Patent Publication Number: US-2022231655-A1

Title: Multilayer filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-218719 filed on Dec. 3, 2019 and is a Continuation application of PCT Application No. PCT/JP2020/040084 filed on Oct. 26, 2020. 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 multilayer filter. 
     2. Description of the Related Art 
     In the related art, a multilayer filter includes a multilayer body including a plurality of dielectric layers. For example, International Publication No. 2002/009225 discloses a laminated bandpass filter. In the laminated bandpass filter, two strip lines forming a resonator are placed at a certain distance from each other on the same layer. The two strip lines are placed in parallel with each other to thus make it possible to produce electromagnetic coupling. As a result, a capacitor between resonators can be rendered unnecessary, and a low-profile laminated body can be provided. 
     Furthermore, International Publication No. 2007/119356 discloses a multilayer bandpass filter in which the direction of a loop formed by an inductor electrode of an input-side LC parallel resonator is opposite to the direction of a loop formed by an inductor electrode of an LC parallel resonator adjacent to the inductor electrode of the input-side LC parallel resonator. Furthermore, there is disclosed a multilayer bandpass filter in which three ground electrodes separated from one another are formed on a ground electrode formation layer. 
     To improve attenuation characteristics (frequency characteristics of insertion loss outside a pass band) of a multilayer filter, electromagnetic coupling (electric field coupling and magnetic field coupling) between resonators has to be balanced, and both an increase in the steepness of a change in attenuation at boundaries of the pass band and the provision of attenuation outside the pass band have to be achieved. In the laminated bandpass filter disclosed in International Publication No. 2002/009225, however, the balance of electromagnetic coupling that occurs between the two strip lines placed in parallel is not considered. 
     As disclosed in International Publication No. 2007/119356, when the directions of loops formed by inductor electrodes of two LC parallel resonators adjacent to each other are opposite to each other, magnetic field coupling can be weakened, but it is difficult to finely adjust magnetic field coupling. Furthermore, when a plurality of ground electrodes are formed on the same dielectric layer, manufacturing variations are likely to occur. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide multilayer filters each having improve attenuation characteristics. 
     A multilayer filter according to a preferred embodiment of the present invention includes a first ground electrode and a second ground electrode, a first LC resonator, and a second LC resonator. The first LC resonator is connected to the first ground electrode. The second LC resonator is connected to the second ground electrode. The first LC resonator includes a first line electrode, a first capacitor electrode, a first via conductor, and a second via conductor. The first capacitor electrode is between the first ground electrode and the first line electrode. The first via conductor connects the first line electrode and the first capacitor electrode. The second via conductor extends from the first line electrode to a first capacitor electrode side and connects the first line electrode and the first ground electrode. The second LC resonator includes a second line electrode, a second capacitor electrode, a third via conductor, and a fourth via conductor. The second capacitor electrode is between the second ground electrode and the second line electrode. The third via conductor connects the second line electrode and the second capacitor electrode. The fourth via conductor extends from the second line electrode to a second capacitor electrode side and connects the second line electrode and the second ground electrode. 
     In a multilayer filter according to preferred embodiment of the present invention, the first LC resonator is connected to the first ground electrode, and the second LC resonator is also connected to the second ground electrode, thus enabling an improvement in attenuation characteristics. 
     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 an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 1 of the present invention. 
         FIG. 2  is an external perspective view of the multilayer filter in  FIG. 1 . 
         FIG. 3  illustrates a plurality of electrodes provided in a multilayer body in  FIG. 2 . 
         FIG. 4  illustrates the multilayer filter in  FIG. 2  as viewed in plan from an X-axis direction (second direction). 
         FIG. 5  illustrates the multilayer filter in  FIG. 2  as viewed in plan from a Y-axis direction (first direction). 
         FIG. 6  is an equivalent circuit diagram of a multilayer filter according to a comparative example. 
         FIG. 7  illustrates a bandpass characteristic (solid line) of the multilayer filter in  FIGS. 1 to 5  and a bandpass characteristic (dotted line) of the multilayer filter in  FIG. 6 . 
         FIG. 8  illustrates changes in insertion loss of the multilayer filter when ground via conductors in  FIG. 4  are moved in the Y-axis direction. 
         FIG. 9  illustrates an electrode structure in a multilayer body of a multilayer filter according to a modification of Preferred Embodiment 1 of the present invention as viewed in plan from the Y-axis direction. 
         FIG. 10  is an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 2 of the present invention. 
         FIG. 11  illustrates an electrode structure in a multilayer body of the multilayer filter in  FIG. 10  as viewed in plan from the Y-axis direction. 
         FIG. 12  illustrates a bandpass characteristic (solid line) of the multilayer filter in  FIG. 11  and a bandpass characteristic (dotted line) of the multilayer filter in  FIG. 5 . 
         FIG. 13  is an enlarged view of a portion ranging from about 4 GHz to about 7 GHz in  FIG. 12 . 
         FIG. 14  is an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 3 of the present invention. 
         FIG. 15  is an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 4 of the present invention. 
         FIG. 16  is an equivalent circuit diagram of a multilayer filter according to a modification of Preferred Embodiment 4 of the present invention. 
         FIG. 17  is an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 5 of the present invention. 
         FIG. 18  is an equivalent circuit diagram of a multilayer filter according to a modification of Preferred Embodiment 5 of the present invention. 
         FIG. 19  is an equivalent circuit diagram of a multilayer filter according to Preferred Embodiment 6 of the present invention. 
         FIG. 20  is an equivalent circuit diagram of a multilayer filter according to a modification of Preferred Embodiment 6 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below with reference to the drawings. The same or corresponding elements or portions in the drawings are denoted by the same reference signs and a repeated description thereof may be omitted. 
     Preferred Embodiment 1 
       FIG. 1  is an equivalent circuit diagram of a multilayer filter  1  according to Preferred Embodiment 1 of the present invention. An equivalent circuit illustrated in  FIG. 1  is the same as or similar to an equivalent circuit of a multilayer filter  1 A according to a modification of Preferred Embodiment 1 to be described below. In the following description, electric field coupling occurring between two circuit elements is represented by a capacitor. In other words, the two circuit elements are connected to each other via the capacitor. Furthermore, among cases where two circuit elements are electrically connected to each other are a case where the two circuit elements are directly connected to each other and a case where the two circuit elements are coupled to each other through electric field coupling (capacitive coupling). 
     As illustrated in  FIG. 1 , the multilayer filter  1  includes an input/output terminal P 1  (first terminal), an input/output terminal P 2  (second terminal), an LC parallel resonator LC 1  (first LC resonator), an LC parallel resonator LC 2  (second LC resonator), an LC parallel resonator LC 3  (fourth LC resonator), an LC parallel resonator LC 4  (third LC resonator), and capacitors C 10 , C 12 , C 14 , C 20 , C 23 , and C 34 . 
     In  FIG. 1 , the LC parallel resonators LC 1  and LC 2  are coupled to each other through electromagnetic field coupling. Magnetic field coupling M 1  and the capacitor C 12  respectively denote magnetic field coupling and electric field coupling between the LC parallel resonators LC 1  and LC 2 . The LC parallel resonators LC 2  and LC 3  are coupled to each other through electromagnetic field coupling. Magnetic field coupling M 2  and the capacitor C 23  respectively denote magnetic field coupling and electric field coupling between the LC parallel resonators LC 2  and LC 3 . The LC parallel resonators LC 3  and LC 4  are coupled to each other through electromagnetic field coupling. Magnetic field coupling M 3  and the capacitor C 34  respectively denote magnetic field coupling and electric field coupling between the LC parallel resonators LC 3  and LC 4 . In some cases, electric field coupling is achieved by a capacitor. 
     The input/output terminal P 1  is electrically connected to the LC parallel resonator LC 1 .  FIG. 1  illustrates the case where the input/output terminal P 1  is connected to the LC parallel resonator LC 1  via the capacitor C 10 . The input/output terminal P 1  may be directly connected to the LC parallel resonator LC 1 . 
     The input/output terminal P 2  is electrically connected to the LC parallel resonator LC 4 .  FIG. 1  illustrates the case where the input/output terminal P 2  is connected to the LC parallel resonator LC 4  via the capacitor C 20 . The input/output terminal P 2  may be directly connected to the LC parallel resonator LC 4 . 
     The LC parallel resonators LC 1  and LC 2  are connected to each other via the capacitor C 12 . The LC parallel resonators LC 2  and LC 3  are connected to each other via the capacitor C 23 . The LC parallel resonators LC 3  and LC 4  are connected to each other via the capacitor C 34 . The LC parallel resonators LC 1  and LC 4  are connected to each other via the capacitor C 14 . 
     The LC parallel resonator LC 1  includes an inductor L 1  and a capacitor C 1 . The inductor L 1  and the capacitor C 1  are connected in parallel between a ground point G 1  and a connection point between the capacitors C 10  and C 12 . 
     The LC parallel resonator LC 2  includes an inductor L 2  and a capacitor C 2 . The inductor L 2  and the capacitor C 2  are connected in parallel between a ground point G 2  and a connection point between the capacitors C 12  and C 23 . 
     The LC parallel resonator LC 3  includes an inductor L 3  and a capacitor C 3 . The inductor L 3  and the capacitor C 3  are connected in parallel between the ground point G 2  and a connection point between the capacitors C 23  and C 34 . 
     The LC parallel resonator LC 4  includes an inductor L 4  and a capacitor C 4 . The inductor L 4  and the capacitor C 4  are connected in parallel between the ground point G 1  and a connection point between the capacitors C 34  and C 20 . 
       FIG. 2  is an external perspective view of the multilayer filter  1  in  FIG. 1 . In  FIG. 2 , the X-axis, the Y-axis, and the Z-axis are orthogonal or substantially orthogonal to each other. The same applies to  FIGS. 3 to 5, 9, and 11  to be described. 
     As illustrated in  FIG. 2 , the multilayer filter  1  includes a multilayer body  100  including a plurality of dielectric layers that are laminated in a Z-axis direction. The multilayer body  100  has, for example, a rectangular or substantially rectangular parallelepiped shape. Surfaces of outermost layers of the multilayer body  100  that are perpendicular or substantially perpendicular to the Z-axis direction are an upper surface UF and a bottom surface BF. The upper surface UF and the bottom surface BF face each other in the Z-axis direction. 
     On the bottom surface BF, the input/output terminals P 1  and P 2 , and a ground terminal  110  are provided. The input/output terminals P 1  and P 2 , and the ground terminal  110  are, for example, LGA (Land Grid Array) terminals in which planar electrodes are regularly provided on the bottom surface BF. The bottom surface BF is connected to a circuit board that is not illustrated. 
       FIG. 3  illustrates a plurality of electrodes provided in the multilayer body  100  in  FIG. 2 .  FIG. 4  illustrates the multilayer filter  1  in  FIG. 2  as viewed in plan from an X-axis direction (second direction).  FIG. 5  illustrates the multilayer filter  1  in  FIG. 2  as viewed in plan from a Y-axis direction (first direction). 
     Referring to  FIGS. 1 and 3 to 5 , in the multilayer body  100 , a ground electrode  111  (first ground electrode) and a ground electrode  112  (second ground electrode) are provided. The ground electrode  111  is disposed between the ground terminal  110  and the ground electrode  112 . The ground electrode  111  is connected to the ground terminal  110  by via conductors V 1 , V 2 , V 3 , V 4 , V 5 , and V 6  (second via conductors). The ground electrode  112  is connected to the ground electrode  111  by each of a via conductor V 81  (first ground via conductor) and a via conductor V 82  (first ground via conductor). The ground electrodes  111  and  112  correspond to the respective ground points G 1  and G 2  in  FIG. 1 . The ground electrodes  111  and  112  are provided as separate conductors. 
     The LC parallel resonator LC 1  includes a line electrode  101  (first line electrode), a capacitor electrode  102  (first capacitor electrode), a capacitor electrode  103 , a via conductor V 11  (first via conductor), and a via conductor V 12  (second via conductor). The line electrode  101  extends in the Y-axis direction. The capacitor electrode  102  is disposed between the ground electrode  112  and the line electrode  101 . The via conductor V 11  connects the line electrode  101  and the capacitor electrode  102 . The via conductor V 12  extends from the line electrode  101  to a capacitor electrode  102  side and connects the line electrode  101  and the ground electrode  111 . The capacitor electrode  103  is disposed between the capacitor electrode  102  and the ground electrode  112 . The capacitor electrode  103  is connected to the ground electrode  112  by a via conductor V 13 . In a direction in which the via conductor V 11  extends, a distance between the line electrode  101  and the ground electrode  112  is shorter than a distance between the line electrode  101  and the ground electrode  111 . 
     A capacitor electrode  104  (fifth capacitor electrode) is disposed between the input/output terminal P 1  and the capacitor electrode  102 . The capacitor electrode  104  is connected to the input/output terminal P 1  by a via conductor V 10 . 
     The capacitor C 10  includes the capacitor electrodes  102  and  104 . The loop-shaped inductor L 1  includes the via conductor V 12 , the line electrode  101 , and the via conductor V 11 . The capacitor C 1  includes the capacitor electrodes  102  and  103 . 
     The LC parallel resonator LC 2  includes a line electrode  201  (second line electrode), a capacitor electrode  202  (second capacitor electrode), a capacitor electrode  203 , a via conductor V 21  (third via conductor), and a via conductor V 22  (fourth via conductor). The line electrode  201  extends in the Y-axis direction. The capacitor electrode  202  is disposed between the ground electrode  112  and the line electrode  201 . The via conductor V 21  connects the line electrode  201  and the capacitor electrode  202 . The via conductor V 22  extends from the line electrode  201  to a capacitor electrode  202  side and connects the line electrode  201  and the ground electrode  112 . The capacitor electrode  203  is disposed between the capacitor electrode  202  and the ground electrode  112 . The capacitor electrode  203  is connected to the ground electrode  112  by a via conductor V 23 . 
     The loop-shaped inductor L 2  includes the via conductor V 22 , the line electrode  201 , and the via conductor V 21 . The capacitor C 2  includes the capacitor electrodes  202  and  203 . 
     A coupling electrode  121  (third coupling electrode) is connected to the via conductor V 11  between the line electrode  101  and the capacitor electrode  102 . The coupling electrode  121  faces each of the capacitor electrodes  102  and  202 . The capacitor C 12  includes the capacitor electrodes  102  and  202  and the coupling electrode  121 . 
     The LC parallel resonator LC 3  includes a line electrode  301  (second line electrode), a capacitor electrode  302  (second capacitor electrode), a capacitor electrode  303 , a via conductor V 31  (third via conductor), and a via conductor V 32  (fourth via conductor). The line electrode  301  extends in the Y-axis direction. The capacitor electrode  302  is disposed between the ground electrode  112  and the line electrode  301 . The via conductor V 31  connects the line electrode  301  and the capacitor electrode  302 . The via conductor V 32  extends from the line electrode  301  to a capacitor electrode  302  side and connects the line electrode  301  and the ground electrode  112 . The capacitor electrode  303  is disposed between the capacitor electrode  302  and the ground electrode  112 . The capacitor electrode  303  is connected to the ground electrode  112  by a via conductor V 33 . 
     The loop-shaped inductor L 3  includes the via conductor V 32 , the line electrode  301 , and the via conductor V 31 . The capacitor C 3  includes the capacitor electrodes  302  and  303 . 
     A coupling electrode  222  is disposed between the line electrode  201  and the capacitor electrode  202 . The coupling electrode  222  faces each of the capacitor electrodes  202  and  302 . The capacitor C 23  includes the capacitor electrodes  202  and  302  and the coupling electrode  222 . 
     The LC parallel resonator LC 4  includes a line electrode  401  (third line electrode), a capacitor electrode  402  (third capacitor electrode), a capacitor electrode  403 , a via conductor V 41  (fifth via conductor), and a via conductor V 42  (sixth via conductor). The line electrode  401  extends in the Y-axis direction. The capacitor electrode  402  is disposed between the ground electrode  112  and the line electrode  401 . The via conductor V 41  connects the line electrode  401  and the capacitor electrode  402 . The via conductor V 42  extends from the line electrode  401  to a capacitor electrode  402  side and connects the line electrode  401  and the ground electrode  111 . The capacitor electrode  403  is disposed between the capacitor electrode  402  and the ground electrode  112 . The capacitor electrode  403  is connected to the ground electrode  112  by a via conductor V 43 . 
     A capacitor electrode  404  (sixth capacitor electrode) is disposed between the input/output terminal P 2  and the capacitor electrode  402 . The capacitor electrode  404  is connected to the input/output terminal P 2  by a via conductor V 20 . 
     The capacitor C 20  includes the capacitor electrodes  402  and  404 . The loop-shaped inductor L 4  includes the via conductor V 42 , the line electrode  401 , and the via conductor V 41 . The capacitor C 4  includes the capacitor electrodes  402  and  403 . 
     A coupling electrode  221  is disposed between the line electrode  201  and the capacitor electrode  202 . The coupling electrode  221  faces each of coupling electrodes  121  and  122 . The capacitor C 14  includes the coupling electrodes  121 ,  122 , and  221 . 
     Referring to  FIG. 4 , in the Y-axis direction, each of the via conductors V 21 , V 31 , and V 41  is closer to the via conductor V 11  than the via conductor V 12  is. In the Y-axis direction, the via conductors V 22 , V 32 , and V 42  are closer to the via conductor V 12  than the via conductor V 11  is. Winding directions of the inductors L 1  to L 4  from the corresponding ground electrodes to the respective capacitor electrodes are the same. Air core portions of the inductors L 1  to L 4  coincide with each other. In the Y-axis direction, the via conductors V 81  and V 82  are closer to the via conductor V 11  than the via conductor V 12  is. 
       FIG. 6  is an equivalent circuit diagram of a multilayer filter  10  according to a comparative example. An equivalent circuit of the multilayer filter  10  is an equivalent circuit obtained by removing the ground point G 2  from the equivalent circuit of the multilayer filter  1  in  FIG. 1  and short-circuiting each of the LC parallel resonators LC 1  to LC 4  to the ground point G 1 . Configurations other than these are the same as or similar to those in the multilayer filter  1  in  FIG. 1 , and thus a repeated description thereof is not provided. 
       FIG. 7  illustrates a bandpass characteristic (solid line) of the multilayer filter  1  in  FIGS. 1 to 5  and a bandpass characteristic (dotted line) of the multilayer filter  10  in  FIG. 6 . A bandpass characteristic refers to a frequency characteristic of insertion loss. In  FIG. 7 , attenuation represented by the vertical axis increases downward from 0 dB. The same applies to  FIGS. 8, 12, and 13  to be described. 
     Referring to  FIGS. 1, 5, and 7 , the ground point G 1  (ground electrode  111 ) to which each of the LC parallel resonators LC 1  and LC 4  is short-circuited and the ground point G 2  (ground electrode  112 ) to which each of the LC parallel resonators LC 2  and LC 3  is short-circuited are physically separated, thus reducing the magnetic field coupling M 1  and the magnetic field coupling M 3 . As a result, attenuation characteristics of the multilayer filter  1  are improved. 
     As illustrated in  FIG. 7 , in a frequency band ranging from about 2 GHz to about 4 GHz, a local minimum of insertion loss of the multilayer filter  1  is larger than that of the multilayer filter  10 . In a lower frequency band (on a lower-frequency side) than a pass band, the multilayer filter  1  can provide larger attenuation than the multilayer filter  10 . That is, the attenuation characteristics of the multilayer filter  1  are improved in comparison with attenuation characteristics of the multilayer filter  10 . 
     In the multilayer filter  1 , when the via conductors V 81  and V 82  in  FIG. 4  are moved in the Y-axis direction, an inductance component between the ground points G 1  and G 2  in  FIG. 1  changes. As a result, the frequency of an attenuation pole that appears on a higher-frequency side can be adjusted. 
       FIG. 8  illustrates changes in insertion loss of the multilayer filter  1  when the via conductors V 81  and V 82  in  FIG. 4  are moved in the Y-axis direction. In  FIG. 8 , a bandpass characteristic A 1  represents a bandpass characteristic when the via conductors V 81  and V 82  are disposed at locations illustrated in  FIG. 4 . A bandpass characteristic A 2  represents a bandpass characteristic when the via conductors V 81  and V 82  are closer to the via conductor V 12  than in the bandpass characteristic A 1 . A bandpass characteristic A 3  represents a bandpass characteristic when the via conductors V 81  and V 82  are closer to the via conductor V 12  than in the bandpass characteristic A 2 . 
     Referring to  FIGS. 4 and 8 , the bandpass characteristics A 1  to A 3  are almost the same in terms of bandpass characteristics in the pass band and on the lower-frequency side. In a higher frequency band (on the higher-frequency side) than the pass band of the bandpass characteristic A 1 , an attenuation pole appears at a frequency f 1 . On the higher-frequency side of the bandpass characteristic A 2 , an attenuation pole appears at a frequency f 2  (&gt;f 1 ). On the higher-frequency side of the bandpass characteristic A 3 , an attenuation pole appears at a frequency f 3  (&gt;f 2 ). When the via conductors V 81  and V 82  are brought close to the via conductor V 12 , the frequency of an attenuation pole that appears on the higher-frequency side can be increased. 
     In the multilayer filter  1 , the case has been described where the capacitor electrodes  102 ,  202 ,  302 , and  402  define the respective capacitors together with the capacitor electrodes  103 ,  203 ,  303 , and  403  connected to the ground electrode  112 . Each of the capacitor electrodes  102 ,  202 ,  302 , and  402  may define a capacitor together with the ground electrode  112 . 
       FIG. 9  illustrates an electrode structure in a multilayer body of the multilayer filter  1 A according to a modification of Preferred Embodiment 1 as viewed in plan from the Y-axis direction. The electrode structure of the multilayer filter  1 A is an electrode structure obtained by removing the capacitor electrodes  103 ,  203 ,  303 , and  403  and the via conductors V 13 , V 23 , V 33 , and V 43  from an electrode structure of the multilayer filter  1  in  FIG. 5 . Configurations other than these are the same as or similar to those in the multilayer filter  1  in  FIG. 5 , and thus a repeated description thereof is not given. 
     Referring to  FIGS. 1 and 9 , the capacitor electrodes  102 ,  202 ,  302 , and  402  face the ground electrode  112 . In the multilayer filter  1 A, the capacitor electrodes  102 ,  202 ,  302 , and  402  define the respective capacitors C 1  to C 4  together with the ground electrode  112 . When Preferred Embodiment 1 or the modification is selected in accordance with desired characteristics demanded of the multilayer filter, a distance between electrodes for each of the capacitors C 1  to C 4  can be adjusted. 
     Thus, the multilayer filters according to Preferred Embodiment 1 and the modification enable an improvement in attenuation characteristics. 
     Preferred Embodiment 2 
     In Preferred Embodiment 2 of the present invention, a configuration will be described in which two terminals of the multilayer filter according to Preferred Embodiment 1 are coupled to each other through electric field coupling to thus improve attenuation characteristics of the multilayer filter further. 
       FIG. 10  is an equivalent circuit diagram of a multilayer filter  2  according to Preferred Embodiment 2. As illustrated in  FIG. 10 , an equivalent circuit of the multilayer filter  2  is an equivalent circuit obtained by adding a capacitor C 22  to the equivalent circuit of the multilayer filter  1  in  FIG. 1 . Configurations other than these are the same as or similar to those in the multilayer filter  1  in  FIG. 1 , and thus a repeated description thereof is not provided. As illustrated in  FIG. 10 , the input/output terminals P 1  and P 2  are connected to each other via the capacitor C 22 . 
       FIG. 11  illustrates an electrode structure in a multilayer body of the multilayer filter  2  in  FIG. 10  as viewed in plan from the Y-axis direction. The electrode structure of the multilayer filter  2  is an electrode structure obtained by adding a coupling electrode  231  (first coupling electrode) and a coupling electrode  232  (second coupling electrode) to the electrode structure of the multilayer filter  1  in  FIG. 5 . Configurations other than these are the same as or similar to those in the multilayer filter  1  in  FIG. 5 , and thus a repeated description thereof is not provided. 
     As illustrated in  FIG. 11 , the coupling electrode  231  is connected to the via conductor V 10  between the ground electrodes  111  and  112  and extends from the input/output terminal P 1  toward the input/output terminal P 2 . The coupling electrode  232  is connected to the via conductor V 20  between the ground electrodes  111  and  112  and extends from the input/output terminal P 2  toward the input/output terminal P 1 . In a direction from the input/output terminal P 1  toward the input/output terminal P 2 , a side of the coupling electrode  231  faces a side of the coupling electrode  232 . The capacitor C 22  in  FIG. 10  includes the coupling electrodes  231  and  232 . 
       FIG. 12  illustrates a bandpass characteristic (solid line) of the multilayer filter  2  in  FIG. 11  and a bandpass characteristic (dotted line) of the multilayer filter  1  in  FIG. 5 . As illustrated in  FIG. 12 , in the multilayer filter  1 , an attenuation pole appears near a frequency f 21  on the lower-frequency side. On the other hand, in the multilayer filter  2 , an attenuation pole appears not only at the frequency f 21  on the lower-frequency side, but also at a frequency f 22  (&lt;f 21 ). As a result, on the lower-frequency side, a local minimum of insertion loss of the multilayer filter  2  is larger than that of the multilayer filter  1 . On the lower-frequency side, the multilayer filter  2  can provide larger attenuation than the multilayer filter  1 . That is, the attenuation characteristics of the multilayer filter  2  are improved in comparison with the attenuation characteristics of the multilayer filter  1 . 
       FIG. 13  is an enlarged view of a portion ranging from about 4 GHz to about 7 GHz in  FIG. 12 . As illustrated in  FIG. 13 , a minimum value of insertion loss of the multilayer filter  2  is the same or almost the same as a minimum value of insertion loss of the multilayer filter  1 . That is, with respect to the bandpass characteristic in the pass band of the multilayer filter  2 , the bandpass characteristic in the pass band of the multilayer filter  1  is maintained. 
     Thus, the multilayer filter according to Preferred Embodiment 2 enables a further improvement in attenuation characteristics while maintaining the bandpass characteristic in the pass band. 
     In Preferred Embodiments 1 and 2, the multilayer filters including four LC resonators have been described. The number of LC resonators included in a multilayer filter according to a preferred embodiment is not limited to four. In the following description, examples where the numbers of LC resonators included in multilayer filters according to preferred embodiments are 3, 5, 6, and 7 will be respectively described in Preferred Embodiments 3, 4, 5, and 6 of the present invention. 
     Preferred Embodiment 3 
       FIG. 14  is an equivalent circuit diagram of a multilayer filter  3  according to Preferred Embodiment 3 of the present invention. An equivalent circuit of the multilayer filter  3  is an equivalent circuit obtained by removing the LC parallel resonator LC 4  from the LC parallel resonators LC 1  to LC 4  included in the multilayer filter  1  in  FIG. 1 . Configurations other than changes based on the removal of the LC parallel resonator LC 4  are the same as or similar to those in the multilayer filter  1  in  FIG. 1 , and thus a repeated description thereof is not provided. 
     As illustrated in  FIG. 14 , the LC parallel resonator LC 3  (third LC resonator) is connected to the ground point G 1 . The LC parallel resonator LC 3  is connected to the input/output terminal P 2  via the capacitor C 20 . The LC parallel resonator LC 3  is connected to the LC parallel resonator LC 1  via a capacitor C 13 . 
     Thus, the multilayer filter according to Preferred Embodiment 3 enables an improvement in attenuation characteristics. 
     Preferred Embodiment 4 
       FIG. 15  is an equivalent circuit diagram of a multilayer filter  4  according to Preferred Embodiment 4 of the present invention. An equivalent circuit of the multilayer filter  4  is an equivalent circuit obtained by adding an LC parallel resonator LC 5  (third LC resonator) to the LC parallel resonators LC 1  to LC 4  included in the multilayer filter  1  in  FIG. 1 . Configurations other than changes based on the addition of the LC parallel resonator LC 5  are the same as or similar to those in the multilayer filter  1  in  FIG. 1 , and thus a repeated description thereof is not given. 
     As illustrated in  FIG. 15 , magnetic field coupling M 4  denotes magnetic field coupling between the LC parallel resonators LC 4  and LC 5 . The LC parallel resonator LC 5  is connected to the LC parallel resonator LC 4  (third LC resonator) via a capacitor C 45 . The LC parallel resonator LC 5  is connected to the LC parallel resonator LC 1  via a capacitor C 15 . The LC parallel resonator LC 5  is connected to the input/output terminal P 2  via the capacitor C 20 . The LC parallel resonator LC 5  is connected to the ground point G 1 . The LC parallel resonator LC 4  is connected to the ground point G 2 . 
     The LC parallel resonator LC 5  includes an inductor L 5  and a capacitor C 5 . The inductor L 5  and the capacitor C 5  are connected in parallel between the ground point G 1  and a connection point between the capacitors C 45  and C 20 . 
       FIG. 16  is an equivalent circuit diagram of a multilayer filter  4 A according to a modification of Preferred Embodiment 4. An equivalent circuit of the multilayer filter  4 A is an equivalent circuit obtained by changing the ground point to which each of the LC parallel resonators LC 1  and LC 5  in  FIG. 15  is connected from G 1  to G 2  and also changing the ground point to which the LC parallel resonator LC 3  is connected from G 2  to G 1 . Configurations other than these are the same as or similar to those in  FIG. 15 , and thus a repeated description thereof is not given. 
     Thus, the multilayer filters according to Preferred Embodiment 4 and the modification enable an improvement in attenuation characteristics. 
     Preferred Embodiment 5 
       FIG. 17  is an equivalent circuit diagram of a multilayer filter  5  according to Preferred Embodiment 5 of the present invention. An equivalent circuit of the multilayer filter  5  is an equivalent circuit obtained by adding an LC parallel resonator LC 6  (third LC resonator) to the LC parallel resonators LC 1  to LC 5  included in the multilayer filter  4  in  FIG. 15 . Configurations other than changes based on the addition of the LC parallel resonator LC 6  are the same as or similar to those in the multilayer filter  4  in  FIG. 15 , and thus a repeated description thereof is not provided. 
     As illustrated in  FIG. 17 , magnetic field coupling M 5  denotes magnetic field coupling between the LC parallel resonators LC 5  and LC 6 . The LC parallel resonator LC 6  is connected to the LC parallel resonator LC 5  (fourth LC resonator) via a capacitor C 56 . The LC parallel resonator LC 6  is connected to the LC parallel resonator LC 1  via a capacitor C 16 . The LC parallel resonator LC 6  is connected to the input/output terminal P 2  via the capacitor C 20 . The LC parallel resonator LC 6  is connected to the ground point G 1 . The LC parallel resonator LC 5  is connected to the ground point G 2 . 
     The LC parallel resonator LC 6  includes an inductor L 6  and a capacitor C 6 . The inductor L 6  and the capacitor C 6  are connected in parallel between the ground point G 1  and a connection point between the capacitors C 56  and C 20 . 
       FIG. 18  is an equivalent circuit diagram of a multilayer filter  5 A according to a modification of Preferred Embodiment 5. An equivalent circuit of the multilayer filter  5 A is an equivalent circuit obtained by changing the ground point to which each of the LC parallel resonators LC 1  and LC 6  in  FIG. 17  is connected from G 1  to G 2  and also changing the ground point to which each of the LC parallel resonators LC 3  and LC 4  is connected from G 2  to G 1 . Configurations other than these are the same as or similar to those in  FIG. 17 , and thus a repeated description thereof is not given. 
     Thus, the multilayer filters according to Preferred Embodiment 5 and the modification enable an improvement in attenuation characteristics. 
     Preferred Embodiment 6 
       FIG. 19  is an equivalent circuit diagram of a multilayer filter  6  according to Preferred Embodiment 6 of the present invention. An equivalent circuit of the multilayer filter  6  is an equivalent circuit obtained by adding an LC parallel resonator LC 7  (third LC resonator) to the LC parallel resonators LC 1  to LC 6  included in the multilayer filter  5  in  FIG. 17 . Configurations other than changes based on the addition of the LC parallel resonator LC 7  are the same as or similar to those in the multilayer filter  5  in  FIG. 17 , and thus a repeated description thereof is not provided. 
     As illustrated in  FIG. 19 , magnetic field coupling M 6  denotes magnetic field coupling between the LC parallel resonators LC 6  and LC 7 . The LC parallel resonator LC 7  is connected to the LC parallel resonator LC 6  (fourth LC resonator) via a capacitor C 67 . The LC parallel resonator LC 7  is connected to the LC parallel resonator LC 1  via a capacitor C 17 . The LC parallel resonator LC 7  is connected to the input/output terminal P 2  via the capacitor C 20 . The LC parallel resonator LC 7  is connected to the ground point G 1 . The LC parallel resonator LC 6  is connected to the ground point G 2 . 
     The LC parallel resonator LC 7  includes an inductor L 7  and a capacitor C 7 . The inductor L 7  and the capacitor C 7  are connected in parallel between the ground point G 1  and a connection point between the capacitors C 67  and C 20 . 
       FIG. 20  is an equivalent circuit diagram of a multilayer filter  6 A according to a modification of Preferred Embodiment 6. An equivalent circuit of the multilayer filter  6 A is an equivalent circuit obtained by changing the ground point to which each of the LC parallel resonators LC 1  and LC 7  in  FIG. 19  is connected from G 1  to G 2  and also changing the ground point to which the LC parallel resonator LC 4  is connected from G 2  to G 1 . Configurations other than these are the same as or similar to those in  FIG. 19 , and thus a repeated description thereof is not given. 
     Thus, the multilayer filters according to Preferred Embodiment 6 and the modification enable an improvement in attenuation characteristics. 
     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.