Patent Publication Number: US-10763561-B2

Title: Band-pass filter and control method thereof

Description:
This application is a National Stage Entry of PCT/JP2017/017253 filed on May 2, 2017, which claims priority from Japanese Patent Application 2016-101808 filed on May 20, 2016, the contents of all of which are incorporated herein by reference, in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a band-pass filter capable of changing a bandwidth of a pass band, and a control method of the band-pass filter. 
     BACKGROUND ART 
     In a wireless communication system that performs transmission and reception using a micro-wave band or a millimeter-wave band, a band-pass filter is used to pass signals in a predetermined frequency band and to remove unnecessary frequency components. Patent Literature 1 discloses a technique relating to a band-pass filter capable of changing a pass band. 
     Specifically, Patent Literature 1 discloses that a semi-coaxial resonant element is arranged inside a cavity resonator, and a movable conductor is arranged in a space between a cover covering the cavity resonator and an open end of the resonant element, and resonance frequency of the cavity resonator is changed by moving the movable conductor. Patent Literature 1 further discloses that a similar movable conductor is arranged in a space between two cavity resonators, and a coupling coefficient between two cavity resonators is changed by moving the movable conductor to change a bandwidth of a pass band. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2014-086839 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the technique disclosed in Patent Literature 1 relates to a filter for changing a frequency. Thus, although the movable conductor for changing a bandwidth of a pass band is independently moved, the variable range of the bandwidth is narrow, and the effect of varying the bandwidth is small. In addition, no mechanism for changing an external Q factor is disclosed, and the filter characteristics are deteriorated as the bandwidth is changed. 
     In view of the above, a purpose of the present disclosure is to provide a band-pass filter that solves the above problems and is capable of easily changing a bandwidth of a pass band, and a control method of the band-pass filter. 
     Solution to Problem 
     In one example aspect, a band-pass filter comprising: 
     a housing; 
     a plurality of resonant plates stored in the housing; 
     a first coupling conductor connecting two adjacent resonant plates of the plurality of resonant plates; and 
     a second coupling conductor arranged at a position affecting a coupling coefficient between the two adjacent resonant plates, 
     wherein a distance from the resonant plate to the second coupling conductor is changeable. 
     In one example aspect, a control method of a band-pass filter including a plurality of resonant plates stored in a housing, the control method comprising: 
     connecting two adjacent resonant plates of the plurality of resonant plates by a first coupling conductor; 
     arranging a second coupling conductor at a position affecting a coupling coefficient between the two adjacent resonant plates; and 
     changing a distance from the resonant plate to the second coupling conductor. 
     Advantageous Effects of Invention 
     According to the above aspects, it is possible to provide a band-pass filter capable of easily changing a bandwidth of a pass band, and a control method of the band-pass filter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing an example of a band-pass filter according to a first embodiment; 
         FIG. 2  is a side view showing an example of a band-pass filter according to the first embodiment; 
         FIG. 3  is a top view showing an example of a band-pass filter according to the first embodiment; 
         FIG. 4  is a diagram showing an example of a distance (sr_dy) from a resonator to a third coupling conductor; 
         FIG. 5  is a perspective view showing an example of the band-pass filter according to the first embodiment to which a substrate is mounted; 
         FIG. 6  is a perspective view of the band-pass filter in  FIG. 5  when viewed from the back side; 
         FIG. 7  is a perspective view showing an example of the band-pass filter according to the first embodiment to which a substrate is mounted; 
         FIG. 8  is a perspective view of the band-pass filter in  FIG. 7  when viewed from the back side; 
         FIG. 9  is a perspective view showing an example of the band-pass filter according to the first embodiment to which a substrate is mounted; 
         FIG. 10  is a perspective view of the band-pass filter in  FIG. 9  when viewed from the back side; 
         FIG. 11  is a diagram showing an example of a filter characteristic of the band-pass filter according to the first embodiment; 
         FIG. 12  is a diagram showing an example of a filter characteristic of the band-pass filter according to the first embodiment; 
         FIG. 13  is a diagram showing an example of a filter characteristic of the band-pass filter according to the first embodiment; 
         FIG. 14  is a diagram showing an example of a filter characteristic of the band-pass filter according to the first embodiment; 
         FIG. 15  is a diagram showing an example of a filter characteristic of the band-pass filter according to the first embodiment; 
         FIG. 16  is a perspective view showing an example of a band-pass filter according to a second embodiment; 
         FIG. 17  is a side view showing an example of a band-pass filter according to the second embodiment; 
         FIG. 18  is a top view showing an example of a band-pass filter according to the second embodiment; 
         FIG. 19  is a perspective view showing an example of the band-pass filter according to the second embodiment to which a substrate is mounted; and 
         FIG. 20  is a perspective view of the band-pass filter in  FIG. 19  when viewed from the back side; 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure are described with reference to the drawings. Note that, specific numerical values to be used in the following embodiments are merely examples in order for the present discloser to be easily understood, and are not limited thereto. 
     (1) First Embodiment 
       FIGS. 1 to 3  are respectively a perspective view, a side view, and a top view showing an example of a band-pass filter  1 A according to a first embodiment. 
     As shown in  FIGS. 1 to 3 , the band-pass filter  1 A according to the first embodiment is a band-pass filter capable of changing a bandwidth of a pass band, and includes a housing  11 , three resonant plates  12 - 1  to  12 - 3 , two input/output ports  13 - 1  and  13 - 2 , two loop antennas  14 - 1  and  14 - 2 , two first coupling conductors  15 - 1  and  15 - 2 , two second coupling conductors  16 - 1  and  16 - 2 , and two third coupling conductors  17 - 1  and  17 - 2 . In the following description, the resonant plates  12 - 1  to  12 - 3  are simply referred to as a “resonant plate  12 ” unless especially distinguished. Similarly, the input/output ports  13 - 1  and  13 - 2  are simply referred to as an “input/output port  13 ”, the loop antennas  14 - 1  and  14 - 2  are simply referred to as a “loop antenna  14 ”, the first coupling conductors  15 - 1  and  15 - 2  are simply referred to as a “first coupling conductor  15 ”, the second coupling conductors  16 - 1  and  16 - 2  are simply referred to as a “second coupling conductor  16 ”, and the third coupling conductors  17 - 1  and  17 - 2  are simply referred to as a “third coupling conductor  17 ”. 
     The band-pass filter  1 A according to the first embodiment has a three-stage configuration including the three resonant plates  12 - 1  to  12 - 3 . However, the number of stages of the band-pass filter  1 A according to the first embodiment is not limited as long as the band-pass filter  1 A has two or more stages. 
     In the first embodiment, it is assumed that, in  FIGS. 1 to 3 , the center position of the contact surface of the second-stage resonant plate  12 - 2  with the bottom face of the housing  11  is the origin point, the direction in which the resonant plates  12 - 1  to  12 - 3  are arranged is an x direction, the direction orthogonal to the principal surfaces (a surface having the largest area) of the resonant plates  12 - 1  to  12 - 3  is a y direction, and the longitudinal direction of the resonant plates  12 - 1  to  12 - 3  is a z direction. 
     The housing  11  has a cavity inside, and is a conductive member for storing the resonant plates  12 - 1  to  12 - 3  in the cavity. The housing  11  is constituted by, for example, a casing having a recessed portion capable of storing the resonant plates  12 - 1  to  12 - 3  and a cover covering the opening of the casing. 
     The resonant plates  12 - 1  to  12 - 3  are semi-coaxial resonators formed of plate-shaped conductors each having one end connected to the bottom face of the housing  11  and the other end which is an open end (that is, the other end is not connected to any member). The resonant plates  12 - 1  to  12 - 3  are arranged in the x direction so that the side faces are opposed to each other. Since the resonant plates  12 - 1  to  12 - 3  each have a plate shape, it is possible for the resonant plates  12 - 1  to  12 - 3  to be formed integrally with the first coupling conductors  15 - 1  and  15 - 2  and to be strongly spacially coupled with the second coupling conductors  16 - 1  and  16 - 2 . The material of the resonant plates  12 - 1  to  12 - 3  is only required to be metal having high conductivity, and is, for example, copper. The resonant plates  12 - 1  to  12 - 3  resonate at a resonance frequency determined by the shape, the length (z direction), and the like. 
     The input/output ports  13 - 1  and  13 - 2  input and output high-frequency signals. The input/output port  13 - 1  is constituted by a coaxial line, and an internal conductor of the coaxial line serves as the loop antenna  14 - 1 . The input/output port  13 - 1  is inserted from the bottom face of the housing  11  at one end of the housing  11  in the x direction (at the side of the resonant plate  12 - 1 ), and connected to the resonant plate  12 - 1  by the loop antenna  14 - 1  by electromagnetic coupling. The input/output port  13 - 2  is constituted by a coaxial line, and an internal conductor of the coaxial line serves as the loop antenna  14 - 2 . The input/output port  13 - 2  is inserted from the bottom face of the housing  11  at the other end of the housing  11  in the x direction (at the side of the resonant plate  12 - 3 ), and connected to the resonant plate  12 - 3  by the loop antenna  14 - 2  by electromagnetic coupling. One of the input/output ports  13 - 1  and  13 - 2  serves as an input port, and the other serves as an output port. For example, when the input/output port  13 - 1  serves as the input port and the input/output port  13 - 2  serves as the output port, high-frequency signals are input to the input/output port  13 - 1 , and the high-frequency signals within the pass band of the band-pass filter  1 A among the input signals are output from the input/output port  13 - 2 . Each of the loop antennas  14 - 1  and  14 - 2  may have a shape other than a loop shape, such as a simple rod shape. 
     The first coupling conductor  15 - 1  is a plate-shaped conductor connecting the two adjacent resonant plates  12 - 1  and  12 - 2 . The first coupling conductor  15 - 2  is a plate-shaped conductor connecting the two adjacent resonant plates  12 - 2  and  12 - 3 . Specifically, the first coupling conductor  15  connects the two adjacent resonant plates  12  on the side faces of the resonant plates  12  at a position other than the open ends of the resonant plates  12 . In the first embodiment, by designing the shape and the position of the first coupling conductor  15  so as to connect the two adjacent resonant plates  12 , it is possible to obtain a desired coupling coefficient (or a coupling amount) between the two adjacent resonant plates  12 . In this manner, the coupling coefficient is determined by the shape and the position of the first coupling conductor  15 , and a partition plate, which is often used for a semi-coaxial filter, for partitioning the two adjacent resonant plates  12  is unnecessary. Since the first coupling conductors  15 - 1  and  15 - 2  each have a plate shape, it is possible for the first coupling conductors  15 - 1  and  15 - 2  to be integrally formed with the resonant plates  12 - 1  to  12 - 3 . The material of the first coupling conductors  15 - 1  and  15 - 2  is only required to be metal having high conductivity, and is, for example, copper. The closer the position (wire_H) of the first coupling conductors  15 - 1  and  15 - 2  in the z direction with respect to the open ends of the resonant plates  12 - 1  to  12 - 3 , the higher the coupling coefficient between the two adjacent resonant plates  12 . Thus, the position of the first coupling conductors  15 - 1  and  15 - 2  in the z direction is at a position where a desired coupling coefficient is obtained. 
     The second coupling conductor  16 - 1  is a plate-shaped conductor arranged at a position affecting the coupling coefficient between the two adjacent resonant plates  12 - 1  and  12 - 2 . The second coupling conductor  16 - 2  is a plate-shaped conductor arranged at a position affecting the coupling coefficient between the two adjacent resonant plates  12 - 2  and  12 - 3 . Since the second coupling conductors  16 - 1  and  16 - 2  each have a plate shape, it is possible for the second coupling conductors  16 - 1  and  16 - 2  to be strongly spacially coupled with the resonant plates  12 - 1  to  12 - 3 . 
     The second coupling conductor  16 - 1  is arranged so that its principal surface is opposed to the principal surfaces of the resonant plates  12 - 1  to  12 - 3 , and extends over the resonant plates  12 - 1  and  12 - 2  in the x direction. The second coupling conductor  16 - 2  is arranged so that its principal surface is opposed to the principal surfaces of the resonant plates  12 - 1  to  12 - 3 , and extends over the resonant plates  12 - 2  and  12 - 3  in the x direction. When the position (cp_H) of the second coupling conductors  16 - 1  and  16 - 2  in the z direction is closer to the bottom face of the housing  11  than that of the first coupling conductors  15 - 1  and  15 - 2 , although the distance (y direction; cp_dy) from the resonant plates  12 - 1  to  12 - 3  to the second coupling conductors  16 - 1  and  16 - 2  is changed, the coupling coefficient between the two adjacent resonant plates  12  is only slightly changed. Thus, the position of the second coupling conductors  16 - 1  and  16 - 2  in the z direction is closer to the open ends of the resonant plates  12 - 1  to  12 - 3  than that of the first coupling conductors  15 - 1  and  15 - 2 . In addition, the closer the position of the second coupling conductors  16 - 1  and  16 - 2  in the z direction with respect to the open ends of the resonant plates  12 - 1  to  12 - 3 , the steeper the change of the coupling coefficient between the two adjacent resonant plates  12  when the distance cp_dy is changed. Thus, the position of the second coupling conductors  16 - 1  and  16 - 2  in the z direction is closer to the open ends of the resonant plates  12 - 1  to  12 - 3  than that of the first coupling conductors  15 - 1  and  15 - 2 , and is a position where the coupling coefficient is changed as desired. 
     In the first embodiment, the distance (y direction; cp_dy) from the resonant plates  12 - 1  to  12 - 3  to the second coupling conductors  16 - 1  and  16 - 2  is changeable as described above. In order to change the distance cp_dy, the position of the resonant plates  12 - 1  to  12 - 3  is fixed, and the position of the second coupling conductors  16 - 1  and  16 - 2  in the y direction is changed (this changing method is to be described later). When the distance cp_dy is changed, the coupling coefficient between the two adjacent resonant plates  12  is changed. Specifically, the shorter the distance cp_dy (the closer the second coupling conductors  16 - 1  and  16 - 2  with respect to the resonant plates  12 - 1  to  12 - 3 ), the smaller the coupling coefficient, and the narrower the bandwidth of the pass band. On the other hand, the longer the distance cp_dy (the farther the second coupling conductors  16 - 1  and  16 - 2  with respect to the resonant plates  12 - 1  to  12 - 3 ), the larger the coupling coefficient, and the broader the bandwidth of the pass band. However, the coupling coefficient does not exceed a coupling coefficient determined by the position of the first coupling conductors  15 - 1  and  15 - 2  in the z direction. Thus, by setting the coupling coefficient determined by the position of the first coupling conductors  15 - 1  and  15 - 2  in the z direction as the maximum value, and changing the coupling coefficient by the distance cp_dy, it is possible to change the bandwidth of the pass band. 
     The material of the second coupling conductors  16 - 1  and  16 - 2  is only required to be metal having high conductivity, and is, for example, copper. The longer the length (x direction; cp_x) of the principal surfaces of the second coupling conductors  16 - 1  and  16 - 2  and the wider the width (z direction; cp_z), the gentler the change of the coupling coefficient between the two adjacent resonant plates  12  when the distance cp_dy is changed. Thus, the length and width of the principal surfaces of the second coupling conductors  16 - 1  and  16 - 2  are a length and width at which the coupling coefficient is changed as desired. 
     The third coupling conductor  17 - 1  is arranged at a position affecting an external Q factor between the input/output port  13 - 1  and the resonant plate  12 - 1 . The third coupling conductor  17 - 2  is arranged at a position affecting the external Q factor between the input/output port  13 - 2  and the resonant plate  12 - 3 . 
     The third coupling conductors  17 - 1  and  17 - 2  are provided to change the reflection delay of the input/output stage of the band-pass filter  1 A. The reflection delay is changed to adjust the external Q factor (so that it is consistent with an external circuit). The reflection delay is the amount of delay of a reflection signal with respect to an incident signal when a measurement object measured by a network analyzer is set as a first-stage resonator, and is uniquely determined according to the external Q factor. For example, when it is assumed that the reflection delay between the resonant plate  12 - 1  and the input/output port  13 - 1  is τ p , that the external Q factor between the resonant plate  12 - 1  and the input/output port  13 - 1  is Q ext , and that the resonance frequency of the resonant plate  12 - 1  is f p , the relationship between τ p  and Q ext  is expressed by the following expression. 
     
       
         
           
             
               
                 
                   
                     Q 
                     ext 
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           f 
                           p 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               τ 
                               p 
                             
                             - 
                             
                               τ 
                               ∞ 
                             
                           
                           ) 
                         
                       
                     
                     4 
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     where, τ ∞ =0 may hold. 
     The position of the third coupling conductor  17 - 1  in the x direction is a position between the input/output port  13 - 1  and the resonant plate  12 - 1 , and the position in the z direction may be an arbitrary position. The position of the third coupling conductor  17 - 2  in the x direction is a position between the input/output port  13 - 2  and the resonant plate  12 - 3 , and the position in the z direction is an arbitrary position. 
     In the first embodiment, the distance (y direction) from the resonant plates  12 - 1  to  12 - 3  to the third coupling conductors  17 - 1  and  17 - 2  is changeable. The distance is represented by sr_dy as shown in  FIG. 4 .  FIG. 4  corresponds to the enlarged diagram around the third coupling conductor  17 - 1  in  FIG. 3 . In order to change the distance sr_dy, the position of the resonant plates  12 - 1  to  12 - 3  is fixed, and the position of the third coupling conductors  17 - 1  and  17 - 2  in the y direction is changed (this changing method is to be described later). When the distance sr_dy is changed, the reflection delay between the resonant plate  12 - 1  and the input/output port  13 - 1 , and the reflection delay between the resonant plate  12 - 3  and the input/output port  13 - 2  are changed. Specifically, the shorter the distance sr_dy (the closer the third coupling conductors  17 - 1  and  17 - 2  with respect to the resonant plates  12 - 1  to  12 - 3 ), the smaller the reflection delay, and the longer the distance sr_dy (the farther the third coupling conductors  17 - 1  and  17 - 2  with respect to the resonant plates  12 - 1  to  12 - 3 ), the larger the reflection delay. Thus, it is possible to change the reflection delay by the distance sr_dy. 
     The material of the third coupling conductors  17 - 1  and  17 - 2  is only required to be metal having high conductivity, and is, for example, copper. The third coupling conductors  17 - 1  and  17 - 2  may each have a plate shape, a cylindrical shape, a polygonal prism shape, or the like. However, when, for example, the third coupling conductors  17 - 1  and  17 - 2  have quadrangular prism shapes compared to plate shapes, the reflection delay between the resonant plate  12 - 1  and the input/output port  13 - 1  and the reflection delay between the resonant plate  12 - 3  and the input/output port  13 - 2  are more gently changed when the distance sr_dy is changed than when the third coupling conductors  17 - 1  and  17 - 2  have plate shapes. Thus, the third coupling conductors  17 - 1  and  17 - 2  have quadrangular prism shapes in  FIGS. 1 to 3 . 
     The method for changing the above distances cp_dy and sr_dy is described below. 
     In the first embodiment, the second coupling conductors  16 - 1  and  16 - 2  and the third coupling conductors  17 - 1  and  17 - 2  are mounted on a substrate, and the principal surface of the substrate is opposed to the principal surfaces of the resonant plates  12 - 1  to  12 - 3 . Then, the positions in the y direction of the second coupling conductors  16 - 1  and  16 - 2  and the third coupling conductors  17 - 1  and  17 - 2  mounted on the substrate are changed to change the distances cp_dy and sr_dy. 
     In the example shown in  FIGS. 5 and 6 , the second coupling conductors  16 - 1  and  16 - 2  are mounted on a surface of a substrate  18 . The surface on the back side of the above surface thereof is opposed to the resonant plates  12 - 1  to  12 - 3 . Since the third coupling conductors  17 - 1  and  17 - 2  have quadrangular prism shapes and are thick, the third coupling conductors  17 - 1  and  17 - 2  are mounted on both surfaces of the substrate  18 , and connect both surfaces via through holes. 
     In the example shown in  FIGS. 7 and 8 , the second coupling conductors  16 - 1  and  16 - 2  are mounted on a surface of the substrate  18  opposed to the resonant plates  12 - 1  to  12 - 3 . The third coupling conductors  17 - 1  and  17 - 2  have thin plate shapes, not quadrangular prism shapes. Thus, the third coupling conductors  17 - 1  and  17 - 2  are mounted only on one surface (the surface on the back side of the one surface opposed to the resonant plates  12 - 1  to  12 - 3 ) of the substrate  18 . 
     In the example shown in  FIGS. 9 and 10 , the second coupling conductors  16 - 1  and  16 - 2  are mounted on a surface of the substrate  18 . The surface on the back side of the above surface thereof is opposed to the resonant plates  12 - 1  to  12 - 3 . Since the third coupling conductors  17 - 1  and  17 - 2  have quadrangular prism shapes and are thick, the third coupling conductors  17 - 1  and  17 - 2  are mounted on both surfaces of the substrate  18 , and connect both surfaces via through holes. 
     As described above, the third coupling conductors  17 - 1  and  17 - 2  may be mounted on one surface of the substrate  18 , or on both surfaces of the substrate  18  and connect both surfaces via through holes. The second coupling conductors  16 - 1  and  16 - 2  may be mounted on one surface of the substrate  18 , or on both surfaces of the substrate  18  and connect both surfaces via through holes when the second coupling conductors  16 - 1  and  16 - 2  are thick. When mounted on one surface of the substrate  18 , the second coupling conductors  16 - 1  and  16 - 2  and the third coupling conductors  17 - 1  and  17 - 2  may be mounted either on the surface opposed to the resonant plates  12 - 1  to  12 - 3  or on the surface on the back side of the above surface. 
     In the case of the substrate  18  shown in  FIGS. 5 to 8 , by attaching a supporting rod (now shown) to the substrate  18  and moving the supporting rod in the y direction using a stepping motor (now shown) provided outside the band-pass filter  1 A, the substrate  18  is configured to be movable in the y direction. Then, by moving the substrate  18  in the y direction to change the positions in the y direction of the second coupling conductors  16 - 1  and  16 - 2  and the third coupling conductors  17 - 1  and  17 - 2 , it is possible to change the distances cp_dy and sr_dy. At this time, by determining the shape of the third coupling conductor  17  so that it corresponds to the inclination of the coupling amount determined by the second coupling conductor  16 , it is possible to consecutively change the distances cp_dy and sr_dy using the stepping motor without deteriorating the filter characteristic. In addition, it is possible to change the distances cp_dy and sr_dy separately using respective stepping motors. 
     On the other hand, in the case of the substrate  18  shown in  FIGS. 9 and 10 , the substrate  18  is configured to be attachable to and detachable from a substrate mounting part (now shown) in the housing  11 . The substrate mounting part is only required to have a structure in which the substrate  18  is attachable and detachable, such as a slot into which the substrate  18  is inserted. Furthermore, a plurality of substrates  18  manufactured so as to have different distances cp_dy and sr_dy from each other when mounted to the housing  11  is to be prepared. Such substrates  18  can be manufactured by, for example, changing the substrate thickness, the mounting height, the mounding surface, or the like. Then, the substrate  18  mounted to the housing  11  is replaced with another substrate  18  having different distances cp_dy and sr_dy. Accordingly, the positions of the second coupling conductors  16 - 1  and  16 - 2  and the third coupling conductors  17 - 1  and  17 - 2  in the y direction are changed, and it is possible to change the distances cp_dy and sr_dy. 
     A filter characteristic (a simulation result) of the band-pass filter  1 A according to the first embodiment is described below. 
     Here, it is assumed that the distances cp_dy and sr_dy are changed by preparing four substrates # 1  to # 4  as the substrates  18 , and sequentially replacing the substrates # 1  to # 4 . 
     Furthermore, in order to obtain a band-pass filter having the center frequency of 2.0 [GHz], it is assumed that the band-pass filter  1 A is configured in the following conditions: 
     the length of the housing  11  (z direction) cav_z: 40 [mm]; 
     the width of the housing  11  (x direction) cav_x: 30 [mm]; 
     the depth of the housing  11  (y direction) cav_y: 20 [mm]; 
     the length of the resonant plate  12  (z direction) reso_z: 35 [mm]; 
     the width of the resonant plate  12  (x direction): 7 [mm]; 
     the thickness of the resonant plate  12  (y direction): 1 [mm]; 
     the position of the first coupling conductor  15  (z direction) wire_H: 9 [mm]; 
     the width of the first coupling conductor  15  (z direction): 1 [mm]; 
     the thickness of the first coupling conductor  15  (y direction): 1 [mm]; 
     the position of the second coupling conductor  16  (z direction) cp_H: 20 [mm]; 
     the length of the second coupling conductor  16  (x direction) cp_x: 28 [mm]; 
     the width of the second coupling conductor  16  (z direction) cp_z: 3 [mm]; 
     the thickness of the second coupling conductor  16  (y direction): 0.018 [mm]; 
     the length of the third coupling conductor  17  (z direction): 15 [mm]; 
     the width of the third coupling conductor  17  (x direction): 1 [mm]; and 
     the depth of the third coupling conductor  17  (y direction): 1 [mm]. 
     Note that, the distance cav_x is equivalent to, when it is assumed that the housing  11  is divided into three housings for storing three respective resonant plates  12 , the length of each housing in the x direction. 
     Furthermore, it is assumed that the distances cp_dy and sr_dy when the substrates # 1  to # 4  are mounted to the housing  11  are as follows: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 sr_dy 
                   
                 cp_dy 
                 bandwidth 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 substrate #1 
                 1 
                 mm 
                 1.9 
                 mm 
                 73 
                 MHz 
               
               
                   
                 substrate #2 
                 0.6 
                 mm 
                 2.25 
                 mm 
                 83 
                 MHz 
               
               
                   
                 substrate #3 
                 0.2 
                 mm 
                 2.9 
                 mm 
                 96 
                 MHz 
               
               
                   
                 substrate #4 
                 0 
                 mm 
                 3.1 
                 mm 
                 102 
                 MHz 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 11  shows an example of a filter characteristic when the substrate # 1  is mounted to the band-pass filter  1 A according to the first embodiment. Similarly,  FIGS. 12 to 14  show respective filter characteristics when the substrates # 2  to # 4  are mounted. In  FIGS. 11 to 14 , each abscissa indicates a frequency [GHz], and each ordinate indicates S 11  and S 21  [dB] of the S parameter. S 11  indicates return loss, and shows the reflection characteristic of a high-frequency signal. S 21  indicates insertion loss, and shows the pass characteristic of a high-frequency signal. 
     In the example shown in  FIG. 11 , S 21  shows the pass characteristic that its lower-limit frequency is 1.95 [GHz] and its peak is in a frequency band having the width of 73 [MHz]. S 11  shows the reflection characteristic in a similar frequency band. Thus, the band-pass filter  1 A, to which the substrate # 1  is mounted, functions as a pass band filter having a pass band the bandwidth of which is 73 [MHz]. Similarly, in the example shown in  FIG. 12 , the band-pass filter  1 A, to which the substrate # 2  is mounted, functions as a pass band filter having a pass band the bandwidth of which is 83 [MHz]. In the example shown in  FIG. 13 , the band-pass filter  1 A, to which the substrate # 3  is mounted, functions as a pass band filter having a pass band the bandwidth of which is 96 [MHz]. In the example shown in  FIG. 14 , the band-pass filter  1 A, to which the substrate # 4  is mounted, functions as a pass band filter having a pass band the bandwidth of which is 102 [MHz]. 
       FIG. 15  is a diagram showing an example of the filter characteristic of the band-pass filter  1 A according to the first embodiment, and shows that respective waveforms S 21  shown in  FIGS. 11 to 14  are superimposed. 
       FIG. 15  shows that the band-pass filter  1 A according to the first embodiment has the filter characteristic that the lower-limit frequency of the pass band is not changed and the upper-limit frequency is changed according to the distance cp_dy. Specifically, the longer the distance cp_dy (the farther the second coupling conductors  16 - 1  and  16 - 2  with respect to the resonant plates  12 - 1  to  12 - 3 ), the higher the coupling coefficient, the higher the upper-limit frequency, and the broader the bandwidth. 
     As described above, the band-pass filter  1 A according to the first embodiment has a configuration in which the two adjacent resonant plates  12  are connected by the first coupling conductor  15 , the second coupling conductor  16  is arranged at a position affecting the coupling coefficient between the two adjacent resonant plates  12 , and the distance (cp_dy) from the resonant plate  12  to the second coupling conductor  16  is changeable. 
     Accordingly, the coupling coefficient between the two adjacent resonant plates  12  is changed according to the distance cp_dy, and it is possible to change the bandwidth of the pass band. Furthermore, although the distance cp_dy is changed, the upper-limit frequency of the pass band is only changed while the lower-limit frequency is not changed, and it is possible to change the bandwidth while the center frequency of the band-pass filter  1 A is hardly changed. Thus, it is possible to easily change the bandwidth of the pass band. 
     Furthermore, the band-pass filter  1 A according to the first embodiment has a configuration in which the third coupling conductor  17  is arranged at a position affecting the external Q factor between the resonant plate  12  on each end and the input/output port  13 , and the distance (sr_dy) from the resonant plate  12  to the third coupling conductor  17  is changeable. 
     Accordingly, it is possible to change, according to the distance sr_dy, the reflection delay uniquely determined according to the external Q factor, and to be consistent with the external circuit. 
     Here, the band-pass filter  1 A according to the first embodiment may have a configuration in which the distances cp_dy and sr_dy are changed by, for example, configuring the substrate  18 , on which the second coupling conductor  16  and the third coupling conductor  17  are mounted, to be attachable to and detachable from the housing  11  and replacing the substrate  18 . 
     Accordingly, it is possible to change the bandwidth of the pass band only by replacing the substrate  18  while the main components, such as the housing  11  and the resonant plate  12 , remain unchanged. 
     Alternatively, the band-pass filter  1 A according to the first embodiment may have a configuration in which the distances cp_dy and sr_dy are changed by moving the substrate  18 , on which the second coupling conductor  16  and the third coupling conductor  17  are mounted, in the y direction using a stepping motor (now shown) provided outside the band-pass filter  1 A. 
     Accordingly, it is possible to change the bandwidth of the pass band while the substrate  18  as well as the main components, such as the housing  11  and the resonant plate  12 , remain unchanged. 
     Furthermore, the band-pass filter  1 A according to the first embodiment has a configuration in which the bandwidth of the pass band is changed by connecting the two adjacent resonant plates  12  by the first coupling conductor  15  and changing the coupling coefficient according to the distance cp_dy. 
     Accordingly, it is possible to determine the coupling coefficient by the shape and the position of the first coupling conductor  15 , and a partition plate, which is often used for a semi-coaxial filter, for partitioning the two adjacent resonant plates  12  is unnecessary. Thus, the housing  11  does not need to be cut to form a partition plate and is easily processed, and which reduces the processing cost. In addition, if a partition plate is provided, the band-pass filter  1 A is lengthened in the x direction by the thickness of the partition plate. However, the band-pass filter  1 A according to the first embodiment does not need a partition plate and can be downsized in the x direction. Furthermore, the shape of a partition plate and the interval between adjacent resonators is normally used as parameters to design the coupling coefficient in a semi-coaxial filter, but a desired coupling coefficient can be designed by selecting the shape and the position of the first coupling conductor  15  without such parameters in the first embodiment. Thus, it is possible not only to downsize a filter in the x direction, but also to manufacture a filter having an arbitrary length in the x direction. 
     (2) Second Embodiment 
       FIGS. 16 to 18  are respectively a perspective view, a side view, and a top view showing an example of a band-pass filter  1 B according to a second embodiment. 
     As shown in  FIGS. 16 to 18 , the band-pass filter  1 B according to the second embodiment has a configuration in which the third coupling conductors  17 - 1  and  17 - 2  are removed from the band-pass filter  1 A according to the first embodiment. The configuration other than this in the second embodiment is similar to that in the first embodiment. 
     The third coupling conductors  17 - 1  and  17 - 2  are provided to adjust an external Q factor (to be consistent with an external circuit) as described above. Thus, when deterioration in return loss is little or allowable and the external Q factor does not need to be adjusted, the third coupling conductors  17 - 1  and  17 - 2  can be removed as in the band-pass filter  1 B according to the second embodiment. 
     In the band-pass filter  1 B according to the second embodiment, second coupling conductors  16 - 1  and  16 - 2  are mounted on a substrate, and the principal surface of the substrate is opposed to the principal surfaces of resonant plates  12 - 1  to  12 - 3 . Then, the position of the second coupling conductors  16 - 1  and  16 - 2  mounted on the substrate is changed in the y direction to change the distance cp_dy. 
     In the example shown in  FIGS. 19 and 20 , the second coupling conductors  16 - 1  and  16 - 2  are mounted on the face of the substrate  18  opposed to the resonant plates  12 - 1  to  12 - 3 . The second coupling conductors  16 - 1  and  16 - 2  may be mounted on one surface of the substrate  18  in this manner or on both surfaces of the substrate  18  and connect both surfaces via through holes when the second coupling conductors  16 - 1  and  16 - 2  are thick. When the second coupling conductors  16 - 1  and  16 - 2  are mounted on one surface of the substrate  18 , the second coupling conductors  16 - 1  and  16 - 2  may be mounted either on the face opposed to the resonant plates  12 - 1  to  12 - 3  or on the surface on the back side of the above surface. 
     Here, in the case of the substrate  18  shown in  FIGS. 19 and 20 , by attaching a supporting rod (now shown) to the substrate  18  and moving the supporting rod in the y direction using a stepping motor (now shown) provided outside the band-pass filter  1 B, the substrate  18  is configured to be movable in the y direction. Then, by moving the substrate  18  in the y direction to change the position of the second coupling conductors  16 - 1  and  16 - 2  in the y direction, and it is possible to change the distance cp_dy. 
     However, the distance cp_dy may be changed by configuring the substrate  18  to be attachable to and detachable from a substrate mounting part (not shown) in the housing  11  and replacing the substrate  18  with another substrate  18  having a different distance cp_dy when mounted to the housing  11 . 
     As described above, the band-pass filter  1 B according to the second embodiment has a configuration in which the two adjacent resonant plates  12  are connected by the first coupling conductor  15 , the second coupling conductor  16  is arranged at a position affecting the coupling coefficient between the two adjacent resonant plates  12 , and the distance (cp_dy) from the resonant plate  12  to the second coupling conductor  16  is changeable, similarly to the band-pass filter  1 A according to the first embodiment. 
     Thus, it is possible to easily change the bandwidth of the pass band similarly to the band-pass filter  1 A according to the first embodiment. The other effects are similar to that in the first embodiment except that an external Q factor is adjustable (to be consistent with an external circuit). 
     While the present disclosure has been particularly shown and described with reference to embodiments thereof, the present disclosure is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the claims. 
     For example, it has been exemplified that second coupling conductor and third coupling conductor are mounted on the same substrate in the first embodiment. However, the band-pass filter according to the present disclosure is not limited thereto, and the second coupling conductor and the third coupling conductor may be separately mounted on two respective substrates. When the distances cp_dy and sr_dy are changed by moving the substrates in the y direction in this configuration, the two substrates may be moved together by the same stepping motor or respective stepping motors, or moved separately by respective stepping motors. Furthermore, when the distances cp_dy and sr_dy are changed by replacing the substrates, the two substrates may be replaced together, or either one may be replaced. 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-101808, filed on May 20, 2016, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           1 A,  1 B BAND-PASS FILTER 
           11  HOUSING 
           12 - 1  to  12 - 3  RESONANT PLATE 
           13 - 1 ,  13 - 2  INPUT/OUTPUT PORT 
           14 - 1 ,  14 - 2  LOOP ANTENNA 
           15 - 1 ,  15 - 2  FIRST COUPLING CONDUCTOR 
           16 - 1 ,  16 - 2  SECOND COUPLING CONDUCTOR 
           17 - 1 ,  17 - 2  THIRD COUPLING CONDUCTOR 
           18  SUBSTRATE