Patent Publication Number: US-2023143899-A1

Title: Filter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filter including a resonator constituted of a distributed constant line. 
     2. Description of the Related Art 
     One of electronic components used in a communication apparatus is a band-pass filter including a plurality of resonators. Each of the plurality of resonators is constituted of, for example, a distributed constant line. The distributed constant line is configured to have a predetermined line length. 
     An example of a resonator configured of such a distributed constant line is a stub resonator. For example, US 2003/0184407 A1 describes a technique in which stub elements are used as a means to adjust the directivity and the coupling degree. JP 2011-119841 A describes a technique in which an open-ended stub is used as a means to reduce spurious components in higher-order resonance frequencies. 
     A band-pass filter is required to have a large absolute value of attenuation (hereinafter also referred to as pass attenuation) on a high-frequency side of a passband in some cases. To achieve this, spurious components to be generated on the high-frequency side of the passband needs to be controlled. 
     Provision of communication services using fifth-generation mobile communication systems (hereinafter referred to as 5G) is currently being started. For 5G, the use of frequency bands of 10 GHz or higher, particularly a quasi-millimeter wave band of 10 GHz to 30 GHz and a millimeter wave band of 30 GHz to 300 GHz, is assumed. When frequency bands that are higher and wider than those in a conventional art are used as described above, band-pass filters are also required to satisfy their characteristics in such frequency bands, which are higher and wider than those in a conventional art. However, it is difficult to obtain sufficient characteristics in a related art. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a filter that can increase pass attenuation in a wide frequency band on a high-frequency side of a passband. 
     A filter of the present invention includes: a first resonator and a second resonator each including a first conductor part and a second conductor part having an impedance smaller than an impedance of the first conductor part; a first stub resonator composed of a distributed constant line and electrically connected to the first conductor part of the first resonator; and a second stub resonator composed of a distributed constant line and electrically connected to the first conductor part of the second resonator. The shape of the first stub resonator and a shape of the second stub resonator are different from each other. 
     In the filter of the present invention, a length of the first stub resonator and the length of the second stub resonator may be different from each other. 
     In the filter of the present invention, each of the first conductor part and the second conductor part may be a distributed constant line. 
     The filter of the present invention may be a band-pass filter that selectively allows a signal of a frequency in a predetermined passband to pass. In this case, the first conductor part of the first resonator may include a first connecting part to which the first stub resonator is connected and a first non-connecting part other than the first connecting part. The first conductor part of the second resonator may include a second connecting part to which the second stub resonator is connected and a second non-connecting part other than the second connecting part. The current density of the first connecting part in a center frequency of the passband may be higher than the current density of the first non-connecting part in the center frequency of the passband. The current density of the second connecting part in the center frequency of the passband may be higher than the current density of the second non-connecting part in the center frequency of the passband. 
     In the filter of the present invention, an impedance ratio being a ratio of an impedance of the second conductor part to an impedance of the first conductor part in each of the first resonator and the second resonator may be 0.3 or smaller. 
     In the filter of the present invention, each of the first conductor part of the first resonator and the first conductor part of the second resonator may include a plurality of portions extending in a plurality of directions different from each other. 
     The filter of the present invention may further include a stack including a plurality of dielectric layers stacked together. The first resonator, the second resonator, the first stub resonator, and the second stub resonator may be integrated with the stack. In this case, the first conductor part and the second conductor part may be arranged at positions different from each other in a stacking direction of the plurality of dielectric layers and electrically connected to each other in each of the first resonator and the second resonator. The filter of the present invention may further include a plurality of through holes connecting the first conductor part and the second conductor part of each of the first resonator and the second resonator. The first conductor part of the first resonator and the first conductor part of the second resonator may be arranged at a same position in the stacking direction. The second conductor part of the first resonator and the second conductor part of the second resonator may be arranged at the same position in the stacking direction. 
     The filter of the present invention may further include a third resonator arranged between the first resonator and the second resonator in a circuit configuration. In this case, the third resonator may include a third conductor part and a fourth conductor part having an impedance smaller than an impedance of the third conductor part. The third conductor part may have an asymmetrical shape. 
     The filter of the present invention includes a first stub resonator electrically connected to the first conductor part of the first resonator, and a second stub resonator electrically connected to the first conductor part of the second resonator. The shape of the first stub resonator and the shape of the second stub resonator are different from each other. With this configuration, according to the present invention, it is possible to provide a filter that can increase pass attenuation in a wide frequency band on a high-frequency side of a passband. 
     Other and further objects, features, and advantages of the present invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram showing a circuit configuration of a filter according to a first embodiment of the present invention. 
         FIG.  2    is a perspective view showing an external appearance of the filter according to the first embodiment of the present invention. 
         FIG.  3 A  to  FIG.  3 C  are explanatory diagrams showing respective patterned surfaces of a first to a third dielectric layer of a stack of the filter according to the first embodiment of the present invention. 
         FIG.  4 A  to  FIG.  4 C  are explanatory diagrams showing respective patterned surfaces of a fourth to a sixth dielectric layer of the stack of the filter according to the first embodiment of the present invention. 
         FIG.  5 A  to  FIG.  5 C  are explanatory diagrams showing respective patterned surfaces of a seventh to a ninth dielectric layer of the stack of the filter according to the first embodiment of the present invention. 
         FIG.  6    is a perspective view showing an inside of the stack of the filter according to the first embodiment of the present invention. 
         FIG.  7    is a perspective view showing part of the inside of the stack of the filter according to the first embodiment of the present invention. 
         FIG.  8    is a perspective view showing part of the inside of the stack of the filter according to the first embodiment of the present invention. 
         FIG.  9    is a circuit diagram showing a circuit configuration of a filter of a first comparative example. 
         FIG.  10    is an explanatory diagram showing a patterned surface of a seventh dielectric layer of a stack of the filter of the first comparative example. 
         FIG.  11    is an explanatory diagram showing a patterned surface of a seventh dielectric layer of a stack of a filter of a second comparative example. 
         FIG.  12    is an explanatory diagram showing a patterned surface of a seventh dielectric layer of a stack of a filter of a third comparative example. 
         FIG.  13    is a characteristic chart showing pass attenuation characteristics of a model of the first comparative example. 
         FIG.  14    is a characteristic chart showing pass attenuation characteristics of a model of the second comparative example. 
         FIG.  15    is a characteristic chart showing pass attenuation characteristics of a model of the third comparative example. 
         FIG.  16    is a characteristic chart showing pass attenuation characteristics of a model of a practical example. 
         FIG.  17    is an explanatory diagram showing a patterned surface of an eighth dielectric layer of a stack of a filter of a fourth comparative example. 
         FIG.  18    is a characteristic chart showing pass attenuation characteristics of a model of the fourth comparative example. 
         FIG.  19    is a circuit diagram showing a circuit configuration of a filter according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Embodiments of the present invention will now be described in detail with reference to the drawings. First, reference is made to  FIG.  1    to describe a configuration of a filter  1  according to a first embodiment of the present invention.  FIG.  1    is a circuit diagram showing a circuit configuration of the filter  1 . The filter  1  is configured to function as a band-pass filter that selectively allows a signal of a frequency in a predetermined passband to pass. 
     The filter  1  according to the present embodiment includes a first resonator  10 , a second resonator  20 , and a third resonator  30  arranged between the first resonator  10  and the second resonator  20  in a circuit configuration. In the present application, the expression of “in the (a) circuit configuration” is used not to indicate a layout in physical configuration but to indicate a layout in a circuit diagram. 
     The first to third resonators  10 ,  20 , and  30  are configured so that the first resonator  10  and the third resonator  30  are adjacent to each other in the circuit configuration to be electromagnetically coupled to each other, and the second resonator  20  and the third resonator  30  are adjacent to each other in the circuit configuration to be electromagnetically coupled to each other. In  FIG.  1   , a curve with a sign K 13  represents an electric field coupling between the first resonator  10  and the third resonator  30 , and a curve with a sign K 23  represents an electric field coupling between the second resonator  20  and the third resonator  30 . 
     The first resonator  10  is magnetically coupled to the second resonator  20  not adjacent to the first resonator  10  in the circuit configuration. Such electromagnetic-field coupling between two resonators not adjacent to each other in the circuit configuration is referred to as cross coupling. In  FIG.  1   , a curve with a sign K 12  represents a magnetic field coupling between the first resonator  10  and the second resonator  20 . 
     The first resonator  10  includes a first conductor part  11  and a second conductor part  12  having an impedance smaller than that of the first conductor part  11 . The first conductor part  11  and the second conductor part  12  are electrically connected to each other. The first conductor part  11  is connected to ground. Each of the first conductor part  11  and the second conductor part  12  is a distributed constant line. In particular, in the present embodiment, the first conductor part  11  is a distributed constant line having a small width, and the second conductor part  12  is a distributed constant line having a width larger than that of the first conductor part  11 . 
     The first resonator  10  further includes a third conductor part  13  electrically connecting the first conductor part  11  and the second conductor part  12 . The third conductor part  13  may include a distributed constant line having a width smaller than that of the distributed constant line constituting the second conductor part  12 . The width of the distributed constant line of the third conductor part  13  may be the same as or different from the width of the distributed constant line constituting the first conductor part  11 . 
     A configuration of the second resonator  20  is basically the same as the configuration of the first resonator  10 . Specifically, the second resonator  20  includes a first conductor part  21  and a second conductor part  22  having an impedance smaller than that of the first conductor part  21 . The first conductor part  21  and the second conductor part  22  are electrically connected to each other. The first conductor part  21  is connected to ground. Each of the first conductor part  21  and the second conductor part  22  is a distributed constant line. In particular, in the present embodiment, the first conductor part  21  is a distributed constant line having a small width, and the second conductor part  22  is a distributed constant line having a width larger than that of the first conductor part  21 . 
     The second resonator  20  further includes a third conductor part  23  electrically connecting the first conductor part  21  and the second conductor part  22 . The third conductor part  23  may include a distributed constant line having a width smaller than that of the distributed constant line constituting the second conductor part  22 . The width of the distributed constant line of the third conductor part  23  may be the same as or different from the width of the distributed constant line constituting the first conductor part  21 . 
     The third resonator  30  includes a first conductor part  31  and a second conductor part  32  having an impedance smaller than that of the first conductor part  31 . The first conductor part  31  corresponds to a “third conductor part” of the present invention, and the second conductor part  32  corresponds to a “fourth conductor part” of the present invention. The first conductor part  31  and the second conductor part  32  are electrically connected to each other. The first conductor part  31  is connected to ground. Each of the first conductor part  31  and the second conductor part  32  is a distributed constant line. In particular, in the present embodiment, the first conductor part  31  is a distributed constant line having a small width, and the second conductor part  32  is a distributed constant line having a width larger than that of the first conductor part  31 . 
     All the first to third resonators  10 ,  20 , and  30  are each a stepped-impedance resonator composed of a distributed constant line having a small width and a distributed constant line having a large width. All the first to third resonators  10 ,  20 , and  30  are each a quarter-wavelength resonator with one end being short-circuited and the other end being open. 
     The impedance of each of the first conductor parts  11 ,  21 , and  31  is within a range from 150 to 350, for example. The impedance of each of the second conductor parts  12 ,  22 , and  32  is within a range from 10 to 50, for example. Here, the ratio of the impedance of the second conductor part to the impedance of the first conductor part in each of the first to third resonators  10 ,  20 , and  30  is referred to as an impedance ratio. In each of the first to third resonators  10 ,  20 , and  30 , the impedance ratio is smaller than 1. 
     From an aspect of making the resonators small, the impedance ratio is preferably small. For example, by adjusting the widths of the distributed constant line configuring the first conductor part and the distributed constant line configuring the second conductor part, the impedance ratio can be adjusted. For a smaller impedance ratio, the width of the distributed constant line configuring the first conductor part is relatively small, and the width of the distributed constant line configuring the second conductor part is relatively large. 
     In particular, in the present embodiment, in each of the first to third resonators  10 ,  20 , and  30 , the impedance ratio is 0.3 or smaller. In one example, the impedance of the second conductor part of each of the first and second resonators  10  and  20  is 2.870, and the impedance of the first conductor part of each of the first and second resonators  10  and  20  is 270. In this case, the impedance ratio in each of the first and second resonators  10  and  20  is 0.106. In one example, the impedance of the second conductor part  32  of the third resonator  30  is 2.550, and the impedance of the first conductor part  31  of the third resonator  30  is 270. In this case, the impedance ratio in the third resonator  30  is 0.094. 
     When the impedance ratio is made too small, desired characteristics are not obtained in some cases. For example, when the impedance ratio is made too small in a stepped-impedance resonator (quarter-wavelength resonator) with one end being short-circuited and the other end being open, this resonator serves substantially as a half-wavelength resonator composed only of a second conductor part with both ends being open. Consequently, desired characteristics cannot be obtained. To prevent this, in the present embodiment, the impedance ratio in each of the first to third resonators  10 ,  20 , and  30  is 0.06 or greater. 
     The filter  1  further includes a first port  2 , a second port  3 , and conductor portions  4  and  5 . The first to third resonators  10 ,  20 , and  30  are arranged between the first port  2  and the second port  3  in the circuit configuration. 
     The conductor portion  4  electrically connects the first port  2  and the first resonator  10 . The conductor portion  4  is connected, at one end thereof, to the first port  2 . The conductor portion  4  is connected, at the other end thereof, to the first resonator  10  between the first conductor part  11  and the third conductor part  13 . 
     The conductor portion  5  electrically connects the second port  3  and the second resonator  20 . The conductor portion  5  is connected, at one end thereof, to the second port  3 . The conductor portion  5  is connected, at the other end thereof, to the second resonator  20  between the first conductor part  21  and the third conductor part  23 . 
     The filter  1  further includes a first stub resonator  91  electrically connected to the first conductor part  11  of the first resonator  10 , and a second stub resonator  92  electrically connected to the first conductor part  21  of the second resonator  20 . Each of the first and second stub resonators  91  and  92  is a distributed constant line. 
     The first stub resonator  91  is connected in the middle of the first conductor part  11 . In  FIG.  1   , for the first conductor part  11 , a portion located between a connecting point with the first stub resonator  91  and the second conductor part  12  in a circuit configuration is indicated by a reference numeral  11 A, and a portion located between the connecting point with the first stub resonator  91  and the ground in the circuit configuration is indicated by a reference numeral  11 B. 
     The second stub resonator  92  is connected in the middle of the first conductor part  21 . In  FIG.  1   , for the first conductor part  21 , a portion located between a connecting point with the second stub resonator  92  and the second conductor part  22  in the circuit configuration is indicated by a reference numeral  21 A, and a portion located between a connecting point with the second stub resonator  92  and the ground in the circuit configuration is indicated by a reference numeral  21 B. 
     As will be described later, the shape of the first stub resonator  91  and the shape of the second stub resonator  92  are different from each other. In particular, in the present embodiment, the length of the first stub resonator  91  and the length of the second stub resonator  92  are different from each other. 
     Each of the first and second stub resonators  91  and  92  may be an open stub with one end being open or may be a short stub with one end being connected to ground.  FIG.  1    shows an example in which each of the first and second stub resonators  91  and  92  is an open stub. 
     Next, other configurations of the filter  1  will be described with reference to  FIG.  2   .  FIG.  2    is a perspective view showing an external appearance of the filter  1 . 
     The filter  1  further includes a stack  50 . The stack  50  includes a plurality of dielectric layers stacked together and a plurality of conductor layers and a plurality of through holes formed in the plurality of dielectric layers. The first to third resonators  10 ,  20 , and  30  and the first and second stub resonators  91  and  92  are integrated with the stack  50 . The first to third resonators  10 ,  20 , and  30  and the first and second stub resonators  91  and  92  are formed by using the plurality of conductor layers. 
     The stack  50  has a first surface  50 A and a second surface  50 B located at both ends in a stacking direction T of the plurality of dielectric layers, and four side surfaces  50 C to  50 F connecting the first surface  50 A and the second surface  50 B. The side surfaces  50 C and  50 D are opposite to each other. The side surfaces  50 E and  50 F are opposite to each other. The side surfaces  50 C to  50 F are perpendicular to the first surface  50 A and the second surface  50 B. 
     Here, X, Y, and Z directions are defined as shown in  FIG.  2   . The X, Y, and Z directions are orthogonal to one another. In the present embodiment, a direction parallel to the stacking direction T will be referred to as the Z direction. The opposite directions to the X, Y, and Z directions are defined as -X, -Y, and -Z directions, respectively. 
     As shown in  FIG.  2   , the first surface  50 A is located at the end of the stack  50  in the -Z direction. The first surface  50 A is also the bottom surface of the stack  50 . The second surface  50 B is located at the end of the stack  50  in the Z direction. The second surface  50 B is also the top surface of the stack  50 . The side surface  50 C is located at the end of the stack  50  in the -X direction. The side surface  50 D is located at the end of the stack  50  in the X direction. The side surface  50 E is located at the end of the stack  50  in the -Y direction. The side surface  50 F is located at the end of the stack  50  in the Y direction. 
     The plane shape of the stack  50  when seen in the Z direction, i.e., the shape of the first surface  50 A or the second surface  50 B, is long in one direction. In particular, in the present embodiment, the plane shape of the stack  50  when seen in the Z direction is a rectangular shape that is long in a direction parallel to the X direction. 
     The filter  1  further includes a plurality of terminals  111 ,  112 ,  113 ,  114 ,  115 , and  116  provided on the first surface  50 A of the stack  50 . The terminal  111  extends in the Y direction near the side surface  50 C. The terminal  112  extends in the Y direction near the side surface  50 D. The terminals  113  to  116  are arranged between the terminal  111  and the terminal  112 . The terminals  113  and  114  are arranged in this order near the side surface  50 E in the X direction. The terminals  115  and  116  are arranged in this order near the side surface  50 F in the X direction. 
     The terminal  111  corresponds to the first port  2 , and the terminal  112  corresponds to the second port  3 . Thus, the first and second ports  2  and  3  are provided on the first surface  50 A of the stack  50 . The terminals  113  to  116  are connected to ground. Hereinafter, the terminal  111  is also referred to as a first terminal  111 , the terminal  112  is also referred to as a second terminal  112 , and the terminals  113  to  116  are also referred to as ground terminals  113  to  116 . 
     Next, an example of the plurality of dielectric layers and the plurality of conductor layers constituting the stack  50  will be described with reference to  FIG.  3 A  to  FIG.  5 C . In this example, the stack  50  includes nine dielectric layers stacked together. The nine dielectric layers will be referred to as a first to a ninth dielectric layer in the order from bottom to top. The first to ninth dielectric layers are denoted by reference numerals  51  to  59 , respectively. 
       FIG.  3 A  shows the patterned surface of the first dielectric layer  51 . The terminals  111 ,  112 ,  113 ,  114 ,  115 , and  116  are formed on the patterned surface of the dielectric layer  51 . Through holes  51 T 1 ,  51 T 2 ,  51 T 3 ,  51 T 4 ,  51 T 5 , and  51 T 6  connected respectively to the terminals  111 ,  112 ,  113 ,  114 ,  115 , and  116  are formed in the dielectric layer  51 . 
       FIG.  3 B  shows the patterned surface of the second dielectric layer  52 . A conductor layer  521  is formed on the patterned surface of the dielectric layer  52 . Further, through holes  52 T 1 ,  52 T 2 ,  52 T 3 ,  52 T 4 ,  52 T 5 , and  52 T 6  are formed in the dielectric layer  52 . The through holes  51 T 1  and  51 T 2  formed in the dielectric layer  51  are connected to the through holes  52 T 1  and  52 T 2 , respectively. The through holes  51 T 3  to  51 T 6  formed in the dielectric layer  51  and the through holes  52 T 3  to  52 T 6  are connected to the conductor layer  521 . 
       FIG.  3 C  shows the patterned surface of the third dielectric layer  53 . Conductor layers  531 ,  532 ,  533 , and  534  are formed on the patterned surface of the dielectric layer  53 . The conductor layer  532  is connected to the conductor layer  531 . The conductor layer  534  is connected to the conductor layer  533 . In  FIG.  3 C , each of the boundary between the conductor layer  531  and the conductor layer  532  and the boundary between the conductor layer  533  and the conductor layer  534  is indicated by a dotted line. 
     Through holes  53 T 1 ,  53 T 2 ,  53 T 3 ,  53 T 4 ,  53 T 5 , and  53 T 6  are formed in the dielectric layer  53 . The through hole  52 T 1  formed in the dielectric layer  52  and the through hole  53 T 1  are connected to the conductor layer  532 . The through hole  52 T 2  formed in the dielectric layer  52  and the through hole  53 T 2  are connected to the conductor layer  534 . The through holes  52 T 3  to  52 T 6  formed in the dielectric layer  52  are connected to the through holes  53 T 3  to  53 T 6 , respectively. 
       FIG.  4 A  shows the patterned surface of the fourth dielectric layer  54 . A conductor layer  541  is formed on the patterned surface of the dielectric layer  54 . Through holes  54 T 1 ,  54 T 2 ,  54 T 3 ,  54 T 4 ,  54 T 5 ,  54 T 6 , and  54 T 7  are formed in the dielectric layer  54 . The through holes  53 T 1  to  53 T 6  formed in the dielectric layer  53  are connected to the through holes  54 T 1  to  54 T 6 , respectively. The through hole  54 T 7  is connected to the conductor layer  541 . 
       FIG.  4 B  shows the patterned surface of the fifth dielectric layer  55 . A conductor layer  551  is formed on the patterned surface of the dielectric layer  55 . Through holes  55 T 1 ,  55 T 2 ,  55 T 7 , and  55 T 8  are formed in the dielectric layer  55 . The through holes  54 T 1 ,  54 T 2 , and  54 T 7  formed in the dielectric layer  54  are connected to the through holes  55 T 1 ,  55 T 2 , and  55 T 7 , respectively. The through holes  54 T 3  to  54 T 6  formed in the dielectric layer  54  and the through hole  55 T 8  are connected to the conductor layer  551 . 
       FIG.  4 C  shows the patterned surface of the sixth dielectric layer  56 . Through holes  56 T 1 ,  56 T 2 ,  56 T 7 , and  56 T 8  are formed in the dielectric layer  56 . The through holes  55 T 1 ,  55 T 2 ,  55 T 7 , and  55 T 8  formed in the dielectric layer  55  are connected to the through holes  56 T 1 ,  56 T 2 ,  56 T 7 , and  56 T 8 , respectively. 
       FIG.  5 A  shows the patterned surface of the seventh dielectric layer  57 . Conductor layers  571 ,  572 ,  573 , and  574  are formed on the patterned surface of the dielectric layer  57 . Each of the conductor layers  571  and  572  has a first end and a second end opposite to each other. The first end of the conductor layer  571  and the first end of the conductor layer  572  are connected to each other. In  FIG.  5 A , the boundary between the conductor layer  571  and the conductor layer  572  is indicated by a dotted line. The through hole  56 T 1  formed in the dielectric layer  56  is connected to a portion near the second end of the conductor layer  571 . The through hole  56 T 2  formed in the dielectric layer  56  is connected to a portion near the second end of the conductor layer  572 . 
     The conductor layer  573  is connected in the middle of the conductor layer  571 . The conductor layer  574  is connected in the middle of the conductor layer  572 . In  FIG.  5 A , each of the boundary between the conductor layer  571  and the conductor layer  573  and the boundary between the conductor layer  572  and the conductor layer  574  is indicated by a dotted line. 
     Through holes  57 T 7  and  57 T 8  are formed in the dielectric layer  57 . The through hole  56 T 7  formed in the dielectric layer  56  is connected to the through hole  57 T 7 . The through hole  56 T 8  formed in the dielectric layer  56  and the through hole  57 T 8  are connected to a portion near the first end of the conductor layer  571  and a portion near the first end of the conductor layer  572 . 
       FIG.  5 B  shows the patterned surface of the eighth dielectric layer  58 . A conductor layer  581  is formed on the patterned surface of the dielectric layer  58 . The conductor layer  581  has a first end and a second end opposite to each other. The through hole  57 T 7  formed in the dielectric layer  57  is connected to a portion near the first end of the conductor layer  581 . 
     A through hole  58 T 8  is formed in the dielectric layer  58 . The through hole  57 T 8  formed in the dielectric layer  57  and the through hole  58 T 8  are connected to a portion near the second end of the conductor layer  581 . 
       FIG.  5 C  shows the patterned surface of the ninth dielectric layer  59 . A conductor layer  591  is formed on the patterned surface of the dielectric layer  59 . The through hole  58 T 8  formed in the dielectric layer  58  is connected to the conductor layer  591 . 
     The stack  50  shown in  FIG.  2    is formed by stacking the first to ninth dielectric layers  51  to  59  such that the patterned surface of the first dielectric layer  51  serves as the first surface  50 A of the stack  50  and the surface of the ninth dielectric layer  59  opposite to the patterned surface thereof serves as the second surface  50 B of the stack  50 . 
       FIG.  6    shows the inside of the stack  50  formed by stacking the first to ninth dielectric layers  51  to  59 . As shown in  FIG.  6   , the plurality of conductor layers and the plurality of through holes shown in  FIG.  3 A to  5 C  are stacked inside the stack  50 . 
     Correspondences between the circuit components of the filter  1  shown in  FIG.  1    and the internal components of the stack  50  shown in  FIG.  3 A  to  FIG.  5 C  will now be described. First, the first resonator  10  will be described. The first conductor part  11  is formed of the conductor layer  571 . The second conductor part  12  is formed of the conductor layer  531 . The third conductor part  13  is formed of the conductor layer  532 . 
     The conductor layer  532  (third conductor part  13 ) and the through holes  53 T 1 ,  54 T 1 ,  55 T 1 , and  56 T 1  connect the conductor layer  571  forming the first conductor part  11  and the conductor layer  531  forming the second conductor part  12 . The conductor layer  571  forming the first conductor part  11  is connected to the ground terminals  113  to  116  via the through holes  51 T 3  to  51 T 6 , the conductor layer  521 , the through holes  52 T 3  to  52 T 6  and  53 T 3  to  53 T 6 , the through holes  54 T 3  to  54 T 6 , the conductor layer  551 , and the through holes  55 T 8  and  56 T 8 . 
     Next, the second resonator  20  will be described. The first conductor part  21  is formed of the conductor layer  572 . The second conductor part  22  is formed of the conductor layer  533 . The third conductor part  23  is formed of the conductor layer  534 . 
     The conductor layer  534  (third conductor part  23 ) and the through holes  53 T 2 ,  54 T 2 ,  55 T 2 , and  56 T 2  connect the conductor layer  572  forming the first conductor part  21  and the conductor layer  533  forming the second conductor part  22 . The conductor layer  572  forming the first conductor part  21  is connected to the ground terminals  113  to  116  via the through holes  51 T 3  to  51 T 6 , the conductor layer  521 , the through holes  52 T 3  to  52 T 6  and  53 T 3  to  53 T 6 , the through holes  54 T 3  to  54 T 6 , the conductor layer  551 , and the through holes  55 T 8  and  56 T 8 . 
     Next, the third resonator  30  will be described. The first conductor part  31  is formed of the conductor layer  581 . The second conductor part  32  is formed of the conductor layer  541 . 
     The conductor layer  581  forming the first conductor part  31  is connected to the ground terminals  113  to  116  via the through holes  51 T 3  to  51 T 6 , the conductor layer  521 , the through holes  52 T 3  to  52 T 6  and  53 T 3  to  53 T 6 , the through holes  54 T 3  to  54 T 6 , the conductor layer  551 , and the through holes  55 T 8 ,  56 T 8 , and  57 T 8 . 
     Next, the first and second stub resonators  91  and  92  will be described. The first stub resonator  91  is formed of the conductor layer  573 . The second stub resonator  92  is formed of the conductor layer  574 . 
     Next, the conductor portions  4  and  5  will be described. The conductor portion  4  is formed of the through holes  51 T 1  and  52 T 1 . The through hole  51 T 1  is connected to the first terminal  111 . The through hole  52 T 1  is connected to the conductor layer  532  forming the third conductor part  13  and is also connected to the conductor layer  571  forming the first conductor part  11  via the through holes  53 T 1 ,  54 T 1 ,  55 T 1 , and  56 T 1 . 
     The conductor portion  5  is formed of the through holes  51 T 2  and  52 T 2 . The through hole  51 T 2  is connected to the second terminal  112 . The through hole  52 T 2  is connected to the conductor layer  534  forming the third conductor part  23  and is also connected to the conductor layer  572  forming the first conductor part  21  via the through holes  53 T 2 ,  54 T 2 ,  55 T 2 , and  56 T 2 . 
     Next, the structural features of the filter  1  according to the present embodiment will be described with reference to  FIG.  2    to  FIG.  8   .  FIG.  7    and  FIG.  8    are each a perspective view showing part of an inside of the stack  50 .  FIG.  7    mainly shows a plurality of conductor layers and a plurality of through holes constituting the first and second resonators  10  and  20  and the first and second stub resonators  91  and  92 .  FIG.  8    mainly shows a plurality of conductor layers and a plurality of through holes constituting the third resonator  30 . 
     The first resonator  10  is arranged in an area on the -X direction side in the stack  50 . In other words, the first resonator  10  is arranged at a position closer to the side surface  50 C than the side surface  50 D. As shown in  FIG.  7   , the first conductor part  11  (conductor layer  571 ) and the second conductor part  12  (conductor layer  531 ) of the first resonator  10  are arranged at positions different from each other in the stacking direction T. The second conductor part  12  is arranged between the first surface  50 A, where the plurality of terminals  111  to  116  are arranged, and the first conductor part  11 . 
     The first conductor part  11  (conductor layer  571 ) includes a plurality of portions extending in a plurality of directions that are orthogonal to the stacking direction T. In particular, in the present embodiment, the first conductor part  11  (conductor layer  571 ) includes four portions each extending in a direction parallel to the X direction and three portions each extending in a direction parallel to the Y direction. 
     The shape of the second conductor part  12  (conductor layer  531 ) is long in a direction crossing the longitudinal direction of the stack  50 . In particular, in the present embodiment, the shape of the second conductor part  12  (conductor layer  531 ) is a rectangular shape that is long in a direction parallel to the Y direction. 
     The second resonator  20  is arranged in an area on the X direction side in the stack  50 . In other words, the second resonator  20  is arranged at a position closer to the side surface  50 D than the side surface  50 C. As shown in  FIG.  7   , the first conductor part  21  (conductor layer  572 ) and the second conductor part  22  (conductor layer  533 ) of the second resonator  20  are arranged at positions different from each other in the stacking direction T. The second conductor part  22  is arranged between the first surface  50 A, where the plurality of terminals  111  to  116  are arranged, and the first conductor part  21 . 
     The first conductor part  21  (conductor layer  572 ) includes a plurality of portions extending in a plurality of directions that are orthogonal to the stacking direction T. In particular, in the present embodiment, the first conductor part  21  (conductor layer  572 ) includes four portions each extending in a direction parallel to the X direction and three portions each extending in a direction parallel to the Y direction. 
     The shape of the second conductor part  22  (conductor layer  533 ) is long in a direction crossing the longitudinal direction of the stack  50 . In particular, in the present embodiment, the shape of the second conductor part  22  (conductor layer  533 ) is a rectangular shape that is long in a direction parallel to the Y direction. 
     At least part of the third resonator  30  is arranged between the first resonator  10  and the second resonator  20  when seen in the Z direction. In particular, in the present embodiment, part of the third resonator  30  is arranged between the first resonator  10  and the second resonator  20 . 
     As shown in  FIG.  8   , the first conductor part  31  (conductor layer  581 ) and the second conductor part  32  (conductor layer  541 ) of the third resonator  30  are arranged at positions different from each other in the stacking direction T. The second conductor part  32  is arranged between the first surface  50 A, where the plurality of terminals  111  to  116  are arranged, and the first conductor part  31 . 
     The first conductor part  31  (conductor layer  581 ) includes a plurality of portions extending in a plurality of directions that are orthogonal to the stacking direction T. In particular, in the present embodiment, the first conductor part  31  (conductor layer  581 ) includes three portions each extending in a direction parallel to the X direction and four portions each extending in a direction parallel to the Y direction. 
     The first conductor part  31  (conductor layer  581 ) has an asymmetrical shape with respect to a given XZ plane crossing the first conductor part  31  and also has an asymmetrical shape with respect to a given YZ plane crossing the first conductor part  31 . Hereinafter, the given XZ plane crossing the first conductor part  31  is referred to as a first virtual plane, and the given YZ plane crossing the first conductor part  31  is referred to as a second virtual plane. The first virtual plane may cross the center of the stack  50  in a direction parallel to the Y direction. The second virtual plane may cross the center of the stack  50  in a direction parallel to the X direction. 
     The shape of the second conductor part  32  (conductor layer  541 ) is long in the longitudinal direction of the stack  50 . In particular, in the present embodiment, the shape of the second conductor part  32  (conductor layer  541 ) is a rectangular shape that is long in a direction parallel to the X direction. 
     As shown in  FIG.  5 A  and  FIG.  6   , the first conductor part  11  (conductor layer  571 ) of the first resonator  10  and the first conductor part  21  (conductor layer  572 ) of the second resonator  20  are arranged at the same position in the stacking direction T. As shown in  FIG.  5 A ,  FIG.  5 B , and  FIG.  6   , the first conductor part  31  (conductor layer  581 ) of the third resonator  30  is arranged at a position different from the positions of the first conductor parts  11  and  21  in the stacking direction T. Part of the first conductor part  11  and part of the first conductor part  21  overlap the first conductor part  31  when seen in the Z direction. The shape of the first conductor part  31  is different from the shape of the first conductor part  11  and the shape of the first conductor part  21 . 
     As shown in  FIG.  3 C  and  FIG.  6   , the second conductor part  12  (conductor layer  531 ) of the first resonator  10  and the second conductor part  22  (conductor layer  533 ) of the second resonator  20  are arranged at the same position in the stacking direction T. As shown in  FIG.  3 C ,  FIG.  4 A , and  FIG.  6   , the second conductor part  32  (conductor layer  541 ) of the third resonator  30  is arranged at a position different from the positions of the second conductor parts  12  and  22  in the stacking direction T. Part of the second conductor part  12  and part of the second conductor part  22  overlap the second conductor part  32  when seen in the Z direction. The shape of the second conductor part  32  is different from the shape of the second conductor part  12  and the shape of the second conductor part  22 . 
     As shown in  FIG.  5 A  to  FIG.  5 C , the shape of the first stub resonator  91  (conductor layer  573 ) and the shape of the second stub resonator  92  (conductor layer  574 ) are different from each other. Specifically, the length of the first stub resonator  91  and the length of the second stub resonator  92  are different from each other. In the example shown in  FIG.  5 A  to  FIG.  5 C , the first stub resonator  91  is longer than the second stub resonator  92 . The first stub resonator  91  includes two portions each extending in a direction parallel to the X direction and one portion extending in a direction parallel to the Y direction. The second stub resonator  92  extends in a direction parallel to the X direction. Note that the width of the first stub resonator  91  and the width of the second stub resonator  92  are the same or approximately the same. 
     The first conductor part  11  of the first resonator  10  includes a first connecting part to which the first stub resonator  91  is connected and a first non-connecting part other than the first connecting part. Specifically, the first connecting part is a portion  571   a  of the conductor layer  571  shown in  FIG.  5 A  near the boundary with the conductor layer  573  indicated by a dotted line. In  FIG.  5 A , an approximate position of the portion  571   a  is indicated by an arrow. The first non-connecting part is a portion of the conductor layer  571  other than the portion  571   a.    
     The current density of the first connecting part (portion  571   a ) in the center frequency of the passband of the filter  1  (band-pass filter) is lower than the current density of the first non-connecting part in the center frequency of the passband of the filter  1  (band-pass filter). In other words, the first stub resonator  91  is connected to or near a portion of the first conductor part  11  with the highest current density. 
     The first conductor part  21  of the second resonator  20  includes a second connecting part to which the second stub resonator  92  is connected and a second non-connecting part other than the second connecting part. Specifically, the first connecting part is a portion  572   a  of the conductor layer  572  shown in  FIG.  5 A  near the boundary with the conductor layer  574  indicated by a dotted line. In  FIG.  5 A , an approximate position of the portion  572   a  is indicated by an arrow. The second non-connecting part is a portion of the conductor layer  572  other than the portion  572   a.    
     The current density of the second connecting part (portion  572   a ) in the center frequency of the passband of the filter  1  (band-pass filter) is lower than the current density of the second non-connecting part in the center frequency of the passband of the filter  1  (band-pass filter). In other words, the second stub resonator  92  is connected to or near a portion of the first conductor part  21  with the highest current density. 
     As described above, in the present embodiment, the first conductor part  11  and the second conductor part  12  of the first resonator  10  are arranged at positions different from each other in the stacking direction T. Thus, according to the present embodiment, the first conductor part  11  and the second conductor part  12  can be arranged while overlapping each other. Hence, according to the present embodiment, the area for arranging the first resonator  10  can be made substantially smaller than that for a case where the first conductor part  11  and the second conductor part  12  are formed in the same dielectric layer to be arranged in the same position in the stacking direction T. 
     The description of the first resonator  10  above is also applicable to the second and third resonators  20  and  30 . In view of these, according to the present embodiment, the filter  1  can be miniaturized. 
     In the present embodiment, part of the first conductor part  11  of the first resonator  10  and part of the first conductor part  21  of the second resonator  20  overlap the first conductor part  31  of the third resonator  30  when seen in the Z direction, and part of the second conductor part  12  of the first resonator  10  and part of the second conductor part  22  of the second resonator  20  overlap the second conductor part  32  of the third resonator  30  when seen in the Z direction. Also in view of this, according to the present embodiment, the filter  1  can be miniaturized. 
     In the present embodiment, each of the first conductor parts  11 ,  21 , and  31  includes the plurality of portions extending in the plurality of directions different from each other. Hence, according to the present embodiment, the area for arranging each of the first conductor parts  11 ,  21 , and  31  can be made substantially smaller than that for a case where each of the first conductor parts  11 ,  21 , and  31  extends in one direction. 
     In the present embodiment, the conductor layer  591  is connected to the ground terminals  113  to  116  via the through holes  51 T 3  to  51 T 6 , the conductor layer  521 , the through holes  52 T 3  to  52 T 6  and  53 T 3  to  53 T 6 , the through holes  54 T 3  to  54 T 6 , the conductor layer  551 , and the through holes  55 T 8 ,  56 T 8 ,  57 T 8 , and  58 T 8 . The first to third resonators  10 ,  20 , and  30  are arranged between the conductor layer  521  and the conductor layer  591 . Each of the conductor layers  521  and  591  overlap the first to third resonators  10 ,  20 , and  30  when seen in the Z direction. The conductor layers  521  and  591  function as shields. 
     In the present embodiment, the impedance of the first conductor part  11  of the first resonator  10  is larger than the impedance of the second conductor part  12  of the first resonator  10 . The first stub resonator  91  is electrically connected to the first conductor part  11  having a large impedance. In particular, in the present embodiment, the first stub resonator  91  is connected to the portion of the first conductor part  11  with the highest current density. Thus, according to the present embodiment, it is possible to control spurious while suppressing an influence of the first stub resonator  91  to the basic resonance of the first resonator  10 . 
     The description of the first resonator  10  and the first stub resonator  91  above is also applicable to the second resonator  20  and the second stub resonator  92 . According to the present embodiment, it is possible to control spurious while suppressing an influence of the second stub resonator  92  to the basic resonance of the second resonator  20 . 
     Next, a description will be given of results of a first simulation indicating that the absolute value of attenuation (hereinafter referred to as pass attenuation) can be increased in a wide frequency band on a high-frequency side of the passband by the first and second stub resonators  91  and  92 . First, models of first to third comparative examples and a model of a practical example used in the first simulation will be described. The model of the first comparative example is a model of a filter of the first comparative example.  FIG.  9    is a circuit diagram showing a circuit configuration of the filter of the first comparative example.  FIG.  10    is an explanatory diagram showing a patterned surface of a seventh dielectric layer of a stack of the filter of the first comparative example. A configuration of the filter of the first comparative example is almost the same as the configuration of the filter  1  according to the present embodiment except that the first and second stub resonators  91  and  92  and the conductor layers  573  and  574  formed on the dielectric layer  57  of the stack  50  are not provided. 
     The model of the second comparative example is a model of a filter of the second comparative example.  FIG.  11    is an explanatory diagram showing a patterned surface of the seventh dielectric layer  57  of the stack  50  of the filter of the second comparative example. In the filter of the second comparative example, a conductor layer  575  is formed on the dielectric layer  57  instead of the conductor layer  573  of the present embodiment. In  FIG.  11   , the boundary between the conductor layer  571  and the conductor layer  575  is indicated by a dotted line. In the filter according to the second comparative example, the first stub resonator  91  is formed of the conductor layer  575 . Other configurations of the filter of the second comparative example are the same as the configurations of the filter  1  according to the present embodiment. 
     In particular, in the model of the second comparative example, the shape of the first stub resonator  91  (conductor layer  575 ) is the same as the shape of the second stub resonator  92  (conductor layer  574 ). In other words, the first stub resonator  91  extends in a direction parallel to the X direction. 
     The model of the third comparative example is a model of a filter according to the third comparative example.  FIG.  12    is an explanatory diagram showing a patterned surface of the seventh dielectric layer  57  of the stack  50  of the filter of the third comparative example. In the filter of the third comparative example, a conductor layer  576  is formed in the dielectric layer  57  instead of the conductor layer  574  of the present embodiment. In  FIG.  12   , the boundary between the conductor layer  572  and the conductor layer  576  is indicated by a dotted line. In the filter of the third comparative example, the second stub resonator  92  is formed of the conductor layer  576 . Other configurations of the filter of the third comparative example are the same as the configurations of the filter  1  according to the present embodiment. 
     In particular, in the model of the third comparative example, the shape of the second stub resonator  92  (conductor layer  576 ) is the same as the shape of the first stub resonator  91  (conductor layer  573 ). In other words, the second stub resonator  92  includes two portions each extending in a direction parallel to the X direction and one portion extending in a direction parallel to the Y direction. 
     The model of the practical example is a model of the filter  1  according to the present embodiment. In the simulation, in each of the models of the first to third comparative examples and the model of the practical example, the impedance ratio in each of the first and second resonators  10  and  20  was set to 0.106, and the impedance ratio in the third resonator  30  was set to 0.094. 
     In the first simulation, each of the models of the first to third comparative examples and the model of the practical example was designed to function as a band-pass filter. Under these conditions, pass attenuation characteristics of each of the models of the comparative examples and the model of the practical example were determined. 
       FIG.  13    is a characteristic chart showing pass attenuation characteristics of the model of the first comparative example.  FIG.  14    is a characteristic chart showing pass attenuation characteristics of the model of the second comparative example.  FIG.  15    is a characteristic chart showing pass attenuation characteristics of the model of the third comparative example.  FIG.  16    is a characteristic chart showing pass attenuation characteristics of the model of the practical example. In each of  FIG.  13    to  FIG.  16   , the horizontal axis represents frequency, and the vertical axis represents attenuation. 
     As shown in  FIG.  13    to  FIG.  16   , a plurality of spurious components are generated on the high-frequency side of the passband in all the models of the first to third comparative examples and the model of the practical example. The frequencies of the plurality of spurious components are different from each other among the models of the first to third comparative examples and the model of the practical example. As described above, in the model of the first comparative example, the first and second stub resonators  91  and  92  are not provided. Among the models of the second and third comparative examples and the model of the practical example, the shapes of the first and second stub resonators  91  and  92  are different from each other. The results of the first simulation shown in  FIG.  13    to  FIG.  16    indicate that a plurality of spurious components can be controlled with the first and second stub resonators  91  and  92 . 
     When the model of the first comparative example ( FIG.  13   ) and the model of the second comparative example ( FIG.  14   ) are compared with each other, the peak where the pass attenuation is relatively low is present in a band having frequencies of 17 GHz to 18 GHz in both the model of the first comparative example and the model of the second comparative example. In the model of the second comparative example, the smallest value of the pass attenuation at the peak is slightly greater than that of the model of the first comparative example. 
     In the model of the third comparative example ( FIG.  15   ), the peak where the pass attenuation is relatively low is present in a band having frequencies of 14 GHz to 18 GHz. In terms of a frequency band at the peak described above and near the peak for each of the models of the first to third comparative examples, the pass attenuation is higher in the model of the third comparative example than those of the models of the first and second comparative examples. In contrast, in terms of the band having frequencies of 24 GHz to 31 GHz for each of the models of the first to third comparative examples, the pass attenuation is lower in the model of the third comparative example than those of the models of the first and second comparative examples. 
     In the model of the practical example ( FIG.  16   ), the peak where the pass attenuation is relatively low is present in a band having frequencies of 14 GHz to 16 GHz. In terms of a frequency band at the peak described above and near the peak for each of the models of the first and second comparative examples and the model of the practical example, the pass attenuation is higher in the model of the practical example than those of the models of the first and second comparative examples. In terms of the band having frequencies of 27 GHz to 31 GHz for each of the model of the third comparative example and the model of the practical example, the pass attenuation is lower in the model of the practical example than that of the model of the third comparative example. 
     As understood from the results of the first simulation shown in  FIG.  13    to  FIG.  16   , spurious to be generated on the high-frequency side of the passband can be controlled with the first and second stub resonators  91  and  92  according to the present embodiment. As understood from the results of the first simulation shown in  FIG.  14    to  FIG.  16   , by the shape of the first stub resonator  91  and the shape of the second stub resonator  92  being made different from each other, the pass attenuation can be increased in a wide frequency band on the high-frequency side of the passband according to the present embodiment. 
     Next, a description will be given of results of a second simulation indicating that the pass attenuation (the absolute value of attenuation) can be increased on the high-frequency side of the passband, based on the shape of the first conductor part  31  of the third resonator  30 . First, a model of a fourth comparative example used in the second simulation will be described. The model of the fourth comparative example is a model of a filter of the fourth comparative example. 
       FIG.  17    is an explanatory diagram showing a patterned surface of an eighth dielectric layer of a stack of the filter of the fourth comparative example. In the filter of the fourth comparative example, a conductor layer  1581  is formed in the eighth dielectric layer  58  instead of the conductor layer  581  of the present embodiment. In the filter of the fourth comparative example, the first conductor part  31  of the third resonator  30  is formed of the conductor layer  1581  shown in  FIG.  17   . In the filter of the fourth comparative example, the first conductor part  31  (conductor layer  1581 ) has a shape symmetrical with respect to a YZ plane crossing the center of the stack  50  in a direction parallel to the X direction. Other configurations of the filter of the fourth comparative example are approximately the same as the configurations of the filter  1  according to the present embodiment. 
       FIG.  18    is a characteristic chart showing pass attenuation characteristics of the model of the fourth comparative example. In  FIG.  18   , the horizontal axis represents frequency, and the vertical axis represents attenuation. In the model of the fourth comparative example, the peak where the pass attenuation is relatively low is present in a band having frequencies of 15 GHz to 18 GHz. In terms of a frequency band at the peak described above and near the peak for each of the model of the fourth comparative example and the model of the practical example (refer to  FIG.  16   ), the pass attenuation is lower in the model of the fourth comparative example than that of the practical example. 
     As described above, in the present embodiment, the first conductor part  31  (conductor layer  581 ) has an asymmetrical shape. As understood from the results of the second simulation, by the first conductor part  31  having an asymmetrical shape, the pass attenuation can be increased on the high-frequency side of the passband according to the present embodiment. 
     Second Embodiment 
     A description will be made on a second embodiment of the present invention with reference to  FIG.  19   .  FIG.  19    is a circuit diagram showing a circuit configuration of a filter according to the present embodiment. 
     A filter  1  according to the present embodiment differs from that of the first embodiment in the following respects. The filter  1  according to the present embodiment includes a fourth resonator  40 . The fourth resonator  40  is arranged between the second resonator  20  and the third resonator  30  in the circuit configuration. In the present embodiment, the first to fourth resonators  10 ,  20 ,  30 , and  40  are configured so that the first resonator  10  and the third resonator  30  are adjacent to each other in the circuit configuration to be electromagnetically coupled to each other, the third resonator  30  and the fourth resonator  40  are adjacent to each other in the circuit configuration to be electromagnetically coupled to each other, and the second resonator  20  and the fourth resonator  40  are adjacent to each other in the circuit configuration to be electromagnetically coupled to each other. In  FIG.  19   , a curve with a sign K 13  represents electric field coupling between the first resonator  10  and the third resonator  30 , a curve with a sign K 34  represents magnetic field coupling between the third resonator  30  and the fourth resonator  40 , and a curve with a sign K 24  represents electric field coupling between the second resonator  20  and the fourth resonator  40 . 
     A configuration of the fourth resonator  40  is basically the same as the configuration of the third resonator  30 . Specifically, the fourth resonator  40  includes a first conductor part  41  and a second conductor part  42  having an impedance smaller than that of the first conductor part  41 . The first conductor part  41  and the second conductor part  42  are electrically connected to each other. The first conductor part  41  is connected to ground. Each of the first conductor part  41  and the second conductor part  42  is a distributed constant line. In particular, in the present embodiment, the first conductor part  41  is a distributed constant line having a small width, and the second conductor part  42  is a distributed constant line having a width larger than that of the first conductor part  41 . 
     The fourth resonator  40 , similarly to the first to third resonators  10 ,  20 , and  30 , is a stepped-impedance resonator composed of a distributed constant line having a small width and a distributed constant line having a large width. 
     Although not shown, the first conductor part  41  and the second conductor part  42  of the fourth resonator  40 , similarly to the first conductor part  31  and the second conductor part  32  of the third resonator  30 , are arranged at positions different from each other in the stacking direction T. The first conductor part  31  and the first conductor part  41  may be arranged at the same position in the stacking direction T or may be arranged at positions different from each other in the stacking direction T. Similarly, the second conductor part  32  and the second conductor part  42  may be arranged at the same position in the stacking direction T or may be arranged at positions different from each other in the stacking direction T. 
     In the present embodiment, at least part of the third resonator  30  and at least part of the fourth resonator  40  are arranged between the first resonator  10  and the second resonator  20  when seen in the Z direction (refer to  FIG.  2   ). 
     In the present embodiment, part of the first conductor part  11  of the first resonator  10  may overlap the first conductor part  31  of the third resonator  30  when seen in the Z direction. In this case, part of the first conductor part  21  of the second resonator  20  may overlap the first conductor part  41  of the fourth resonator  40  when seen in the Z direction. 
     In the present embodiment, part of the second conductor part  12  of the first resonator  10  may overlap the second conductor part  32  of the third resonator  30  when seen in the Z direction. In this case, part of the second conductor part  22  of the second resonator  20  may overlap the second conductor part  42  of the fourth resonator  40  when seen in the Z direction. 
     The filter  1  according to the present embodiment further includes a third stub resonator  93  electrically connected to the first conductor part  31  of the third resonator  30 , and a fourth stub resonator  94  electrically connected to the first conductor part  41  of the fourth resonator  40 . Each of the third and fourth stub resonators  93  and  94  is a distributed constant line. 
     The third stub resonator  93  is connected in the middle of the first conductor part  31 . In  FIG.  19   , for the first conductor part  31 , a portion located between a connecting point with the third stub resonator  93  and the second conductor part  32  in the circuit configuration is indicated by a reference numeral  31 A, and a portion located between a connecting point with the third stub resonator  93  and the ground in the circuit configuration is indicated by a reference numeral  31 B. 
     The fourth stub resonator  94  is connected in the middle of the first conductor part  41 . In  FIG.  19   , for the first conductor part  41 , a portion located between a connecting point with the fourth stub resonator  94  and the second conductor part  42  in the circuit configuration is indicated by a reference numeral  41 A, and a portion located between a connecting point with the fourth stub resonator  94  and the ground in the circuit configuration is indicated by a reference numeral  41 B. 
     The third and fourth stub resonators  93  and  94  are used, for example, to control spurious to be generated in a higher frequency region than a passband. Each of the third and fourth stub resonators  93  and  94  may be an open stub with one end being open or may be a short stub with one end being connected to ground. 
     The configuration, operation, and effects of the present embodiment are otherwise the same as those of the first embodiment. 
     The present invention is not limited to the foregoing embodiments, and various modifications may be made thereto. For example, the number and configuration of resonators are not limited to those shown in the embodiments, and any number and configuration of resonators may be employed as long as the scope of the claims is satisfied. The number of resonators may be one, two, or five or more. 
     Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims and equivalents thereof, the invention may be practiced in other embodiments than the foregoing most preferable embodiments.