Abstract:
Disclosed is a surface acoustic wave filter having a first serial-arm SAW resonator coupled to an input terminal and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and an output terminal, wherein the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third comb-shaped interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal. Also disclosed is a branching filter utilizing the surface acoustic wave filter.

Description:
BACKGROUND OF THE INVENTION 
     (i) Field of the Invention 
     The present invention relates to a surface acoustic wave filter (hereinbelow, referred to as an SAW filter) for use in a radio frequency unit of mobile communication equipment, particularly, a portable telephone and relates to a branching filter comprising the filter. This application is a counterpart application of Japanese Application Serial Number 160636/1999, filed Jun. 8, 1999, the subject matter of which is incorporated herein by reference. 
     (ii) Description of the Related Art 
     Hitherto, as an art regarding an SAW filter, for example, there is one disclosed in “Structure of transmitting SAW filter for branching filter” of Japanese Patent Application Laid-Open No. 10-256869 (1998). FIG. 8 shows the structure of a branching filter comprising conventional SAW filters. As shown in FIG. 8, the branching filter comprising the SAW filters is constructed as described hereinbelow. An antenna-side matching circuit  7  is coupled to an antenna terminal  1  and an earth terminal  2 . Between a transmitting terminal  3 , an earth terminal  4 , and the antenna-side matching circuit  7 , a transmission-side branching circuit  8 , a transmission-side SAW resonator type filter  9 , and a transmission-side matching circuit  10  are coupled. On the other hand, between a receiving terminal  5 , an earth terminal  6 , and the antenna-side matching circuit  7 , a reception-side branching circuit  11 , a reception-side SAW resonator type filter  12 , and a reception-side matching circuit  13  are coupled. 
     FIG. 9 shows the construction of a radio frequency unit (RF unit) including a branching filter constructed by using general SAW resonators. As shown in FIG. 9, a branching filter  15  is coupled to an antenna  14 . An amplifier  16  and a reception-side SAW filter  17  are coupled to the reception side of the branching filter  15 . A power amplifier  18  and a transmission-side SAW filter  19  are coupled to the transmission side of the branching filter  15 . 
     In this instance, FIG. 10 shows the structure of the reception-side SAW resonator type filter  12  as an example. The reception-side SAW filter  12  shown in FIG. 10 has a ladder-shaped six-stage structure using SAW resonators. Between an input terminal  21  and an output terminal  22 , a first serial-arm SAW resonator  23 , a second serial-arm SAW resonator  24 , and a third serial-arm SAW resonator  25  are coupled. Between the input terminal  21  and the ground potential, a first parallel-arm SAW resonator  26  is coupled. Between the ground potential and node between the first and second serial-arm SAW resonators  23  and  24 , a second parallel-arm SAW resonator  27  is coupled. Between the ground potential and node between the second and third serial-arm SAW resonators  24  and  25 , a third parallel-arm SAW resonator  28  is coupled. Between the output terminal  22  and the ground potential, a fourth parallel-arm SAW resonator  29  is coupled. 
     As kinds of the branching filters, two kinds of branching filters, namely, a branching filter comprising dielectric filters for use in a communication terminal such as a portable telephone and an SAW branching filter for use in a card terminal can be mainly mentioned so far. Since a demand for miniaturization of a communication terminal device itself is increased in association with the spread of communication terminals such as portable telephones in recent years, a demand for miniaturization of high frequency parts built in the communication terminal device is also increased. Consequently, the SAW branching filter is used in place of the dielectric filer also in the communication terminal such as a portable telephone. Requested standards for the SAW branching filter as a device come to be equivalent to those for the branching filter comprising the dielectric filters. 
     When the SAW branching filter is used in the communication terminal such as a portable telephone, as compared with the case where it is used in the card terminal, the electric power resistance of comb teeth electrodes of the SAW filter in the SAW branching filter is regarded as important. It is strongly desired that the SAW branching filter satisfies standards regarding an amount of attenuation in the communication terminal such as a portable telephone. Further, the above-mentioned electric power resistance of the SAW branching filter is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter, namely, in a frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter. Also in the SAW branching filter using a cascade-connecting dual-mode SAW filter, while the electric power resistance of comb teeth electrodes is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of the frequency pass band regarding the reception-side SAW filter, namely, in the frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter. 
     SUMMARY OF THE INVENTION 
     According to the present invention, it is an object to provide an SAW filer in which while realizing the miniaturization of the chip size of a SAW filter, the electric power resistance of comb teeth electrodes in the SAW filter can be obtained, and an amount of attenuation in the vicinity of a frequency pass band regarding one SAW filter, namely, in a frequency pass band regarding the invention other SAW filter can be obtained as much as possible and provide a branching filter utilizing it. 
     To accomplish the above object, according to the present invention, there is provided an SAW filter comprising: a first serial-arm SAW resonator coupled to an input terminal; and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and an output terminal, wherein the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal. 
     To accomplish the above object, according to the present invention, there is provided an SAW branching filter comprising: a reception-side surface acoustic wave filter which is coupled between an antenna terminal and an amplifier; and a transmission-side surface acoustic wave filter which is coupled between the antenna terminal and a power amplifier, wherein an input terminal of the reception-side surface acoustic wave filter is coupled to the antenna terminal and an output terminal (of the reception-side surface acoustic wave filter) is coupled to the amplifier, the reception-side surface acoustic wave filter has a first serial-arm SAW resonator coupled to the input terminal and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and the output terminal, the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third comb-shaped interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the construction of an SAW filter in a first embodiment of the present invention; 
     FIG. 2 is a diagram showing the construction of an SAW filter in a second embodiment of the present invention; 
     FIG. 3 is a diagram showing the construction of an SAW filter in a third embodiment of the present invention; 
     FIG. 4 is a diagram showing the construction of an SAW filter in a fourth embodiment of the present invention; 
     FIG. 5 is a diagram showing the construction of an SAW filter in a fifth embodiment of the present invention; 
     FIG. 6 is a diagram showing the construction of an SAW filter in a sixth embodiment of the present invention; 
     FIG. 7 is a diagram showing the construction of an SAW filter in a seventh embodiment of the present invention; 
     FIG. 8 is a diagram showing the construction of a branching filter comprising conventional SAW filters; 
     FIG. 9 is a diagram showing the construction of a radio frequency unit including a branching filter comprising general SAW resonators; and 
     FIG. 10 is a diagram showing the construction of a conventional reception-side SAW resonator type filter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described hereinbelow with reference to the drawings. 
     FIG. 1 is a diagram showing the construction of a reception-side SAW filter  100  in an SAW branching filter according to a first embodiment of the present invention. As shown in FIG. 1, between an input terminal  101  and an output terminal  102 , a serial-arm SAW resonator  104  and a two-stage cascade-connecting dual-mode SAW filter  107  are coupled. Between the input terminal  101  and a ground potential V SS , a parallel-arm SAW resonator  103  is coupled. 
     The structure of the two-stage cascade-connecting dual-mode SAW filter  107  will now be described. The cascade-connecting dual-mode SAW filter  107  is constructed by cascading two dual-mode SAW filters  105  and  106 . The dual-mode SAW filter  105  is constituted of three comb-shaped interdigital transducers (hereinbelow, abbreviated to IDTs)  105 A to  105 C and reflectors  105 D and  105 E. The IDT  105 B is arranged between the IDTs  105 A and  105 C. One comb-shaped electrode of the IDT  105 B is coupled to the serial-arm SAW resonator  104  and the other comb-shaped electrode of the IDT  105 B is coupled to the ground potential V SS . One electrode of each of the IDTs  105 A and  105 C is coupled to the ground potential V SS . The other electrodes of the IDTs  105 A and  105 C are coupled to each other and also coupled to the dual-mode SAW filter  106 . The dual-mode SAW filter  106  is also constituted of three IDTs  106 A to  106 C and reflectors  106 D and  106 E. The IDT  106 B is arranged between the IDTs  106 A and  106 C. One comb-shaped electrode of the IDT  106 B is coupled to the ground potential V SS  and the other one of the IDT  106 B is coupled to the output terminal  102 . One electrode of each of the IDTs  106 A and  106 C is coupled to the ground potential V SS . The other electrodes of the IDTs  106 A and  106 C are coupled to each other and also coupled to the dual-mode SAW filter  105 . 
     The reception-side SAW filter  100  constructed as mentioned above is built in the branching filter  15  that is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, the input terminal  101  of the reception-side SAW filter  100  is coupled to the antenna side and the output terminal  102  is coupled to the amplifier  16  side. The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the reception-side SAW filter  100  constructed as mentioned above is built will now be described hereinbelow. First, the flow of the reception signal in the radio frequency unit of the communication terminal will now be explained. A radio wave signal received by the antenna  14  is branched by the branching filter  15 . After that, the branched signal is amplified by the amplifier  16  on the reception side and transmitted to the reception-side SAW filter  17 . The flow of the transmission signal in the radio frequency unit of the communication terminal will now be described. The transmission signal from the transmission-side SAW filter  19  is amplified by the power amplifier  18  and, after that, the amplified signal is transmitted as a radio wave signal from the antenna  14  via the branching filter  15 . 
     In this instance, we now pay attention to the branching filter  15 . When the communication terminal transmits the transmission signal as a radio wave signal, the amplified transmission signal is generated from the power amplifier  18 . Therefore, a large electric power is applied to the branching filter  15 . At that time, since the amplified transmission signal is directed to the reception-side SAW filter  100 , the large electric power is also applied to the reception-side SAW filter  100  in the branching filter  15  through the input terminal  101  of the reception-side SAW filter  100 . 
     In the input terminal  101  of the reception-side SAW filter  100  according to the first embodiment of the present invention, however, in order to favorably maintain the electric power resistance of the reception-side SAW filter  100 , the parallel-arm SAW resonator  103  and the serial-arm SAW resonator  104  are coupled in a ladder form. Therefore, even if the amplified transmission signal from the power amplifier  18  is directed and inputted to the reception-side SAW filter  100 , the destruction of the comb-shaped electrodes in the reception-side SAW filter  100  can be suppressed. Since the cascade-connecting dual-mode SAW filter  107  in which the dual-mode SAW filters  105  and  106  are cascaded is coupled to the output terminal  102  of the reception-side SAW filter  100 , it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter  100  in the branching filter  15 , namely, in the frequency pass band regarding the transmission-side SAW filter in the branching filter  15 . In the reception-side SAW filter  100  having the above-mentioned construction, since the number of IDTs is smaller than that of a conventional SAW filter, a degree of freedom regarding the pattern design of comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     The SAW filter  100  having the construction shown in FIG. 1 can be used as a transmission-side SAW filter in the branching filter  15  shown in FIG.  9 . That is, the input terminal  101  is coupled to the power amplifier  18  and used as an input terminal for the transmission signal amplified by the power amplifier  18 . The output terminal  102  is coupled to the antenna side and used as an output terminal of the amplified transmission signal. 
     When the transmission signal amplified by the power amplifier  18  is inputted to the branching filter  15 , a large electric power is applied to the branching filter  15 . However, since the first stage of the transmission-side SAW filter, to which the amplified transmission signal is inputted, is constructed by the parallel-arm SAW resonator  103  and the serial-arm SAW resonator  104  which are coupled in a ladder form, even if an electric power caused by the amplified transmission signal is applied to the transmission-side SAW filter, the destruction of the comb-shaped electrodes in the transmission-side SAW filter can be suppressed. Since the cascade-connecting dual-mode SAW filter  107  in which the dual-mode SAW filters  105  and  106  are cascaded is coupled on the output side of the transmission-side SAW filter, it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the transmission-side SAW filter in the branching filter  15 , namely, in a frequency pass band regarding the reception-side SAW filter in the branching filter  15 . 
     A reception-side SAW filter  200  according to a second embodiment of the present invention will now be described with reference to FIG.  2 . As shown in FIG. 2, between an input terminal  201  and an output terminal  202 , a serial-arm SAW resonator  203  and a two-stage cascade-connecting dual-mode SAW filter  207  are coupled. Between the ground potential V SS  and node between the serial-arm SAW resonator  203  and the cascade-connecting dual-mode SAW filter  207 , a parallel-arm SAW resonator  204  is coupled. The cascade-connecting dual-mode SAW filter  207  is constructed by cascading two dual-mode SAW filters  205  and  206 . The dual-mode SAW filter  205  comprises three IDTs  205 A to  205 C and two reflectors  205 D and  205 E. The IDT  205 B is arranged between the IDTs  205 A and  205 C. One comb-shaped electrode of the IDT  205 B is coupled to the serial-arm SAW resonator  203  and the parallel-arm SAW resonator  204 . The other comb-shaped electrode of the IDT  205 B is coupled to the ground potential V SS . One electrode of each of the IDTs  205 A and  205 C is connected to the ground potential V SS . The other electrodes of the IDTs  205 A and  205 C are coupled to each other and also coupled to the dual-mode SAW filter  206 . The dual-mode SAW filter  206  also comprises three IDTs  206 A to  206 C and two reflectors  206 D and  206 E. The IDT  206 B is arranged between the IDTs  206 A and  206 C. One comb-shaped electrode of the IDT  206 B is coupled to the ground potential V SS  and the other one of the IDT  206 B is coupled to the output terminal  202 . One electrode of each of the IDTs  206 A and  206 C is connected to the ground potential V SS . The other electrodes of the IDTs  206 A and  206 C are coupled to each other and also coupled to the dual-mode SAW filter  205 . 
     The reception-side SAW filter  200  constructed as mentioned above is built in the branching filter  15  that is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, the input terminal  201  of the reception-side SAW filter  200  is coupled to the antenna side and the output terminal  202  is coupled to the amplifier  16  side. The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the reception-side SAW filter  100  constructed as mentioned above is built is similar to that of the first embodiment. 
     In this instance, we now pay attention to the branching filter  15 . When the communication terminal transmits the transmission signal as a radio wave signal, the amplified transmission signal is generated from the power amplifier  18 . Therefore, a large electric power is applied to the branching filter  15 . At that time, the amplified transmission signal is directed to the reception-side SAW filter  200 , so that the large electric power is also applied to the reception-side SAW filter  200  in the branching filter  15  through the input terminal  201  of the reception-side SAW filter  200 . 
     In the reception-side SAW filter  200  according to the second embodiment of the present invention, however, in order to favorably maintain the electric power resistance of the reception-side SAW filter  200 , the parallel-arm SAW resonator  204  and the serial-arm SAW resonator  203  are coupled in a ladder form to the input terminal  201 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is directed and inputted to the reception-side SAW filter  200 , the destruction of the comb-shaped electrodes in the reception-side SAW filter  200  can be suppressed. Since the cascade-connecting dual-mode SAW filter  207  comprising the dual-mode SAW filters  205  and  206  which are cascaded is coupled to the output terminal  202  of the reception-side SAW filter  200 , it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter  200  in the branching filter  15 , namely, in a frequency pass band regarding the transmission-side SAW filter in the branching filter  15 . Also in the reception-side SAW filter  200  having the above-mentioned construction, similar to the construction of the reception-side SAW filter  100  in the first embodiment, since the number of IDTs is smaller than that of the conventional SAW filter, a degree of freedom regarding the pattern design of comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     The SAW filter  200  having the construction shown in FIG. 2 can be used as a transmission-side SAW filter in the branching filter  15  shown in FIG.  9 . That is, the input terminal  201  is coupled to the power amplifier  18  and used as an input terminal for the transmission signal amplified by the power amplifier  18 . The output terminal  202  is coupled to the antenna side and used as an output terminal of the amplified transmission signal. 
     When the transmission signal amplified by the power amplifier  18  is inputted to the branching filter  15 , a large electric power is applied to the branching filter  15 . However, since the first stage of the transmission-side SAW filter, to which the amplified transmission signal is inputted, is constructed by the parallel-arm SAW resonator  204  and the serial-arm SAW resonator  203  which are coupled in a ladder form, even if an electric power caused by the amplified transmission signal is applied to the transmission-side SAW filter, the destruction of the comb-shaped electrodes in the transmission-side SAW filter can be suppressed. Since the cascade-connecting dual-mode SAW filter  207  in which the dual-mode SAW filters  205  and  206  are cascaded is coupled on the output side of the transmission-side SAW filter, it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the transmission-side SAW filter in the branching filter  15 , namely, in a frequency pass band regarding the reception-side SAW filter in the branching filter  15 . 
     A reception-side SAW filter  300  according to a third embodiment of the present invention will now be described with reference to FIG.  3 . 
     As shown in FIG. 3, between an input terminal  301  and an output terminal  302 , a serial-arm SAW resonator  303  and a two-stage cascade-connecting dual-mode SAW filter  306  are coupled. The two-stage cascade-connecting dual-mode SAW filter  306  is constructed by cascading two dual-mode SAW filters  304  and  305 . The dual-mode SAW filter  304  is constituted of three IDTs  304 A to  304 C and two reflectors  304 D and  304 E. The IDT  304 B is arranged between the IDTs  304 A and  304 C. One comb-shaped electrode of the IDT  304 B is coupled to the serial-arm SAW resonator  303 . The other one of the IDT  304 B is coupled to the ground potential V SS . One electrode of each of the IDTs  304 A and  304 C is connected to the ground potential V SS . The other electrodes of the IDTs  304 A and  304 C are coupled to each other and also coupled to the dual-mode SAW filter  305 . The dual-mode SAW filter  305  also comprises three IDTs  305 A to  305 C and two reflectors  305 D and  305 E. The IDT  305 B is arranged between the IDTs  305 A and  305 C. One comb-shaped electrode of the IDT  305 B is coupled to the ground potential V SS  and the other one of the IDT  305 B is coupled to the output terminal  302 . One electrode of each of the IDTs  305 A and  305 C is coupled to the ground potential V SS . The other electrodes of the IDTs  305 A and  305 C are coupled to each other and also coupled to the dual-mode SAW filter  304 . 
     The reception-side SAW filter  300  constructed as mentioned above is built in the branching filter  15  that is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, the input terminal  301  of the reception-side SAW filter  300  is coupled to the antenna side and the output terminal  302  is coupled to the amplifier  16  side. The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the reception-side SAW filter  300  constructed as mentioned above is built is similar to those of the first and second embodiments. 
     In this instance, we now pay attention to the branching filter  15 . When the communication terminal transmits a transmission signal as a radio wave signal, an amplified transmission signal is generated from the power amplifier  18 . Therefore, a large electric power is applied to the branching filter  15 . At that time, the amplified transmission signal is directed to the reception-side SAW filter  300 , so that the large electric power is also applied to the reception-side SAW filter  300  in the branching filter  15  through the input terminal  301  of the reception-side SAW filter  300 . 
     In the reception-side SAW filter  300  according to the third embodiment of the present invention, however, in order to preferably maintain the electric power resistance of the reception-side SAW filter  300 , the one serial-arm SAW resonator  303  is coupled to the input terminal  301 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is directed and inputted to the reception-side SAW filter  300 , the destruction of the comb-shaped electrodes in the reception-side SAW filter  300  can be suppressed. Since the cascade-connecting dual-mode SAW filter  306  comprising the dual-mode SAW filters  304  and  305  which are cascaded is coupled to the output terminal  302  of the reception-side SAW filter  300 , it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter  300  in the branching filter  15 , namely, in a frequency pass band regarding the transmission-side SAW filter in the branching filter  15 . Also in the reception-side SAW filter  300  having the above-mentioned construction, similar to the constructions of the reception-side SAW filters  100  and  200  in the first and second embodiments, since the number of IDTs is smaller than that of the conventional SAW filter, a degree of freedom regarding the pattern design of comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     The SAW filter  300  having the construction shown in FIG. 3 can also be used as a transmission-side SAW filter in the branching filter  15  shown in FIG.  9 . In other words, the input terminal  301  is coupled to the power amplifier  18  and used as an input terminal for the transmission signal amplified by the power amplifier  18 . The output terminal  302  is coupled to the antenna side and used as an output terminal of the amplified transmission signal. 
     When the transmission signal amplified by the power amplifier  18  is inputted to the branching filter  15 , a large electric power is applied to the branching filter  15 . However, since the first stage of the transmission-side SAW filter, to which the amplified transmission signal is inputted, is constructed by the serial-arm SAW resonator  303 , even if an electric power caused by the amplified transmission signal is applied to the transmission-side SAW filter, the destruction of the comb-shaped electrodes in the transmission-side SAW filter can be suppressed. Since the cascade-connecting dual-mode SAW filter  306  in which the dual-mode SAW filters  304  and  305  are cascaded is coupled on the output side of the transmission-side SAW filter, it is possible to increase an amount of attenuation in the vicinity of a frequency pass band regarding the transmission-side SAW filter in the branching filter  15 , namely, in a frequency pass band regarding the reception-side SAW filter in the branching filter  15 . 
     An SAW filter  400  according to a fourth embodiment of the present invention will now be described with reference to FIG.  4 . The SAW filter  400  is also applicable to both of the transmission filter and the reception filter in the branching filter. 
     As shown in FIG. 4, between an input terminal  401  and an output terminal  402 , a serial-arm SAW resonator  404 , a one-stage cascade-connecting dual-mode SAW filter  405 , and a serial-arm SAW resonator  406  are coupled. A parallel-arm SAW resonator  403  is coupled between the input terminal  401  and the ground potential V SS . A parallel-arm SAW resonator  407  is coupled between the output terminal  402  and the ground potential. The one-stage cascade-connecting dual-mode SAW filter  405  is constituted of three IDTs  405 A to  405 C and two reflectors  405 D and  405 E. The IDT  405 B is arranged between the IDTs  405 A and  405 C. One comb-shaped electrode of the IDT  405 B is coupled to the serial-arm SAW resonator  404 . The other one of the IDT  405 B is coupled to the ground potential V SS . One electrode of each of the IDTs  405 A and  405 C is connected to the ground potential V SS . The other electrodes of the IDTs  405 A and  405 C are coupled to each other and also coupled to the serial-arm SAW resonator  406 . 
     The SAW filter  400  constructed as mentioned above is built as a transmission-side SAW filter and a reception-side SAW filter in the branching filter  15  which is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, when the SAW filter  400  is built as a reception-side SAW filter in the branching filter  15 , the input terminal  401  is coupled to the antenna side and the output terminal  402 , is coupled to the amplifier  16  side. On the other hand, when the SAW filter  400  is built as a transmission-side SAW filter in the branching filter  15 , the input terminal  401  is coupled to the power amplifier  18  and the output terminal  402  is coupled to the antenna side. 
     The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the transmission-side and reception-side SAW filter constructed as mentioned above is built is similar to those of the above-described first to third embodiments. 
     In this instance, we now pay attention to the branching filter  15  and consider the case where the communication terminal transmits a transmission signal as a radio wave signal. A transmission signal amplified by the power amplifier  18  is generated as a radio wave signal from the antenna through the transmission-side SAW filter. At that time, the amplified transmission signal is also directed and inputted to the reception-side SAW filter. That is, in the branching filter  15 , the amplified transmission signal is inputted not only to the transmission-side SAW filter but also to the reception side SAW filter. Therefore, a large electric power is applied to both of the transmission-side and reception-side SAW filters. 
     In the SAW filter  400  which is used as a transmission-side and reception-side SAW filter according to the fourth embodiment of the present invention, however, in order to preferably maintain the electric power resistance of the reception-side SAW filter  400 , the parallel-arm SAW resonator  403  and the serial-arm SAW resonator  404  constructed in a ladder form are coupled to the input terminal  401 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is inputted to the transmission-side and reception-side SAW filter, such a possibility that the comb-shaped electrodes in the SAW filter are broken is reduced. 
     Since the cascade-connecting dual-mode SAW filter  405  is coupled after the first stage of the SAW filter  400  that is used as a transmission-side and reception-side SAW filter, in the transmission-side filter and reception-side filter in the branching filter  15 , it is possible to increase an amount of attenuation in the vicinity of each of frequency pass bands of both the filters, namely, both of the attenuation amount of the reception-side filter in the frequency pass band regarding the transmission-side SAW filter  300  in the branching filter  15  and the attenuation amount of the transmission-side filter in the frequency pass band regarding the reception-side filter. 
     Further, also in the SAW filter  400  having the above-mentioned construction, similar to the constructions of the SAW filters in the first to third embodiments, since the number of IDTs is smaller than that of the conventional SAW filter, a degree of freedom regarding the pattern design of the comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     An SAW filter  500  according to a fifth embodiment of the present invention will now be described with reference to FIG.  5 . The SAW filter  500  is also applicable to both of the transmission filter and the reception filter in the branching filter. 
     As shown in FIG. 5, between an input terminal  501  and an output terminal  502 , a serial-arm SAW resonator  504 , a one-stage cascade-connecting dual-mode SAW filter  505 , and a serial-arm SAW resonator  507  are coupled. A parallel-arm SAW resonator  503  is coupled between the input terminal  501  and the ground potential V SS . A parallel-arm SAW resonator  506  is coupled between the one-stage cascade-connecting dual-mode SAW filter  505 , serial-arm SAW resonator  507 , and ground potential V SS . The one-stage cascade-connecting dual-mode SAW filter  505  comprises three IDTs  505 A to  505 C and two reflectors  505 D and  505 E. The IDT  505 B is arranged between the IDTs  505 A and  505 C. One comb-shaped electrode of the IDT  505 B is coupled to the serial-arm SAW resonator  504 . The other one of the IDT  505 B is coupled to the ground potential V SS . One electrode of each of the IDTs  505 A and  505 C is connected to the ground potential V SS . The other electrodes of the IDTs  505 A and  505 C are coupled to each other and also coupled to the parallel-arm SAW resonator  506  and the serial-arm SAW resonator  507 . 
     The SAW filter  500  constructed as mentioned above is built as a transmission-side SAW filter and a reception-side SAW filter in the branching filter  15  which is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, when the SAW filter  500  is built as a reception-side SAW filter in the branching filter  15 , the input terminal  501  is coupled to the antenna side and the output terminal  502  is coupled to the amplifier  16  side. On the other hand, when the SAW filter  500  is built as a transmission-side SAW filter in the branching filter  15 , the input terminal  501  is coupled to the power amplifier  18  and the output terminal  502  is coupled to the antenna side. 
     The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the transmission-side and reception-side SAW filter constructed as mentioned above is built is similar to those of the above-described first to fourth embodiments. 
     In this instance, we now pay attention to the branching filter  15  and consider the case where the communication terminal transmits a transmission signal as a radio wave signal. A transmission signal amplified by the power amplifier  18  is generated as a radio wave signal from the antenna through the transmission-side SAW filter. At that time, the amplified transmission signal is also directed and inputted to the reception-side SAW filter. In other words, in the branching filter  15 , the amplified transmission signal is inputted not only to the transmission-side SAW filter but also to the reception-side SAW filter. Accordingly, a large electric power is applied to both of the transmission-side and reception-side SAW filters. 
     In the SAW filter  500  which is used as a transmission-side and reception-side SAW filter according to the fifth embodiment of the present invention, however, in order to favorably maintain the electric power resistance of the SAW filter  500 , the parallel-arm SAW resonator  503  and the serial-arm SAW resonator  504  constructed in a ladder form are coupled to the input terminal  501 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is inputted to the transmission-side and reception-side SAW filter, such a possibility that the comb-shaped electrodes in the SAW filter are broken is reduced. 
     Since the cascade-connecting dual-mode SAW filter  505  is coupled after the first stage of the SAW filter  500  which is used as a transmission-side and reception-side SAW filter, in the transmission-side filter and the reception-side filter in the branching filter  15 , it is possible to increase an amount of attenuation in the vicinity of each of frequency pass bands of both the filters, namely, both of the attenuation amount of the reception-side filter in the frequency pass band regarding the transmission-side filter in the branching filter  15  and the attenuation amount of the transmission-side filter in the frequency pass band regarding the reception-side filter. 
     Further, also in the SAW filter  500  having the above-mentioned construction, similar to the SAW filters of the first to fourth embodiments, the number of IDTs is smaller than that of the conventional SAW filter. Accordingly, a degree of freedom regarding the pattern design of the comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     An SAW filter  600  according to a sixth embodiment of the present invention will now be described with reference to FIG.  6 . The SAW filter  600  is also applicable to both of the transmission filter and the reception filter in the branching filter. 
     As shown in FIG. 6, between an input terminal  601  and an output terminal  602 , a serial-arm SAW resonator  603 , a one-stage cascade-connecting dual-mode SAW filter  605 , and a serial-arm SAW resonator  607  are coupled. A parallel-arm SAW resonator  604  is coupled between the serial-arm SAW resonator  603 , cascade-connecting dual-mode SAW filter  605 , and ground potential V SS . On the other hand, a parallel-arm SAW resonator  606  is coupled between the cascade-connecting dual-mode SAW filter  605 , serial-arm SAW resonator  607 , and ground potential V SS . The one-stage cascade-connecting dual-mode SAW filter  605  is constituted of three IDTs  605 A to  605 C and two reflectors  605 D and  605 E. The IDT  605 B is arranged between the IDTs  605 A and  605 C. One comb-shaped electrode of the IDT  605 B is coupled to the serial-arm SAW resonator  603  and the parallel-arm SAW resonator  604 . The other one of the IDT  605 B is coupled to the ground potential V SS . One electrode of each of the IDTs  605 A and  605 C is coupled to the ground potential V SS . The other electrodes of the IDTs  605 A and  605 C are coupled to each other and also coupled to the parallel-arm SAW resonator  606  and the serial-arm SAW resonator  607 . 
     The SAW filter  600  constructed as mentioned above is built as a transmission-side SAW filter and a reception-side SAW filter in the branching filter  15  which is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, when the SAW filter  600  is built as a reception-side SAW filter in the branching filter  15 , the input terminal  601  is coupled to the antenna side and the output terminal  602  is coupled to the amplifier  16  side. On the other hand, when the SAW filter  600  is built as a transmission-side SAW filter in the branching filter  15 , the input terminal  601  is coupled to the power amplifier  18  and the output terminal  602  is coupled to the antenna side. 
     The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the transmission-side and reception-side SAW filter constructed as mentioned above is built is similar to those of the above-described first to fifth embodiments. 
     In this instance, we now pay attention to the branching filter  15  and consider the case where the communication terminal transmits a transmission signal as a radio wave signal. A transmission signal amplified by the power amplifier  18  is generated as a radio wave signal from the antenna through the transmission-side SAW filter. At that time, the amplified transmission signal is also directed and inputted to the reception-side SAW filter. In other words, in the branching filter  15 , the amplified transmission signal is inputted not only to the transmission-side SAW filter but also to the reception-side SAW filter. Accordingly, a large electric power is applied to both of the transmission-side and reception-side SAW filters. 
     In the SAW filter  600  which is used as a transmission-side and reception-side SAW filter according to the sixth embodiment of the present invention, however, in order to preferably maintain the electric power resistance of the SAW filter  600 , the serial-arm SAW resonator  603  and the parallel-arm SAW resonator  604  which are constructed in a ladder form are coupled to the input terminal  601 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is inputted to the transmission-side and reception-side SAW filter, such a possibility that the comb-shaped electrodes in the SAW filter are broken is reduced. 
     Since the cascade-connecting dual-mode SAW filter  605  is coupled after the first stage of the SAW filter  600  which is used as a transmission-side and reception-side SAW filter, in the transmission-side filter and the reception-side filter in the branching filter  15 , it is possible to increase an amount of attenuation in the vicinity of each of frequency pass bands of both the filters, namely, both the attenuation amount of the reception-side filter in the frequency pass band regarding the transmission-side filter in the branching filter  15  and the attenuation amount of the transmission-side filter in the frequency pass band regarding the reception-side filter. 
     Further, also in the SAW filter  600  having the above-mentioned construction, similar to the SAW filters of the first to fifth embodiments, the number of IDTs is smaller than that of the conventional SAW filter. Accordingly, a degree of freedom regarding the pattern design of the comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed. 
     An SAW filter  700  according to a seventh embodiment of the present invention will now be described with reference to FIG.  7 . The SAW filter  700  is also applicable to both of the transmission filter and the reception filter in the branching filter. 
     As shown in FIG. 7, between an input terminal  701  and an output terminal  702 , a serial-arm SAW resonator  703 , a one-stage cascade-connecting dual-mode SAW filter  705 , and a serial-arm SAW resonator  706  are coupled. A parallel-arm SAW resonator  704  is coupled between the serial-arm SAW resonator  703 , cascade-connecting dual-mode SAW filter  705 , and ground potential V SS . On the other hand, a parallel-arm SAW resonator  707  is coupled between the output terminal  702  and the ground potential V SS . 
     The one-stage cascade-connecting dual-mode SAW filter  705  comprises three IDTs  705 A to  705 C and two reflectors  705 D and  705 E. The IDT  705 B is arranged between the IDTs  705 A and  705 C. One comb-shaped electrode of the IDT  705 B is coupled to the serial-arm SAW resonator  703  and the parallel-arm SAW resonator  704 . The other one of the IDT  705 B is coupled to the ground potential V SS . One electrode of each of the IDTs  705 A and  705 C is connected to the ground potential V SS . The other electrodes of the IDTs  705 A and  705 C are coupled to each other and also coupled to the serial-arm SAW resonator  706 . 
     The SAW filter  700  constructed as mentioned above is built as a transmission-side SAW filter and a reception-side SAW filter in the branching filter  15  which is used for the radio frequency unit (RF unit) of the communication terminal as shown in FIG.  9 . In other words, when the SAW filter  700  is built as a reception-side SAW filter in the branching filter  15 , the input terminal  701  is coupled to the antenna side and the output terminal  702  is coupled to the amplifier  16  side. On the other hand, when the SAW filter  700  is built as a transmission-side SAW filter in the branching filter  15 , the input terminal  701  is coupled to the power amplifier  18  and the output terminal  702  is coupled to the antenna side. 
     The flow of transmission/reception signals in the communication terminal in which the branching filter  15  comprising the transmission-side and reception-side SAW filter constructed as mentioned above is built is similar to those of the above-described first to sixth embodiments. 
     In this instance, we now pay attention to the branching filter  15  and consider the case where the communication terminal transmits a transmission signal as a radio wave signal. A transmission signal amplified by the power amplifier  18  is generated as a radio wave signal from the antenna through the transmission-side SAW filter. At that time, the amplified transmission signal is also directed and inputted to the reception-side SAW filter. In other words, in the branching filter  15 , the amplified transmission signal is inputted not only to the transmission-side SAW filter but also to the reception-side SAW filter. Accordingly, a large electric power is applied to both of the transmission-side and reception-side SAW filters. 
     In the SAW filter  700  which is used as a transmission-side and reception-side SAW filter according to the seventh embodiment of the present invention, however, in order to preferably maintain the electric power resistance of the SAW filter  700 , the serial-arm SAW resonator  703  and the parallel-arm SAW resonator  704  which are constructed in a ladder form are coupled to the input terminal  701 . Accordingly, even if the amplified transmission signal from the power amplifier  18  is inputted to the transmission-side and reception-side SAW filter, such a possibility that the comb-shaped electrodes in the SAW filter are broken is reduced. 
     Since the cascade-connecting dual-mode SAW filter  705  is coupled after the first stage of the SAW filter  700  which is used as a transmission-side and reception-side SAW filter, in the transmission-side filter and the reception-side filter in the branching filter  15 , it is possible to increase an amount of attenuation in the vicinity of each of frequency pass bands of both the filters, namely, both the attenuation amount of the reception-side filter in the frequency pass band regarding the transmission-side filter in the branching filter  15  and the attenuation amount of the transmission-side filter in the frequency pass band regarding the reception-side filter. 
     Further, also in the SAW filter  700  having the above-mentioned construction, similar to the SAW filters of the first to sixth embodiments, the number of IDTs is smaller than that of the conventional SAW filter. Accordingly, a degree of freedom regarding the pattern design of the comb teeth electrodes can be improved and the miniaturization of the branching filter can be also realized, so that the manufacturing cost of the branching filter can be suppressed.