Patent Publication Number: US-2020280303-A1

Title: Multiplexer, high frequency front-end circuit, and communication apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-234486 filed on Dec. 6, 2017 and is a Continuation Application of PCT Application No. PCT/JP2018/044589 filed on Dec. 4, 2018. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multiplexer including an acoustic wave filter, a high frequency front-end circuit, and a communication apparatus. 
     2. Description of the Related Art 
     In a multiplexer configured to demultiplex or multiplex multi-band high frequency signals, the band width of a frequency band in use varies from a narrow band to a broadband, and the multiplexer includes a plurality of band pass filters corresponding to these frequency bands. For example, an acoustic wave filter corresponding to Band  30  (transmission band: 2305 MHz to 2315 MHz, reception band: 2350 MHz to 2360 MHz) of Long Term Evolution (LTE) is required to support a pass band of a narrow band (fractional band: about 0.43%). For example, an acoustic wave resonator with a resonance band width (an interval between a resonant frequency and an anti-resonant frequency) being narrow may be applied to achieve a band pass filter of a narrow band. 
     Japanese Unexamined Patent Application Publication No. 2001-203556 discloses that, as a method of narrowing a resonance band width, a thinning electrode is formed in a portion of an interdigital transducer (IDT) electrode of a surface acoustic wave resonator, so that an electromechanical coupling coefficient of the surface acoustic wave resonator is substantially reduced to narrow the resonance band width. 
     However, as in Japanese Unexamined Patent Application Publication No. 2001-203556, when a thinning electrode is formed in a portion of the IDT electrode to achieve a narrow band filter, a periodic structure by the thinning electrode is formed separately from a periodic structure of the IDT electrode pitch. Accordingly, a frequency response corresponding to the periodic structure by the thinning electrode, which is different from a frequency response of the main mode corresponding to the periodic structure of the IDT electrode pitch, is generated, and the generated frequency response appears as a spurious signal (unwanted wave) outside the pass band. 
     When a multiplexer is formed by an acoustic wave filter including an acoustic wave resonator including an IDT electrode in which the above-described thinning electrode is formed, the spurious signal appears in the acoustic wave filter. There is a problem that the spurious signal causes a ripple to be generated in a pass band of another filter connected to a common terminal, and thus insertion loss in the band of another filter is increased. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide multiplexers, high frequency front-end circuits, and communication apparatuses that are each able to provide, while narrowing a band of one filter, low loss of another filter in the multiplexer in which a plurality of filters are electrically connected to a common terminal. 
     A multiplexer according to a preferred embodiment of the present invention includes a common terminal, a first input-output terminal, and a second input-output terminal; a first filter electrically connected to the common terminal and the first input-out terminal; and a second filter having a pass band different from that of the first filter, and electrically connected to the common terminal and the second input-output terminal. The first filter includes a plurality of series arm resonators provided on a path connecting the common terminal and the first input-output terminal, and a plurality of parallel arm resonators provided between the above-mentioned path and the ground. Each of the plurality of series arm resonators and the plurality of parallel arm resonators is an acoustic wave resonator including an interdigital transducer (IDT) electrode provided on a substrate having piezoelectricity. A capacitance element is electrically connected in parallel to at least one of a first series arm resonator, which is a series arm resonator connected most proximately to the common terminal among the plurality of series arm resonators, and a first parallel arm resonator, which is a parallel arm resonator connected most proximately to the common terminal among the plurality of parallel arm resonators, and the IDT electrode included in the at least one of the first series arm resonator and the first parallel arm resonator does not include a thinning electrode. The IDT electrode included in at least one of the series arm resonators excluding the first series arm resonator among the plurality of series arm resonators and the parallel arm resonators excluding the first parallel arm resonator among the plurality of parallel arm resonators, includes a thinning electrode. 
     Accordingly, a thinning electrode is provided in at least one of the acoustic wave resonators excluding the first series arm resonator and the first parallel arm resonator in close proximity to the common terminal in the first filter, and a capacitance element is electrically connected in parallel to at least one of the first series arm resonator and the first parallel arm resonator. As described above, by providing the thinning electrode in the acoustic wave resonator or connecting the capacitance element in parallel to the acoustic wave resonator, the band of the first filter is able to be narrowed. 
     On the other hand, it is assumed that a spurious signal is generated outside the pass band due to the thinning electrode provided in the acoustic wave resonator of the first filter, and that a frequency of the spurious signal generation is included in the pass band of the second filter. In contrast, since a thinning electrode is not provided in at least one of the first series arm resonator and the first parallel arm resonator, which are branches near the common terminal of the first filter, an increase in return loss in the pass band of the second filter is able to be significantly reduced or prevent when the first filter is seen from the common terminal. This is because an influence on reflection characteristics becomes larger when the first filter is seen from the common terminal, as a branch becomes closer to the common terminal of the first filter. Accordingly, the deterioration in bandpass characteristics of the second filter caused by an unwanted wave generated by the first filter is able to be significantly reduced or prevented. Accordingly, in a multiplexer in which a plurality of filters are electrically connected to the common terminal, while narrowing the band of one filter, low loss of another filter is able to be provided. 
     No capacitance element may be electrically connected in parallel to each of the series arm resonators excluding the first series arm resonator among the plurality of series arm resonators, and each of the parallel arm resonators excluding the first parallel arm resonator among the plurality of parallel arm resonators. 
     By connecting the capacitance element in parallel to an acoustic wave resonator, the resonance band width of the acoustic wave resonator may be reduced, but a Q-value of the acoustic wave resonator decreases caused by a Q-value of the capacitance element. Accordingly, the location of the capacitance element that narrows the band of the first filter is limited to at least one of the first series arm resonator and the first parallel arm resonator, and no capacitance element is provided in each of the series arm resonators excluding the first series arm resonator and each of the parallel arm resonators excluding the first parallel arm resonator. Accordingly, in a multiplexer in which a plurality of filters are electrically connected to a common terminal, while reducing the loss of and narrowing the band of one filter, low loss of another filter is able to be provided. 
     Each of the series arm resonators excluding the first series arm resonator among the plurality of series arm resonators and each of the parallel arm resonators excluding the first parallel arm resonator among the plurality of parallel arm resonators, may include a thinning electrode. 
     By connecting the capacitance element in parallel to the acoustic wave resonator, the resonance band width of the acoustic wave resonator may be reduced, but the Q-value of the acoustic wave resonator decreases caused by the Q-value of the capacitance element. Accordingly, the location of the capacitance element that narrows the band of the first filter is limited to at least one of the first series arm resonator and the first parallel arm resonator, and a thinning electrode is provided in each of the series arm resonators excluding the first series arm resonator and each of the parallel arm resonators excluding the first parallel arm resonator. Accordingly, in a multiplexer in which a plurality of filters is electrically connected to a common terminal, while reducing the loss of and further narrowing the band of one filter, low loss of another filter is able to be provided. 
     The capacitance element may include a comb-tooth electrode provided on the substrate. 
     Accordingly, a circuit in which the acoustic wave resonator and the capacitance element are electrically connected in parallel is able to be significantly reduced in size, and thus the multiplexer may be significantly reduced in size. 
     The comb-tooth electrode may include a plurality of electrode fingers in parallel or substantially in parallel to one another and a pair of busbars that oppose each other across the plurality of electrode fingers. The plurality of electrode fingers may be provided along a propagation direction of the acoustic wave in the IDT electrode, and may also be provided periodically along a direction orthogonal or substantially orthogonal to the propagation direction. 
     Accordingly, a situation in which an unwanted wave generated in the capacitance element interferes with the acoustic wave propagating through the IDT electrode is able to be significantly reduced or prevented. 
     The substrate may include a piezoelectric film including the IDT electrode provided on one surface of the piezoelectric film, a high acoustic velocity support substrate through which a bulk wave propagates at a higher acoustic velocity than an acoustic velocity of the acoustic wave that propagates through the piezoelectric film, and a low acoustic velocity film which is provided between the high acoustic velocity support substrate and the piezoelectric film, and through which a bulk wave propagates at a lower acoustic velocity than an acoustic velocity of a bulk wave that propagates through the piezoelectric film. 
     When a capacitance element is electrically connected in parallel to an acoustic wave resonator, it is assumed that the Q-value of the acoustic wave resonator is decreased equivalently. However, according to a laminated structure of the substrate, the Q-value of the acoustic wave resonator is able to be maintained at a high value. Therefore, an acoustic wave filter exhibiting low loss in the pass band is able to be provided. 
     The multiplexer may include a first duplexer provided with two filters including the first filter, and a second duplexer provided with two filters including the second filter. 
     Thus, in the multiplexer including a plurality of duplexers, the deterioration in bandpass characteristics of the second filter caused by the unwanted wave generated by the first filter is able to be significantly reduced or prevented while narrowing the band of the first filter. 
     The pass band of the first filter may be an uplink frequency band in Band  30  of Long Term Evolution (LTE), and the pass band of the second filter may be an uplink frequency band in Band  25  of the LTE. 
     When the pass band of the first filter is an uplink frequency band in Band  30  of the LTE, and the pass band of the second filter is an uplink frequency band in Band  25  of the LTE, it is assumed that a spurious signal generation frequency of the first filter is located within the pass band of the second filter in some cases. Accordingly, the increase in the return loss in the pass band of the second filter is able to be significantly reduced or prevented when the first filter is seen from the common terminal. Accordingly, the deterioration in bandpass characteristics of the second filter caused by the spurious signal generated by the first filter is able to be significantly reduced or prevented while applying the first filter to Band  30 , which is a narrow band. 
     A high frequency front-end circuit according to a preferred embodiment of the present invention includes a multiplexer according to a preferred embodiment of the present invention, and an amplifier circuit electrically connected to the multiplexer. 
     Accordingly, a high frequency front-end circuit is able to be provided in which, while narrowing the band of one filter defining a multiplexer, low loss of another filter defining the multiplexer is provided. 
     A communication apparatus according to a preferred embodiment of the present invention includes an RF signal processing circuit that processes a high frequency signal transmitted and received by an antenna element, and a high frequency front-end circuit according to a preferred embodiment of the present invention that transmits the high frequency signal between the antenna element and the RF signal processing circuit. 
     Accordingly, a communication apparatus is able to be provided in which, while narrowing the band of one filter defining a multiplexer, low loss of another filter defining the multiplexer is provided. 
     According to the multiplexers, the high frequency front-end circuits, and the communication apparatuses according to preferred embodiments of the present invention, while narrowing the band of one filter, low loss of another filter is able to be provided. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a multiplexer and its peripheral circuit according to a first preferred embodiment of the present invention. 
         FIG. 2A  includes a plan view and a cross-sectional view showing an example of an acoustic wave resonator according to the first preferred embodiment of the present invention. 
         FIG. 2B  is a cross-sectional view showing an acoustic wave resonator according to a first modification of the first preferred embodiment of the present invention. 
         FIG. 3  is a circuit diagram of a transmission-side filter of Band A defining a multiplexer according to a working example. 
         FIG. 4  is a circuit diagram of a transmission-side filter of Band A defining a multiplexer according to a second modification of the first preferred embodiment of the present invention. 
         FIG. 5A  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of a transmission-side filter of Band A according to a working example. 
         FIG. 5B  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of a transmission-side filter of Band A according to a third modification of the first preferred embodiment of the present invention. 
         FIG. 5C  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of a transmission-side filter of Band A according to a fourth modification of the first preferred embodiment of the present invention. 
         FIG. 6  is a plan view showing electrodes of a first stage-side acoustic wave resonator of a transmission-side filter of Band A according to a working example. 
         FIG. 7  is a circuit diagram of a transmission-side filter of Band A defining a multiplexer according to a comparative example. 
         FIG. 8A  is a graph showing bandpass characteristics of a transmission-side filter of Band A defining a multiplexer according to a working example. 
         FIG. 8B  is a graph showing bandpass characteristics of a transmission-side filter of Band A defining a multiplexer according to a comparative example. 
         FIG. 9A  is a graph showing bandpass characteristics of a transmission-side filter of Band B defining a multiplexer according to a working example. 
         FIG. 9B  is a graph showing bandpass characteristics of a transmission-side filter of Band B defining a multiplexer according to a comparative example. 
         FIG. 10A  includes graphs showing impedance characteristics and reflection characteristics, respectively, of an acoustic wave resonator including a thinning electrode and having no capacitance element connected in parallel. 
         FIG. 10B  includes graphs showing impedance characteristics and reflection characteristics, respectively, of an acoustic wave resonance circuit including no thinning electrode and having a capacitance element connected in parallel. 
         FIG. 11  is a diagram describing a relationship between a branch of a ladder acoustic wave filter and reflection characteristics thereof. 
         FIG. 12  is a diagram of a communication apparatus according to a second preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with respect to the preferred embodiments and drawings. It is to be noted that any of the preferred embodiments described below represents a general or specific example. Numeric values, shapes, materials, elements, structure, locations, and connections of the elements, and the like described in the following preferred embodiments are mere examples and are not intended to limit the present invention. Of the elements in the following preferred embodiments, elements that are not described in the independent claims are described as arbitrary or optional elements. The sizes or the size ratios of the components and elements shown in the drawings are not necessarily strict. 
     First Preferred Embodiment 
     1. Basic Configuration of Multiplexer 
       FIG. 1  is a diagram of a multiplexer  1  and its peripheral circuit according to a first preferred embodiment of the present invention. As shown in  FIG. 1 , the multiplexer  1  includes transmission-side filters  10  and  12 , reception-side filters  11  and  13 , a common terminal  90 , transmission input terminals  91  and  93 , and reception output terminals  92  and  94 . The multiplexer  1  is connected to an antenna element  2  at the common terminal  90 . An inductance element  30  providing impedance matching is connected between a connection path between the common terminal  90  and the antenna element  2 , and the ground defining and functioning as a reference terminal. The inductance element  30  may be connected in series between the common terminal  90  and the antenna element  2 . The multiplexer  1  may not include the inductance element  30 . The inductance element  30  may be included in the multiplexer  1 , or may be externally attached to the multiplexer  1 . 
     The transmission-side filter  10  is a first filter that is connected to the common terminal  90  and the transmission input terminal  91  (first input-output terminal), inputs a transmission wave generated by a transmission circuit (for example, an RFIC) via the transmission input terminal  91 , and performs filtering on the transmission wave in a transmission pass band of Band A to output the filtered transmission wave to the common terminal  90 . The transmission-side filter  10  is preferably, for example, a surface acoustic wave filter defined by an acoustic wave resonator, and includes a plurality of series arm resonators provided on a path connecting the common terminal  90  and the transmission input terminal  91 , and a plurality of parallel arm resonators provided between the above path and the ground. 
     The reception-side filter  11  is a filter that is connected to the common terminal  90  and the reception output terminal  92 , inputs a reception wave inputted from the common terminal  90 , and performs filtering on the reception wave in a reception pass band of Band A to output the filtered reception wave to the reception output terminal  92 . The reception-side filter  11  is not particularly limited, and may preferably be, for example, an acoustic wave filter, or may be an LC filter including an inductance element and a capacitance element. 
     The transmission-side filter  12  is a second filter that is connected to the common terminal  90  and the transmission input terminal  93  (second input-output terminal), inputs a transmission wave generated by a transmission circuit (for example, an RFIC) via the transmission input terminal  93  (second input-output terminal), and performs filtering on the transmission wave in a transmission pass band of Band B, which is different from Band A), to output the filtered transmission wave to the common terminal  90 . The transmission-side filter  12  is not particularly limited, and may preferably be, for example, an acoustic wave filter, or may be an LC filter including an inductance element and a capacitance element. 
     The reception-side filter  13  is a filter that is connected to the common terminal  90  and the reception output terminal  94 , is inputted with a reception wave inputted from the common terminal  90 , and performs filtering on the reception wave in a reception pass band of Band B to output the filtered reception wave to the reception output terminal  94 . The reception-side filter  13  is not particularly limited, and may preferably be, for example, an acoustic wave filter, or may be an LC filter including an inductance element and a capacitance element. 
     At least one of an inductance element and a capacitance element providing impedance matching may be connected between the common terminal  90  and each of the above-described filters. 
     Hereinafter, the structure of an acoustic wave resonator defining the transmission-side filter  10  is described. 
     2. Structure of Acoustic Wave Resonator 
       FIG. 2A  is a diagram showing an example of an acoustic wave resonator according to the first preferred embodiment, where part (a) of  FIG. 2A  is a plan view, and parts (b) and (c) of  FIG. 2A  are each a cross-sectional view taken along a dot-dash line shown in part (a) of  FIG. 2A . In  FIG. 2A , there are exemplified a plan view and cross-sectional views showing the structure of a series arm resonator  101 , which is connected most proximately to the common terminal  90  among the plurality of series arm resonators and the plurality of parallel arm resonators defining the transmission-side filter  10 . The series arm resonator  101  shown in  FIG. 2A  provides an example of a structure of the plurality of acoustic wave resonators, and the number, length, and the like of electrode fingers defining the electrode are not limited thereto. 
     The series arm resonator  101  includes a substrate  5  having piezoelectricity and comb-shaped electrodes  101   a  and  101   b.    
     As shown in part (a) of  FIG. 2A , a pair of comb-shaped electrodes  101   a  and  101   b  opposing each other is provided on the substrate  5 . The comb-shaped electrode  101   a  is defined by a plurality of electrode fingers  121   a  parallel or substantially parallel to one another and a busbar electrode  111   a  connecting the plurality of electrode fingers  121   a . The comb-shaped electrode  101   b  is defined by a plurality of electrode fingers  121   b  parallel or substantially parallel to one another and a busbar electrode  111   b  connecting the plurality of electrode fingers  121   b . The plurality of electrode fingers  121   a  and the plurality of electrode fingers  121   b  extend along a direction orthogonal or substantially orthogonal to an acoustic wave propagation direction (X-axis direction). 
     An interdigital transducer (IDT) electrode  54  defined by the plurality of electrode fingers  121   a , the plurality of electrode fingers  121   b , and the busbar electrodes  111   a  and  111   b  has, for example, a laminated structure of a close contact layer  541  and a main electrode layer  542 , as shown in  FIG. 2A (b). 
     The close contact layer  541  is a layer that significantly improves a close contact property between the substrate  5  and the main electrode layer  542 , and Ti, for example, is preferably used as a material. The close contact layer  541  preferably has a film thickness of, for example, about 12 nm. 
     The main electrode layer  542  preferably includes, as a material thereof, Al including about 1% of Cu, for example. The film thickness of the main electrode layer  542  is preferably, for example, about 162 nm. 
     A protective layer  55  covers the comb-shaped electrodes  101   a  and  101   b . The protective layer  55  is a layer to protect the main electrode layer  542  from the external environment, to adjust frequency temperature characteristics, to significantly increase moisture resistance, and the like, and is preferably, for example, a dielectric film mainly including silicon dioxide. The protective layer  55  preferably has a thickness of, for example, about 25 nm. 
     Materials that define the close contact layer  541 , the main electrode layer  542 , and the protective layer  55  are not limited to those described above. The IDT electrode  54  may not have the above-described laminated structure. The IDT electrode  54  may be made of a metal, for example, Ti, Al, Cu, Pt, Au, Ag, Pd or the like, or an alloy thereof, and may include a plurality of multilayer bodies including the above-mentioned metal or alloy, for example. Note that the protective layer  55  may not be provided. 
     Next, a laminated structure of the substrate  5  is described. 
     As shown in part (c) of  FIG. 2A , the substrate  5  includes a high acoustic velocity support substrate  51 , a low acoustic velocity film  52  and a piezoelectric film  53 , and has a structure in which the high acoustic velocity support substrate  51 , the low acoustic velocity film  52 , and the piezoelectric film  53  are laminated in this order. 
     The piezoelectric film  53  preferably includes a 50° Y-cut X-propagation LiTaO 3  piezoelectric single crystal or piezoelectric ceramics (a lithium tantalate single crystal or ceramics being cut with a surface which takes, as its normal line, an axis rotated by about 50° from a Y-axis about an X-axis, that is, a single crystal or ceramics through which surface acoustic waves propagate in the X-axis direction). The piezoelectric film  53  preferably has, for example, a thickness of about 600 nm. In accordance with predetermined specifications of each of the filters, a material and cut-angles of a piezoelectric single crystal to be used as the piezoelectric film  53  are appropriately selected. 
     The high acoustic velocity support substrate  51  supports the low acoustic velocity film  52 , the piezoelectric film  53 , and the IDT electrode  54 . The high acoustic velocity support substrate is also a substrate where acoustic velocity of a bulk wave propagating therein is higher than that of an acoustic wave, for example, a surface acoustic wave, a boundary wave and the like propagating in the piezoelectric film  53 , and confines the surface acoustic wave in a portion where the piezoelectric film  53  and the low acoustic velocity film  52  are laminated, thus preventing the surface acoustic wave from leaking toward a lower side relative to the high acoustic velocity support substrate  51 . The high acoustic velocity support substrate  51  is preferably, for example, a silicon substrate, and has a thickness of, for example, about 200 μm. 
     The low acoustic velocity film  52  is a film where the acoustic velocity of a bulk wave propagating therein is lower than that of a bulk wave propagating in the piezoelectric film  53 , and is provided between the piezoelectric film  53  and the high acoustic velocity support substrate  51 . Due to the above-described structure and the energy of the acoustic wave concentrating into a low acoustic velocity medium, leakage of surface acoustic wave energy to an outside of the IDT electrode is significantly reduced or prevented. The low acoustic velocity film  52  is preferably, for example, a film mainly including silicon dioxide, and has a thickness of, for example, about 670 nm. 
     According to the above-described laminated structure of the substrate  5 , the Q-value at the resonant frequency and the anti-resonant frequency may be significantly increased as compared to a known structure in which a piezoelectric substrate is used in a single layer. That is, since the acoustic wave resonator having a high Q-value may be provided, a filter with low insertion loss is able to be provided by using the acoustic wave resonator. 
     As described herein, when a capacitance element is connected in parallel to the series arm resonator  101  in order to narrow the band of the transmission-side filter  10 , it is assumed that the Q-value of the series arm resonator  101  is lowered equivalently caused by the Q-value of the capacitance element in some cases. However, according to the laminated structure of the substrate described above, the Q-value of the series arm resonator  101  is able to be maintained at a high value. Therefore, the acoustic wave filter exhibiting low loss in the pass band is able to be provided. 
     The high acoustic velocity support substrate  51  may have a structure in which a support substrate and a high acoustic velocity film where the acoustic velocity of the bulk wave propagating therein is higher than that of the acoustic wave, for example, a surface acoustic wave, a boundary wave or the like, propagating in the piezoelectric film  53 , are laminated. In this case, the support substrate may include a piezoelectric body, for example, lithium tantalate, lithium niobate, quartz or the like, various ceramics, for example, alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, forsterite or the like, a dielectric, for example, sapphire, glass or the like, or a semiconductor, for example, silicon or gallium nitride, and a resin substrate or the like. For the high acoustic velocity film, various high acoustic velocity materials may be used, for example, aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, a DLC film or diamond, a medium including the above-described material as a main component, a medium including a mixture of the above materials as a main component and the like. 
       FIG. 2B  is a cross-sectional view showing an acoustic wave resonator according to a first modification of the first preferred embodiment. In the series arm resonator  101  shown in  FIG. 2A , an example in which the IDT electrode  54  is provided on the substrate  5  having the piezoelectric film  53  is described, but the substrate on which the IDT electrode  54  is provided may be a piezoelectric single crystal substrate  57  including a single layer of a piezoelectric body layer as shown in  FIG. 2B . The piezoelectric single crystal substrate  57  preferably includes, for example, a piezoelectric single crystal of LiNbO 3 . A series arm resonator  101  according to the present modification includes the piezoelectric single crystal substrate  57  of LiNbO 3 , an IDT electrode  54 , and a protective layer  55  provided on the piezoelectric single crystal substrate  57  and on the IDT electrode  54 . 
     In the piezoelectric film  53  and the piezoelectric single crystal substrate  57  described above, the laminated structure, material, cut-angles, and thickness may be appropriately changed in accordance with predetermined bandpass characteristics or the like of an acoustic wave filter device. Even in the series arm resonator  101  using, for example, a LiTaO 3  piezoelectric substrate having cut-angles other than the cut-angles described above, the same or similar effect as that of the series arm resonator  101  using the above-described piezoelectric film  53  may be provided. 
     Hereinafter, an example (working example) of electrode parameters of an IDT electrode defining an acoustic wave resonator is described. 
     The wavelength of an acoustic wave resonator is defined by a wavelength λ, which is a cycle period of the plurality of electrode fingers  121   a  or  121   b  defining the IDT electrode  54  as shown in part (b) of  FIG. 2A . An electrode pitch is half the wavelength λ, and is defined as (W+S) when a line width of each of the electrode fingers  121   a  and  121   b  defining the comb-shaped electrodes  101   a  and  101   b  is W, and a space width between the adjacent electrode fingers  121   a  and  121   b  is S. An intersecting width L of the pair of comb-shaped electrodes  101   a  and  101   b  is a length of overlapping electrode fingers when viewed from the acoustic wave propagation direction (X-axis direction) of the electrode fingers  121   a  and  121   b , as shown in part (a) of  FIG. 2A . An electrode duty of each acoustic wave resonator is a line width occupancy rate of the plurality of electrode fingers  121   a  and the plurality of electrode fingers  121   b , is a ratio of the line widths of the plurality of electrode fingers  121   a  and plurality of electrode fingers  121   b  to the addition value of the line widths and the space widths, and is defined as W/(W+S). The height of the comb-shaped electrodes  101   a  and  101   b  is defined as h. Parameters that determine the shape and size of the acoustic wave resonator, for example, the wavelength λ, the intersecting width L, the electrode duty, the height h of the IDT electrode  54  and the like, are referred to as resonator parameters. 
     3. Configuration of Transmission-Side Filter  10  According to Embodiment 
     Hereinafter, circuitry of a transmission-side filter  10  is described with reference to  FIG. 3 . 
       FIG. 3  is a circuit diagram of the transmission-side filter  10  according to a working example. As shown in the drawing, the transmission-side filter  10  includes series arm resonators  101 ,  102 ,  103  and  104 , parallel arm resonators  201 ,  202  and  203 , capacitance elements C 1  and C 2 , and inductance elements L 1  and L 2 . 
     The series arm resonators  101  to  104  are provided on a path connecting the common terminal  90  and the transmission input terminal  91 , and are connected in series to each other. The parallel arm resonators  201  to  203  are provided between nodes on the above-described path and a reference terminal (ground). The transmission-side filter  10  defines a ladder band pass filter by the above-described connections of the series arm resonators  101  to  104  and the parallel arm resonators  201  to  203 . 
     The inductance element L 1  is connected between the parallel arm resonator  202  and the ground, and the inductance element L 2  is connected between the parallel arm resonator  203  and the ground. An attenuation pole near the pass band of the transmission-side filter  10  is adjusted by the inductance elements L 1  and L 2 . 
     Note that the transmission-side filter  10  has a pass band as a narrow band, and the fractional band is substantially equal to or less than about 1%, for example. In order to support such narrow band filter characteristics, the transmission-side filter  10  is defined by an acoustic wave resonance circuit having a narrow resonance band width (a narrow interval between the resonant frequency and the anti-resonant frequency). 
     More specifically, the capacitance element C 1  is connected in parallel to the series arm resonator  101 , and the capacitance element C 2  is connected in parallel to the parallel arm resonator  201 . In other words, the series arm resonator  101  and the capacitance element C 1  are connected in parallel at nodes n 1  and n 2 , and the parallel arm resonator  201  and the capacitance element C 2  are connected in parallel at nodes n 3  and n 4 . Accordingly, the resonance band width of the series arm circuit where the series arm resonator  101  and the capacitance element C 1  are connected in parallel is smaller than the resonance band width of the series arm resonator  101  alone. In addition, the resonance band width of the parallel arm circuit where the parallel arm resonator  201  and the capacitance element C 2  are connected in parallel is smaller than the resonance band width of the parallel arm resonator  201  alone. Note that, as shown in  FIG. 2A , the IDT electrodes included in the series arm resonator  101  and the parallel arm resonator  201  are provided on the substrate  5  and do not include a thinning electrode. 
     On the other hand, the IDT electrodes included in the series arm resonators  102 ,  103  and  104 , and the parallel arm resonators  202  and  203  are provided on the substrate  5  and include a thinning electrode as shown in  FIGS. 5A to 5C , which is described herein. Accordingly, the resonance band widths of the series arm resonators  102 ,  103  and  104 , and the parallel arm resonators  202  and  203  are smaller than those of the acoustic wave resonators in which a thinning electrode is not provided. 
     Accordingly, since the resonance band width of the acoustic wave resonance circuit (or the acoustic wave resonator) defining the transmission-side filter  10  becomes relatively small, the band of the transmission-side filter  10  is able to be narrowed. 
     Features of the transmission-side filter  10  are described below. 
     (1) The capacitance element C 1  is connected in parallel to the series arm resonator  101  (a first series arm resonator: referred to as a first stage-side acoustic wave resonator in some cases), which is connected most proximately to the common terminal  90  among the plurality of series arm resonators  101  to  104 , and the IDT electrode of the series arm resonator  101  does not include a thinning electrode. 
     (2) The capacitance element C 2  is connected in parallel to the parallel arm resonator  201  (a first parallel arm resonator: referred to as the first stage-side acoustic wave resonator in some cases), which is connected most proximately to the common terminal  90  among the plurality of parallel arm resonators  201  to  203 , and the IDT electrode of the parallel arm resonator  201  does not include a thinning electrode. 
     (3) The IDT electrodes included in the series arm resonators  102  to  104  (referred to as subsequent-side acoustic wave resonators in some cases) among the plurality of series arm resonators  101  to  104  excluding the series arm resonator  101 , include a thinning electrode. 
     (4) The IDT electrodes included in the parallel arm resonators  202  and  203  (referred to as the subsequent-side acoustic wave resonators in some cases) among the plurality of parallel arm resonators  201  to  203  excluding the parallel arm resonator  201 , include a thinning electrode. 
     According to the features (1) to (4) described above, since the capacitance elements are connected in parallel to, or the thinning electrodes are provided in the acoustic wave resonators defining the transmission-side filter  10 , the band of the transmission-side filter  10  is able to be narrowed. 
     On the other hand, a spurious signal (unwanted wave) is generated outside the pass band (Band ATx) caused by the thinning electrodes provided in the series arm resonators  102  to  104  and the parallel arm resonators  202  to  203  of the transmission-side filter  10 . In this case, it is assumed that the spurious signal generation frequency is included in the pass band (Band BTx) of the transmission-side filter  12 , for example. In contrast, since the thinning electrodes are not provided in the series arm resonator  101  and the parallel arm resonator  201 , which are branches (first stage-side acoustic wave resonators) of the transmission-side filter  10  near the common terminal  90 , an increase in return loss in the pass band of the transmission-side filter  12  is able to be significantly reduced or prevented when the transmission-side filter  10  is seen from the common terminal  90 . Accordingly, the deterioration in bandpass characteristics of the transmission-side filter  12  caused by the spurious signal generated by the transmission-side filter  10  is able to be significantly reduced or prevented. Accordingly, in the multiplexer  1  in which the plurality of filters is connected to the common terminal  90 , low loss of the transmission-side filter is able to be provided while narrowing the band of the transmission-side filter  10 . 
     Note that in the present working example, the capacitance element is connected in parallel to both the series arm resonator  101  and the parallel arm resonator  201  connected most proximately to the common terminal  90 . However, the capacitance element may be connected in parallel to only one of the series arm resonator  101  and the parallel arm resonator  201 . 
       FIG. 4  is a circuit diagram of a transmission-side filter  14  defining a multiplexer according to a second modification of the first preferred embodiment. The multiplexer according to the second modification includes the transmission-side filter  14  instead of the transmission-side filter  10  of Band A defining the multiplexer  1  according to the working example. 
     The transmission-side filter  14  according to the present modification is a first filter that is connected to the common terminal  90  and the transmission input terminal  91  (first input-output terminal), inputs a transmission wave generated by a transmission circuit (for example, an RFIC) via the transmission input terminal  91 , and performs filtering on the transmission wave in a transmission pass band of Band A to output the filtered transmission wave to the common terminal  90 . As shown in  FIG. 4 , the transmission-side filter  14  includes the series arm resonators  101 ,  102 ,  103  and  104 , the parallel arm resonators  201 ,  202  and  203 , a capacitance element C 3 , and the inductance elements L 1  and L 2 . The transmission-side filter  14  according to the present modification is different from the transmission-side filter  10  according to the working example in that the capacitance element C 3  is connected in parallel to only the series arm resonator  101 , and a capacitor element is not connected in parallel to the parallel arm resonator  201 . Even with the structure described above, low loss of the transmission-side filter  12  is able to be provided while narrowing the band of the transmission-side filter  10 , in comparison with a capacitance element being connected in parallel to neither the series arm resonator  101  nor the parallel arm resonator  201 . 
     In the present working example and modification, the thinning electrode is provided in each of the series arm resonators  102  to  104  and the parallel arm resonators  202  and  203 . However, the thinning electrode may be provided in at least one of the series arm resonators  102  to  104  and the parallel arm resonators  202  and  203 . Accordingly, low loss of the transmission-side filter is able to be provided while narrowing the band of the transmission-side filter  10 , in comparison with a thinning electrode not being provided in any of the series arm resonators  102  to  104  and the parallel arm resonators  202  and  203 . 
     In the present working example and modification, no capacitance elements are connected in parallel to the series arm resonators  102  to  104  and the parallel arm resonators  202  and  203 . That is, the capacitance element is not connected in parallel to any of the series arm resonators excluding the series arm resonator  101  connected most proximately to the common terminal  90  (subsequent-side acoustic wave resonators), and to any of the parallel arm resonators excluding the parallel arm resonator  201  connected most proximately to the common terminal  90  (subsequent-side acoustic wave resonators). 
     By connecting the capacitance element in parallel to each of the acoustic wave resonators, the resonance band width of the acoustic wave resonance circuit where the acoustic wave resonator and the capacitance element are connected in parallel may be reduced, but the Q-value of the acoustic wave resonance circuit is lowered caused by the Q-value of the capacitance element, so that the insertion loss of the circuit increases. Accordingly, the location of a parallel capacitance element that narrows the band of the transmission-side filter  10  is limited to the series arm resonator  101  and the parallel arm resonator  201  (first stage-side acoustic wave resonators), and the parallel capacitance element is not provided in each of the series arm resonators excluding the series arm resonator  101  and each of the parallel arm resonators excluding the parallel arm resonator  201  (subsequent-side acoustic wave resonators). Accordingly, in the multiplexer  1  in which the plurality of filters is connected to the common terminal  90 , low loss of the transmission-side filter is able to be provided while reducing the loss of the transmission-side filter  10  and narrowing the band of the transmission-side filter  10 . 
     4. Configuration of Thinning Electrode and Capacitance Element 
     Hereinafter, a thinning electrode and a capacitance element are described with respect to  FIG. 5A  to  FIG. 6 . 
       FIG. 5A  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of the transmission-side filter  10  according to the working example.  FIG. 5A  exemplifies a plan view showing an IDT electrode structure of the series arm resonator  102  as a representative of the resonators (the subsequent-side acoustic wave resonators) excluding both the series arm resonator  101  and the parallel arm resonator  201  connected most proximately to the common terminal  90 . Note that the series arm resonator  102  shown in  FIG. 5A  provides an example of a structure of the subsequent-side acoustic wave resonator, and the number, length, and the like of electrode fingers defining the electrode are not limited thereto. 
     The series arm resonator  102  includes the substrate  5  having piezoelectricity, comb-shaped electrodes  102   a  and  102   b  provided on the substrate  5 , and reflectors  142 . 
     As shown in  FIG. 5A , the comb-shaped electrode  102   a  includes a plurality of electrode fingers  122   a  parallel or substantially parallel to one another, and a busbar electrode  112   a  connecting the plurality of electrode fingers  122   a . The comb-shaped electrode  102   b  includes a plurality of electrode fingers  122   b  parallel or substantially parallel to one another, and a busbar electrode  112   b  connecting the plurality of electrode fingers  122   b . The plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b  extend along a direction orthogonal or substantially orthogonal to the acoustic wave propagation direction (X-axis direction). The comb-shaped electrodes  102   a  and  102   b  oppose each other, and the plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b  interdigitate each other. Note that the comb-shaped electrode  102   a  includes dummy electrodes that face one another in a longitudinal direction of the plurality of electrode fingers  122   b , but the dummy electrodes may not be provided. Also, the comb-shaped electrode  102   b  includes dummy electrodes that face one another a longitudinal direction of the plurality of electrode fingers  122   a , but the dummy electrodes may not be provided. 
     The reflectors  142  each include a plurality of electrode fingers parallel or substantially parallel to one another and a busbar electrode connecting the plurality of electrode fingers, and are provided at both ends of the comb-shaped electrodes  102   a  and  102   b.    
     The IDT electrode defined by the plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b , and the busbar electrodes  112   a  and  112   b  has a laminated structure including the close contact layer  541  and the main electrode layer  542 , as shown in part (b) of  FIG. 2A . 
     Electrode fingers  132  are provided in the IDT electrode of the series arm resonator  102 . The electrode fingers  132  are not connected to any of the busbar electrodes  112   a  and  112   b , and are thinning electrodes (floating electrodes) provided in parallel or substantially in parallel to the plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b  at the same or substantially the same pitch. The plurality of electrode fingers  122   a  and plurality of electrode fingers  122   b  are provided between adjacent two electrode fingers  132 . That is, the pitch of the electrode fingers  132  is larger than the pitch of the plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b.    
     Note that a thinning electrode may also include the IDT electrode in which, instead of providing the electrode fingers  132  (floating electrodes), electrode fingers are not provided in portions where the electrode fingers  132  are to be provided. 
     With the IDT electrode including the electrode fingers  132  as shown in  FIG. 5A , the resonance band width of the acoustic wave resonator is able to be significantly reduced. However, a periodic structure by the pitch of the electrode fingers  132  may be provided, separately from a periodic structure of the pitch (IDT electrode pitch) of the plurality of electrode fingers  122   a  and the plurality of electrode fingers  122   b . Accordingly, there is a case where a frequency response corresponding to the periodic structure of the electrode fingers  132 , which is different from a frequency response of the main mode corresponding to the periodic structure of the IDT electrode pitch, is generated, and the generated frequency response appears as a spurious signal (unwanted wave) outside the pass band. 
       FIG. 5B  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of the transmission-side filter  10  according to a third modification of the first preferred embodiment.  FIG. 5B  exemplifies a plan view showing an IDT electrode structure of the series arm resonator  102  as a representative of the acoustic wave resonators (the subsequent-side acoustic wave resonators) excluding both the series arm resonator  101  and the parallel arm resonator  201  connected most proximately to the common terminal  90 . Note that the series arm resonator  102  shown in  FIG. 5B  provides an example of a structure of the subsequent-side acoustic wave resonator, and the number, length, and the like of electrode fingers defining an electrode are not limited thereto. 
     The series arm resonator  102  includes the substrate  5  having piezoelectricity, comb-shaped electrodes  102   c  and  102   d  provided on the substrate  5 , and the reflectors  142 . 
     As shown in  FIG. 5B , the comb-shaped electrode  102   c  includes a plurality of electrode fingers  122   c  parallel or substantially parallel to one another, and a busbar electrode  112   c  connecting the plurality of electrode fingers  122   c . The comb-shaped electrode  102   d  includes a plurality of electrode fingers  122   d  parallel or substantially parallel to one another, and a busbar electrode  112   d  connecting the plurality of electrode fingers  122   d . The plurality of electrode fingers  122   c  and the plurality of electrode fingers  122   d  extend along a direction orthogonal or substantially orthogonal to the acoustic wave propagation direction (X-axis direction). The comb-shaped electrodes  102   c  and  102   d  oppose each other, and the plurality of electrode fingers  122   c  and the plurality of electrode fingers  122   d  interdigitate each other. The comb-shaped electrode  102   c  includes dummy electrodes that face one another a longitudinal direction of the plurality of electrode fingers  122   d , but the dummy electrodes may not be provided. The comb-shaped electrode  102   d  includes dummy electrodes that face one another a longitudinal direction of the plurality of electrode fingers  122   c , but the dummy electrodes may not be provided. 
     The reflectors  142  each include a plurality of electrode fingers parallel or substantially parallel to each other and a busbar electrode connecting the plurality of electrode fingers, and are provided at both ends of the comb-shaped electrodes  102   c  and  102   d.    
     Note that the IDT electrode defined by the plurality of electrode fingers  122   c  and the plurality of electrode fingers  122   d , and the busbar electrodes  112   c  and  112   d  has a laminated structure including the close contact layer  541  and the main electrode layer  542 , as shown in  FIG. 2A (b). 
     Electrode fingers  152  are provided in the IDT electrode of the series arm resonator  102 . The electrode finger  152  is a thinning electrode (filled electrode) in which the plurality of electrode fingers  122   c  and plurality of electrode fingers  122   d  adjacent to each other and spaces between the plurality of electrode fingers adjacent to each other are united into a single electrode finger, the single electrode finger is connected to any one of the busbar electrodes  112   c  and  112   d , and the electrode finger width of the single electrode finger is wider than that of the plurality of electrode fingers  122   c  and plurality of electrode fingers  122   d . Additionally, the plurality of electrode fingers  122   c  and the plurality of electrode fingers  122   d  are provided between two electrode fingers  152  adjacent to each other. That is, the pitch of the electrode fingers  152  is larger than the pitch of the plurality of electrode fingers  122   c  and plurality of electrode fingers  122   d . Note that it is sufficient for the electrode finger width of the electrode fingers  152  to be larger than that of the plurality of electrode fingers  122   c  or  122   d.    
     With the IDT electrode including the electrode fingers  152  as shown in  FIG. 5B , the resonance band width of the acoustic wave resonator is able to be significantly reduced. However, a periodic structure by the pitch of the electrode fingers  152  may be provided, separately from a periodic structure of the pitch (IDT electrode pitch) of the plurality of electrode fingers  122   c  and the plurality of electrode fingers  122   d . Accordingly, there is a case where a frequency response corresponding to the periodic structure of the electrode fingers  152 , which is different from a frequency response of the main mode corresponding to the periodic structure of the IDT electrode pitch, is generated, and the generated frequency response appears as a spurious signal (unwanted wave) outside the pass band. 
       FIG. 5C  is a plan view showing electrodes of a subsequent-side acoustic wave resonator of the transmission-side filter  10  according to a fourth modification of the first preferred embodiment.  FIG. 5C  exemplifies a plan view showing an IDT electrode structure of the series arm resonator  102  as a representative of the acoustic wave resonators (the subsequent-side acoustic wave resonators) excluding both the series arm resonator  101  and the parallel arm resonator  201  connected most proximately to the common terminal  90 . Note that the series arm resonator  102  shown in  FIG. 5C  is provided as an example of a structure of the subsequent-side acoustic wave resonator, and the number, length, and the like of electrode fingers defining the electrode are not limited thereto. 
     The series arm resonator  102  includes the substrate  5  having piezoelectricity, comb-shaped electrodes  102   e  and  102   f  provided on the substrate  5 , and reflectors  142 . 
     As shown in  FIG. 5C , the comb-shaped electrode  102   e  includes a plurality of electrode fingers  122   e  parallel or substantially parallel to one another, and a busbar electrode  112   e  connecting the plurality of electrode fingers  122   e . The comb-shaped electrode  102   f  includes a plurality of electrode fingers  122   f  parallel or substantially parallel to one another, and a busbar electrode  112   f  connecting the plurality of electrode fingers  122   f . The plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f  extend along a direction orthogonal or substantially orthogonal to the acoustic wave propagation direction (X-axis direction). The comb-shaped electrodes  102   e  and  102   f  oppose each other, and the plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f  interdigitate each other. The comb-shaped electrode  102   e  includes dummy electrodes that face one another in a longitudinal direction of the plurality of electrode fingers  122   f , but the dummy electrodes may not be provided. The comb-shaped electrode  102   f  includes dummy electrodes that face one another in a longitudinal direction of the plurality of electrode fingers  122   e , but the dummy electrodes may not be provided. 
     The reflectors  142  each include a plurality of electrode fingers parallel or substantially parallel to one another and a busbar electrode connecting the plurality of electrode fingers, and are provided at both ends of the comb-shaped electrodes  102   e  and  102   f.    
     Note that the IDT electrode defined by the plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f , and the busbar electrodes  112   e  and  112   f  has a laminated structure including the close contact layer  541  and the main electrode layer  542 , as shown in  FIG. 2A (b). 
     Electrode fingers  162  are provided in the IDT electrode of the series arm resonator  102 . The electrode finger  162  is connected to the same busbar electrode as the busbar electrode to which the electrode fingers adjacent to the electrode finger  162  are connected, and is a thinning electrode provided in parallel or substantially in parallel to the plurality of electrode fingers  122   e  and plurality of electrode fingers  122   f  and at the same or substantially the same pitch. Also, the plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f  are provided between adjacent two electrode fingers  162 . That is, the pitch of the electrode fingers  162  is larger than the pitch of the plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f.    
     With the IDT electrode including the electrode fingers  162  as shown in  FIG. 5C , the resonance band width of the acoustic wave resonator is able to be significantly reduced. However, a periodic structure by the pitch of the electrode fingers  162  is provided, separately from a periodic structure of the pitch (IDT electrode pitch) of the plurality of electrode fingers  122   e  and the plurality of electrode fingers  122   f . Accordingly, there is a case where a frequency response corresponding to the periodic structure of the electrode fingers  162 , which is different from a frequency response of the main mode corresponding to the periodic structure of the IDT electrode pitch, is generated, and the generated frequency response appears as a spurious signal (unwanted wave) outside the pass band. 
     Note that the IDT electrodes shown in  FIGS. 5A to 5C  may each be a tilting IDT electrode in which, in addition to the thinning electrode being included, the busbar electrode is tilted with respect to the acoustic wave propagation direction. 
     Additionally, three electrode fingers  132  (floating electrodes) shown in  FIG. 5A  include the electrode finger connected to the busbar electrode  112   a  in the original periodic configuration and the electrode finger connected to the busbar electrode  112   b  in the original periodic configuration alternately provided in the X direction. On the other hand, the structure and location of the plurality of electrode fingers  132  is not limited thereto, and only the electrode fingers connected to the busbar electrode  112   a  in the original periodic structure and location may be used as the plurality of electrode fingers  132 , or only the electrode fingers connected to the busbar electrode  112   b  in the original periodic structure and location may be used as the plurality of electrode fingers  132 . 
     All of the three electrode fingers  152  (filled electrodes) shown in  FIG. 5B  are connected to the busbar electrode  112   d . On the other hand, the structure and location of the plurality of electrode fingers  152  is not limited thereto. All of the plurality of electrode fingers  152  may be connected to the busbar electrodes  112   c , or the electrode finger  152  connected to the busbar electrodes  112   d  and the electrode finger  152  connected to the busbar electrodes  112   c  may be alternately provided in the X direction. 
     All of the three electrode fingers  162  shown in  FIG. 5C  are connected to the busbar electrode  112   f . On the other hand, the structure and location of the plurality of electrode fingers  162  is not limited thereto. All the plurality of electrode fingers  162  may be connected to the busbar electrodes  112   e , or the electrode finger  162  connected to the busbar electrodes  112   f  and the electrode finger  162  connected to the busbar electrodes  112   e  may be alternately provided in the X direction. 
       FIG. 6  is a plan view showing an example of electrodes of a first stage-side acoustic wave resonator of the transmission-side filter  10  according to the working example.  FIG. 6  exemplifies a plan view showing an electrode structure of a series arm resonator  101  and the capacitance element C 1  as a representative of the series arm resonator  101  and the parallel arm resonator  201  connected most proximately to the common terminal  90 . As shown in  FIG. 6 , the capacitance element C 1  may include a comb-tooth electrode provided on the substrate  5 . Although not shown, the capacitance element C 2  may include a comb-tooth electrode provided on the substrate  5 . Accordingly, since a series arm circuit in which the capacitance element C 1  and the series arm resonator  101  are connected in parallel and a parallel arm circuit in which the capacitance element C 2  and the parallel arm resonator  201  are connected in parallel may be significantly reduced in size, the multiplexer  1  is able to be significantly reduced in size. 
     As shown in  FIG. 6 , the comb-tooth electrode of the capacitance element C 1  includes a plurality of electrode fingers  301   a  and the plurality of electrode fingers  301   b  that are parallel or substantially parallel to each other and to interdigitate each other, and a pair of busbar electrodes  311   a  and  311   b  oppose each other across the plurality of electrode fingers  301   a  and the plurality of electrode fingers  301   b . The plurality of electrode fingers  301   a  are connected to the busbar electrode  311   a , and the plurality of electrode fingers  301   b  are connected to the busbar electrode  311   b.    
     As shown in  FIG. 6 , the plurality of electrode fingers  301   a  and the plurality of electrode fingers  301   b  may be provided along a propagation direction of the surface acoustic wave in the IDT electrode of the series arm resonator  101 , and may also be provided periodically along a direction orthogonal or substantially orthogonal to the propagation direction. 
     Accordingly, a situation in which an unwanted wave generated in the capacitance element C 1  interferes with the surface acoustic wave propagating through the IDT electrode of the series arm resonator  101  is able to be significantly reduced or prevented. 
     5. Bandpass Characteristics and Reflection Characteristics of Multiplexer  1   
     Bandpass characteristics and reflection characteristics of the multiplexer  1  according to the working example are compared with bandpass characteristics and reflection characteristics of a multiplexer according to a comparative example to describe the effects exhibited by the multiplexer  1 . In the multiplexer  1  according to the working example, Band A, which is a pass band of the transmission-side filter  10  and the reception-side filter  11 , is applied to Band  30  (transmission band: about 2305 MHz to about 2315 MHz, reception band: about 2350 MHz to about 2360 MHz) of Long Term Evolution (LTE), and Band B, which is a pass band of the transmission-side filter  12  and the reception-side filter  13 , is applied to Band  25  (transmission band: about 1850 MHz to about 1915 MHz, reception band: about 1930 MHz to about 1995 MHz) of LTE. That is, the transmission-side filter  10  and the reception-side filter  11  are applied to a duplexer of Band  30  (first duplexer), and the transmission-side filter  12  and the reception-side filter  13  are applied to a duplexer of Band  25  (second duplexer). That is, the multiplexer  1  according to the working example is applied as a quadplexer of Band  30  and Band  25 . 
     Note that in the multiplexer according to the comparative example, in comparison with the multiplexer according to the working example, the structures and features of the transmission-side filter  12  and the reception-side filters  11  and  13  are the same as or similar to those of the working example, and only the structures and features of a transmission-side filter  510  for Band A is different. 
       FIG. 7  is a circuit diagram of the transmission-side filter  510  of Band A defining the multiplexer according to the comparative example. As shown in  FIG. 7 , the transmission-side filter  510  includes series arm resonators  701 ,  702 ,  703  and  704 , parallel arm resonators  801 ,  802  and  803 , and inductance elements L 3  and L 4 . The series arm resonators  701  to  704  are provided on a path connecting the common terminal  90  and the transmission input terminal  91 , and are connected in series to one another. The parallel arm resonators  801  to  803  are provided between nodes on the above-described path and a reference terminal (ground). A capacitance element is not connected in parallel to any of the series arm resonator  701  and the parallel arm resonator  801  of the transmission-side filter  510 . In each of the series arm resonators  701  to  704  and the parallel arm resonators  801  to  803 , a thinning electrode is provided. 
     Table 1 shows electrode parameters of each acoustic wave resonator of the transmission-side filter  10  according to the working example. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Transmission-side filter 10 
               
               
                   
                 (working example) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Series arm 
                 Series arm 
                 Series arm 
                 Series arm 
               
               
                   
                 resonator 
                 resonator 
                 resonator 
                 resonator 
               
               
                   
                 101 
                 102 
                 103 
                 104 
               
               
                   
               
               
                 Wavelength λ 
                 1.6247 
                 1.6915 
                 1.7010 
                 1.6912 
               
               
                 (μm) 
               
               
                 Intersecting 
                 33.2 
                 24.5 
                 34.0 
                 27.9 
               
               
                 width (μm) 
               
               
                 Number of pairs 
                 165 
                 62 
                 169 
                 62 
               
               
                 (pairs) 
               
               
                 Electrode duty 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Number of 
                 3 
                 4 
                 1 
                 3 
               
               
                 segments 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Parallel arm 
                 Parallel arm 
                 Parallel arm 
               
               
                   
                 resonator 
                 resonator 
                 resonator 
               
               
                   
                 201 
                 202 
                 203 
               
               
                   
               
               
                 Wavelength λ 
                 1.6832 
                 1.7328 
                 1.7290 
               
               
                 (μm) 
               
               
                 Intersecting 
                 32.2 
                 34.1 
                 68.6 
               
               
                 width (μm) 
               
               
                 Number of pairs 
                 80 
                 62 
                 197 
               
               
                 (pairs) 
               
               
                 Electrode duty 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Number of 
                 3 
                 1 
                 1 
               
               
                 segments 
               
               
                   
               
            
           
         
       
     
     Note that in Table 1, “number of segments” refers to the number of series segments in a series arm resonator, and the number of parallel segments in a parallel arm resonator. 
     In the transmission-side filter  10  according to the working example, in addition to the acoustic wave resonators shown in Table 1, capacitance elements C 1  (about 0.63 pF) and C 2  (about 0.28 pF) and inductance elements L 1  (about 0.65 nH) and L 2  (about 0.33 nH) are added. Further, in the transmission-side filter  10  according to the working example, a thinning electrode is provided in the IDT electrode of each of the series arm resonators  102  to  104  and the parallel arm resonators  202  to  203 , with respect to the electrode parameters of the acoustic wave resonators shown in Table 1. 
     On the other hand, in the transmission-side filter  510  according to the comparative example, electrode parameters of the series arm resonators  701  to  704  and the parallel arm resonators  801  to  803  are the same as or similar to the electrode parameters of the series arm resonators  101  to  104  and the parallel arm resonators  201  to  203  shown in Table 1, respectively. Also, an inductance value of the inductance element L 3  is equal or substantially equal to an inductance value of the inductance element L 1 , and an inductance value of the inductance element L 4  is equal or substantially equal to an inductance value of the inductance element L 2 . A thinning electrode is provided in the IDT electrode of each of the acoustic wave resonators of the series arm resonators  701  to  704  and the parallel arm resonators  801  to  803 . 
       FIG. 8A  is a graph showing bandpass characteristics of the transmission-side filter  10  defining the multiplexer  1  according to the working example.  FIG. 8B  is a graph showing bandpass characteristics of the transmission-side filter  510  defining the multiplexer according to the comparative example. When the graphs of  FIGS. 8A and 8B  are compared with each other, in the transmission band of Band  30  (about 2305 MHz to about 2315 MHz), the insertion loss of the transmission-side filter  10  (maximum insertion loss: about 2.72 dB) and insertion loss of the transmission-side filter  510  (maximum insertion loss: about 2.70 dB) are substantially equal to each other. From this, it is understood that, in the transmission-side filter  10  according to the working example, even when capacitance elements are connected in parallel in place of the thinning electrodes in the series arm resonator  101  and the parallel arm resonator  201 , a narrowed band is able to be provided without worsening the insertion loss. 
       FIG. 9A  is a graph showing bandpass characteristics of the transmission-side filter  12  for Band  25  defining the multiplexer  1  according to the working example.  FIG. 9B  is a graph showing bandpass characteristics of the transmission-side filter for Band  25  defining the multiplexer according to the comparative example. When the graphs of  FIGS. 9A and 9B  are compared with each other, it is observed that a ripple is generated (a broken-line portion in  FIG. 9B ) in the transmission band of Band  25  (about 1850 MHz to about 1915 MHz) in the comparative example. Because of this, the insertion loss in the pass band of the transmission-side filter  12  according to the comparative example is larger than the insertion loss in the pass band of the transmission-side filter  12  according to the working example. That is, in the comparative example, since the thinning electrodes are provided in the series arm resonator  101  and the parallel arm resonator  201  of the transmission-side filter  510 , the insertion loss of the transmission-side filter  12  is worsened. 
       FIG. 10A  includes graphs showing impedance characteristics and reflection characteristics, respectively, of an acoustic wave resonator (series arm resonator  701  according to the comparative example) including a thinning electrode and to which a capacitance element is not connected in parallel.  FIG. 10B  includes graphs showing impedance characteristics and reflection characteristics, respectively, of an acoustic wave resonance circuit (series arm circuit in which the series arm resonator  101  and the capacitance element C 1  according to the working example are connected in parallel) without a thinning electrode and to which the capacitance element connected in parallel. The acoustic wave resonator having the characteristics shown in  FIG. 10A  (the series arm resonator  701 ) and the acoustic wave resonance circuit having the characteristics shown in  FIG. 10B  (the series arm circuit in which the series arm resonator  101  and the capacitance element C 1  are connected in parallel) both reduce the resonance band width. 
     In  FIG. 10A , in the series arm resonator  701  according to the comparative example, since the thinning electrode is provided, a point where the reflection loss is increased (a point where a spurious signal is generated: about 1800 MHz to about 2100 MHz in  FIG. 10A ) appears, and the frequency thereof is included in the transmission band of Band  25  (about 1850 MHz to about 1915 MHz). When a thinning electrode is provided in a portion of the IDT electrode to provide a narrow band filter, a periodic structure by the thinning electrode is provided separately from the periodic structure of the IDT electrode pitch. Accordingly, a frequency response corresponding to the periodic structure by the thinning electrode, which is different from a frequency response of the main mode corresponding to the periodic structure of the IDT electrode pitch, is generated, and the generated frequency response appears as a spurious signal (unwanted wave) outside the pass band. 
     On the other hand, in  FIG. 10B , in the series arm circuit in which the series arm resonator  101  and the capacitance element C 1  according to the working example are connected in parallel, a point where the return loss is increased (a point where a spurious signal is generated: a low frequency side relative to about 1600 MHz in  FIG. 10B ) caused by the comb-tooth capacitance of the capacitance element C 1  appears, but it is easy to shift the spurious signal appearing frequency; for example, the frequency is shifted toward a lower frequency side by, for example, adjusting the pitch of the comb-tooth capacitance or the like. However, when the capacitance element C 1  is connected in parallel to the series arm resonator  101 , the Q-value of the series arm circuit in which the series arm resonator  101  and the capacitance element C 1  are connected in parallel is adversely affected by a low Q-value of the capacitance element C 1  defined by a comb-tooth electrode or the like. 
       FIG. 11  is a diagram describing a relationship between a branch of a ladder acoustic wave filter and reflection characteristics thereof.  FIG. 11  shows a ladder acoustic wave filter including five series arm resonators s 1  to s 5  and four parallel arm resonators p 1  to p 4 . When the reflection characteristics seen from a common terminal are evaluated, in return loss seen from the common terminal, the return loss of the branch which is closest to the common terminal (the series arm resonator s 1  in  FIG. 11 ), is almost affected as it is, the return loss of the branch which is second closest to the common terminal (the parallel arm resonator p 1  in  FIG. 11 ), is affected by substantially one sixth of the return loss, and the return loss of the branch which is third closest to the common terminal (the series arm resonator s 2  in  FIG. 11 ), is hardly affected. The return loss seen from the common terminal is hardly affected by the return loss of the branches including the fourth closest branch and the subsequent branches (the parallel arm resonators p 2 , p 3  and p 4 , and the series arm resonators s 3 , s 4  and s 5  in  FIG. 11 ). 
     Note that the term “branch” described in the present specification is used to represent one unit of a series arm resonator or a parallel arm resonator that defines a ladder filter. When a plurality of series arm resonators are continuously connected, in a case where a node to be connected to the parallel arm resonator is not provided between the plurality of series arm resonators, the plurality of series arm resonators are defined as one branch. When a plurality of nodes to be connected to the plurality of parallel arm resonators respectively is continuously connected, in a case where a series arm resonator is not provided between the plurality of nodes, the plurality of parallel arm resonators are defined as one branch. 
     When this is applied to the multiplexer  1  according to the working example, the acoustic wave resonators with the thinning electrodes provided therein whose acoustic wave resonance circuits, which may cause spurious signals to appear, affect the return loss seen from the common terminal  90 , are the series arm resonator  101 , which is the first branch, and the parallel arm resonator  201 , which is the second branch. On the other hand, even when the thinning electrodes are provided in the acoustic wave resonators as the third and subsequent branches, spurious signals generated due to the thinning electrode formation hardly affect the return loss seen from the common terminal  90 . In consideration of balance of the above characteristics and the characteristics that the insertion loss is worsened in the acoustic wave resonance circuit in which the capacitance element is connected in parallel, the multiplexer  1  according to the working example includes the following structure and feature. In the transmission-side filter  10 , the capacitance element is connected in parallel to each of the series arm resonator  101  as the first branch and the parallel arm resonator  201  as the second branch, and the thinning electrodes are provided in the other acoustic wave resonators. 
     Accordingly, an increase in insertion loss of the transmission-side filter  12  caused by spurious signals (unwanted waves) generated in the subsequent-side acoustic wave resonator is able to be significantly reduced or prevented while narrowing the band of the transmission-side filter  10 . 
     As in the present working example, a capacitance element is preferably not connected in parallel to the subsequent-side acoustic wave resonator, for example, because the Q-value is lowered to increase the insertion loss of the filter as the number of acoustic wave resonance circuits having the capacitance elements connected in parallel is larger. That is, as in the present working example, by not connecting the capacitance element in parallel to the subsequent-side acoustic wave resonator, an increase in the insertion loss of the transmission-side filter  12  caused by spurious signals (unwanted waves) generated in the subsequent-side acoustic wave resonator is able to be significantly reduced or prevented while reducing the loss of the transmission-side filter  10  and narrowing the band of the transmission-side filter  10 . 
     Note that the multiplexer  1  according to the present working example may be a hexaplexer in which, for example, a duplexer of Band  66  (transmission band: about 1710 MHz to about 1780 MHz, reception band: about 2110 MHz to about 2200 MHz) is added to a quadplexer of Band  30  and Band  25 . Since the reception band of Band  66  (about 2110 MHz to about 2200 MHz) is located in a vicinity of a lower frequency side of the transmission band of Band  30  (about 2305 MHz to about 2315 MHz), attenuation in a vicinity of the lower frequency side of the transmission-side filter  10  for Band  30  is preferably provided, for example. Accordingly, the transmission-side filter  10  has a narrower band to ensure the attenuation in the reception band of Band  66 . Therefore, in a case of providing the above-described hexaplexer, the first preferred embodiment is able to provide both a narrower band of the transmission-side filter  10  for Band  30  and a decrease in loss of the transmission-side filter  12  for Band  25 . 
     Second Preferred Embodiment 
     The multiplexer  1  according to the first preferred embodiment may also be applied to a high frequency front-end circuit, and further to a communication apparatus including the high frequency front-end circuit. In a present preferred embodiment of the present invention, a high frequency front-end circuit and a communication apparatus are described. 
       FIG. 12  is a diagram of a communication apparatus  70  according to the second preferred embodiment. The communication apparatus  70  includes a high frequency front-end circuit  60 , an RF signal processing circuit  3 , and a baseband signal processing circuit  4 . In  FIG. 12 , an antenna element  2  connected to the communication apparatus  70  is also shown. 
     The high frequency front-end circuit  60  includes the multiplexer  1  according to a preferred embodiment of the present invention, an inductance element  30 , a reception-side switch  22 , a transmission-side switch  21 , a low-noise amplifier circuit  42 , and a power amplifier circuit  41 . 
     The transmission-side switch  21  is a switch circuit including two selection terminals individually connected to transmission input terminals  91  and  93  of the multiplexer  1 , and a common terminal connected to the power amplifier circuit  41 . 
     The reception-side switch  22  is a switch circuit including two selection terminals individually connected to reception output terminals  92  and  94  of the multiplexer  1 , and a common terminal connected to the low-noise amplifier circuit  42 . 
     Each of the transmission-side switch  21  and the reception-side switch  22  connects the common terminal and a signal path corresponding to a predetermined band in accordance with a control signal from a controller (not shown), and preferably includes, for example, a single pole double throw (SPDT) type switch. The number of selection terminals connected to the common terminal is not limited to one, and a plurality of selection terminals may be connected thereto. That is, the high frequency front-end circuit  60  may support carrier aggregation. 
     The power amplifier circuit  41  is a transmission amplifier circuit that amplifies a high frequency signal (in this case, a high frequency transmission signal) outputted from the RF signal processing circuit  3  and output the amplified high frequency signal to the antenna element  2  via the transmission-side switch  21  and the multiplexer  1 . 
     The low-noise amplifier circuit  42  is a reception amplifier circuit that amplifies a high frequency signal (in this case, a high frequency reception signal) having passed through the antenna element  2 , the multiplexer  1  and the reception-side switch  22 , and output the amplified high frequency signal to the RF signal processing circuit  3 . 
     Note that the power amplifier circuit may include amplification elements individually corresponding to Band A and Band B. In this case, the transmission-side switch  21  may not be provided. The low-noise amplifier circuit may include amplification elements individually corresponding to Band A and Band B, respectively. In this case, the reception-side switch  22  may not be provided. 
     The RF signal processing circuit  3  performs signal processing on a high frequency reception signal inputted from the antenna element  2  via the low-noise amplifier circuit  42  by down-conversion or the like, and outputs a reception signal generated by the signal processing to the baseband signal processing circuit  4 . In addition, the RF signal processing circuit  3  performs signal processing on a transmission signal inputted from the baseband signal processing circuit  4  by up-conversion or the like, and outputs a high frequency transmission signal generated by the signal processing to the power amplifier circuit  41 . The RF signal processing circuit  3  is preferably, for example, an RFIC. 
     The signal processed by the baseband signal processing circuit  4  is used, for example, as an image signal for image display, or as a sound signal for conversation. 
     Note that the communication apparatus  70  may not include the baseband signal processing circuit (BBIC)  4  in accordance with a processing scheme of the high frequency signal. 
     Note that the high frequency front-end circuit  60  may include another circuit element between each of the elements described above. 
     According to the high frequency front-end circuit  60  and the communication apparatus  70  provided as described above, since the multiplexer  1  according to the first preferred embodiment described above is provided, low loss of the transmission-side filter  12  is able to be provided while narrowing the band of the transmission-side filter  10 . 
     Other Modifications 
     The multiplexer  1  according to the first preferred embodiment, and the high frequency front-end circuit  60  and the communication apparatus  70  according to the second preferred embodiment have been described while referring to the preferred embodiments. However, the present invention is not limited to the above-described preferred embodiments. For example, aspects in which the following modifications are applied to the above-described preferred embodiments may also be included in the present invention. 
     For example, in the above description, a quadplexer applied to Band A and Band B is exemplified and explained as the multiplexer  1 . However, the present invention may also be applied to, for example, a triplexer in which an antenna connection terminal is shared by three filters, or a hexaplexer in which three duplexers are common-connected at a common terminal. That is, it is sufficient for the multiplexer to include at least two filters. 
     The multiplexer according to the preferred embodiments of the present invention is not limited to both a transmission-side filter and a reception-side filter, and may include only a plurality of transmission-side filters or only a plurality of reception-side filters. 
     Also, in the first preferred embodiment, it is described that the transmission-side filter  10  applied to Band A corresponds to the first filter, and the transmission-side filter  12  corresponds to the second filter. That is, in the first preferred embodiment, the first filter and the second filter are both the transmission-side filters. However, without being limited to the usage for the first and second filters or the like, the present invention may be applied to any multiplexer as long as the frequency of the unwanted wave generated by the narrow-band first filter is located within the pass band of the second filter. 
     Preferred embodiments of the present invention may be widely applied to communication equipment, for example, mobile phones, as a low-loss multiplexer, a high frequency front-end circuit, a communication apparatus, or the like that is applicable to multi-band and multi-mode frequency standards. 
     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 from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.