Patent Publication Number: US-2021194460-A1

Title: Composite filter device and band pass filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-231795 filed on Dec. 23, 2019. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a composite filter device and a band pass filter. 
     2. Description of the Related Art 
     In radio communication devices or other devices, composite filter devices have recently been used and each are provided with various filters including a band pass filter for passing a signal in a predetermined frequency band. For example, Japanese Patent No. 6489294 discloses a technique for a multiplexer including three or more acoustic wave filters with which to reduce the insertion loss of each of the acoustic wave filters in the pass band by reducing a capacitance generated between the ground and a wiring line between the acoustic wave filter and an antenna terminal to which an antenna element is connected. 
     However, in the case of the technique disclosed in Japanese Patent No. 6489294, there is sometimes a need to increase the length of a wiring electrode forming an inductor as required to provide a predetermined inductance. Accordingly, the insertion loss in the pass band may be deteriorated because a stray capacitance is generated by the wiring electrode. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a composite filter device and a band pass filter with which the insertion loss of a filter in the pass band is able to be reduced or prevented while providing a predetermined inductance. 
     A composite filter device according to a preferred embodiment of the present invention includes a connection portion connected to a common terminal, a first filter between the connection portion and a first terminal, a second filter between the connection portion and a second terminal, a first inductor that is connected in series between the second filter and the connection portion, includes a first wiring electrode wound in a predetermined direction, and is provided at an internal layer of a multilayer board, and a second inductor that is connected in series between the common terminal and the connection portion, includes a second wiring electrode wound in the predetermined direction, and is provided at an internal layer of the multilayer board. The first wiring electrode and the second wiring electrode at least partially overlap when the multilayer board is viewed in plan. 
     According to a preferred embodiment of the present invention, there can be provided a composite filter device and a band pass filter with which the insertion loss of a filter in the pass band is able to be reduced or prevented while providing a predetermined inductance. 
     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 illustrating a communication device according to a first preferred embodiment of the present invention. 
         FIG. 2  is a diagram illustrating the positional relationship between wiring electrodes in a composite filter device according to the first preferred embodiment of the present invention when a first layer in a multilayer board is viewed in plan. 
         FIG. 3  is a diagram illustrating the positional relationship between wiring electrodes in the composite filter device according to the first preferred embodiment of the present invention when a second layer in the multilayer board is viewed in plan. 
         FIG. 4  is a diagram illustrating the positional relationship between wiring electrodes in the composite filter device according to the first preferred embodiment of the present invention when a third layer in the multilayer board is viewed in plan. 
         FIG. 5  is a diagram illustrating the positional relationship between a wiring electrode defining an inductor provided at the first layer and a wiring electrode defining an inductor provided at the second layer. 
         FIG. 6  is a diagram illustrating the positional relationship between the wiring electrode defining the inductor provided at the second layer and a wiring electrode defining an inductor provided at the third layer. 
         FIG. 7  is a diagram illustrating simulation results of bandpass characteristics of a band elimination filter in Example 1 and Comparative Example 1. 
         FIG. 8  is a diagram illustrating simulation results of bandpass characteristics of a band elimination filter in Example 1 and Comparative Example 1 in a frequency band near a stop band. 
         FIG. 9  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter in Example 1 and Comparative Example 1. 
         FIG. 10  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter of Example 1 and Comparative Example 1 in a frequency band near a pass band. 
         FIG. 11  is a Smith chart illustrating the impedance characteristics of a composite filter device when the composite filter devices are viewed from a common terminal in the frequency band of about 600 MHz to about 1000 MHz in Example 1 and Comparative Example 1. 
         FIG. 12  is a Smith chart illustrating the impedance characteristics of a composite filter device when the composite filter devices are viewed from the common terminal in the frequency band of about 1700 MHz to about 2700 MHz in Example 1 and Comparative Example 1. 
         FIG. 13  is a Smith chart illustrating the impedance characteristics of a composite filter device when the composite filter devices are viewed from the common terminal in the frequency band of about 3300 MHz to about 4200 MHz in Example 1 and Comparative Example 1. 
         FIG. 14  is a Smith chart illustrating the impedance characteristics of a band elimination filter when the band elimination filters are viewed from a second terminal in the frequency band of about 600 MHz to about 1000 MHz in Example 1 and Comparative Example 1. 
         FIG. 15  is a Smith chart illustrating the impedance characteristics of a band elimination filter when the band elimination filters are viewed from the second terminal in the frequency band of about 1700 MHz to about 2700 MHz in Example 1 and Comparative Example 1. 
         FIG. 16  is a Smith chart illustrating the impedance characteristics of a band elimination filter when the band elimination filters are viewed from the second terminal in the frequency band of about 3300 MHz to about 4200 MHz in Example 1 and Comparative Example 1. 
         FIG. 17  is a diagram illustrating a band pass filter according to a second preferred embodiment of the present invention. 
         FIG. 18  is a diagram illustrating the layout of a wiring electrode defining a band pass filter according to the second preferred embodiment of the present invention. 
         FIG. 19  is a diagram illustrating the layout of a wiring electrode defining a band pass filter according to the second preferred embodiment of the present invention. 
         FIG. 20  is a diagram illustrating the layout of a wiring electrode defining a band pass filter according to the second preferred embodiment of the present invention. 
         FIG. 21  is a diagram illustrating the positional relationship between a wiring electrode defining an inductor provided at a first layer and a wiring electrode defining an inductor provided at a third layer in the second preferred embodiment of the present invention. 
         FIG. 22  is a diagram illustrating the positional relationship between a wiring electrode defining an inductor provided at a second layer and the wiring electrode defining the inductor provided at the third layer in the second preferred embodiment of the present invention. 
         FIG. 23  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter in Example 2 and Comparative Example 2 in a frequency band near a pass band. 
         FIG. 24  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter in Example 2 and Comparative Example 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the drawings, the components represented by the same reference numeral have the same features, components, and elements or similar features, components, and elements. 
       FIG. 1  is a diagram illustrating a communication device according to a first preferred embodiment of the present invention. The communication device  1  according to this preferred embodiment may be installed into, for example, a mobile communication device, such as a cellular phone and performs the transmission and reception of a signal. 
     As illustrated in  FIG. 1 , the communication device  1  includes an antenna  10  and a composite filter device  12 . The antenna  10  and the composite filter device  12  are connected to each other via a common terminal T 0 . 
     The antenna  10  performs the transmission and reception of a radio-frequency (RF) signal to/from a base station and the reception of a signal from a global positioning system (GPS). 
     The composite filter device  12  separates a transmission signal to be transmitted from the antenna  10  and a reception signal received by the antenna  10  in accordance with frequencies. The composite filter device  12  includes a band pass filter  14  (a first filter), a band elimination filter  18  (a second filter), an inductor L 1  (a first inductor), and an inductor L 2  (a second inductor). A node  102  (a connection portion) is connected to the common terminal T 0  via the inductor L 2 . The band pass filter  14  is provided between the node  102  and a first terminal T 1 . The band elimination filter  18  is provided between the node  102  and a second terminal T 2 . The band elimination filter  18  is connected to the node  102  via the inductor L 1 . The band pass filter  14  is a filter that passes signals of a predetermined frequency and attenuates signals of frequencies other than the predetermined frequency. The band elimination filter  18  is a filter that attenuates signals of a predetermined signal and passes signals of frequencies other than the predetermined frequency. The band elimination filter  18  may be, for example, a notch filter having a relatively narrow stop band. 
     In this preferred embodiment, the pass band in which the band pass filter  14  passes signals overlaps the stop band in which the band elimination filter  18  attenuates signals. For example, when a GPS reception signal is to be extracted from various signals received by the antenna  10 , the pass band of the band pass filter  14  and the stop band of the band elimination filter  18  are set to be the frequency of the GPS reception signal. Accordingly, the GPS reception signal passes through the band pass filter  14  and reception signals other than the GPS reception signal pass through the band elimination filter  18 . Thus, the composite filter device  12  is able to extract a signal of a specific frequency from signals of various frequencies. The type of a signal to be extracted is not limited to GPS. 
     The composite filter device  12  according to this preferred embodiment will be described below. The band pass filter  14  outputs, from the first terminal T 1 , a reception signal supplied from the antenna  10  via the common terminal T 0  and the node  102 . The band elimination filter  18  outputs, from the second terminal T 2 , a reception signal supplied from the antenna  10  via the common terminal T 0  and the node  102 . The band pass filter  14  passes a reception signal in this preferred embodiment, but may pass a transmission signal instead of, or in addition to, a reception signal. 
     The band pass filter  14  is a ladder filter in which a plurality of resonators are provided at series and parallel arms. Specifically, the band pass filter  14  includes seven series arm resonators S 11  to S 17 , three parallel arm resonators P 11  to P 13 , and an inductor L 3 . The numbers of series arm resonators, parallel arm resonators, and inductors are merely examples and are not limited to the above-described numbers. 
     Components defining the series arm resonators S 11  to S 17  and the parallel arm resonators P 11  to P 13  are not particularly limited, and may be, for example, surface acoustic wave (SAW) filters, filters such as piezoelectric thin film resonators, or bulk acoustic wave (BAW) filters. The resonators S 21  to S 23 , the series arm resonators S 31  to S 34 , and the parallel arm resonators P 31  to P 33 , which are described below, may include similar filters. 
     The seven series arm resonators S 11  to S 17  are connected in series with each other in order of increasing distance from the node  102  at the series arm connecting the node  102  and the first terminal T 1 . The three parallel arm resonators P 11  to P 13  are connected in parallel with each other in order of increasing distance from the node  102 , and the three parallel arm resonators P 11  to P 13  branch off from the series arm. First ends of the three parallel arm resonators P 11  to P 13  are connected to the node between the series arm resonators S 14  and S 15 , the node between the series arm resonators S 15  and S 16 , and the node between the series arm resonators S 16  and S 17 , respectively. A reference potential (for example, a ground potential) is supplied to second ends of the three parallel arm resonators P 11  to P 13 . 
     Between the series arm resonator S 17  and the first terminal T 1 , the inductor L 3  is connected in series with the series arm resonator S 17 . 
     The band elimination filter  18  includes a plurality of resonators that are connected in series with each other. Specifically, the band elimination filter  18  includes the three resonators S 21  to S 23  and an inductor L 4 . The numbers of the resonators and inductors are merely examples and are not limited to the above-described numbers. 
     The resonators S 21  to S 23  are adjacently connected in series with each other in order of increasing distance from the node  102  on the line connecting the node  102  and the second terminal T 2 . A first end of the inductor L 4  is connected to the node between the resonators S 22  and S 23 . A reference potential (for example, a ground potential) is supplied to a second end of the inductor L 4 . 
     Between the node  102  and the resonator S 21 , the inductor L 1  (the first inductor) is connected in series with the node  102 . Between the common terminal T 0  and the node  102 , the inductor L 2  (the second inductor) is connected in series with the node  102 . 
     In this preferred embodiment, the pass band and pass band width of the band pass filter  14  and the stop band and stop band width of the band elimination filter  18  are substantially the same. 
     In the composite filter device  12  according to this preferred embodiment, the wiring electrodes connecting the various resonators and the wiring electrodes defining the inductors are provided at the internal layers of a multilayer board having a layered structure. The layout of the wiring electrodes defining the inductors L 1  to L 3  according to this preferred embodiment will be described with reference to  FIGS. 2 to 4 .  FIGS. 2 to 4  are diagrams illustrating the positional relationship between the wiring electrodes in the composite filter device  12  according to this preferred embodiment when a multilayer board is viewed in plan. 
     When a direction extending from the back to the front of the drawing sheet is defined as a direction from bottom to top, a first layer  110  illustrated in  FIG. 2 , a second layer  120  illustrated in  FIG. 3 , and a third layer  130  illustrated in  FIG. 4  are laminated in this order from the top. Although not illustrated, respective adjacent layers are laminated on the upper side of the first layer  110  and the underside of the third layer  130 . 
     As illustrated in  FIG. 2 , at the first layer  110 , a wiring electrode  112 , a wiring electrode  114  (a first wiring electrode), and a wiring electrode  116  are provided. The wiring electrode  114  defines a portion of the inductor L 1 . The wiring electrode  116  defines a portion of the inductor L 3 . A terminal C 1  of the wiring electrode  112  is connected to a wiring electrode that is provided at a layer on the upper side of the first layer  110  and is connected to the series arm resonator S 11 . A terminal E 1  of the wiring electrode  114  is connected to a wiring electrode that is provided at a layer on the upper side of the first layer  110  and is connected to the resonator S 21 . A terminal F 1  of the wiring electrode  116  is connected to wiring electrodes that are provided at a layer on the upper side of the first layer  110  and are connected to the resonators S 22  and S 23 . The wiring electrodes connected to the resonators S 22  and S 23  are connected to the ground. 
     As illustrated in  FIG. 3 , a wiring electrode  122  (a second wiring electrode) and wiring electrodes  124  and  126  are provided at the second layer  120 . The wiring electrode  122  defines a portion of the inductor L 2 . The wiring electrode  126  defines a portion of the inductor L 3 . A terminal C 2  of the wiring electrode  122  is connected to the terminal C 1  of the wiring electrode  112  at the first layer  110 . A terminal D 2  of the wiring electrode  124  is connected to a terminal D 1  of the wiring electrode  114  at the first layer  110 . A terminal G 2  of the wiring electrode  126  is connected to a terminal G 1  of the wiring electrode  116  at the first layer  110 . 
     As illustrated in  FIG. 4 , a wiring electrode  132 , a wiring electrode  134  (the first wiring electrode), and a wiring electrode  136  are provided at the third layer  130 . The wiring electrode  134  defines a portion of the inductor L 1 . The wiring electrode  136  defines a portion of the inductor L 3 . A terminal A 2  of the wiring electrode  132  is connected to a terminal A 1  of the wiring electrode  122  at the second layer  120 . The terminal A 2  of the wiring electrode  132  is also connected to a wiring electrode that is provided at a layer on the underside of the third layer  130  and is connected to the common terminal T 0 . A terminal B 2  of the wiring electrode  134  is connected to a terminal B 1  of the wiring electrode  122  at the second layer  120 . Accordingly, the inductors L 1  and L 2  are connected by the terminals B 1  and B 2 . 
     A terminal D 3  of the wiring electrode  134  is connected to the terminal D 2  of the wiring electrode  124  at the second layer  120 . As described above, the wiring electrode  124  at the second layer  120  is connected to the wiring electrode  114  at the first layer  110 . Accordingly, the wiring electrode  134  at the third layer  130  is connected to the wiring electrode  114  at the first layer  110  via the wiring electrode  124  at the second layer  120 . The second layer  120  is sandwiched between the first layer  110  and the third layer  130 . The wiring electrode  122  at the second layer  120 , which defines a portion of the inductor L 2 , is sandwiched between the wiring electrode  114  at the first layer  110  and the wiring electrode  134  at the third layer  130  which define the inductor L 1 . 
     A terminal H 2  of the wiring electrode  136  is connected to a terminal H 1  of the wiring electrode  126  at the second layer  120 . As described above, the wiring electrode  126  is connected to the wiring electrode  116  at the first layer  110 . Accordingly, the wiring electrode  116  at the first layer  110 , the wiring electrode  126  at the second layer  120 , and the wiring electrode  136  at the third layer  130  define a portion of the inductor L 3  in an integrated manner. A terminal I 1  of the wiring electrode  136  at the third layer  130  is connected to a wiring electrode that is provided at a layer on the underside of the third layer  130  and is connected to the ground. 
     Next, the positional relationship between each of the wiring electrodes  114  and  134  defining a portion of the inductor L 1  and the wiring electrode  122  defining a portion of the inductor L 2  will be described with reference to  FIGS. 5 and 6 .  FIG. 5  is a diagram illustrating the positional relationship between the wiring electrode  114  defining the inductor L 1  at the first layer  110  and the wiring electrode  122  defining the inductor L 2  at the second layer  120 .  FIG. 6  is a diagram illustrating the positional relationship between the wiring electrode  122  defining the inductor L 2  at the second layer  120  and the wiring electrode  134  defining the inductor L 1  at the third layer  130 . 
     Referring to  FIG. 5 , the direction of a current at the wiring electrode  114  in the inductor L 1  is represented by a solid arrow and the direction of a current at the wiring electrode  122  in the inductor L 2  is represented by a broken arrow. At both the wiring electrodes  114  and  122 , the direction of a current is a clockwise direction. That is, the winding directions of the wiring electrodes  114  and  122  are the same or substantially the same. The wiring electrode  114  in the inductor L 1  and the wiring electrode  122  in the inductor L 2  partially overlap. In the overlapping portion, the directions of currents flowing through the wiring electrodes  114  and  122  are the same or substantially the same. Accordingly, magnetic fields generated at the wiring electrodes  114  and  122  are mutually strengthened and the wiring electrodes  114  and  122  are electromagnetically coupled. 
     In this preferred embodiment, the first layer  110  at which the wiring electrode  114  is provided and the second layer  120  at which the wiring electrode  122  is provided are adjacent to each other. Accordingly, the degree of electromagnetic coupling between the wiring electrodes  114  and  122  is able to be increased. 
     Referring to  FIG. 6 , the direction of a current at the wiring electrode  122  in the inductor L 2  is represented by a broken arrow and the direction of a current at the wiring electrode  134  in the inductor L 1  is represented by a solid arrow. At both the wiring electrodes  122  and  134 , the direction of a current is a clockwise direction. That is, the winding directions of the wiring electrodes  122  and  134  are the same or substantially the same. The wiring electrode  122  in the inductor L 2  and the wiring electrode  134  in the inductor L 1  partially overlap. In the overlapping portion, the directions of currents flowing through the wiring electrodes  122  and  134  are substantially the same. Accordingly, magnetic fields generated at the wiring electrodes  122  and  134  are mutually strengthened and the wiring electrodes  122  and  134  are electromagnetically coupled. 
     In this preferred embodiment, the second layer  120  at which the wiring electrode  122  is provided and the third layer  130  at which the wiring electrode  134  is provided are adjacent to each other. Accordingly, the degree of electromagnetic coupling between the wiring electrodes  122  and  134  is able to be increased. 
       FIGS. 7 and 8  are diagrams illustrating simulation results of bandpass characteristics of a band elimination filter in Example 1 and Comparative Example 1. Example 1 of a composite filter device is the composite filter device  12  described with reference to  FIGS. 1 to 6 . In contrast, Comparative Example 1 of a composite filter device includes a wiring electrode in an inductor corresponding to the inductor L 1  and a wiring electrode in an inductor corresponding to the inductor L 2  are spaced apart from each other not to overlap each other. That is, in Comparative Example 1, the two inductors corresponding to the inductors L 1  and L 2  are not electromagnetically coupled. The bandpass characteristics of a band elimination filter refer to bandpass characteristics provided by including the inductor L 1  (an inductor corresponding to the inductor L 1  in Comparative Example 1) in addition to the band elimination filter  18 . 
       FIG. 7  is a diagram illustrating the bandpass characteristics of a band elimination filter in the frequency band of about 0 MHz to about 6000 MHz. A solid line represents a simulation result of Example 1 and a broken line represents a simulation result of Comparative Example 1. Similarly, a solid line represents a simulation result of Example 1 and a broken line represents a simulation result of Comparative Example 1 in  FIGS. 8 to 10 . Referring to  FIG. 7 , LB (Low Band) corresponds to the frequency band of about 600 MHz to about 1000 MHz, MB (Middle Band) corresponds to the frequency band of about 1700 MHz to about 2000 MHz, HB (High Band) corresponds to the frequency band of about 2000 MHz to about 2700 MHz, and UHB (Ultra High Band) represents the frequency band of about 3300 MHz to about 4200 MHz.  FIG. 7  shows that the insertion loss is reduced in the frequency band equal to or higher than about 3000 MHz which includes UHB. 
     In Example 1, as described above, the inductors L 1  and L 2  are electromagnetically coupled to each other. Accordingly, in Example 1, a predetermined inductance is able to be provided because the wiring electrode is shorter than the electrode wiring in Comparative Example 1 in which the inductors L 1  and L 2  are not electromagnetically coupled to each other. By reducing the length of a wiring electrode, stray capacitance is able be significantly reduced and the self-resonant frequency of an inductor is able to be increased. An inductor connected in series with a stray capacitance operates as a low pass filter. Accordingly, when a self-resonant frequency becomes relatively high, a cutoff frequency also becomes relatively high. For this reason, by reducing a stray capacitance, the insertion loss of a band elimination filter in a high frequency band including UHB can be reduced while a predetermined inductance is provided. 
       FIG. 8  is a diagram illustrating simulation results of bandpass characteristics in a frequency band near a stop band in Example 1 and Comparative Example 1. A signal is attenuated in the stop band in Example 1, similar to Comparative Example 1. Thus, in Example 1, the insertion loss of the band elimination filter in the high frequency band is able to be significantly improved while the band elimination filter performs signal attenuation. 
       FIG. 9  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter in Example 1 and Comparative Example 1.  FIG. 10  is a diagram illustrating simulation results of bandpass characteristics of a band pass filter of Example 1 and Comparative Example 1 in a frequency band near the pass band. As illustrated in  FIGS. 9 and 10 , there is no significant difference between the bandpass characteristics of a band pass filter in Example 1 and Comparative Example 1 and both of the band pass filter are able to pass a signal in the pass band. Thus, in Example 1, the insertion loss of the band elimination filter in the high frequency band can be reduced while the characteristics of the band pass filter are maintained. 
     Next, the impedance characteristics of a band elimination filter in Example 1 and Comparative Example 1 will be described with reference to  FIGS. 11 to 16 . The impedance characteristics of a band elimination filter refer to impedance characteristics provided by including the inductor L 1  (an inductor corresponding to the inductor L 1  in Comparative Example 1) and the inductor L 2  (an inductor corresponding to the inductor L 2  in Comparative Example 1) in addition to the band elimination filter  18 .  FIGS. 11 to 13  are Smith charts illustrating the impedance characteristics of the composite filter device  12  when they are viewed from the common terminal T 0  in Example 1 and Comparative Example 1.  FIGS. 14 to 16  are Smith charts illustrating the impedance characteristics of a band elimination filter when they are viewed from the second terminal T 2  in Example 1 and Comparative Example 1. More specifically,  FIGS. 11 and 14  are Smith charts in the frequency band of LB,  FIGS. 12 and 15  are Smith charts in the frequency bands of MB and HB, and  FIGS. 13 and 16  are Smith charts in the frequency band of UHB. In all of these drawings, a solid line represents the impedance characteristics of Example 1 and a broken line represents the impedance characteristics of Comparative Example 1.  FIGS. 11, 12, 14, and 15  show that there is no significant difference between the impedance characteristics of Example 1 and Comparative Example 1 in the frequency bands of LB, MB, and HB. 
     However,  FIGS. 13 and 16  show that an impedance is converged at about 50Ω and is comparatively matched to about 50Ω in Example 1, in contrast to Comparative Example 1. The reason for this is that the inductors L 1  and L 2  have been electromagnetically coupled, the length of a wiring electrode required to provide a predetermined inductance has been able to be significantly reduced, and the occurrence of a stray capacitance has been able to be reduced or prevented as described above. Thus, according to this preferred embodiment, impedance matching can be performed in the high frequency band including UHB and the insertion loss in the pass band are able to be significantly improved. 
     In the second preferred embodiment, descriptions of features, components, and elements that common to those in the first preferred embodiment will be omitted and only different points will be described. 
       FIG. 17  is a diagram illustrating a band pass filter  2  according to the second preferred embodiment. The band pass filter  2  according to the second preferred embodiment includes a plurality of series arm resonators provided at a series arm connecting an input terminal T 3  and an output terminal T 4  and a plurality of parallel arm resonators provided at parallel arms. More specifically, the band pass filter  2  according to the second preferred embodiment includes the four series arm resonators S 31  to S 34 , the three parallel arm resonators P 31  to P 33 , an inductor L 5  (a third inductor), and an inductor L 6  (fourth inductor). The four series arm resonators S 31  to S 34  are provided at the series arm connecting the input terminal T 3  and the output terminal T 4  in order of increasing distance from the input terminal T 3 . The three parallel arm resonators P 31  to P 33  are provided at respective parallel arms, and a reference potential (for example, a ground potential) is supplied to the parallel arm resonators P 31  to P 33 . The numbers of the series arm resonators, the parallel arm resonators, and the inductors included in the band pass filter  2  are examples and are not limited to the above-described numbers. 
     In this preferred embodiment, the inductor L 5  is connected in series with the input terminal T 3 . The inductor L 6  is connected in parallel with the series arm resonator S 31 . Various wiring electrodes including wiring electrodes defining portions of the inductors L 5  and L 6  are provided at the internal layers of a multilayer board having a layered structure.  FIGS. 18 to 20  are diagrams illustrating the layout of wiring electrodes in the band pass filter  2  according to the second preferred embodiment. When a direction extending from the back to the front of the drawing sheet is defined as a direction from bottom to top in  FIGS. 18 to 20 , a first layer  210  illustrated in  FIG. 18 , a second layer  220  illustrated in  FIG. 19 , and a third layer  230  illustrated in  FIG. 20  are laminated in this order from the top. Respective adjacent layers are laminated on the upper side of the first layer  210  and the underside of the third layer  230 . 
     At the first layer  210  illustrated in  FIG. 18 , a wiring electrode  212  and a wiring electrode  214  (a fourth wiring electrode) are provided. The wiring electrode  214  defines a portion of the inductor L 6 . A terminal W 1  of the wiring electrode  212  is connected to a wiring electrode that is provided at a layer on the upper side of the first layer  210  and is connected to the input terminal T 3 . A terminal Y 1  of the wiring electrode  214  is connected to a wiring electrode that is provided at a layer on the upper side of the first layer  210  and is connected to a node  202  illustrated in  FIG. 17 . 
     At the second layer  220  illustrated in  FIG. 19 , a wiring electrode  222  (the fourth wiring electrode) is provided. The wiring electrode  222  defines a portion of the inductor L 6 . A terminal W 2  of the wiring electrode  222  is connected to the terminal W 1  of the wiring electrode  212  at the first layer  210 . A terminal X 2  of the wiring electrode  222  is connected to a terminal X 1  of the wiring electrode  214  at the first layer  210 . Thus, in this preferred embodiment, the wiring electrode  214  at the first layer  210  and the wiring electrode  222  at the second layer  220  define a portion of the inductor L 6  in an integrated manner. 
     At the third layer  230  illustrated in  FIG. 20 , a wiring electrode  232  (a third wiring electrode) is provided. The wiring electrode  232  defines a portion of the inductor L 5 . A terminal V 2  of the wiring electrode  232  is connected to a terminal V 1  of the wiring electrode  222  at the second layer  220 . Accordingly, the inductors L 5  and L 6  are connected by the terminals V 1  and V 2 . A terminal U 1  of the wiring electrode  232  at the third layer  230  is connected to a wiring electrode that is provided at a layer on the underside of the third layer  230  and is connected to the input terminal T 3 . 
     The positional relationship between the wiring electrode defining the inductor L 5  and the wiring electrode defining the inductor L 6  will be described with reference to  FIGS. 21 and 22 .  FIG. 21  is a diagram illustrating the positional relationship between the wiring electrode  214  defining the inductor L 6  at the first layer  210  and the wiring electrode  232  defining the inductor L 5  at the third layer  230  when a multilayer board is viewed in plan. Referring to  FIG. 21 , a solid arrow represents the direction of a current flowing through the wiring electrode  214  and a broken arrow represents the direction of a current flowing through the wiring electrode  232 . At both the wiring electrodes  214  and  232 , the direction of a current is a clockwise direction. Accordingly, the winding directions of the wiring electrodes  214  and  232  are the same or substantially the same. 
     The wiring electrodes  214  and  232  partially overlap. In the overlapping portion, the directions of currents flowing through the wiring electrodes  214  and  232  are substantially the same. Accordingly, the degree of electromagnetic coupling between the wiring electrodes  214  and  232  is relatively high. 
       FIG. 22  is a diagram illustrating the positional relationship between the wiring electrode  222  defining the inductor L 6  at the second layer  220  and the wiring electrode  232  defining the inductor L 5  at the third layer  230  when a multilayer board is viewed in plan. Referring to  FIG. 22 , a solid arrow represents the direction of a current flowing through the wiring electrode  222  and a broken arrow represents the direction of a current flowing through the wiring electrode  232 . At both the wiring electrodes  222  and  232 , the direction of a current is a clockwise direction. Accordingly, the winding directions of the wiring electrodes  222  and  232  are the same or substantially the same. 
     The wiring electrodes  222  and  232  partially overlap. In the overlapping portion, the directions of currents flowing through the wiring electrodes  222  and  232  are substantially the same. Accordingly, the degree of electromagnetic coupling between the wiring electrodes  222  and  232  is relatively high. 
       FIGS. 23 and 24  are diagrams illustrating simulation results of bandpass characteristics of a band pass filter of Example 2 and Comparative Example 2. Example 2 of a band pass filter is the band pass filter  2  described with reference to  FIGS. 17 to 22 . In contrast, Comparative Example 2 shows a band pass filter in which the wiring electrode defining the inductor L 5  and the wiring electrode defining the inductor L 6  do not overlap. Accordingly, in Comparative Example 2, two inductors corresponding to the inductors L 5  and L 6  are not electromagnetically coupled. 
     As illustrated in  FIG. 23 , the insertion loss on the higher-frequency side is significantly reduced in Example 2 as compared with Comparative Example 2. In Example 2, the degree of the electromagnetic coupling between the inductors L 5  and L 6  is relatively high. Accordingly, the length of a wiring electrode required to provide a predetermined inductance is able to be significantly reduced and the occurrence of a stray capacitance is able to be reduced or prevented. Accordingly, the self-resonant frequencies of the inductors are increased and bandpass characteristics in the pass band are improved. 
     As illustrated in  FIG. 24 , there is no significant difference in bandpass characteristics in a frequency band outside the pass band between Example 2 and Comparative Example 2 of a band pass filter and they both pass a signal in the pass band. The pole at about 1900 MHz is an attenuation pole. 
     The preferred embodiments described above are intended to help easily understand the present invention and are not to be used to limit the present invention. Elements included in the preferred embodiments and the arrangements, materials, conditions, shapes, sizes, and the like are not limited to those illustrated as examples but may be modified as appropriate. Furthermore, the features, components, and elements described in the different preferred embodiments can be partially replaced or combined. 
     For example, in the preferred embodiments described above, two wiring electrodes overlapping when viewed in plan are provided at adjacent layers, but do not necessarily have to be provided at adjacent layers. 
     In the first preferred embodiment, the wiring electrode  122  defining a portion of the inductor L 2  is sandwiched between the wiring electrodes  114  and  134  defining the inductor L 1 . The wiring electrode defining at least a portion of the inductor L 1  may be sandwiched between the wiring electrodes defining at least a portion of the inductor L 2 . In a band pass filter according to the second preferred embodiment, the wiring electrode defining at least a portion of the inductor L 5  may be sandwiched between the wiring electrodes defining at least a portion of the inductor L 6 . Alternatively, the wiring electrode defining at least a portion of the inductor L 6  may be sandwiched between the wiring electrodes defining at least a portion of the inductor L 5 . 
     In the first preferred embodiment, the inductor L 1  is connected in series between the band elimination filter  18  and the node  102 . However, an inductor (i.e., an inductor corresponding to the inductor L 1 ) including a wiring electrode at least partially overlapping the wiring electrode in the inductor L 2  when a multilayer board is viewed in plan may be connected in series between a filter other than the band elimination filter  18  (for example, the band pass filter  14 ) and the node  102 . Accordingly, the inductor L 1  does not necessarily have to be connected in series between the band elimination filter  18  and the node  102 . 
     In the second preferred embodiment, the inductor L 6  is connected in parallel with the series arm resonator S 31  nearest to the input terminal T 3 . However, an inductor (i.e., an inductor corresponding to the inductor L 6 ) including a wiring electrode at least partially overlapping the wiring electrode in the inductor L 5  when a multilayer board is viewed in plan may be connected in parallel with any one of the series arm resonators included in the band pass filter. 
     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.