Abstract:
A balun transformer includes: a drum-shaped core having a core unit and a pair of flanges arranged on both sides of the core unit; a plurality of terminal electrodes arranged on the flanges; a primary winding wound around the core unit, both ends of the primary winding being connected to the terminal electrodes; and a secondary winding wound around the core unit, both ends and a center tap of the secondary winding being connected to the terminal electrodes, wherein the secondary winding includes a first wire extending from one end to the center tap, and a second wire extending from the other end to the center tap, and the first wire and the second wire are wound around the core unit so as to extend along each other.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of copending and commonly assigned U.S. patent application Ser. No. 12/368,795 filed Feb. 10, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a balun transformer, and more particularly relates to a balun transformer using a drum-shaped core. 
     BACKGROUND OF THE INVENTION 
     Transmission lines connected to an antenna or the like are generally unbalanced transmission lines, while transmission lines connected to a high-frequency circuit, such as a semiconductor IC, are balanced transmission lines. Accordingly, when connecting the unbalanced transmission line and the balanced transmission line, a balun transformer that mutually converts an unbalanced signal and a balanced signal is inserted between these lines. In this case, the unbalanced signal means a single ended signal with a fixed electric potential (such as a ground electric potential) as a reference, and the balanced signal means a differential signal. 
     A balun transformer using a spectacle-shaped core as described in Japanese Patent Application Laid-open No. H11-135330, and a balun transformer using a toroidal core as described in Japanese Patent Application Laid-open No. H8-115820 are examples of general balun transformers. However, there is a problem in the balun transformer using the spectacle-shaped core or the toroidal core in that not only it has a comparatively large overall size, but also it poses difficulties in the automation of the winding operation of a winding and in surface mounting. 
     Meanwhile, a balun transformer using a drum-shaped core as described in Japanese Patent Application Laid-open No. 2005-39446 has advantages that downsizing is easy and is suitable for the automation of the winding operation of a wiring and for surface mounting. 
     In the balun transformer using a drum-shaped core, however, its characteristics are greatly changed depending on a winding method of a secondary winding, and thus it is difficult to obtain a good high-frequency characteristic. Particularly in the high frequency area, it is difficult to obtain a good amplitude balance (amplitude balance in the balanced signal) and phase balance (phase balance in the balanced signal). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a balun transformer using a drum-shaped core, capable of obtaining a good high-frequency characteristic. 
     Another object of the present invention is to provide a balun transformer using a drum-shaped core, having a good amplitude balance and phase balance in high frequency areas. 
     As a result of extensive studies by the present inventors, it has been found that the cause for deterioration in the amplitude balance and the phase balance in the high frequency area of a balun transformer using a drum-shaped core is a disturbance in the symmetry of two wires configuring a secondary wiring. The present invention has been completed based on such technical findings. 
     That is, a balun transformer according to the present invention includes: a drum-shaped core having a core unit and a pair of flanges arranged on both sides of the core unit; a plurality of terminal electrodes arranged on the flanges; a primary winding wound around the core unit with both ends connected to the terminal electrodes; and a secondary winding wound around the core unit with both ends and a center tap connected to the terminal electrodes. The secondary winding includes a first wire extending from one end to the center tap, and a second wire extending from the other end to the center tap, and the first wire and the second wire are wound around the core unit so as to extend along each other. 
     According to the present invention, the first wire and the second wire configuring the secondary winding are wound such that the both wires extend along each other, and thus a remarkably high level of symmetry is secured between these two wires. As a result, particularly in high frequency areas, it is possible to achieve favorable values for an amplitude balance and a phase balance. In the present invention, the “primary winding” and “secondary winding” do not define an input side and an output side. That is, a side connected to the unbalanced transmission line is defined as the “primary winding” and a side connected to the balanced transmission line is defined as the “secondary winding”, for the convenient sake, however, any one of the input side and the output side can be the “primary winding” and the “secondary winding”. 
     A preferable method for winding the two wires around the core unit such that the both wires extend along each other is a so-called bifilar winding. The bifilar winding is often adopted as a winding method for a common mode filter or the like. However, in the common mode filter, the primary winding and secondary winding are simply wound by bifilar winding. In contrast thereto, the present invention focuses on the symmetry of the two wires configuring the secondary winding, and these two wires are wound in a state of extending along each other as in the bifilar winding. Thereby, the symmetry between the secondary windings, which has not been paid attention to in the technical field, can be improved significantly. Note that the “state of extending along each other” is not limited to a state that the two wires are wound in contact with each other, but also includes a state that the two wires are wound by providing a constant space in between. 
     In the present invention, it is preferable that one end of the primary winding is connected to the terminal electrode arranged on one flange, and the other end of the primary winding is connected to the terminal electrode arranged on the other flange. Accordingly, it is not necessary to wind, while crossing the primary winding, and thus it becomes possible to suppress the occurrence of short circuits, thereby enabling improvement on the reliability of the product. 
     In this case, it is preferable that, as viewed from one direction, first to third terminal electrodes are arranged in this order on the one flange, and as viewed from the one direction, fourth to sixth terminal electrodes are arranged in this order on the other flange, one end of the primary winding is connected to the first terminal electrode, the other end of the primary winding is connected to the fourth terminal electrode, one end of the secondary winding is connected to the third terminal electrode, and the other end of the secondary winding is connected to the sixth terminal electrode. It is also preferable that out of the center tap of the secondary winding, a part belonging to the first wire is connected to the fifth terminal electrode, and a part belonging to the second wire is connected to the second terminal electrode. Accordingly, with the axis of the core unit as the center, the unbalanced transmission line can be connected to the first and fourth terminal electrodes positioned on one side, and with the axis of the core unit as the center, the balanced transmission line can be connected to the third and sixth terminal electrodes positioned on the other side. Thus, it becomes unnecessary, for example, to detour a wiring pattern configuring the transmission line, thereby making it possible to achieve a highly linear and symmetrical transmission line. 
     Further, in this case, it is preferable that the primary winding include a third wire from the one end to a relay point and a fourth wire from the other end to the relay point, a seventh terminal electrode located between the first and second terminal electrodes is further arranged on the one flange, and an eighth terminal electrode located between the fourth and fifth terminal electrodes is further arranged on the other flange. It is also preferable that out of the relay point, a part belonging to the third wire is connected to the eighth terminal electrode, a part belonging to the fourth wire is connected to the seventh terminal electrode, and the third and fourth wires are wound around the core unit so as to extend along each other. This results in a configuration such that the primary winding and the secondary winding are adjoined at parts where the number of times of turns from the corresponding terminal electrodes is equal to each other, which enables the improvement of the magnetic coupling of the primary winding and the secondary winding. 
     In the present invention, it is also preferable that the first and second terminal electrodes are arranged on one flange, and the third and fourth terminal electrodes are arranged on the other flange; one end of the primary winding is connected to the first terminal electrode, and the other end of the primary winding is connected to the second terminal electrode; the one end of the secondary winding is connected to the third terminal electrode, and the other end of the secondary winding is connected to the fourth terminal electrode, and the center tap of the secondary winding is connected to the second terminal electrode. Accordingly, the number of terminal electrodes can be reduced. Further, the unbalanced transmission line can be connected to the first and second terminal electrodes arranged on one flange, and the balanced transmission line can be connected to the third and fourth terminal electrodes arranged on the other flange. Thus, it becomes unnecessary, for example, to detour a wiring pattern configuring the transmission line, thereby making it possible to achieve a highly linear and symmetrical transmission line. 
     In this case, it is preferable that the primary winding is wound on an outer circumferential side of the core unit, and the secondary winding is wound on an inner circumferential side of the core unit. Accordingly, no excessive stress is applied to an intersecting part of the primary winding, and the reliability of the product can be improved. 
     In the present invention, it is also preferable that, as viewed from one direction, first to third terminal electrodes are arranged in this order on the one flange, and as viewed from one direction, fourth to sixth terminal electrodes are arranged in this order on the other flange, the one end of the primary winding is connected to the first terminal electrode, the other end of the primary winding is connected to the sixth terminal electrode; the one end of the secondary winding is connected to the third terminal electrode, and the other end of the secondary winding is connected to the fourth terminal electrode, and out of the center tap of the secondary winding, a part belonging to the first wire is connected to the fifth terminal electrode, and a part belonging to the second wire is connected to the second terminal electrode. Accordingly, the directionality at the time of mounting is nullified, and thus it becomes unnecessary to control amounting direction, thereby decreasing mounting costs. Further, it is not necessary to intersect the first and second wires, and thus the production is simplified. 
     In the present invention, it is also preferable that as viewed from one direction, first to third terminal electrodes are arranged in this order on the one flange, as viewed from one direction, fourth to sixth terminal electrodes are arranged in this order on the other flange, the one end of the primary winding is connected to the first terminal electrode, the other end of the primary winding is connected to the fourth terminal electrode, the one end of the secondary winding is connected to the third terminal electrode, the other end of the secondary winding is connected to the fifth terminal electrode, and out of a center tap of the secondary winding, a part belonging to the first wire is connected to the sixth terminal electrode, and a part belonging to the second wire is connected to the second terminal electrode. Accordingly, it is not necessary to intersect the first and second wires, and thus the production is simplified. Further, because there is almost no difference in the length and winding conditions between the wire configuring the primary winding and the first and second wires configuring the secondary winding, these wires can be maintained at a uniform state. 
     In the present invention, it is also preferable that as viewed from one direction, first to third terminal electrodes are arranged in this order on the one flange, and as viewed from one direction, fourth to sixth terminal electrodes are arranged in this order on the other flange, the one end of the primary winding is connected to the second terminal electrode, the other end of the primary winding is connected to the fifth terminal electrode, the one end of the secondary winding is connected to the third terminal electrode, the other end of the secondary winding is connected to the fourth terminal electrode, and out of a center tap of the secondary winding, a part belonging to the first wire is connected to the sixth terminal electrode, and a part belonging to the second wire is connected to the first terminal electrode. Accordingly, the directionality at the time of mounting is nullified, and it is not necessary to control the mounting direction, thereby decreasing mounting costs. Further, it is not necessary to intersect the first and second wires, and thus the production is simplified. 
     In the present invention, it is also preferable that as viewed from one direction, first to third terminal electrodes are arranged in this order on the one flange, and as viewed from one direction, fourth to sixth terminal electrodes are arranged in this order on the other flange, the one end of the primary winding is connected to the second terminal electrode, the other end of the primary winding is connected to the fifth terminal electrode, the one end of the secondary winding is connected to the third terminal electrode, the other end of the secondary winding is connected to the sixth terminal electrode, and out of a center tap of the secondary winding, a part belonging to the first wire is connected to the fourth terminal electrode, and a part belonging to the second wire is connected to the first terminal electrode. Accordingly, a pair of balanced transmission lines connected to the secondary winding can be formed in parallel and linearly, and accordingly, the symmetry between the pair of balanced transmission lines can be secured. Further, it is not necessary to intersect the first and second wires, and thus the production is simplified. 
     Thus, according to the present invention, the symmetry between the two wires configuring the secondary winding is high, and thereby it is possible to provide a balun transformer with a good high-frequency characteristic, particularly with a good amplitude balance and phase balance in high frequency areas. 
     According to another embodiment thereof, the present invention is a balun transformer that includes a drum-shaped core having a core unit and first and second flanges arranged on both sides of the core unit; first to third terminal electrodes arranged on the first flange; fourth to sixth terminal electrodes arranged on the second flange; a first wire wound in a first number of turns around the core unit, the first wire having one end connected to the first terminal electrode and other end connected to the fourth terminal electrode; a second wire wound in a second number of turns around the core unit, the second wire having one end connected to the second terminal electrode and other end connected to the fifth terminal electrode, and a third wire wound in a third number of turns around the core unit, the third wire having one end connected to the third terminal electrode and other end connected to the sixth terminal electrode. The first number is different from the second and third numbers, and the second and third numbers are same as each other. 
     The first number may be larger than the second and third numbers. The first to third terminal electrodes may be arranged in this order as viewed from a predetermined direction on the first flange, and the fourth to sixth terminal electrodes may be arranged in different from this order as viewed from the predetermined direction on the second flange. The fourth, sixth, and fifth terminal electrodes may be arranged in this order as viewed from the predetermined direction on the second flange. The fifth, sixth, and fourth terminal electrodes may be arranged in this order as viewed from the predetermined direction on the second flange. The sixth, fifth, and fourth terminal electrodes may be arranged in this order as viewed from the predetermined direction on the second flange. The first to third terminal electrodes may be arranged in this order as viewed from a predetermined direction on the first flange, and the fourth to sixth terminal electrodes may be arranged in this order as viewed from the predetermined direction on the second flange. The second, first, and third terminal electrodes may be arranged in this order as viewed from a predetermined direction on the first flange, and the fifth, fourth, and sixth terminal electrodes may be arranged in this order as viewed from the predetermined direction on the second flange. The second wire and the third wire may be wound around the core unit so as to extend along each other. 
     Yet another embodiment of the present invention is a balun transformer that includes a drum-shaped core having a core unit and first and second flanges arranged on both sides of the core unit; a first electrode group arranged on the first flange constituted of a plurality of terminal electrodes arranged in line including at least a first terminal electrode located at a near end of the first electrode group viewed from a predetermined direction and a second terminal electrode located at a far end of the first electrode group viewed from the predetermined direction; a second electrode group arranged on the second flange constituted of a plurality of terminal electrodes arranged in line including at least a third terminal electrode located at a near end of the second electrode group viewed from the predetermined direction and a fourth terminal electrode located at a far end of the second electrode group viewed from the predetermined direction; a primary winding wound around the core unit, the primary winding having first and second ends; and a secondary winding wound around the core unit, the secondary winding having third and fourth ends and a center tap, the secondary winding including a first wire extending from the third end to the center tap and a second wire extending from the fourth end to the center tap. The first end of the primary winding is connected to the first terminal electrode, the second end of the primary winding is connected to one of the second and third terminal electrodes, the third end of the secondary winding is connected to other of the second and third terminal electrodes, the fourth end of the secondary winding is connected to the fourth terminal electrode, and the center tap of the secondary winding is connected to one or more of the terminal electrodes. 
     According to further embodiments, the first electrode group may further include a fifth terminal electrode located between the first and second terminal electrode, the second electrode group may further include a sixth terminal electrode located between the third and fourth terminal electrode, the second end of the primary winding may be connected to the third terminal electrode, the third end of the secondary winding may be connected to the second terminal electrode, the center tap belonging to the first wire may be connected to the sixth terminal electrode, and the center tap belonging to the second wire may be connected to the fifth terminal electrode. The primary winding may include a third wire extending from the first end to a relay point and a fourth wire extending from the second end to the relay point, the first electrode group may further include a seventh terminal electrode located between the first and fifth terminal electrode, the second electrode group may further include an eighth terminal electrode located between the third and sixth terminal electrode. The relay point belonging to the third wire may be connected to the eighth terminal electrode, and the relay point belonging to the fourth wire may be connected to the seventh terminal electrode. The second end of the primary winding may be connected to the second terminal electrodes, the third end of the secondary winding may be connected to the third terminal electrode, and the center tap of the secondary winding may be connected to the second terminal electrode. The primary winding may be wound on an outer circumferential side of the core unit, and the secondary winding may be wound on an inner circumferential side of the core unit. The first wire and the second wire may be wound around the core unit so as to extend along each other. The third wire and the fourth wire may be wound around the core unit so as to extend along each other. 
     According to a still further embodiment thereof, the present invention is a device having a circuit board and a balun transformer mounted on the circuit board, wherein the circuit board includes at least first to fourth land patterns, the balun transformer includes: a drum-shaped core having a core unit and a pair of flanges arranged on both sides of the core unit; a plurality of terminal electrodes arranged on the flanges; a primary winding wound around the core unit, both ends of the primary winding being connected to the terminal electrodes; and a secondary winding wound around the core unit, both ends and a center tap of the secondary winding being connected to the terminal electrodes, the terminal electrode connected to one end of the primary winding is connected to the first land pattern, the terminal electrode connected to other end of the primary winding is connected to the second land pattern, the terminal electrode connected to one end of the secondary winding is connected to the third land pattern, the terminal electrode connected to other end of the secondary winding is connected to the fourth land pattern, and the terminal electrode connected to the center tap of the secondary winding is connected to the second land pattern. 
     The secondary winding may include a first wire extending from the one end to the center tap, and a second wire extending from the other end to the center tap, and the terminal electrodes connected to the center tap of the secondary winding belonging to the first and second wires, respectively, may be connected via the fourth land pattern. The secondary winding may include a first wire extending from the one end to the center tap, and a second wire extending from the other end to the center tap, the center tap of the secondary winding belonging to the first wire and the center tap of the secondary winding belonging to the second wire may be electrically and physically connected to a same terminal electrode. The first wire and the second wire may be wound around the core unit so as to extend along each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic perspective view showing an appearance of a balun transformer according to a first embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view of the balun transformer according to the first embodiment; 
         FIG. 3  is a schematic bottom view of the balun transformer according to the first embodiment, as viewed from a mounting surface side; 
         FIG. 4  is a schematic diagram for explaining a connection relationship among the wires  131  to  133  and the terminal electrodes  141  to  146 ; 
         FIG. 5  is an equivalent circuit diagram of the balun transformer  100  according to the first embodiment; 
         FIG. 6  is a schematic cross-sectional view of a balun transformer according to a comparative example; 
         FIG. 7  is a diagram showing a wiring pattern on a printed-circuit board for mounting the balun transformer  100 ; 
         FIG. 8  is a schematic perspective view showing an appearance of a balun transformer according to the second embodiment; 
         FIG. 9  is a schematic cross-sectional view of the balun transformer according to the second embodiment; 
         FIG. 10  is a schematic bottom view of the balun transformer according to the second embodiment, as viewed from a mounting surface side; 
         FIG. 11  is a schematic diagram for explaining a connection relationship among the wires  231  to  234  and the terminal electrodes  241  to  248 ; 
         FIG. 12  is an equivalent circuit diagram of the balun transformer  200  according to the second embodiment; 
         FIG. 13A  is a circuit diagram showing a relationship between each turn of the wires  231  to  234  and the terminals; 
         FIG. 13B  is a schematic partial sectional view showing the arrangement of the wires  231  to  234  in each turn; 
         FIG. 14  shows a wiring pattern on a printed-circuit board for mounting the balun transformer  200 ; 
         FIG. 15  is a schematic perspective view showing an appearance of a balun transformer according to the third embodiment; 
         FIG. 16  is a schematic cross-sectional view of the balun transformer according to the third embodiment; 
         FIG. 17  is a schematic bottom view of the balun transformer according to the third embodiment, as viewed from the mounting surface side; 
         FIG. 18  is a schematic diagram for explaining a connection relationship among the wires  331  to  333  and the terminal electrodes  341  to  344 ; 
         FIG. 19  is an equivalent circuit diagram of the balun transformer  300  according to the third embodiment; 
         FIG. 20  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  300  according to the third embodiment; 
         FIG. 21  is a schematic diagram for explaining a connection relationship between the wires and the terminal electrodes of a balun transformer  400  according to the fourth embodiment; 
         FIG. 22  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  400  according to the fourth embodiment; 
         FIG. 23  is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer  500  according to the fifth embodiment; 
         FIG. 24  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  500  according to the fifth embodiment; 
         FIG. 25  is a schematic diagram for explaining a connection relationship between wires and terminal electrodes of a balun transformer  600  according to the sixth embodiment; 
         FIG. 26  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  600  according to the sixth embodiment; 
         FIG. 27  is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer  700  according to the seventh embodiment; 
         FIG. 28  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  700 ; 
         FIG. 29  shows a twisted wire  10  which is utilizable as the secondary winding; 
         FIG. 30  shows measurement results for the amplitude unbalance; and 
         FIG. 31  shows measurement results for the phase unbalance. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic perspective view showing an appearance of a balun transformer according to a first embodiment of the present invention,  FIG. 2  is a schematic cross-sectional view of the balun transformer according to the first embodiment, and  FIG. 3  is a schematic bottom view of the balun transformer according to the first embodiment, as viewed from a mounting surface side. 
     As shown in  FIG. 1  to  FIG. 3 , a balun transformer  100  according to the first embodiment is configured by a drum-shaped core  110 , a plate-shaped core  120 , and three wires  131  to  133 . The drum-shaped core  110  includes a core unit  111 , and a pair of flanges  112  and  113  arranged on both ends of the core unit  111 . As viewed from one direction (from an arrow A shown in  FIG. 3 ), three terminal electrodes  141  to  143  arranged in this order are positioned on one flange  112 . As viewed from the same direction (from the arrow A shown in  FIG. 3 ), three terminal electrodes  144  to  146  arranged in this order are positioned on the other flange  113 . 
     The plate-shaped core  120  is located to link the top of the flanges  112  and  113  of the drum-shaped core  110 . In the present invention, it is not essential to use the plate-shaped core  120 , however, when a closed magnetic circuit is formed by using the plate-shaped core  120 , high magnetic coupling can be obtained. The drum-shaped core  110  and the plate-shaped core  120  are made from magnetic materials, and although not particularly limited, it is preferable to use a NiZn ferrite material. The reason for the use of the NiZn ferrite is that it provides not only a comparatively high magnetic permeability, but also has low electro-conductivity. Thus, with this material, it becomes possible to directly form the terminal electrodes. However, in a case of the plate-shaped core  120  on which the terminal electrodes are not formed, it is also possible to use a MgZn ferrite material, which has an even higher magnetic permeability. 
     As shown in  FIG. 3 , all the three wires  131  to  133  are wound in a clock-wise direction (right turn) towards an arrow B.  FIG. 4  is a schematic diagram for explaining a connection relationship among the wires  131  to  133  and the terminal electrodes  141  to  146 . As shown in  FIG. 4 , one end  131   a  of the wire  131  is connected to the terminal electrode  141 , and the other end  131   b  is connected to the terminal electrode  144 . In the first embodiment, the wire  131  is wound in eight turns. Further, one end  132   a  of the wire  132  is connected to the terminal electrode  143 , and the other end  132   b  is connected to the terminal electrode  145 . In the first embodiment, the wire  132  is wound in four turns. Further, one end  133   a  of the wire  133  is connected to the terminal electrode  142 , and the other end  133   b  is connected to the terminal electrode  146 . In the first embodiment, the wire  133  is wound in four turns. 
       FIG. 5  is an equivalent circuit diagram of the balun transformer  100  according to the first embodiment. 
     As shown in  FIG. 5 , the balun transformer  100  is configured by primary windings L 11  and L 12  connected between a primary-side terminal P and a ground terminal GND, and secondary windings L 21  and L 22  connected between a secondary-side positive electrode terminal ST and a secondary-side negative electrode terminal SB. A connecting point of the secondary windings L 21  and L 22  is used as a center tap CT. 
     In the first embodiment, the four turns on the one end  131   a  side of the wire  131  configure the primary winding L 11 , and the four turns on the other end  131   b  side configure the primary winding L 12 . Further, the wire  132  configures the secondary winding L 21 , while the wire  133  configures the secondary winding L 22 . Accordingly, the terminal electrode  141  is used as the primary-side terminal P, the terminal electrodes  143  and  146  are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB, the terminal electrode  144  is used as the ground terminal GND, and the terminal electrodes  142  and  145  are used as the center tap CT. 
     As shown in  FIG. 2  and  FIG. 3 , in the first embodiment, the wire  131  that configures the primary winding is wound on the inner circumferential side, and the wires  132  and  133  configuring the secondary winding are wound on the outer circumferential side. Note that these wires can be wound in the opposite manner. The wires  132  and  133  configuring the secondary winding are wound by bifilar winding around the core unit  111 . In  FIG. 2 , a wire that is hatched on the cross section is the wire  132 , and a wire that is marked with “x” on the cross section is the wire  133 . That is, the wires  132  and  133  are wound alternately from one flange  112  towards the other flange  113  (or towards the opposite direction). Accordingly, parts coinciding with an n-th turn (n=1 to 4) of the wires  132  and  133  are adjoined to each other. 
     According to such a winding method, a remarkably high level of symmetry can be secured between these two wires  132  and  133 , as compared to a case of a so-called sector winding, i.e., the wire  132  is collectively wound in an area  111   a  on the flange  112  side in the core unit  111  and the wire  133  is collectively wound in an area  111   b  on the flange  113  side in the core unit  111  as shown in a comparative example shown in  FIG. 6  is performed. This is because in contrast to the bifilar winding in which the two wires are wound almost equally, in the sector winding, a part that works as the center tap CT is positioned at the center of the core unit  111 , and accordingly, the symmetry becomes disturbed at the wiring part, which is used for connecting the center tap CT to the terminal electrodes. 
       FIG. 7  shows a wiring pattern on a printed-circuit board for mounting the balun transformer  100  according to the first embodiment. 
     A mount region  150  on a printed-circuit board shown in  FIG. 7  is a region for mounting the balun transformer  100 , and is arranged thereon with four land patterns  151  to  154 . The land pattern  151  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  141  (the primary-side terminal P) of the balun transformer  100 . The land pattern  152  is a pattern connected to the ground wiring GNDL, and is commonly connected to the terminal electrode  144  (the ground terminal GND) and the terminal electrodes  142  and  145  (the center tap CT) of the balun transformer  100 . The land patterns  153  and  154  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  143  (the secondary-side positive electrode terminal ST) and the terminal electrode  146  (the secondary-side negative electrode terminal SB) of the balun transformer  100 . 
     Because of such a layout, the unbalanced transmission line PL can be formed linearly in the direction of an arrow C, as viewed from the mount region  150 , and at the same time, the pair of balanced transmission lines STL and SBL can be formed in parallel and linearly to each other in the direction of an arrow D, as viewed from the mount region  150 . Thereby, it becomes unnecessary, for example, to detour the wiring pattern on the printed-circuit board, and thus the area occupied by the wiring pattern does not increase beyond the required limit. Further, the symmetry of the wiring pattern can be secured. This enables downsizing of the entire device, as well as the improvement in the signal quality. 
     Thus, the balun transformer  100  employs bifilar winding for the two wires  132  and  133  configuring the secondary winding, and accordingly, as compared to a case that these are wound by the sector winding, a remarkably high level of symmetry can be secured between these two wires configuring the secondary winding. As a result, particularly in high frequency areas, it is possible to achieve a good amplitude balance and phase balance. 
     Further, because all the wires  131  to  133  are wound in the same direction, it is not necessary to wind while intersecting the wires in the core unit  111 . Thereby, short circuits hardly occur, and improvement in the reliability of the product can be also achieved. 
     A second embodiment of the present invention is described next. 
       FIG. 8  is a schematic perspective view showing an appearance of a balun transformer according to the second embodiment,  FIG. 9  is a schematic cross-sectional view of the balun transformer according to the second embodiment, and  FIG. 10  is a schematic bottom view of the balun transformer according to the second embodiment, as viewed from a mounting surface side. 
     As shown in  FIG. 8  to  FIG. 10 , a balun transformer  200  according to the second embodiment is configured by a drum-shaped core  210 , a plate-shaped core  220 , and four wires  231  to  234 . The drum-shaped core  210  includes a core unit  211 , and a pair of flanges  212  and  213  arranged on both ends of the core unit  211 . The drum-shaped core  210  and the plate-shaped core  220  correspond to the drum-shaped core  110  and the plate-shaped core  120  in the balun transformer  100 , and thus the materials are also the same as those described above. 
     As viewed from one direction (from an arrow E shown in  FIG. 10 ), four terminal electrodes  241 ,  247 ,  242 , and  243  located in this order are arranged on one flange  212  of the drum-shaped core  210 . As viewed from the same direction (from the arrow E shown in  FIG. 10 ), four terminal electrodes  244 ,  248 ,  245 , and  246  located in this order are arranged on the other flange  213 . Among these, the terminal electrodes  241  to  246  correspond to the terminal electrodes  141  to  146  in the balun transformer  100 . Accordingly, the balun transformer  200  has a configuration in which the two terminal electrodes  247  and  248  are added to the balun transformer  100 . 
     As shown in  FIG. 10 , all the four wires  231  to  234  are wound in a clock-wise direction (right turn) towards an arrow F.  FIG. 11  is a schematic diagram for explaining a connection relationship among the wires  231  to  234  and the terminal electrodes  241  to  248 . As shown in  FIG. 11 , one end  231   a  of the wire  231  is connected to the terminal electrode  241 , and the other end  231   b  is connected to the terminal electrode  248 . One end  232   a  of the wire  232  is connected to the terminal electrode  247 , and the other end  232   b  is connected to the terminal electrode  244 . One end  233   a  of the wire  233  is connected to the terminal electrode  243 , and the other end  233   b  is connected to the terminal electrode  245 . Further, one end  234   a  of the wire  234  is connected to the terminal electrode  242 , and the other end  234   b  is connected to the terminal electrode  246 . In the second embodiment, all the wires  231  to  234  are wound in four turns. 
       FIG. 12  is an equivalent circuit diagram of the balun transformer  200  according to the second embodiment. 
     As shown in  FIG. 12 , the equivalent circuit of the balun transformer  200  is basically the same as that shown in  FIG. 5 . However, the primary windings L 11  and L 12  are configured by the wires  231  and  232  different from each other and these are connected by terminal electrodes  247  and  248  that act as the relay points. Further, like in the equivalent circuit shown in  FIG. 5 , the terminal electrode  241  is used as the primary-side terminal P, the terminal electrodes  243  and  246  are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB, the terminal electrode  244  is used as the ground terminal GND, and the terminal electrodes  242  and  245  are used as the center tap CT. 
     As shown in  FIG. 9  and  FIG. 10 , also in the second embodiment, the wires  231  and  232  configuring the primary winding are wound on the inner circumferential side, and the wires  233  and  234  configuring the secondary winding are wound on the outer circumferential side. Note that these wires are wound in the opposite manner. In the second embodiment, not only the wires  233  and  234  configuring the secondary winding but also the wires  231  and  232  configuring the primary winding are wound by bifilar winding around the core unit  211 . In  FIG. 9 , a wire that is neither hatched nor marked with a symbol on the cross section is the wire  231 , a wire that is marked with “•” (solid circle) on the cross section is the wire  232 , a wire that is hatched on the cross section is the wire  233 , and a wire that is marked with “x” on the cross section is the wire  234 . That is, the balun transformer  200  has a configuration such that the wires  231  and  232  are wound alternately from one flange  212  towards the other flange  213  (towards the opposite direction), and at the same time, the wires  233  and  234  are wound alternately. 
       FIG. 13A  and  FIG. 13B  explain the arrangement of the wires  231  to  234  in more detail, where  FIG. 13A  is a circuit diagram showing a relationship between each turn of the wires  231  to  234  and the terminals, and  FIG. 13B  is a schematic partial sectional view showing the arrangement of the wires  231  to  234  in each turn. In  FIGS. 13A  and  13 B, numbers displayed before hyphens indicate types of wire, and numbers displayed after the hyphen indicate the number of turns. For example, a part assigned with reference numeral  231 - 1  indicates a first turn of the wire  231 . 
     As shown in  FIG. 13A , the number of times of turns for the wire  231  is defined by assuming the terminal electrode  241  (the primary-side terminal P) as a starting point, the number of times of turns for the wire  232  is defined by assuming the terminal electrode  247  (relay point) as a starting point, the number of times of turns for the wire  233  is defined by assuming the terminal electrode  243  (the secondary-side positive electrode terminal ST) as a starting point, and the number of times of turns for the wire  234  is defined by assuming the terminal electrode  242  (the center tap CT) as a starting point. Thereby, as viewed from the corresponding terminal electrodes ( 241  and  243 ), each turn  231 - 1  to  231 - 4  of the wire  231  and each turn  233 - 1  to  233 - 4  of the wire  233  configure a pair PA to each other. Similarly, as viewed from the corresponding terminal electrodes ( 244  and  246 ), each turn  232 - 1  to  232 - 4  of the wire  232  and each turn  234 - 1  to  234 - 4  of the wire  234  configure a pair PA to each other. In this case, the pair PA is the corresponding turn for a pair of wires, and is a portion in which the phases of transmitted signals should coincide. 
     As shown in  FIG. 13B , it is understood that in the parts in which the number of times of turns is the same with each other (that is, a pair PA) as viewed from the corresponding terminal electrodes, the primary and secondary windings are adjoining at the top and bottom. That is, each wire is adjoining in, the portion in which the phases of transmitted signals should coincide, and thus the magnetic coupling of the primary and secondary windings can be enhanced, and a better high-frequency characteristic can be obtained. 
       FIG. 14  shows a wiring pattern on a printed-circuit board for mounting the balun transformer  200 . 
     Amount region  250  on the printed-circuit board shown in  FIG. 14  is a region for mounting the balun transformer  200 , and is arranged with five land patterns  251  to  255 . The land pattern  251  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  241  (the primary-side terminal P) of the balun transformer  200 . The land pattern  252  is a pattern connected to the ground wiring GNDL, and is commonly connected to the terminal electrode  244  (the ground terminal GND) and the terminal electrodes  242  and  245  (the center tap CT) of the balun transformer  200 . The land patterns  253  and  254  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  243  (the secondary-side positive electrode terminal ST) and the terminal electrode  246  (the secondary-side negative electrode terminal SB) of the balun transformer  200 . Further, the land pattern  255  is a pattern connected to a relay point of the primary winding, and is commonly connected to the terminal electrodes  247  and  248  of the balun transformer  200 . 
     According to such a layout, similarly to the balun transformer  100  according to the first embodiment, it becomes unnecessary, for example, to detour the wiring pattern on the printed-circuit board, and thus the area occupied by the wiring pattern does not increase beyond the required limit, and further, the symmetry of the wiring pattern can be secured. This enables the downsizing of the entire device, as well as the improvement in signal quality. 
     Thus, according to the balun transformer  200  of the second embodiment, in addition to the same effects as that of the balun transformer  100  according to the first embodiment, the magnetic coupling of the primary and secondary windings can be further enhanced, which enables the achievement of a better high-frequency characteristic. Further, because the number of times of windings of the wires  231  to  234  is the same with each other, all these four wires  231  to  234  can be wound simultaneously. 
     A third embodiment of the present invention is described next. 
       FIG. 15  is a schematic perspective view showing an appearance of a balun transformer according to the third embodiment.  FIG. 16  is a schematic cross-sectional view of the balun transformer according to the third embodiment, and  FIG. 17  is a schematic bottom view of the balun transformer according to the third embodiment, as viewed from the mounting surface side. 
     As shown in  FIG. 15  to  FIG. 17 , a balun transformer  300  according to the third embodiment is configured by a drum-shaped core  310 , a plate-shaped core  320 , and three wires  331  to  333 . The drum-shaped core  310  includes a core unit  311 , and a pair of flanges  312  and  313  arranged on both ends of the core unit  311 . The drum-shaped core  310  and the plate-shaped core  320  correspond to the drum-shaped core  110  and the plate-shaped core  120  in the balun transformer  100 , and accordingly, the materials are also the same as those described above. 
     Two terminal electrodes  341  and  342  are arranged on one flange  312  of the drum-shaped core  310 , and two terminal electrodes  343  and  344  are arranged on the other flange  313 . As shown in  FIG. 17 , all the three wires  331  to  333  are wound in a clock-wise direction (right turn) towards an arrow G. Note that, with respect to the wire  331 , after four turns are wound from one end  331   a  in the direction of an arrow G, four turns are wound in the direction of an arrow H, in the form of return winding. Thus, the wire  331  intersects itself at some parts. 
       FIG. 18  is a schematic diagram for explaining a connection relationship among the wires  331  to  333  and the terminal electrodes  341  to  344 . As shown in  FIG. 18 , one end  331   a  of the wire  331  is connected to the terminal electrode  341 , and the other end  331   b  is connected to the terminal electrode  342 . One end  332   a  of the wire  332  is connected to the terminal electrode  343 , and the other end  332   b  is connected to the terminal electrode  342 . Further, one end  333   a  of the wire  333  is connected to the terminal electrode  344 , and the other end  333   b  is connected to the terminal electrode  342 . In the third embodiment, the wire  331  is wound in eight turns, while the wires  332  and  333  are wound in four turns each. 
       FIG. 19  is an equivalent circuit diagram of the balun transformer  300  according to the third embodiment. 
     As shown in  FIG. 19 , the equivalent circuit of the balun transformer  300  is basically the same as that shown in  FIG. 5 . However, the terminal electrode  342  is used as both the ground terminal GND and the center tap CT. Further, the terminal electrode  341  is used as the primary-side terminal P, and the terminal electrodes  343  and  344  are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB. 
     As shown in  FIG. 16  and  FIG. 17 , also in the third embodiment, the wire  331  configuring the primary winding is wound on the outer circumferential side, and the wires  332  and  333  configuring the secondary winding are wound on the inner circumferential side. This is because the wire  331  intersects itself at some parts, and accordingly, the surface after winding is roughened, and when the secondary winding (the wires  332  and  333 ) is wound on such a roughened surface, stress is applied to the intersecting part. 
     Also in the third embodiment, the wires  332  and  333  configuring the secondary winding are wound by bifilar winding around the core unit  311 . In  FIG. 16 , a wire that is hatched on the cross section is the wire  332 , and a wire that is marked with “x” on the cross section is the wire  333 . That is, the wires  332  and  333  are wound alternately from one flange  312  towards the other flange  313  (or towards the opposite direction). 
       FIG. 20  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  300  according to the third embodiment. 
     A mount region  350  on the printed-circuit board shown in  FIG. 20  is a region for mounting the balun transformer  300 , and arranged with four land patterns  351  to  354 . The land pattern  351  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  341  (the primary-side terminal P) of the balun transformer  300 . The land pattern  352  is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrode  342  (that serves both the ground terminal GND and the center tap CT) of the balun transformer  300 . The land patterns  353  and  354  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  343  (the secondary-side positive electrode terminal ST) and the terminal electrode  344  (the secondary-side negative electrode terminal SB) of the balun transformer  300 . 
     According to such a layout, similarly to the balun transformer  100  and the balun transformer  200 , it becomes unnecessary, for example, to detour the wiring pattern on the printed-circuit board, and thus the area occupied by the wiring pattern does not increase beyond the required limit, and further, the symmetry of the wiring pattern can be secured. This enables the downsizing of the entire device, as well as the improvement in the signal quality. 
     As described above, according to the balun transformer  300 , in addition to the effects identical to that of the balun transformer  100  according to the first embodiment, the number of terminal electrodes can be reduced to four, and thus the further downsizing can be achieved. 
     A fourth embodiment of the present invention is described next. 
       FIG. 21  is a schematic diagram for explaining a connection relationship between the wires and the terminal electrodes of a balun transformer  400  according to the fourth embodiment. The appearance and the cross section of the balun transformer  400  according to the fourth embodiment are substantially identical to those of the balun transformer  100  according to the first embodiment shown in  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 21 , three wires  431  to  433  are connected to the terminal electrodes  441  to  446  in the fourth embodiment. Among these, the wire  431  configures the primary winding, and the wires  432  and  433  configure the secondary winding. One end  431   a  of the wire  431  is connected to the terminal electrode  441 , and the other end  431   b  is connected to the terminal electrode  446 . One end  432   a  of the wire  432  is connected to the terminal electrode  442 , and the other end  432   b  is connected to the terminal electrode  444 . One end  433   a  of the wire  433  is connected to the terminal electrode  443 , and the other end  433   b  is connected to the terminal electrode  445 . In the fourth embodiment, the wire  431  is wound in eight turns, while the wires  432  and  433  are wound in four turns each. Further, the equivalent circuit of the balun transformer  400  is the same as that shown in  FIG. 5 . 
       FIG. 22  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  400  according to the fourth embodiment. 
     A mount region  450  on the printed-circuit board shown in  FIG. 22  is a region for mounting the balun transformer  400 , and is arranged with four land patterns  451  to  454 . The land pattern  451  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  441  of the balun transformer  400 . The land pattern  452  is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes  442 ,  445 , and  446  of the balun transformer  400 . Thereby, the terminal electrodes  442  and  445  configure the center tap of the secondary winding. The land patterns  453  and  454  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  443  and the terminal electrode  444  of the balun transformer  400 . 
     The balun transformer  400  does not have any directionality, and therefore the same wire-connection state can be obtained even when switching the position of a pair of flanges  412  and  413  arranged on both ends of the core unit  411 . That is, even when the balun transformer  400  is rotated by 180° at the time of mounting, the correct operation can be performed. Reference numerals of the terminal electrodes connected to the land patterns  451  to  454  at the time of rotating the balun transformer  400  by 180° are as shown within brackets in  FIG. 22 . Thus, because the balun transformer  400  does not have any directionality, it is not necessary to control the mounting direction, thereby decreasing mounting costs. 
     Further, in the balun transformer  400 , the wires  432  and  433  wound by bifilar winding do not intersect each other at any location (any location where positions of the wires  432  and  433  are switched). Accordingly, it is not necessary to intersect the wires  432  and  433  during the wire-winding operation, thereby enabling production without utilizing any complex winding machine. 
     Further, in the balun transformer  400 , each of the wirings (PL, STL, STB, and GNDL) can be connected to the terminal electrodes  441 ,  443 ,  444 , and  446  positioned at the corners, and accordingly, it becomes easy to connect the wiring on the printed-circuit board with the balun transformer  400 . 
     A fifth embodiment of the present invention is described next. 
       FIG. 23  is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer  500  according to the fifth embodiment. The appearance and the cross section of the balun transformer  500  according to the fifth embodiment are also substantially identical to those of the balun transformer  100  according to the first embodiment shown in  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 23 , three wires  531  to  533  are connected to the terminal electrodes  541  to  546  according to the fifth embodiment. Among these, the wire  531  configures the primary winding, and the wires  532  and  533  configure the secondary winding. One end  531   a  of the wire  531  is connected to the terminal electrode  541 , and the other end  531   b  is connected to the terminal electrode  554 . One end  532   a  of the wire  532  is connected to the terminal electrode  542 , and the other end  532   b  is connected to the terminal electrode  545 . One end  533   a  of the wire  533  is connected to the terminal electrode  543 , and the other end  533   b  is connected to the terminal electrode  546 . In the fifth embodiment, the wire  531  is wound in eight turns, while the wires  532  and  533  are wound in four turns each. Further, the equivalent circuit of the balun transformer  500  is the same as that shown in  FIG. 5 . 
       FIG. 24  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  500  according to the fifth embodiment. 
     Amount region  550  on the printed-circuit board shown in  FIG. 24  is a region for mounting the balun transformer  500 , and is arranged with four land patterns  551  to  554 . The land pattern  551  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  541  of the balun transformer  500 . The land pattern  552  is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes  542 ,  544 , and  546  of the balun transformer  500 . Thereby, the terminal electrodes  542  and  546  configure the center tap of the secondary winding. The land patterns  553  and  554  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  543  and the terminal electrode  545  of the balun transformer  500 . 
     Similarly to the balun transformer  400  according to the fourth embodiment, also in the balun transformer  500  according to the fifth embodiment, the wires  532  and  533  wound by bifilar winding do not interest each other at any position. Thus, it is not necessary to intersect the wires  532  and  533  during the wire-winding operation, thereby enabling production without utilizing any complex winding machine. 
     Further, in the balun transformer  500 , both ends of all the wires  531  to  533  are connected to terminal electrodes that are opposite to each other, and accordingly, these three wires can be maintained in a uniform state, with substantially no difference in the lengths and winding conditions. 
     A sixth embodiment of the present invention is described next. 
       FIG. 25  is a schematic diagram for explaining a connection relationship between wires and terminal electrodes of a balun transformer  600  according to the sixth embodiment. The appearance and the cross section of the balun transformer  600  according to the sixth embodiment are substantially identical to those of the balun transformer  100  according to the first embodiment shown in  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 25 , three wires  631  to  633  are connected to terminal electrodes  641  to  646  according to the sixth embodiment. Among these wires, the wire  631  configures the primary winding, and the wires  632  and  633  configure the secondary winding. One end  631   a  of the wire  631  is connected to the terminal electrode  642 , and the other end  631   b  is connected to the terminal electrode  645 . One end  632   a  of the wire  632  is connected to the terminal electrode  641 , and the other end  632   b  is connected to the terminal electrode  644 . Further, one end  633   a  of the wire  633  is connected to the terminal electrode  643 , and the other end  633   b  is connected to the terminal electrode  646 . In the sixth embodiment, the wire  631  is wound in eight turns, while the wires  632  and  633  are wound in four turns each. Further, the equivalent circuit of the balun transformer  600  is the same as that shown in  FIG. 5 . 
       FIG. 26  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  600  according to the sixth embodiment. 
     A mount region  650  on the printed-circuit board shown in  FIG. 26  is a region for mounting the balun transformer  600 , and is arranged with four land patterns  651  to  654 . The land pattern  651  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  642  of the balun transformer  600 . The land pattern  652  is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes  641 ,  645 , and  646  of the balun transformer  600 . Thereby, the terminal electrodes  645  and  646  configure the center tap of the secondary winding. The land patterns  653  and  654  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  643  and the terminal electrode  644  of the balun transformer  600 . 
     The balun transformer  600  does not have any directionality, and accordingly, the same wire-connection state can be obtained even when switching the position of a pair of flanges  612  and  613  arranged on both ends of the core unit  611 . That is, even when the balun transformer  600  is rotated by 180° at the time of mounting, the correct operation can be performed. Thus, due to the fact that the balun transformer  600  does not have any directionality, it is not necessary to control the mounting direction, thereby decreasing mounting costs. 
     Further, in the balun transformer  600 , the wires  632  and  633  wound by bifilar winding do not intersect each other at any location (any location where positions of the wires  632  and  633  are switched). Thus, the wires  632  and  633  do not need to be intersected during the wire-winding operation, thereby enabling production without utilizing any complex winding machine. 
     A seventh embodiment of the present invention is described next. 
       FIG. 27  is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer  700  according to the seventh embodiment. The appearance and the cross section of the balun transformer  700  according to the seventh embodiment are substantially identical to those of the balun transformer  100  according to the first embodiment shown in  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 27 , three wires  731  to  733  are connected to terminal electrodes  741  to  746  according to the seventh embodiment. Among these wires, the wire  731  configures the primary winding, and the wires  732  and  733  configure the secondary winding. One end  731   a  of the wire  731  is connected to the terminal electrode  742 , and the other end  731   b  is connected to the terminal electrode  745 . One end  732   a  of the wire  732  is connected to the terminal electrode  741 , and the other end  732   b  is connected to the terminal electrode  746 . One end  733   a  of the wire  733  is connected to the terminal electrode  743 , and the other end  733   b  is connected to the terminal electrode  744 . In the seventh embodiment, the wire  731  is wound in eight turns, while the wires  732  and  733  are wound in four turns each. Further, the equivalent circuit of the balun transformer  700  is the same as that shown in  FIG. 5 . 
       FIG. 28  shows a wiring pattern on the printed-circuit board for mounting the balun transformer  700 . 
     Amount region  750  on the printed-circuit board shown in  FIG. 28  is a region for mounting the balun transformer  700 , and is arranged with four land patterns  751  to  754 . The land pattern  751  is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode  742  of the balun transformer  700 . The land pattern  752  is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes  741 ,  744 , and  745  of the balun transformer  700 . Thereby, the terminal electrodes  741  and  744  configure the center tap of the secondary winding. The land patterns  753  and  754  are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode  743  and the terminal electrode  746  of the balun transformer  700 . 
     The balun transformer  700  does not have any directionality, and therefore the same wire-connection state can be obtained even when switching the position of a pair of flanges  712  and  713  arranged on both ends of the core unit  711 . That is, even when the balun transformer  700  is rotated by 180° at the time of mounting, the correct operation can be performed. Thus, because the balun transformer  700  does not have any directionality, it is not necessary to control the mounting direction, thereby decreasing mounting costs. 
     Further, the pair of balanced transmission lines STL and SBL can be formed in parallel and linearly, and accordingly, it becomes unnecessary to detour the balanced transmission lines STL and SBL on the printed-circuit board, thereby making it possible to secure the symmetry between the pair of balanced transmission lines STL and SBL. 
     While a preferred embodiment of the present invention has been described hereinbefore, the present invention is not limited to the aforementioned embodiment and various modifications can be made without departing from the spirit of the present invention. It goes without saying that such modifications are included in the scope of the present invention. 
     For example, in each of the first to seventh embodiments, the bifilar winding is performed for the two wires configuring the secondary winding. However, the winding method is not limited to the bifilar winding as long as the two wires are wound along each other. Accordingly, as shown in  FIG. 29 , the two wires  11  and  12  are twisted to use a twisted wire  10 , and such a twisted wire  10  can be wound around the core unit to use it as the secondary winding. 
     EXAMPLES 
     While Examples of the present invention are explained below, the present invention is not limited thereto. 
     First, a balun transformer according to an Example having the configuration shown in  FIG. 1  to  FIG. 3 , and a balun transformer according to a comparative example having a configuration shown in  FIG. 6  were prepared. As explained above, the wires  132  and  133  configuring the secondary winding in the balun transformer according to the Example are wound by bifilar winding, while the wires  132  and  133  configuring the secondary winding in the balun transformer of the comparative example are wound by sector winding. Only the winding method of the secondary winding differs between the two examples, and all of the remaining features are the same. Note that a NiZn ferrite was used as the material for the drum-shaped core and the plate-shaped core in both the cases. 
     Next, the frequency characteristics of the amplitude unbalance and phase unbalance were measured for the balun transformers according to the Example and the comparative example.  FIG. 30  shows measurement results for the amplitude unbalance, and  FIG. 31  shows measurement results for the phase unbalance. 
     As shown in  FIG. 30 , the amplitude unbalance of the balun transformer according to the Example is almost 0 dB in the measured frequency range (0 to 200 MHz). It was confirmed that the amplitude balance of the balanced signals was equal. In contrast thereto, in the balun transformer of the comparative example, as the frequency is higher, the amplitude balance collapses, and thus it was confirmed that the amplitude balance of balanced signals was further lowered in higher frequency areas. 
     As shown in  FIG. 31 , the phase unbalance of the balun transformer according to the Example is almost 180° in the measured frequency range, and thus it was confirmed that the phase of the balanced signals was correctly reversed. In contrast thereto, in the balun transformer of the comparative example, as the frequency is higher, the phase unbalance shifts away from the 180-degree level, and it was confirmed that the phase of balanced signals was further deviated in higher frequency areas.