Balun transformer using a drum-shaped core

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.

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 a 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, 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.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a schematic perspective view showing an appearance of a balun transformer according to a first embodiment of the present invention,FIG. 2is a schematic cross-sectional view of the balun transformer according to the first embodiment, andFIG. 3is a schematic bottom view of the balun transformer according to the first embodiment, as viewed from a mounting surface side.

As shown inFIG. 1toFIG. 3, a balun transformer100according to the first embodiment is configured by a drum-shaped core110, a plate-shaped core120, and three wires131to133. The drum-shaped core110includes a core unit111, and a pair of flanges112and113arranged on both ends of the core unit111. As viewed from one direction (from an arrow A shown inFIG. 3), three terminal electrodes141to143arranged in this order are positioned on one flange112. As viewed from the same direction (from the arrow A shown inFIG. 3), three terminal electrodes144to146arranged in this order are positioned on the other flange113.

The plate-shaped core120is located to link the top of the flanges112and113of the drum-shaped core110. In the present invention, it is not essential to use the plate-shaped core120, however, when a closed magnetic circuit is formed by using the plate-shaped core120, high magnetic coupling can be obtained. The drum-shaped core110and the plate-shaped core120are 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 core120on 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 inFIG. 3, all the three wires131to133are wound in a clock-wise direction (right turn) towards an arrow B.FIG. 4is a schematic diagram for explaining a connection relationship among the wires131to133and the terminal electrodes141to146. As shown inFIG. 4, one end131aof the wire131is connected to the terminal electrode141, and the other end131bis connected to the terminal electrode144. In the first embodiment, the wire131is wound in eight turns. Further, one end132aof the wire132is connected to the terminal electrode143, and the other end132bis connected to the terminal electrode145. In the first embodiment, the wire132is wound in four turns. Further, one end133aof the wire133is connected to the terminal electrode142, and the other end133bis connected to the terminal electrode146. In the first embodiment, the wire133is wound in four turns.

FIG. 5is an equivalent circuit diagram of the balun transformer100according to the first embodiment.

As shown inFIG. 5, the balun transformer100is configured by primary windings L11and L12connected between a primary-side terminal P and a ground terminal GND, and secondary windings L21and L22connected between a secondary-side positive electrode terminal ST and a secondary-side negative electrode terminal SB. A connecting point of the secondary windings L21and L22is used as a center tap CT.

In the first embodiment, the four turns on the one end131aside of the wire131configure the primary winding L11, and the four turns on the other end131bside configure the primary winding L12. Further, the wire132configures the secondary winding L21, while the wire133configures the secondary winding L22. Accordingly, the terminal electrode141is used as the primary-side terminal P, the terminal electrodes143and146are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB, the terminal electrode144is used as the ground terminal GND, and the terminal electrodes142and145are used as the center tap CT.

As shown inFIG. 2andFIG. 3, in the first embodiment, the wire131that configures the primary winding is wound on the inner circumferential side, and the wires132and133configuring the secondary winding are wound on the outer circumferential side. Note that these wires can be wound in the opposite manner. The wires132and133configuring the secondary winding are wound by bifilar winding around the core unit111. InFIG. 2, a wire that is hatched on the cross section is the wire132, and a wire that is marked with “×” on the cross section is the wire133. That is, the wires132and133are wound alternately from one flange112towards the other flange113(or towards the opposite direction). Accordingly, parts coinciding with an n-th turn (n=1 to 4) of the wires132and133are adjoined to each other.

According to such a winding method, a remarkably high level of symmetry can be secured between these two wires132and133, as compared to a case of a so-called sector winding, i.e., the wire132is collectively wound in an area111aon the flange112side in the core unit111and the wire133is collectively wound in an area111bon the flange113side in the core unit111as shown in a comparative example shown inFIG. 6is 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 unit111, and accordingly, the symmetry becomes disturbed at the wiring part, which is used for connecting the center tap CT to the terminal electrodes.

FIG. 7shows a wiring pattern on a printed-circuit board for mounting the balun transformer100according to the first embodiment.

A mount region150on a printed-circuit board shown inFIG. 7is a region for mounting the balun transformer100, and is arranged thereon with four land patterns151to154. The land pattern151is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode141(the primary-side terminal P) of the balun transformer100. The land pattern152is a pattern connected to the ground wiring GNDL, and is commonly connected to the terminal electrode144(the ground terminal GND) and the terminal electrodes142and145(the center tap CT) of the balun transformer100. The land patterns153and154are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode143(the secondary-side positive electrode terminal ST) and the terminal electrode146(the secondary-side negative electrode terminal SB) of the balun transformer100.

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 region150, 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 region150. 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 transformer100employs bifilar winding for the two wires132and133configuring 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 wires131to133are wound in the same direction, it is not necessary to wind while intersecting the wires in the core unit111. 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. 8is a schematic perspective view showing an appearance of a balun transformer according to the second embodiment,FIG. 9is a schematic cross-sectional view of the balun transformer according to the second embodiment, andFIG. 10is a schematic bottom view of the balun transformer according to the second embodiment, as viewed from a mounting surface side.

As shown inFIG. 8toFIG. 10, a balun transformer200according to the second embodiment is configured by a drum-shaped core210, a plate-shaped core220, and four wires231to234. The drum-shaped core210includes a core unit211, and a pair of flanges212and213arranged on both ends of the core unit211. The drum-shaped core210and the plate-shaped core220correspond to the drum-shaped core110and the plate-shaped core120in the balun transformer100, and thus the materials are also the same as those described above.

As viewed from one direction (from an arrow E shown inFIG. 10), four terminal electrodes241,247,242, and243located in this order are arranged on one flange212of the drum-shaped core210. As viewed from the same direction (from the arrow E shown inFIG. 10), four terminal electrodes244,248,245, and246located in this order are arranged on the other flange213. Among these, the terminal electrodes241to246correspond to the terminal electrodes141to146in the balun transformer100. Accordingly, the balun transformer200has a configuration in which the two terminal electrodes247and248are added to the balun transformer100.

As shown inFIG. 10, all the four wires231to234are wound in a clock-wise direction (right turn) towards an arrow F.FIG. 11is a schematic diagram for explaining a connection relationship among the wires231to234and the terminal electrodes241to248. As shown inFIG. 11, one end231aof the wire231is connected to the terminal electrode241, and the other end231bis connected to the terminal electrode248. One end232aof the wire232is connected to the terminal electrode247, and the other end232bis connected to the terminal electrode244. One end233aof the wire233is connected to the terminal electrode243, and the other end233bis connected to the terminal electrode245. Further, one end234aof the wire234is connected to the terminal electrode242, and the other end234bis connected to the terminal electrode246. In the second embodiment, all the wires231to234are wound in four turns.

FIG. 12is an equivalent circuit diagram of the balun transformer200according to the second embodiment.

As shown inFIG. 12, the equivalent circuit of the balun transformer200is basically the same as that shown inFIG. 5. However, the primary windings L11and L12are configured by the wires231and232different from each other and these are connected by terminal electrodes247and248that act as the relay points. Further, like in the equivalent circuit shown inFIG. 5, the terminal electrode241is used as the primary-side terminal P, the terminal electrodes243and246are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB, the terminal electrode244is used as the ground terminal GND, and the terminal electrodes242and245are used as the center tap CT.

As shown inFIG. 9andFIG. 10, also in the second embodiment, the wires231and232configuring the primary winding are wound on the inner circumferential side, and the wires233and234configuring 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 wires233and234configuring the secondary winding but also the wires231and232configuring the primary winding are wound by bifilar winding around the core unit211. InFIG. 9, a wire that is neither hatched nor marked with a symbol on the cross section is the wire231, a wire that is marked with “●” (solid circle) on the cross section is the wire232, a wire that is hatched on the cross section is the wire233, and a wire that is marked with “×” on the cross section is the wire234. That is, the balun transformer200has a configuration such that the wires231and232are wound alternately from one flange212towards the other flange213(towards the opposite direction), and at the same time, the wires233and234are wound alternately.

FIG. 13AandFIG. 13Bexplain the arrangement of the wires231to234in more detail, whereFIG. 13Ais a circuit diagram showing a relationship between each turn of the wires231to234and the terminals, andFIG. 13Bis a schematic partial sectional view showing the arrangement of the wires231to234in each turn. InFIGS. 13A and 13B, 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 numeral231-1indicates a first turn of the wire231.

As shown inFIG. 13A, the number of times of turns for the wire231is defined by assuming the terminal electrode241(the primary-side terminal P) as a starting point, the number of times of turns for the wire232is defined by assuming the terminal electrode247(relay point) as a starting point, the number of times of turns for the wire233is defined by assuming the terminal electrode243(the secondary-side positive electrode terminal ST) as a starting point, and the number of times of turns for the wire234is defined by assuming the terminal electrode242(the center tap CT) as a starting point. Thereby, as viewed from the corresponding terminal electrodes (241and243), each turn231-1to231-4of the wire231and each turn233-1to233-4of the wire233configure a pair PA to each other. Similarly, as viewed from the corresponding terminal electrodes (244and246), each turn232-1to232-4of the wire232and each turn234-1to234-4of the wire234configure 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 inFIG. 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. 14shows a wiring pattern on a printed-circuit board for mounting the balun transformer200.

A mount region250on the printed-circuit board shown inFIG. 14is a region for mounting the balun transformer200, and is arranged with five land patterns251to255. The land pattern251is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode241(the primary-side terminal P) of the balun transformer200. The land pattern252is a pattern connected to the ground wiring GNDL, and is commonly connected to the terminal electrode244(the ground terminal GND) and the terminal electrodes242and245(the center tap CT) of the balun trans former200. The land patterns253and254are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode243(the secondary-side positive electrode terminal ST) and the terminal electrode246(the secondary-side negative electrode terminal SB) of the balun transformer200. Further, the land pattern255is a pattern connected to a relay point of the primary winding, and is commonly connected to the terminal electrodes247and248of the balun transformer200.

According to such a layout, similarly to the balun transformer100according 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 transformer200of the second embodiment, in addition to the same effects as that of the balun transformer100according 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 wires231to234is the same with each other, all these four wires231to234can be wound simultaneously.

A third embodiment of the present invention is described next.

FIG. 15is a schematic perspective view showing an appearance of a balun transformer according to the third embodiment.FIG. 16is a schematic cross-sectional view of the balun transformer according to the third embodiment, andFIG. 17is a schematic bottom view of the balun transformer according to the third embodiment, as viewed from the mounting surface side.

As shown inFIG. 15toFIG. 17, a balun transformer300according to the third embodiment is configured by a drum-shaped core310, a plate-shaped core320, and three wires331to333. The drum-shaped core310includes a core unit311, and a pair of flanges312and313arranged on both ends of the core unit311. The drum-shaped core310and the plate-shaped core320correspond to the drum-shaped core110and the plate-shaped core120in the balun transformer100, and accordingly, the materials are also the same as those described above.

Two terminal electrodes341and342are arranged on one flange312of the drum-shaped core310, and two terminal electrodes343and344are arranged on the other flange313. As shown inFIG. 17, all the three wires331to333are wound in a clock-wise direction (right turn) towards an arrow G. Note that, with respect to the wire331, after four turns are wound from one end331ain 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 wire331intersects itself at some parts.

FIG. 18is a schematic diagram for explaining a connection relationship among the wires331to333and the terminal electrodes341to344. As shown inFIG. 18, one end331aof the wire331is connected to the terminal electrode341, and the other end331bis connected to the terminal electrode342. One end332aof the wire332is connected to the terminal electrode343, and the other end332bis connected to the terminal electrode342. Further, one end333aof the wire333is connected to the terminal electrode344, and the other end333bis connected to the terminal electrode342. In the third embodiment, the wire331is wound in eight turns, while the wires332and333are wound in four turns each.

FIG. 19is an equivalent circuit diagram of the balun transformer300according to the third embodiment.

As shown inFIG. 19, the equivalent circuit of the balun transformer300is basically the same as that shown inFIG. 5. However, the terminal electrode342is used as both the ground terminal GND and the center tap CT. Further, the terminal electrode341is used as the primary-side terminal P, and the terminal electrodes343and344are respectively used as the secondary-side positive electrode terminal ST and the secondary-side negative electrode terminal SB.

As shown inFIG. 16andFIG. 17, also in the third embodiment, the wire331configuring the primary winding is wound on the outer circumferential side, and the wires332and333configuring the secondary winding are wound on the inner circumferential side. This is because the wire331intersects itself at some parts, and accordingly, the surface after winding is roughened, and when the secondary winding (the wires332and333) is wound on such a roughened surface, stress is applied to the intersecting part.

Also in the third embodiment, the wires332and333configuring the secondary winding are wound by bifilar winding around the core unit311. InFIG. 16, a wire that is hatched on the cross section is the wire332, and a wire that is marked with on the cross section is the wire333. That is, the wires332and333are wound alternately from one flange312towards the other flange313(or towards the opposite direction).

FIG. 20shows a wiring pattern on the printed-circuit board for mounting the balun transformer300according to the third embodiment.

A mount region350on the printed-circuit board shown inFIG. 20is a region for mounting the balun transformer300, and arranged with four land patterns351to354. The land pattern351is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode341(the primary-side terminal P) of the balun transformer300. The land pattern352is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrode342(that serves both the ground terminal GND and the center tap CT) of the balun transformer300. The land patterns353and354are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode343(the secondary-side positive electrode terminal ST) and the terminal electrode344(the secondary-side negative electrode terminal SB) of the balun transformer300.

According to such a layout, similarly to the balun transformer100and the balun transformer200, 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 transformer300, in addition to the effects identical to that of the balun transformer100according 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. 21is a schematic diagram for explaining a connection relationship between the wires and the terminal electrodes of a balun transformer400according to the fourth embodiment. The appearance and the cross section of the balun transformer400according to the fourth embodiment are substantially identical to those of the balun transformer100according to the first embodiment shown inFIG. 1andFIG. 2.

As shown inFIG. 21, three wires431to433are connected to the terminal electrodes441to446in the fourth embodiment. Among these, the wire431configures the primary winding, and the wires432and433configure the secondary winding. One end431aof the wire431is connected to the terminal electrode441, and the other end431bis connected to the terminal electrode446. One end432aof the wire432is connected to the terminal electrode442, and the other end432bis connected to the terminal electrode444. One end433aof the wire433is connected to the terminal electrode443, and the other end433bis connected to the terminal electrode445. In the fourth embodiment, the wire431is wound in eight turns, while the wires432and433are wound in four turns each. Further, the equivalent circuit of the balun transformer400is the same as that shown inFIG. 5.

FIG. 22shows a wiring pattern on the printed-circuit board for mounting the balun transformer400according to the fourth embodiment.

A mount region450on the printed-circuit board shown inFIG. 22is a region for mounting the balun transformer400, and is arranged with four land patterns451to454. The land pattern451is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode441of the balun transformer400. The land pattern452is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes442,445, and446of the balun transformer400. Thereby, the terminal electrodes442and445configure the center tap of the secondary winding. The land patterns453and454are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode443and the terminal electrode444of the balun transformer400.

The balun transformer400does not have any directionality, and therefore the same wire-connection state can be obtained even when switching the position of a pair of flanges412and413arranged on both ends of the core unit411. That is, even when the balun transformer400is rotated by 180° at the time of mounting, the correct operation can be performed. Reference numerals of the terminal electrodes connected to the land patterns451to454at the time of rotating the balun transformer400by 180° are as shown within brackets inFIG. 22. Thus, because the balun transformer400does not have any directionality, it is not necessary to control the mounting direction, thereby decreasing mounting costs.

Further, in the balun transformer400, the wires432and433wound by bifilar winding do not intersect each other at any location (any location where positions of the wires432and433are switched). Accordingly, it is not necessary to intersect the wires432and433during the wire-winding operation, thereby enabling production without utilizing any complex winding machine.

Further, in the balun transformer400, each of the wirings (PL, STL, STB, and GNDL) can be connected to the terminal electrodes441,443,444, and446positioned at the corners, and accordingly, it becomes easy to connect the wiring on the printed-circuit board with the balun transformer400.

A fifth embodiment of the present invention is described next.

FIG. 23is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer500according to the fifth embodiment. The appearance and the cross section of the balun transformer500according to the fifth embodiment are also substantially identical to those of the balun transformer100according to the first embodiment shown inFIG. 1andFIG. 2.

As shown inFIG. 23, three wires531to533are connected to the terminal electrodes541to546according to the fifth embodiment. Among these, the wire531configures the primary winding, and the wires532and533configure the secondary winding. One end531aof the wire531is connected to the terminal electrode541, and the other end531bis connected to the terminal electrode554. One end532aof the wire532is connected to the terminal electrode542, and the other end532bis connected to the terminal electrode545. One end533aof the wire533is connected to the terminal electrode543, and the other end533bis connected to the terminal electrode546. In the fifth embodiment, the wire531is wound in eight turns, while the wires532and533are wound in four turns each. Further, the equivalent circuit of the balun transformer500is the same as that shown inFIG. 5.

FIG. 24shows a wiring pattern on the printed-circuit board for mounting the balun transformer500according to the fifth embodiment.

A mount region550on the printed-circuit board shown inFIG. 24is a region for mounting the balun transformer500, and is arranged with four land patterns551to554. The land pattern551is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode541of the balun transformer500. The land pattern552is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes542,544, and546of the balun transformer500. Thereby, the terminal electrodes542and546configure the center tap of the secondary winding. The land patterns553and554are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode543and the terminal electrode545of the balun transformer500.

Similarly to the balun transformer400according to the fourth embodiment, also in the balun transformer500according to the fifth embodiment, the wires532and533wound by bifilar winding do not interest each other at any position. Thus, it is not necessary to intersect the wires532and533during the wire-winding operation, thereby enabling production without utilizing any complex winding machine.

Further, in the balun transformer500, both ends of all the wires531to533are 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. 25is a schematic diagram for explaining a connection relationship between wires and terminal electrodes of a balun transformer600according to the sixth embodiment. The appearance and the cross section of the balun transformer600according to the sixth embodiment are substantially identical to those of the balun transformer100according to the first embodiment shown inFIG. 1andFIG. 2.

As shown inFIG. 25, three wires631to633are connected to terminal electrodes641to646according to the sixth embodiment. Among these wires, the wire631configures the primary winding, and the wires632and633configure the secondary winding. One end631aof the wire631is connected to the terminal electrode642, and the other end631bis connected to the terminal electrode645. One end632aof the wire632is connected to the terminal electrode641, and the other end632bis connected to the terminal electrode644. Further, one end633aof the wire633is connected to the terminal electrode643, and the other end633bis connected to the terminal electrode646. In the sixth embodiment, the wire631is wound in eight turns, while the wires632and633are wound in four turns each. Further, the equivalent circuit of the balun transformer600is the same as that shown inFIG. 5.

FIG. 26shows a wiring pattern on the printed-circuit board for mounting the balun transformer600according to the sixth embodiment.

A mount region650on the printed-circuit board shown inFIG. 26is a region for mounting the balun transformer600, and is arranged with four land patterns651to654. The land pattern651is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode642of the balun transformer600. The land pattern652is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes641,645, and646of the balun transformer600. Thereby, the terminal electrodes645and646configure the center tap of the secondary winding. The land patterns653and654are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode643and the terminal electrode644of the balun transformer600.

The balun transformer600does not have any directionality, and accordingly, the same wire-connection state can be obtained even when switching the position of a pair of flanges612and613arranged on both ends of the core unit611. That is, even when the balun transformer600is rotated by 180° at the time of mounting, the correct operation can be performed. Thus, due to the fact that the balun transformer600does not have any directionality, it is not necessary to control the mounting direction, thereby decreasing mounting costs.

Further, in the balun transformer600, the wires632and633wound by bifilar winding do not intersect each other at any location (any location where positions of the wires632and633are switched). Thus, the wires632and633do 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. 27is a schematic diagram for explaining a connection relationship between the wires and terminal electrodes of a balun transformer700according to the seventh embodiment. The appearance and the cross section of the balun transformer700according to the seventh embodiment are substantially identical to those of the balun transformer100according to the first embodiment shown inFIG. 1andFIG. 2.

As shown inFIG. 27, three wires731to733are connected to terminal electrodes741to746according to the seventh embodiment. Among these wires, the wire731configures the primary winding, and the wires732and733configure the secondary winding. One end731aof the wire731is connected to the terminal electrode742, and the other end731bis connected to the terminal electrode745. One end732aof the wire732is connected to the terminal electrode741, and the other end732bis connected to the terminal elect rode746. One end733aof the wire733is connected to the terminal electrode743, and the other end733bis connected to the terminal electrode744. In the seventh embodiment, the wire731is wound in eight turns, while the wires732and733are wound in four turns each. Further, the equivalent circuit of the balun transformer700is the same as that shown inFIG. 5.

FIG. 28shows a wiring pattern on the printed-circuit board for mounting the balun transformer700.

A mount region750on the printed-circuit board shown inFIG. 28is a region for mounting the balun transformer700, and is arranged with four land patterns751to754. The land pattern751is a pattern connected to the unbalanced transmission line PL, and is connected to the terminal electrode742of the balun transformer700. The1and pattern752is a pattern connected to the ground wiring GNDL, and is connected to the terminal electrodes741,744, and745of the balun transformer700. Thereby, the terminal electrodes741and744configure the center tap of the secondary winding. The land patterns753and754are patterns connected to a pair of balanced transmission lines STL and SBL, and are respectively connected to the terminal electrode743and the terminal electrode746of the balun transformer700.

The balun transformer700does not have any directionality, and therefore the same wire-connection state can be obtained even when switching the position of a pair of flanges712and713arranged on both ends of the core unit711. That is, even when the balun transformer700is rotated by 180° at the time of mounting, the correct operation can be performed. Thus, because the balun transformer700does 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 inFIG. 29, the two wires11and12are twisted to use a twisted wire10, and such a twisted wire10can be wound around the core unit to use it as the secondary winding.

EXAMPLES

While Examples of the present invention are explained be low, the present invention is not limited thereto.

First, a balun transformer according to an Example having the configuration shown inFIG. 1toFIG. 3, and a balun transformer according to a comparative example having a configuration shown inFIG. 6were prepared. As explained above, the wires132and133configuring the secondary winding in the balun transformer according to the Example are wound by bifilar winding, while the wires132and133configuring 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. 30shows measurement results for the amplitude unbalance, andFIG. 31shows measurement results for the phase unbalance.

As shown inFIG. 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 inFIG. 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.