Patent Publication Number: US-2023156910-A1

Title: Circuit board and electronic device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-128597 filed on Jul. 29, 2020 and is a Continuation Application of PCT Application No. PCT/JP2021/022052 filed on Jun. 10, 2021. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit board and an electronic device including a signal conductor and a power supply conductor. 
     2. Description of the Related Art 
     As an invention related to a conventional circuit board, for example, a power transmission line described in International Publication No. 2016/163436 is known. The power transmission line includes a signal conductor pattern, power transmission conductor patterns, a first reference conductor, and a second reference conductor, as shown in  FIG.  13    of International Publication No. 2016/163436. The first reference conductor is arranged above the signal conductor pattern. The second reference conductor is arranged below the signal conductor pattern. With this configuration, the signal conductor pattern, the first reference conductor, and the second reference conductor have a stripline structure. The power transmission conductor patterns are arranged to the right of the signal conductor pattern. The power transmission conductor patterns are connected to the first reference conductor. With this configuration, the first reference conductor functions as a power supply line. As a result, reduction in an insertion loss of the power supply line is achieved in the power transmission line described in International Publication No. 2016/163436. 
     In the power transmission line described in International Publication No. 2016/163436, a power supply potential is connected to the first reference conductor. For this reason, noise may come into the first reference conductor. In this case, noise may affect a high frequency signal which is transmitted through the signal conductor pattern. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide circuit boards and electronic devices that are each able to reduce or prevent noise from affecting a high frequency signal which is transmitted through a signal conductor. 
     A circuit board according to a preferred embodiment of the present invention includes a board body, a signal conductor in the board body and through which a high frequency signal is transmitted, a power supply conductor in the board body and connected to a power supply potential, the power supply conductor extending along at least a portion of the signal conductor, and a first reference conductor in the board body and insulated from the signal conductor and the power supply conductor. 
     Definition of terms in the present specification will be described below. Hereinafter, a first member to a third member each mean a member, element, portion, body, or the like included in a signal transmission line. In the present specification, portions of the first member are defined in the following manner unless otherwise specified. A front portion of the first member means a front half of the first member. A rear portion of the first member means a rear half of the first member. A left portion of the first member means a left half of the first member. A right portion of the first member means a right half of the first member. An upper portion of the first member means an upper half of the first member. A lower portion of the first member means a lower half of the first member. A front end of the first member means an end in a forward direction of the first member. A rear end of the first member means an end in a backward direction of the first member. A left end of the first member means an end in a leftward direction of the first member. A right end of the first member means an end in a rightward direction of the first member. An upper end of the first member means an end in an upward direction of the first member. A lower end of the first member means an end in a downward direction of the first member. A front end portion of the first member means the front end of the first member and its vicinity. A rear end portion of the first member means the rear end of the first member and its vicinity. A left end portion of the first member means the left end of the first member and its vicinity. A right end portion of the first member means the right end of the first member and its vicinity. An upper end portion of the first member means the upper end of the first member and its vicinity. A lower end portion of the first member means the lower end of the first member and its vicinity. 
     Circuit boards according to preferred embodiments of the present invention are each able to reduce or prevent noise from affecting a high frequency signal which is transmitted through a signal conductor. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    an external perspective view of a circuit board  10  according to a preferred embodiment of the present invention. 
         FIG.  2    is an exploded perspective view of the circuit board  10 . 
         FIG.  3    is an equivalent circuit diagram of the circuit board  10 . 
         FIG.  4    is an equivalent circuit diagram of a circuit board  10   a  according to a preferred embodiment of the present invention. 
         FIG.  5    is an equivalent circuit diagram of a circuit board  10   b  according to a preferred embodiment of the present invention. 
         FIG.  6    is a sectional view of a circuit board  10   c  according to a preferred embodiment of the present invention. 
         FIG.  7    is an exploded perspective view of a circuit board  10   d  according to a preferred embodiment of the present invention. 
         FIG.  8    is a view showing an electronic device  1  according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred Embodiment 
     Structure of Circuit Board 
     A structure of a circuit board  10  according to a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  1    an external perspective view of the circuit board  10 .  FIG.  2    is an exploded perspective view of the circuit board  10 .  FIG.  3    is an equivalent circuit diagram of the circuit board  10 . 
     In the present specification, directions are defined in the following manner. A stacking direction of a board body  12  of the circuit board  10  is defined as a circuit board upward-downward direction. A longitudinal direction of the circuit board  10  is defined as a circuit board leftward-rightward direction. A lateral direction of the circuit board  10  is defined as a circuit board forward-backward direction. The circuit board upward-downward direction, the circuit board forward-backward direction, and the circuit board leftward-rightward direction are orthogonal to one another. The circuit board upward-downward direction, the circuit board forward-backward direction, and the circuit board leftward-rightward direction need not coincide with an upward-downward direction, a forward-backward direction, and a leftward-rightward direction when the circuit board  10  is actually used. 
     The circuit board  10  is used to connect two circuits in an electronic device, such as a cellular phone, for example. As shown in  FIGS.  1  and  2   , the circuit board  10  includes the board body  12 , a signal conductor  18 , a first power supply conductor  20   a , a second power supply conductor  20   b , a third power supply conductor  20   c , a first reference conductor  22   a , a second reference conductor  22   b , a first signal electrode  24   a , a second signal electrode  24   b , first reference electrodes  26   a  (see  FIG.  1   ), second reference electrodes  26   b  (see  FIG.  1   ), first power supply electrodes  28   a  (see  FIG.  1   ), second power supply electrodes  28   b  (see  FIG.  1   ), a first chip capacitor  30   a , a second chip capacitor  30   b , reference electrodes  222   a  and  222   b , power supply electrodes  224   a  and  224   b , a first interlayer connection conductor  v   1 , a second interlayer connection conductor  v   2 , a plurality of third interlayer connection conductors  v   3 , and a plurality of fourth interlayer connection conductors  v   4 . A typical interlayer connection conductor of the plurality of third interlayer connection conductors  v   3  and a typical interlayer connection conductor of the plurality of fourth interlayer connection conductors  v   4  are denoted by reference characters in  FIG.  2   . 
     As shown in  FIGS.  1  and  2   , the board body  12  has a plate shape. As shown in  FIGS.  1  and  2   , the board body  12  has a rectangular or substantially rectangular shape including long sides extending in the circuit board leftward-rightward direction as viewed in the circuit board upward-downward direction. The board body  12  has flexibility. Thus, the board body  12  can be used in a state of being bent in an electronic device. 
     As shown in  FIG.  2   , the board body  12  has a structure including a resist layer  17   a  and insulating resin layers  16   a  to  16   c  (a plurality of insulator layers) that are stacked in the circuit board upward-downward direction. The resist layer  17   a  and the insulating resin layers  16   a  to  16   c  are stacked so as to be aligned in this order from top to bottom in the circuit board upward-downward direction. The insulating resin layers  16   a  to  16   c  are dielectric sheets having flexibility. A material for each of the insulating resin layers  16   a  to  16   c  is, for example, a thermoplastic resin, such as polyimide or a liquid crystal polymer. The insulating resin layers  16   a  to  16   c  have the same or substantially the same rectangular or substantially rectangular shape as the board body  12  as viewed in the circuit board upward-downward direction. The resist layer  17   a  will be described later. 
     As shown in  FIG.  2   , the signal conductor  18  is provided in the board body  12 . More specifically, the signal conductor  18  is provided at an upper principal surface of the insulating resin layer  16   b . With this configuration, the signal conductor  18  is provided inside the board body  12 . The signal conductor  18  is a conductor layer having a linear shape extending in the circuit board leftward-rightward direction. The signal conductor  18  is arranged midway in the circuit board forward-backward direction of the upper principal surface of the insulating resin layer  16   b . A left end of the signal conductor  18  is located at a left end portion of the insulating resin layer  16   b . A right end of the signal conductor  18  is located at a right end portion of the insulating resin layer  16   c . A high frequency signal is transmitted through the signal conductor  18 . 
     The first power supply conductor  20   a  is provided in the board body  12 . The first power supply conductor  20   a  is positioned above the signal conductor  18  in the circuit board upward-downward direction so as to overlap with the signal conductor  18  as viewed in the circuit board upward-downward direction. In the present specification, the expression “the first power supply conductor  20   a  is positioned above the signal conductor  18  in the circuit board upward-downward direction” refers to the following state. At least a portion of the first power supply conductor  20   a  is positioned in a region through which the signal conductor  18  extends when the signal conductor  18  extends parallel or substantially parallel in a circuit board upward direction. Thus, the first power supply conductor  20   a  may be within the region, through which the signal conductor  18  passes when the signal conductor  18  extends parallel or substantially parallel in the circuit board upward direction or may protrude from the region, through which the signal conductor  18  passes when the signal conductor  18  extends parallel or substantially parallel in the circuit board upward direction. In the present preferred embodiment, the first power supply conductor  20   a  is protruding from the region, through which the signal conductor  18  passes when the signal conductor  18  extends parallel or substantially parallel in the circuit board upward direction. 
     The first power supply conductor  20   a  is provided at an upper principal surface of the insulating resin layer  16   a . As shown in  FIG.  2   , the first power supply conductor  20   a  is a conductor layer having a rectangular or substantially rectangular shape including long sides extending in the circuit board leftward-rightward direction as viewed in the circuit board upward-downward direction. The first power supply conductor  20   a  has a shape corresponding or substantially corresponding to the board body  12  as viewed in the circuit board upward-downward direction. The first power supply conductor  20   a  is slightly smaller than the board body  12  as viewed in the circuit board upward-downward direction. 
     As described earlier, the first power supply conductor  20   a  overlaps with the signal conductor  18  as viewed in the circuit board upward-downward direction. With this configuration, the first power supply conductor  20   a  extends along at least a portion of the signal conductor  18 . More specifically, the first power supply conductor  20   a  overlaps with at least a portion of the signal conductor  18  as viewed in the circuit board upward-downward direction (the board body stacking direction). In the present preferred embodiment, the first power supply conductor  20   a  overlaps with a portion excluding a left end portion and a right end portion of the signal conductor  18  as viewed in the circuit board upward-downward direction. Thus, the first power supply conductor  20   a  extends in the circuit board leftward-rightward direction along the signal conductor  18  in the portion excluding the left end portion and the right end portion of the signal conductor  18 . With this configuration, the first power supply conductor  20   a  is adjacent to the portion excluding the left end portion and the right end portion of the signal conductor  18 . 
     The third power supply conductor  20   c  is provided in the board body  12 . The third power supply conductor  20   c  is positioned below the signal conductor  18  in the circuit board upward-downward direction so as to overlap with the signal conductor  18  as viewed in the circuit board upward-downward direction. The third power supply conductor  20   c  is provided at an upper principal surface of the insulating resin layer  16   c . As shown in  FIG.  2   , the third power supply conductor  20   c  is a conductor layer having a rectangular or substantially rectangular shape including long sides extending in the circuit board leftward-rightward direction as viewed in the circuit board upward-downward direction. The third power supply conductor  20   c  has a shape corresponding or substantially corresponding the board body  12  as viewed in the circuit board upward-downward direction. The third power supply conductor  20   c  is slightly smaller than the board body  12  as viewed in the circuit board upward-downward direction. 
     As described earlier, the third power supply conductor  20   c  overlaps with the signal conductor  18  as viewed in the circuit board upward-downward direction. With this configuration, the third power supply conductor  20   c  extends along at least a portion of the signal conductor  18 . More specifically, the third power supply conductor  20   c  overlaps with at least a portion of the signal conductor  18  as viewed in the circuit board upward-downward direction (the board body stacking direction). In the present preferred embodiment, the third power supply conductor  20   c  overlaps with a portion excluding a left end portion and a right end portion of the signal conductor  18  as viewed in the circuit board upward-downward direction. Thus, the third power supply conductor  20   c  extends in the circuit board leftward-rightward direction along the signal conductor  18  in the portion excluding the left end portion and the right end portion of the signal conductor  18 . With this configuration, the third power supply conductor  20   c  is adjacent to the portion excluding the left end portion and the right end portion of the signal conductor  18 . 
     The second power supply conductor  20   b  is provided in the board body  12 . The second power supply conductor  20   b  is positioned at the same or substantially the same position as the signal conductor  18  in the circuit board upward-downward direction. The second power supply conductor  20   b  is provided at an upper principal surface of the insulating resin layer  16   b . As shown in  FIG.  2   , the second power supply conductor  20   b  is a conductor layer having a rectangular or substantially rectangular shape including long sides extending in the circuit board leftward-rightward direction as viewed in the circuit board upward-downward direction. The second power supply conductor  20   b  has a shape corresponding or substantially corresponding to the board body  12  as viewed in the circuit board upward-downward direction. The second power supply conductor  20   b  is slightly smaller than the board body  12  as viewed in the circuit board upward-downward direction. 
     The second power supply conductor  20   b  extends along at least a portion of the signal conductor  18 . In the present preferred embodiment, the second power supply conductor  20   b  extends along the entire or substantially the entire signal conductor  18 . More specifically, the second power supply conductor  20   b  is not provided in immediate surroundings of the signal conductor  18  such that the second power supply conductor  20   b  is insulated from the signal conductor  18 . With this configuration, the signal conductor  18  is surrounded by the second power supply conductor  20   b  as viewed in the circuit board upward-downward direction. Thus, the second power supply conductor  20   b  extends along the entire or substantially the entire signal conductor  18 . As a result, the second power supply conductor  20   b  is adjacent to the entire or substantially the entire signal conductor  18 . 
     The first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c  as described above are connected to a power supply potential. The power supply potential is, for example, about 1.8 V, about 3 V, or the like. 
     The first reference conductor  22   a  is provided in the board body  12 . The first reference conductor  22   a  is provided at a left end portion of the upper principal surface of the insulating resin layer  16   a . The signal conductor  18  is provided at the upper principal surface of the insulating resin layer  16   b . For this reason, the first reference conductor  22   a  is insulated from the signal conductor  18 . The first reference conductor  22   a  is a conductor layer having a rectangular or substantially rectangular shape as viewed in the circuit board upward-downward direction. The first reference conductor  22   a  includes a protruding portion  122   a  which protrudes from a right side of the rectangular or substantially rectangular shape to a circuit board rightward direction as viewed in the circuit board upward-downward direction. 
     The first reference conductor  22   a  is insulated from the first power supply conductor  20   a . For this reason, the first power supply conductor  20   a  is not provided in immediate surroundings of the first reference conductor  22   a . With this configuration, the first reference conductor  22   a  is surrounded by the first power supply conductor  20   a  as viewed in the circuit board upward-downward direction. The first reference conductor  22   a  is not in contact with the first power supply conductor  20   a . The second reference conductor  22   b  is structured to be a mirror image of the first reference conductor  22   a . Thus, a description of the second reference conductor  22   b  will be omitted. 
     The first reference conductor  22   a  and the second reference conductor  22   b  as described above are connected to a reference potential. The reference potential is, for example, a ground potential (that is, 0 V) . In this case, the first reference conductor  22   a  and the second reference conductor  22   b  are ground conductors. 
     The first signal electrode  24   a  is used as an interface to external circuits. The first signal electrode  24   a  is provided at the left end portion of the upper principal surface of the insulating resin layer  16   a . The first signal electrode  24   a  overlaps with the left end portion of the signal conductor  18  as viewed in the circuit board upward-downward direction. The first signal electrode  24   a  has a rectangular or substantially rectangular shape as viewed in the circuit board upward-downward direction. The first reference conductor  22   a  is not provided in immediate surroundings of the first signal electrode  24   a  such that the first signal electrode  24   a  is insulated from the first reference conductor  22   a . 
     The first interlayer connection conductor  v   1  is provided at a left end portion of the insulating resin layer  16   a . The first interlayer connection conductor  v   1  extends through the insulating resin layer  16   a  in an upward-downward direction. An upper end of the first interlayer connection conductor  v   1  is connected to the first signal electrode  24   a . A lower end of the first interlayer connection conductor  v   1  is connected to the left end portion of the signal conductor  18 . With this configuration, the first interlayer connection conductor  v   1  connects the signal conductor  18  and the first signal electrode  24   a  together. That is, the first signal electrode  24   a  is connected to the left end portion (a first end portion) of the signal conductor  18 . The first interlayer connection conductor  v   1  is, for example, a via hole conductor. The via hole conductor is formed by filling a through-hole extending through the insulating resin layer  16   a  in the upward-downward direction with a conductive paste and sintering the conductive paste. A high frequency signal is input to or output from the signal conductor  18  via the first signal electrode  24   a . 
     The second signal electrode  24   b  and the second interlayer connection conductor  v   2  are structured to be a mirror image of the first signal electrode  24   a  and the first interlayer connection conductor  v   1 . Thus, a description of the second signal electrode  24   b  and the second interlayer connection conductor  v   2  will be omitted. 
     The resist layer  17   a  is an insulative protective layer having flexibility. The resist layer  17   a  covers the entire or substantially the entire upper principal surface of the insulating resin layer  16   a . With this configuration, the resist layer  17   a  protects the first power supply conductor  20   a , the first reference conductor  22   a , and the second reference conductor  22   b . 
     Openings  h   1  to  h   10  are provided in the resist layer  17   a . The opening  h   1  overlaps with the first signal electrode  24   a  as viewed in the circuit board upward-downward direction. With this configuration, the first signal electrode  24   a  is exposed from the circuit board  10  to the outside via the opening  h   1 . 
     The opening  h   2  is provided in front of the opening  h   1  in the circuit board forward-backward direction. The opening  h   3  is provided to the right of the opening  h   1  in the circuit board leftward-rightward direction. The opening  h   4  is provided behind the opening  h   1  in the circuit board forward-backward direction. The opening  h   5  is provided to the left of the opening  h   1  in the circuit board leftward-rightward direction. With this configuration, the first reference conductor  22   a  is exposed from the circuit board  10  to the outside via the openings  h   2  to  h   5 . Portions exposed via the openings  h   2  to  h   5  in the first reference conductor  22   a  are the first reference electrodes  26   a , as shown in  FIG.  1   . The first reference electrodes  26   a  are used as an interface to external circuits and are connected to the first reference conductor  22   a . 
     The opening  h   6  is provided in front of the opening  h   2  in the circuit board forward-backward direction. The opening  h   7  is provided behind the opening  h   4  in the circuit board forward-backward direction. The opening  h   8  is provided to the left of the opening  h   5  in the circuit board leftward-rightward direction. With this configuration, the first power supply conductor  20   a  is exposed from the circuit board  10  to the outside via the openings  h   6  to  h   8 . Portions exposed via the openings  h   6  to  h   8  in the first power supply conductor  20   a  are the first power supply electrodes  28   a , as shown in  FIG.  1   . The first power supply electrodes  28   a  are used as an interface to external circuits and are connected to the first power supply conductor  20   a . 
     The opening  h   9  is provided to the right of the opening  h   3  in the circuit board leftward-rightward direction. With this configuration, the protruding portion  122   a  of the first reference conductor  22   a  is exposed to the outside via the opening  h   9 . A portion exposed via the opening  h   9  in the protruding portion  122   a  of the first reference conductor  22   a  is the reference electrode  222   a . The reference electrode  222   a  is connected to the first reference conductor  22   a . 
     The opening  h   10  is provided to the right of the opening  h   3  in the circuit board leftward-rightward direction. The opening  h   10  is provided in front of the opening  h   9  in the circuit board forward-backward direction. With this configuration, the first power supply conductor  20   a  is exposed to the outside via the opening  h   10 . A portion exposed via the opening  h   10  in the first power supply conductor  20   a  is the power supply electrode  224   a . The power supply electrode  224   a  is connected to the first power supply conductor  20   a . 
     Openings  h   11  to  h   20 , the second reference electrodes  26   b , the second power supply electrodes  28   b , the reference electrode  222   b , and the power supply electrode  224   b  are structured to be a mirror image of the openings  h   1  to  h   10 , the first reference electrodes  26   a , the first power supply electrodes  28   a , the reference electrode  222   a , and the power supply electrode  224   a . Thus, a description of the openings  h   11  to  h   20 , the second reference electrodes  26   b , the second power supply electrodes  28   b , the reference electrode  222   b , and the power supply electrode  224   b  will be omitted. 
     The signal conductor  18 , the first power supply conductor  20   a , the second power supply conductor  20   b , the third power supply conductor  20   c , the first reference conductor  22   a , the second reference conductor  22   b , the first signal electrode  24   a , and the second signal electrode  24   b  as described above are formed by, for example, etching copper foil provided on the upper principal surfaces or lower principal surfaces of the insulating resin layers  16   a  to  16   c . 
     The plurality of third interlayer connection conductors  v   3  are provided in the board body  12  so as to be located in front of the signal conductor  18  in the circuit board forward-backward direction. The plurality of third interlayer connection conductors  v   3  are aligned at equal or substantially equal intervals in the circuit board leftward-rightward direction. The plurality of third interlayer connection conductors  v   3  extend through the insulating resin layers  16   a  and  16   b  in the circuit board upward-downward direction. Upper ends of the plurality of third interlayer connection conductors  v   3  are connected to the first power supply conductor  20   a . Intermediate portions of the plurality of third interlayer connection conductors  v   3  are connected to the second power supply conductor  20   b . Lower ends of the plurality of third interlayer connection conductors  v   3  are connected to the third power supply conductor  20   c . With this configuration, the plurality of third interlayer connection conductors  v   3  connect the first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c  together. 
     The plurality of fourth interlayer connection conductors  v   4  are provided in the board body  12  so as to be located behind the signal conductor  18  in the circuit board forward-backward direction. The plurality of fourth interlayer connection conductors  v   4  are aligned up at equal or substantially equal intervals in the circuit board leftward-rightward direction. The plurality of fourth interlayer connection conductors  v   4  extend through the insulating resin layers  16   a  and  16   b  in the circuit board upward-downward direction. Upper ends of the plurality of fourth interlayer connection conductors  v   4  are connected to the first power supply conductor  20   a . Intermediate portions of the plurality of fourth interlayer connection conductors  v   4  are connected to the second power supply conductor  20   b . Lower ends of the plurality of fourth interlayer connection conductors  v   4  are connected to the third power supply conductor  20   c . With this configuration, the plurality of fourth interlayer connection conductors  v   4  connect the first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c  together. The plurality of third interlayer connection conductors  v   3  and the plurality of fourth interlayer connection conductors  v   4  are, for example, via hole conductors. Each via hole conductor is formed by, for example, filling a through-hole extending through the insulating resin layers  16   a  and  16   b  in the upward-downward direction with a conductive paste and sintering the conductive paste. 
     As shown in  FIG.  3   , the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via a first capacitance C 1 . The first chip capacitor  30   a  in  FIGS.  1  and  2    is the first capacitance C 1 . More specifically, the first chip capacitor  30   a  is mounted on the reference electrode  222   a  and the power supply electrode  224   a  with, for example, solder, as shown in  FIG.  2   . Thus, a first capacitor electrode of the first chip capacitor  30   a  is connected to the first reference conductor  22   a . A second capacitor electrode of the first chip capacitor  30   a  is connected to the first power supply conductor  20   a . 
     Here, the signal conductor  18  includes the left end portion (first end portion), the right end portion (a second end portion), and a center point C, as shown in  FIG.  2   . The center point C is located midway between the left end portion (first end portion) and the right end portion (second end portion) in a path provided by the signal conductor  18 . As shown in  FIGS.  1  and  2   , the reference electrode  222   a  and the power supply electrode  224   a   are disposed nearer to the left end portion (first end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . In the present preferred embodiment, the reference electrode  222   a  and the power supply electrode  224   a  are disposed in the vicinity of the left end portion of the signal conductor  18 . The first chip capacitor  30   a  is mounted on the reference electrode  222   a  and the power supply electrode  224   a . For this reason, the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first capacitance C 1  at a point nearer to the left end portion (first end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . In the present preferred embodiment, the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first capacitance C 1  in the vicinity of the left end portion (first end portion) of the signal conductor  18 . 
     As shown in  FIG.  3   , the second reference conductor  22   b  is connected to the first power supply conductor  20   a  via a second capacitance C 2 . The second chip capacitor  30   b  in  FIGS.  1  and  2    is the second capacitance C 2 . More specifically, the second chip capacitor  30   b  is mounted on the reference electrode  222   b  and the power supply electrode  224   b  with, for example, solder, as shown in  FIG.  2   . Thus, a first capacitor electrode of the second chip capacitor  30   b  is connected to the second reference conductor  22   b . A second capacitor electrode of the second chip capacitor  30   b  is connected to the first power supply conductor  20   a . 
     As shown in  FIG.  2   , the reference electrode  222   b  and the power supply electrode  224   b  are located nearer to the right end portion (second end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . In the present preferred embodiment, the reference electrode  222   b  and the power supply electrode  224   b  are located in the vicinity of the right end portion of the signal conductor  18 . The second chip capacitor  30   b   is mounted on the reference electrode  222   b  and the power supply electrode  224   b . For this reason, the second reference conductor  22   b  is connected to the first power supply conductor  20   a  via the second capacitance C 2  at a point nearer to the right end portion (first end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . In the present preferred embodiment, the second reference conductor  22   b  is connected to the first power supply conductor  20   a  via the second capacitance C 2  in the vicinity of the right end portion (second end portion) of the signal conductor  18 . 
     Advantageous Effects 
     The circuit board  10  can define and function as a high frequency signal transmission line. More specifically, a return current flows through the first power supply conductor  20   a  when a high frequency signal is transmitted through the signal conductor  18 . The return current is preferably guided to the reference potential. In the circuit board  10 , the first reference conductor  22   a  is insulated from the signal conductor  18  and the first power supply conductor  20   a . For this reason, when the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first capacitance C 1 , a return current flows from the first power supply conductor  20   a  to a ground potential via the first capacitance C 1 , the first reference conductor  22   a , and the first reference electrodes  26   a . With this configuration, the circuit board  10  can define and function as a high frequency signal transmission line. 
     According to the circuit board  10 , both reducing or preventing an increase in size of the circuit board  10  and reducing insertion loss of the circuit board  10  are achieved. More specifically, in a general circuit board, a strip line includes a signal conductor and two reference conductors. Sometimes such a circuit board further includes a power supply conductor. It may be difficult to provide space for a power supply conductor having a large area when reducing or preventing an increase in size of a circuit board. This results in increase in resistance value of a power supply conductor. 
     In the circuit board  10 , the first power supply conductor  20   a  extends along at least a portion of the signal conductor  18 . With this configuration, a capacitance which is generated between the first power supply conductor  20   a  and the signal conductor  18  is used in the circuit board  10  to define a high frequency signal transmission line through which a high frequency signal is transmitted. For this reason, in the circuit board  10 , areas of the first reference conductor  22   a  and the second reference conductor  22   b  can be reduced, and an area of the first power supply conductor  20   a  can be increased. This achieves reducing or preventing an increase in size of the circuit board  10 . Additionally, the increase in area of the first power supply conductor  20   a  leads to a reduction in resistance value of the first power supply conductor  20   a . Thus, a return current is likely to flow through the first power supply conductor  20   a  when a high frequency signal is transmitted through the signal conductor  18 . As a result, a reduction in insertion loss of the circuit board  10  is achieved. 
     According to the circuit board  10 , noise can be reduced or prevented from affecting a high frequency signal which is transmitted through the signal conductor  18 . More specifically, the capacitance that is generated between the first power supply conductor  20   a  and the signal conductor  18  is used in the circuit board  10  to define the high frequency signal transmission line, through which a high frequency signal is transmitted. In this case, noise passing through the first power supply electrodes  28   a  and the second power supply electrodes  28   b  into the circuit board  10  may affect a high frequency signal which is transmitted through the signal conductor  18 . 
     Under the circumstances, the first reference conductor  22   a  is insulated from the signal conductor  18  and the first power supply conductor  20   a  in the circuit board  10 . With this configuration, when the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first capacitance C 1 , noise passing through the first power supply electrodes  28   a  and the second power supply electrodes  28   b  into the circuit board  10  flows to the reference potential via the first power supply conductor  20   a , the first capacitance C 1 , and the first reference electrodes  26   a . As a result, noise can be reduced or prevented from affecting a high frequency signal which is transmitted through the signal conductor  18 . 
     According to the circuit board  10 , a reduction in insertion loss of the circuit board  10  is further achieved. More specifically, the first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first capacitance C 1  at a point nearer to the left end portion (first end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . With this configuration, a portion where the first reference conductor  22   a  and the first power supply conductor  20   a  are connected via the first capacitance C 1  is spaced away from the center point C of the signal conductor  18 . For this reason, a portion over which the signal conductor  18  extends along the first power supply conductor  20   a  is long. In the high frequency signal transmission line, a portion over which the first power supply conductor  20   a  having the low resistance value is used is long. As a result, according to the circuit board  10 , a reduction in insertion loss of the circuit board  10  is further achieved. 
     According to the circuit board  10 , a reduction in insertion loss of the circuit board  10  is further achieved. More specifically, the second reference conductor  22   b  is connected to the first power supply conductor  20   a  via the second capacitance C 2  at a point nearer to the right end portion (second end portion) of the signal conductor  18  than to the center point C of the signal conductor  18 . With this configuration, a portion where the second reference conductor  22   b  and the first power supply conductor  20   a  are connected via the second capacitance C 2  is spaced away from the center point C of the signal conductor  18 . For this reason, a section over which the signal conductor  18  extends along the first power supply conductor  20   a  is long. In the high frequency signal transmission line, a section over which the first power supply conductor  20   a  having the low resistance value is used is long. As a result, according to the circuit board  10 , a reduction in insertion loss of the circuit board  10  is further achieved. 
     According to the circuit board  10 , a reduction in size of the circuit board  10  is achieved. More specifically, the first chip capacitor  30   a  is the first capacitance C 1 . The first chip capacitor  30   a  easily achieves a large capacitance value. Thus, a larger reduction in size is achieved in the circuit board  10  than in a circuit board which does not include a chip capacitor to obtain the first capacitance C 1 . 
     First Modification 
     A circuit board  10   a  according to a first modification of a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  4    is an equivalent circuit diagram of the circuit board  10   a . 
     The circuit board  10   a  is different from the circuit board  10  in that the circuit board  10   a  includes a first high pass filter HPF 1  and a second high pass filter HPF 2 , instead of the first capacitance C 1  and the second capacitance C 2 . This aspect will be described below. 
     The first reference conductor  22   a  is connected to the first power supply conductor  20   a  via the first high pass filter HPF 1 . The first high pass filter HPF 1  includes a third capacitance C 3  and an inductor L 1 . The third capacitance C 3  is connected to the first power supply conductor  20   a  and the first reference conductor  22   a , similar to the first capacitance C 1 . The inductor L 1  is connected to the first power supply conductor  20   a  and the first power supply electrodes  28   a . A pass band of the first high pass filter HPF 1  includes a frequency of a high frequency signal which is transmitted through the signal conductor  18 . A return current has the same or substantially the same frequency as a high frequency signal which is transmitted through the signal conductor  18 . Thus, the return current can pass between the first power supply conductor  20   a  and the first reference conductor  22   a . Noise can pass between the first power supply conductor  20   a  and the first reference conductor  22   a . The inductor L 1  may be, for example, a chip inductor or wiring inside the circuit board  10   a . 
     The second reference conductor  22   b  is connected to the first power supply conductor  20   a  via the second high pass filter HPF 2 . The second high pass filter HPF 2  includes a fourth capacitance C 4  and an inductor L 2 . The fourth capacitance C 4  is connected to the first power supply conductor  20   a  and the second reference conductor  22   b , similar to the second capacitance C 2 . The inductor L 2  is connected to the first power supply conductor  20   a  and the second power supply electrodes  28   b . A pass band of the second high pass filter HPF 2  includes a frequency of a high frequency signal which is transmitted through the signal conductor  18 . A return current has the same or substantially the same frequency as a high frequency signal which is transmitted through the signal conductor  18 . Thus, the return current can pass between the first power supply conductor  20   a  and the second reference conductor  22   b . Noise can pass between the first power supply conductor  20   a  and the second reference conductor  22   b . The inductor L 2  may be, for example, a chip inductor or wiring inside the circuit board  10   a . Since the remainder of the structure of the circuit board  10   a  is the same or substantially the same as the circuit board  10 , a description thereof will be omitted. 
     In the circuit board  10   a , the inductor L 1  is included. With this configuration, if a desired pass band is difficult to obtain only with the third capacitance C 3 , the desired pass band can be obtained with the first high pass filter HPF 1  including the third capacitance C 3  and the inductor L 1 . For the same reason, a desired pass band can be obtained with the second high pass filter HPF 2 . 
     Second Modification 
     A circuit board  10   b  according to a second modification of a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  5    is an equivalent circuit diagram of the circuit board  10   b . 
     The circuit board  10   b  is different from the circuit board  10  in that the circuit board  10   b  includes an antenna ANT, instead of the second capacitance C 2 , the second reference conductor  22   b , and the second reference electrodes  26   b . This aspect will be described below. 
     The circuit board  10   b  further includes the antenna ANT. A right end portion of the signal conductor  18  is connected to the antenna ANT for transmission and/or reception of a high frequency signal. The antenna ANT is defined by, for example, a conductor layer of the circuit board  10   b . In this case, the antenna ANT is a part of the circuit board  10   b . Note that the antenna ANT need not be a part of the circuit board  10   b . Thus, the circuit board  10   b  is connected to the antenna ANT or an integrated circuit including the antenna ANT. 
     If the signal conductor  18  is connected to the antenna ANT, the circuit board  10   b  need not transmit a reference potential connected to the first reference electrodes  26   a  to a subsequent circuit board. For this reason, the circuit board  10   b  does not include the second capacitance C 2 , the second reference conductor  22   b , and the second reference electrodes  26   b . Since the remainder of the structure of the circuit board  10   b  is the same or substantially the same as the circuit board  10 , a description thereof will be omitted. 
     Third Modification 
     A circuit board  10   c  according to a third modification of a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  6    is a sectional view of the circuit board  10   c . A section orthogonal or substantially orthogonal to a leftward-rightward direction is shown in  FIG.  6   . 
     The circuit board  10   c  is different from the circuit board  10  in that the circuit board  10   c  further includes power supply conductors  20   d  to  20   g , reference conductors  22   c  to  22   e , and interlayer connection conductors  v   5  to  v   8  and that the board body  12  further includes a resist layer  17   b . The differences will be described below. 
     The power supply conductor  20   d , the reference conductor  22   c , the power supply conductor  20   e , the reference conductor  22   d , the power supply conductor  20   f , the reference conductor  22   e , and the power supply conductor  20   g  are aligned in this order from back to front in a circuit board forward-backward direction. The power supply conductors  20   d  to  20   g  and the reference conductors  22   c  to  22   e  extend in the circuit board forward-backward direction. The reference conductors  22   c  to  22   e  described above are connected to the third power supply conductor  20   c  via first capacitances C 1  and are insulated from the signal conductor  18 . 
     The interlayer connection conductor  v   5  connects the third power supply conductor  20   c  and the power supply conductor  20   d  together. The interlayer connection conductor  v   6  connects the third power supply conductor  20   c  and the power supply conductor  20   e  together. The interlayer connection conductor  v   7  connects the third power supply conductor  20   c  and the power supply conductor  20   f  together. The interlayer connection conductor  v   8  connects the third power supply conductor  20   c  and the power supply conductor  20   g  together. 
     The resist layer  17   b  is an insulative protective layer having flexibility. The resist layer  17   b  covers an entire or substantially an entire lower principal surface of the insulating resin layer  16   c . With this configuration, the resist layer  17   b  protects the power supply conductors  20   d  to  20   g  and the reference conductors  22   c  to  22   e . 
     openings are provided in the resist layer  17   b . The openings overlap with the reference conductors  22   c  to  22   e  as viewed in a circuit board upward-downward direction. With this configuration, the reference conductors  22   c  to  22   e  are exposed from the circuit board  10   c  to the outside via the openings. In the present preferred embodiment, gold-plated layers  50   c  to  50   e  are provided on the reference conductors  22   c  to  22   e . 
     The circuit board  10   c  as described above is fixed to a metal housing  200  which is connected to a reference potential, as shown in  FIG.  6   . In this case, the gold-plated layers  50   c  to  50   e  are connected to the metal housing  200 . Thus, the reference conductors  22   c  to  22   e  are connected to the reference potential. Each of the reference conductors  22   c  to  22   e  may be connected to the metal housing  200  at a plurality of points. That is, the circuit board  10   c  may have the structure of the section in  FIG.  6    at each of the plurality of points. Since the remainder of the structure of the circuit board  10   c  is the same or substantially the same as the circuit board  10 , a description thereof will be omitted. 
     According to the circuit board  10   c , noise can be reduced or prevented from affecting a high frequency signal which is transmitted through the signal conductor  18 . More specifically, the reference conductors  22   c  to  22   e  are capacitively coupled to the third power supply conductor  20   c . Thus, noise flows from the third power supply conductor  20   c  to the reference conductors  22   c  to  22   e . The noise flows to the metal housing  200  via the gold-plated layers  50   c  to  50   e . As a result, according to the circuit board  10   c , noise can be reduced or prevented from affecting a high frequency signal which is transmitted through the signal conductor  18 . Since each of the reference conductors  22   c  to  22   e  is connected to the metal housing  200  at a plurality of points, noise can be further reduced or prevented from affecting a high frequency signal which is transmitted through the signal conductor  18 . In the circuit board  10   c , the gold-plated layers  50   c  to  50   e  and the metal housing  200  may be joined to each other with a conductive jointing material, such as solder, for example. 
     Fourth Modification 
     A circuit board  10   d  according to a fourth modification of a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  7    is an exploded perspective view of the circuit board  10   d . 
     The circuit board  10   d  is different from the circuit board  10  in that the circuit board  10   d  does not include the first chip capacitor  30   a  and the second chip capacitor  30   b . In the circuit board  10   d , the first capacitance C 1  is a capacitance which is generated between the first reference conductor  22   a  and the second power supply conductor  20   b . The second capacitance C 2  is a capacitance which is generated between the second reference conductor  22   b  and the second power supply conductor  20   b . The remainder of the structure of the circuit board  10   d  is the same or substantially the same as the circuit board  10 , and a description thereof will be omitted. 
     According to the circuit board  10   d , the first capacitance C 1  and the second capacitance C 2  are provided by capacitances. For this reason, the circuit board  10   d  needs no chip capacitor. Additionally, a chip capacitor mounting process is unnecessary at the time of manufacture of the circuit board  10   d . This results in a reduction in manufacturing costs of the circuit board  10   d . 
     Electronic Device 
     An electronic device  1  according to a preferred embodiment of the present invention will be described below with reference to the drawings.  FIG.  8    is a view showing the electronic device  1 . Note that the first chip capacitor  30   a  and the second chip capacitor  30   b  are not shown in  FIG.  8   . 
     The electronic device  1  is, for example, a portable wireless communication terminal, such as a smartphone. The electronic device  1  includes the circuit board  10 , and circuit boards  300  and  302 . A left end portion of the circuit board  10  is connected to the circuit board  300 . More specifically, the first signal electrode  24   a , the first reference electrodes  26   a , and the first power supply electrodes  28   a  are fixed to an electrode of the circuit board  300  with solder, for example. 
     A right end portion of the circuit board  10  is connected to the circuit board  302 . More specifically, the second signal electrode  24   b , the second reference electrodes  26   b , and the second power supply electrodes  28   b  are fixed to an electrode of the circuit board  302  with solder, for example. With this configuration, a high frequency signal is transmitted through the signal conductor  18 . The first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c  are connected to a power supply potential. The first reference conductor  22   a  and the second reference conductor  22   b  are connected to a reference potential. 
     A position in an upward-downward direction of the circuit board  300  is different from a position in the upward-downward direction of the circuit board  302 . For this reason, the circuit board  10  is bent. Since a material for each of the insulating resin layers  16   a  to  16   c  is, for example, a thermoplastic resin, the circuit board  10  can be bent. 
     The circuit board  10  may be connected to the circuit board  300  and the circuit board  302  via connectors. The electronic device  1  may include any of the circuit boards  10   a  to  10   d , instead of the circuit board  10 . 
     Other Preferred Embodiments 
     Signal transmission lines according to preferred embodiments of the present invention is not limited to the circuit boards  10  and  10   a  to  10   d , and changes can be made within the scope of the present invention. The configurations of the circuit boards  10  and  10   a  to  10   d  may be combined. 
     In each of the circuit boards  10  and  10   a  to  10   d , the signal conductor  18  extends linearly in a circuit board leftward-rightward direction. However, the signal conductor  18  may be bent as viewed in a circuit board upward-downward direction. 
     Each of the circuit boards  10  and  10   a  to  10   d  only needs to include any one of the first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c . In this case, the first reference conductor  22   a  may be connected to any one of the first power supply conductor  20   a , the second power supply conductor  20   b , and the third power supply conductor  20   c  via the first capacitance C 1  and be insulated from the signal conductor  18 . 
     In each of the circuit boards  10  and  10   a  to  10   d , the first power supply conductor  20   a  and/or the third power supply conductor  20   c  may extend along the entire or substantially the entire signal conductor  18 . 
     In each of the circuit boards  10  and  10   a  to  10   d , the first reference conductor  22   a  may be connected to the first power supply conductor  20   a  via the first capacitance C 1  at a point nearer to the center point C of the signal conductor  18  than to the left end portion (first end portion) of the signal conductor  18 . Further, in each of the circuit boards  10 ,  10   a ,  10   c , and  10   d , the second reference conductor  22   b  may be connected to the first power supply conductor  20   a  via the second capacitance C 2  at a point nearer to the center point C of the signal conductor  18  than to the right end portion (second end portion) of the signal conductor  18 . 
     Each of the circuit boards  10  and  10   b  to  10   d  may include the first high pass filter HPF 1 . instead of the first capacitance C 1 . Further, each of the circuit boards  10 ,  10   c , and  10   d  may include the second high pass filter HPF 2 , instead of the second capacitance C 2 . 
     The circuit board  10   c  need not include the first reference conductor  22   a  and the second reference conductor  22   b . 
     In each of the circuit boards  10  and  10   b  to  10   d , only one insulating resin layer may be provided. In this case, the signal conductor  18  and power supply conductors have a coplanar structure. Alternatively, two insulating resin layers may be provided. In this case, the signal conductor  18  and a power supply conductor have a micro-stripline structure. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.