terminal  9 , the second ground wiring section  36  is arranged nearer to the first and second balanced terminals  10  and  11  relative to the 5-IDT acoustic wave filter element. The terminal of the IDT  612  and the terminal of the IDT  613  which are to be connected to the ground potential, the IDTs  612  and  613  being connected to the unbalanced terminal  9 , are connected to the second ground wiring section  36 . Furthermore, the reflectors  616  and  617  are also connected to the second ground wiring section  36 . As described above, since the reflector  616  is electrically connected to the terminal of the fourth IDT  614  to be connected to the ground potential, the terminal of the fourth IDT  614  to be connected to the ground potential is electrically connected to the ground wiring section  36 . 
     In this preferred embodiment, the first ground terminal  37  is arranged nearer to the unbalanced terminal  9  relative to a region including the first to fifth IDTs  611  to  615 , and arranged nearer to the fourth IDT  614  relative to the center among the first to fifth IDTs  611  to  615 . The fourth IDT  614 , between the first divided IDT section  611   a  and the fourth IDT  614 , which is arranged nearest to the first ground terminal  37  is connected to the first ground terminal  37  through a first ground line. Note that the first ground line extends from the terminal of the IDT  614  to be connected to the ground potential to the first ground terminal  37  through the resonator  616 , the ground wiring section  36 , the reflector  617 , a bus bar shared by the reflector  617  and the IDT  615 , and the ground wiring section A. 
     On the other hand, in this preferred embodiment, a second ground line extends from the terminal of the second divided IDT  611   b  to be connected to the ground potential, the IDT  611   b  being arranged nearer to the first ground terminal  37  than the IDT  615 , to the first ground terminal  37  through the ground wiring section A. 
     Accordingly, also in this preferred embodiment, since the first ground line is longer than the second ground line, balancing between out-of-band attenuation characteristics may be controlled by a difference of the lengths of patterns of the ground lines whereby the balancing is improved. 
     Note that,  FIG. 12  is a plan view schematically illustrating an electrode configuration of a conventional acoustic wave filter device for reference corresponding to the acoustic wave filter device of the preferred embodiment shown in  FIG. 11 . In an acoustic wave filter device  701 , one terminal of a fourth IDT  714  to be connected to the ground potential is electrically connected to a ground line unit A through a ground wiring section B. 
     Accordingly, an out-of-band attenuation characteristic of a first balanced terminal is different from an out-of-band attenuation characteristic of a second balanced terminal, and consequently, balancing between the out-of-band attenuation characteristics may be deteriorated. 
     Note that in the foregoing preferred embodiments and the modifications, the surface acoustic wave filter elements are preferably used as acoustic wave filter elements. However, instead of a surface acoustic wave, a boundary acoustic wave may be included in this preferred embodiment. That is, the acoustic wave filter devices of this preferred embodiment are preferably configured using boundary acoustic wave filter elements. 
     While preferred embodiments of the present 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 present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.