Patent Publication Number: US-11049649-B2

Title: Magnetic transformer having increased bandwidth for high speed data communications

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/725,047 filed on Oct. 4, 2017, which claims priority under 35 USC 119(e) from U.S. Provisional Application No. 62/480,757 filed on Apr. 3, 2017; and the contents both priority applications are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This application is directed to conductors and circuit elements for use in high speed data communications, and, more particularly, to improvements in baluns and twisted wire cables. 
     Description of the Related Art 
     Transformers are devices that transfer electrical energy from one electrical circuit to another electrical circuit through the use of inductively coupled conductors. As is well understood, a varying current in a primary winding creates a varying magnetic flux and thus a varying magnetic field through a secondary winding. This varying magnetic field induces a varying electromotive force (“EMF”) or voltage in the secondary winding. An ideal transformer assumes that all the magnetic flux generated by the primary winding is coupled to every secondary winding of the transformer. In practice however, some of the magnetic flux generated by the primary winding exists outside the secondary windings, thereby giving the appearance that the transformer has an inductance in series with the transformer windings. This non-ideal operating characteristic is known as leakage inductance. 
     Leakage inductance is caused by an imperfect coupling of the windings, which creates a leakage flux that does not link with all the turns of the secondary transformer windings. As a result, the voltage drops across the leakage reactance of the circuit resulting in a less than ideal voltage regulation, especially when the transformer is placed under load. This is particularly problematic in high frequency applications where the high frequency of the electrical current exacerbates the non-ideal parasitic effects seen in the transformer. 
     For years, engineers have recognized that reducing the amount of leakage inductance seen on a transformer increases the high frequency performance of the transformer. Heretofore, the most commonly used methods to reduce the amount of leakage inductance seen in a transformer has traditionally been by twisting the primary and secondary wires together, interleaving the windings (e.g., interspersing individual or layers of primary windings with secondary windings), or alternatively implementing a combination of both twisting and interleaving of the windings in order to increase the coupling between windings. The purpose of both twisting and interleaving techniques is to attempt to distribute electromagnetic energy (both internal energy and externally generated energy) to each of the primary and secondary windings as equally and as completely as possible. However, while it is possible to implement a combination of twisting and interleaving, twisting is often extremely difficult to accomplish when interleaving more than one set of windings. This is primarily a result of the fact that once you have more than one interleaved winding, the order of the wires in the bundle needs to be carefully controlled in order to obtain the best coupling. This is often difficult to achieve when using both interleaving in combination with wire twisting. 
     For high frequency communications, small transformers with relatively few windings are used to electrically isolate network data lines from local circuitry so that any potential differences to ground between the network data lines and the local circuitry do not result in current flow between the data lines and the local circuitry. For example,  FIG. 1  illustrates a known transformer  100  that may be used for isolation. Such an isolation transformer is often referred to as a “balun.” As illustrated, the transformer includes a core  102  that comprises a magnetically permeable material having a relative magnetic permeability (μ/μ 0 ) of, for example, 1,500 to 5,000. A plurality of wires  104  are wound onto the core to form the windings of the transformer. In the illustrated embodiment, the wires are grouped in multi-wire (e.g., three-wire) cables. For example, a first three-wire cable  106  may include two primary wires and one secondary wire and a second three-wire cable  108  may include two additional primary wires another secondary wire. The three wires in each cable are twisted together to cause the three wires in each cable to encounter similar perturbations causes by electromagnetic noise. 
     The transformer core  102  is formed as an oval-shaped (e.g., racetrack-shaped) body  110  with a first cylindrical through-bore  112  spaced apart from a second cylindrical through-bore  114 . An example of such a transformer is described in detail in U.S. Pat. No. 7,924,130 for “Isolation Magnetic Devices Capable of Handling High-Speed Communications,” which is incorporated by reference herein in its entirety. As described in U.S. Pat. No. 7,924,130, the completed transformer is formed by threading the wires (cables)  104  through the first through-bore and through the second through-bore to form the windings of the transformer. The ends of wires are selectively interconnected to define the primary and secondary windings of the transformer. One skilled in the art will appreciate that the circular through-bores that receive the wires cause the wires threaded through the through-bores to be spaced apart differently along the circumferences of the through-bores. For example, the turns of the wires positioned near the center of the core are closer together across the thickness of the core between the through-bores than the turns of the wires that are farther from the center of the core. As further shown in the cross-sectional view of  FIG. 2 , the wires (cables) tend to bunch up within the through-bores rather than being evenly distributed within the through-bores. In some configurations, the bunching of the wires may cause the start of a particular winding to be positioned near the finish of the particular winding, which may increase the parasitic capacitance between the start and the finish of the winding. 
     SUMMARY OF THE INVENTION 
     Although the previously described cable and transformers are adequate for high-speed data communications up to certain data transmission rates (e.g., up to 400 MHz frequency range), the need for higher data transmission rates has resulted in a need for improvements in the coupling between the primary and secondary windings of the transformer. 
     In view of the foregoing, a need exists for a system and method that provides enhanced coupling between the windings of an isolation transformer in a high speed data communications coupler system. 
     One aspect of the embodiments disclosed herein is an isolation transformer that includes a transformer core. First and second through-bores extend through the transformer core from a first surface to a second surface. Each through-bore has an elongated profile with at least a portion of the elongated profile providing a respective flat winding surface. The flat winding surfaces are spaced apart by a central portion of the transformer core. The transformer is wound with a six-wire cable having a central non-conductive core. First, second, third, fourth, fifth and sixth conductive wires are positioned around and adjacent to the central non-conductive core in a substantially equally spaced angular relationship. The second conductive wire is positioned between the first conductive wire and the third conductive wire; and the fifth conductive wire is positioned between the fourth conductive wire and the sixth conductive wire. The conductive wires are twisted about the central non-conductive core at a selected twist density. 
     Another aspect of the embodiments disclosed herein is an isolation transformer comprising a transformer core having a first surface and a second surface. A first through-bore extends through the transformer core from the first surface to the second surface. The first through-bore has an elongated profile with at least a portion of the elongated profile providing a first flat winding surface. A second through-bore extends through the transformer core from the first surface to the second surface. The second through-bore has an elongated profile with at least a portion of the elongated profile providing a second flat winding surface. The second flat winding surface is spaced apart from the first flat winding surface by a central portion of the transformer core. The transformer further includes at least one multi-wire cable comprising a first conductive wire, a second conductive wire, a third conductive wire, a fourth conductive wire, a fifth conductive wire, and a sixth conductive wire. The second conductive wire is positioned between the first conductive wire and the third conductive wire. The fifth conductive wire is positioned between the fourth conductive wire and the sixth conductive wire. In certain embodiments, each of the first and second through-bores has an oval-shaped profile having a central rectangular portion, a first semicircular end portion and a second semicircular end portion. Each of the first and second flat winding portions is defined by a respective side of the central rectangular portion of the respective through-bore. In certain embodiments, the at least one multi-wire cable includes a first three-wire cable that includes the first conductive wire, the second conductive wire and the third conductive wire, wherein the first, second and third conductive wires twisted together; and further includes a second three-wire cable that includes the fourth conductive wire, the fifth conductive wire and the sixth conductive wire, wherein the fourth, fifth and sixth conductive wires twisted together. In certain embodiments, the first three-wire cable and the second three-wire cable are wound onto the transformer core with one turn of the first three-wire cable positioned between adjacent turns of the second three-wire core. In other certain embodiments, the at least one multi-wire cable comprises a six-wire cable that includes the first conductive wire, the second conductive wire, the third conductive wire, the fourth conductive wire, the fifth conductive wire and the sixth conductive wire, wherein the first, second, third, fourth, fifth and sixth conductive wires are helically wound about a central non-conductive core. 
     Another aspect of the embodiments disclosed herein is a transformer core comprising a magnetic material formed into a solid having at least a first surface and a second surface. A first through-bore extends through the magnetic material from the first surface to the second surface. The first through-bore has an elongated profile with at least a portion of the elongated profile providing a first flat winding surface. A second through-bore extends through the magnetic material from the first surface to the second surface. The second through-bore has an elongated profile with at least a portion of the elongated profile providing a second flat winding surface. The second flat winding surface is spaced apart from the first flat winding surface by a central portion of the magnetic material. In certain embodiments in accordance with this aspect, each of the first and second through-bores has an oval-shaped profile having a central rectangular portion, a first semicircular end portion and a second semicircular end portion. Each of the first and second flat winding portions is defined by a respective side of the central rectangular portion of the respective through-bore. 
     Another aspect of the embodiments disclosed herein is a multi-wire cable for a transformer winding. The cable comprises a central non-conductive core. At least a first conductive wire, a second conductive wire, a third conductive wire, a fourth conductive wire, a fifth conductive wire, and a sixth conductive wire are positioned around and adjacent to the central non-conductive core in a substantially equally spaced angular relationship. The second conductive wire is positioned between the first conductive wire and the third conductive wire. The fifth conductive wire is positioned between the fourth conductive wire and the sixth conductive wire. The conductive wires are twisted about the central non-conductive core at a selected twist density. In certain embodiments in accordance with this aspect, each conductive wire has a common diameter corresponding to a selected wire gauge. The central non-conductive core has a diameter at least as great as the common diameter of the conducive wires. In certain embodiments in accordance with this aspect, the central non-conductive core comprises a monofilament material. In certain embodiments in accordance with this aspect, the multi-wire cable comprises only six conductive wires and the central non-conductive wire. In certain embodiments in accordance with this aspect, the multi-wire cable comprises eight conductive wires and the central non-conductive wire. In certain embodiments in accordance with this aspect, the multi-wire cable comprises nine conductive wires and the central non-conductive wire. 
     Another aspect of the embodiments disclosed herein is high data rate coupler system comprising an isolation transformer and a choke. The isolation transformer includes a core having a first surface and a second surface. A first through-bore extends through the transformer core from the first surface to the second surface. The first through-bore has an elongated profile with at least a portion of the elongated profile providing a first flat winding surface. A second through-bore extends through the transformer core from the first surface to the second surface. The second through-bore has an elongated profile with at least a portion of the elongated profile providing a second flat winding surface. The second flat winding surface is spaced apart from the first flat winding surface by a central portion of the transformer core. The transformer further includes at least one multi-wire cable comprising a central non-conductive core, a first conductive wire, a second conductive wire, a third conductive wire, a fourth conductive wire, a fifth conductive wire, and a sixth conductive wire. The second conductive wire is positioned between the first conductive wire and the third conductive wire. The fifth conductive wire is positioned between the fourth conductive wire and the sixth conductive wire. The first and third conductive wires form a first primary winding of the isolation transformer; and the fourth and sixth conductive wires form a second primary winding of the isolation transformer. The first and second primary windings are connected in series to form a center-tapped primary winding. The second wire forms a first secondary winding of the isolation transformer, and the fifth wire forms a second secondary winding of the isolation transformer. The first and second secondary windings are connected in series to form a center-tapped secondary winding. The choke is wound with respective end segments of the second conductive wire and the fifth conductive wire. In certain embodiments in accordance with this aspect, the at least one multi-wire cable comprises six conductive wires and a central non-conductive wire. In other embodiments in accordance with this aspect, the at least one multi-wire cable comprises a first three-wire cable and a second three-wire cable. In certain embodiments having the first three-wire cable and the second three-wire cable, the first, second and third conductive wires are in the first three-wire cable, and wherein the fourth, fifth and sixth conductive wires are in the second three-wire cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other aspects of this disclosure are described in detail below in connection with the accompanying drawing figures in which: 
         FIG. 1  illustrates a perspective view of a known isolation transformer; 
         FIG. 2  illustrates a cross-sectional view of the isolation transformer of  FIG. 1  taken along the line  2 - 2  in  FIG. 2 ; 
         FIG. 3  illustrates a perspective view of a transformer core having elongated through-bores, the view showing the front, top and right sides of the transformer core; 
         FIG. 4  illustrates a rotated perspective view of the transformer core of  FIG. 3 , the view showing the rear, bottom and left sides of the transformer core; 
         FIG. 5  illustrates a perspective view of a transformer incorporating the core of  FIGS. 3 and 4 , the transformer including first and second coils comprising three turns each of first and second three-wire cables; 
         FIG. 6  illustrates a cross-sectional view of the transformer of  FIG. 5  taken along the line  6 - 6  of  FIG. 5 ; 
         FIG. 7  illustrates a schematic diagram of the transformer of  FIGS. 5 and 6 ; 
         FIG. 8  illustrates a segment of a six-wire cable having a central non-conductive core around which the six conductive wires are wound in a twisted pattern; 
         FIG. 9  illustrates a cross-sectional view of the six-wire cable of  FIG. 8  taken along the line  9 - 9  in  FIG. 8 ; 
         FIG. 10  illustrates a perspective view of a transformer incorporating the transformer core of  FIGS. 3 and 4  and the six-wire cable of  FIGS. 8 and 9 ; 
         FIG. 11  illustrates a cross-sectional view of the transformer of  FIG. 10  taken along the line  11 - 11  in  FIG. 10 ; 
         FIG. 12  illustrates a schematic diagram of the transformer of  FIGS. 10 and 11  showing the six wires of the six-wire cable as windings about the core of the transformer; 
         FIG. 13  illustrates a perspective view of a transformer in which the six-wire cable of  FIG. 8  is wound onto a toroidal core structure; 
         FIG. 14  illustrates a perspective view of a high data rate coupler system that incorporates the transformer of  FIGS. 10 and 11  with the six-wire cable and a toroidal core wound with a three-wire cable; 
         FIG. 15  illustrates an enlarged perspective view of the transformer of  FIG. 14  showing the interconnections to the primary windings of the transformer in more detail; 
         FIG. 16  illustrates an enlarged perspective view of the transformer of  FIG. 14  showing the interconnections to the secondary windings of the transformer in more detail; 
         FIG. 17  illustrates a schematic diagram of the high data rate coupler system of  FIGS. 14-16  showing the interconnections of the primary windings and the interconnections of the secondary windings and the toroidal coil; 
         FIG. 18  illustrates a schematic diagram of a high data rate coupler system similar to the system of  FIG. 17  which incorporates the transformer of  FIGS. 5 and 6  in place of the transformer of  FIGS. 10 and 11 ; 
         FIG. 19  illustrates a perspective view of a high data rate coupler system that incorporates the transformer of  FIGS. 5 and 6  with the two three-wire cables and a toroidal core wound with a three-wire cable; 
         FIG. 20  illustrates a cross-sectional view similar to the view of  FIG. 8  wherein the multi-wire cable comprises eight conductive wires around a non-conductive core; and 
         FIG. 21  illustrates a cross-sectional view similar to the view of  FIG. 8  wherein the multi-wire cable comprises nine conductive wires around a non-conductive core. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An improved high data rate isolation transformer is disclosed in the attached drawings and is described below. The embodiment is disclosed for illustration of the transformer and is not limiting except as defined in the appended claims. 
       FIGS. 3 and 4  illustrate a transformer core  300  in accordance with a disclosed implementation. Unlike the core of the previously described oval-shaped transformer  100  of  FIGS. 1 and 2 , the transformer core  300  in  FIGS. 3 and 4  has an overall box-like (parallelepiped) appearance having six generally rectangular sides. In the illustrated orientation referenced to X, Y and Z coordinates, the core has a top surface  310 , a bottom surface  312 , a left surface  314 , a right surface  316 , a front surface  318  and a rear surface  320 . A first (top-bottom) central axis  330  passes through the center of the core from the top surface to the bottom surface parallel to the Y axis. A second (left-right) central axis  332  passes through the center of the core from the left surface to the right surface parallel to the X axis. A third (front-rear) central axis  334  passes through the center of the core from the front surface to the rear surface parallel to the Z axis. The three central axes intersect at the center of the core. The references to top, bottom, left, right, front and rear are for convenience in providing the following description. One skilled in the art will appreciate that the transformer core can be oriented in a variety of different orientations during construction and in use. 
     In the illustrated embodiment, the transformer core  300  has a height along the top-bottom central axis  330  of approximately 0.136 inch, a width along the left-right central axis  332  of approximately 0.120 inch and a thickness (depth) along the front-rear axis  334  of approximately 0.120 inch. The dimensions are for example only and are not intended to be limiting. As further shown in  FIG. 3 , the edges between the top surface  310  and the bottom surface  312  and the adjacent left surface  314  and right surface  316  may be filleted (e.g., rounded) to remove the sharp edges. 
     As further illustrated in  FIGS. 3 and 4 , the transformer core  300  includes a first elongated through-bore  340  and a second elongated through-bore  342 . Each elongated through-bore extends through the core from the front surface  318  to the rear surface  320  in parallel with the front-rear central axis  334 . In the illustrated embodiment, the two elongated through-bores are spaced substantially equally distant from the front-rear central axis and are also spaced equal distant from the left-right central axis  332  of the core. 
     Unlike the previously described circular through-bores  110 ,  112  of the core  100  of  FIG. 1 , the elongated through-bores  340 ,  342  of the transformer core  300  of  FIGS. 3 and 4  are generally oval-shaped (e.g., racetrack-shaped). Each through-bore is wider in a left-to-right direction parallel to the left-right central axis  332  and is narrower in a top-to-bottom direction parallel to the top-bottom central axis  330 . Each elongated through-bore has a generally rectangular central portion  350 . A first semicircular end portion  352  extends from the left end of the rectangular central portion. A second semicircular end portion  354  extends from the right end of the rectangular central portion. Each elongated through-bore has a respective inner flat surface  356  that is nearest to the center of the core and a respective outer flat surface  358  that is farthest from the center of the core. A central portion  360  of the core extends from the front surface  318  to the rear surface  320  of the core between the two through-bores. The central portion of the core has a nominal height between the respective flat surfaces of the two through-bores. 
     In the illustrated embodiment, each elongated through-bore  340 ,  342  has an overall width (W) from the outer perimeter of the respective first semicircular end portion  352  to the outer perimeter of the second semicircular portion  354  of approximately 0.065 inch. In the illustrated embodiment, each elongated through-bore has a height (H) from the respective inner flat surface to the respective outer flat surface of approximately 0.034 inch, which corresponds to the diameter of each semicircular end portion. The rectangular central portion  350  of each elongated through-bore has a width of approximately 0.31 inch. The inner flat surfaces of the through-bores are spaced apart from each other by approximately 0.23 inch, which corresponds to the height of the central portion  360  of the core. The foregoing dimensions and the spacing of the elongated through-bores are examples only and are not intended to be limiting. 
       FIG. 5  illustrates a perspective view of the transformer core  300  of  FIGS. 3 and 4  configured as part of a transformer  500  with a plurality of turns of wires wound through the elongated through-bores  340 ,  342  and around the central portion  360  of the core.  FIG. 6  is a cross-sectional view of the transformer of  FIG. 5 . In the illustrated embodiment, a first three-wire cable  510  and a second three-wire cable  512  are wound around the central portion of the core in an interleaved fashion such that three turns of the first cable are interleaved with three turns of the second cable. The resulting transformer is illustrated schematically in  FIG. 7 . For convenience in the following description, two of the wires in the first cable are labeled as N 1  and N 2 , and the third wire in the first cable is labeled as G. Two of the wires in the second cable are labeled in  FIGS. 5-7  as B 1  and B 2 , and the third wire in the second cable is labeled as R. In  FIGS. 5-7 , the start of each wire (upper left) in  FIG. 6  is further identified with an S suffix, and the finish of each wire is labeled with an F suffix. The start of each wire is threaded first through the second (lower) elongated through-bore and out through the first (upper) through bore. The finish of each wire extends from the second (lower) elongated through-bore. The start and finish identifications can be interchanged. 
     As illustrated schematically in  FIG. 7 , in a particular application of the transformer  500  of  FIGS. 5 and 6 , the starts (N 1 S and N 2 S) of the N 1  and N 2  wires of the first cable  510  are connected together, and the finishes (N 1 F and N 2 F) of the N 1  and N 2  wires are connected together such that the N 1  and N 2  wires are connected in parallel for winding about the central portion of the core. The starts (B 1 S and B 2 S) of the B 1  and B 2  wires in the second cable  512  are connected together, and the finishes (B 1 F and B 2 F) of the B 1  and B 2  wires are connected together such that the B 1  and B 2  wires are connected in parallel for winding about the central portion of the core. The interconnected finishes (N 1 F and N 2 F) of the N 1  and N 2  wires of the first cable are further connected to the starts (B 1 S and B 2 S) of the B 1  and B 2  wires of the second cable such that the parallel connected N 1  and N 2  wires and the parallel connected B 1  and B 2  wires are connected in series as a continuous six-turn primary winding  700  of the transformer. The interconnected finishes N 1 F, N 2 F of the N 1  and N 2  wires and the starts B 1 S, B 2 S of the B 1  and B 2  wires form a center-tap  702  of the primary winding as shown in the schematic diagram. The interconnected N 1 S and N 2 S end segments of the N 1  and N 2  wires form a first outer lead  704  of the primary winding. The interconnected B 1 F and B 2 F end segments of the B 1  and B 2  wires form a second outer lead  706  of the primary winding. 
     As further illustrated in  FIG. 7 , the finish (RF) of the R wire in the second cable  512  is connected to the start (GS) of the G wire in the first cable  510  such that the R wire and the G wire are connected in series as a six-turn secondary winding. The common connection of the finish (RF) of the R wire and the start (GS) of the G wire forms a center-tap  712  of a secondary winding  710  of the transformer  500  as shown in the schematic diagram. The RS end segment of the R wire forms a first outer lead  714  of the secondary winding. The GF end segment of the G wire forms a second outer lead  716  of the secondary winding. In the illustrated embodiment, the secondary windings are interconnected in a cross-coupled configuration as shown to further improve impedance matching in the passband by adding half of the interwinding capacitance and reducing the leakage inductance. 
     As shown in a cross-sectional view in  FIG. 6 , the two cables  510 ,  512  are positioned against the respective inner flat surfaces  356  (see element number  356  in  FIGS. 3 and 4 ) of the elongated through-bores  340 ,  342  such that each turn of each cable is positioned adjacent the central portion  360  of the transformer core  300 . If the sum of the diameters of the adjacent turns of the wire exceed the extent of the flat inner surfaces, the turns of the wires at one or both ends of the flat inner surfaces may extend into the semicircular end portions  352 ,  354  as shown; however, the small difference in the height of the central portion of the core between the respective end turns relative to the nominal height of the central portion of the core between the flat inner surfaces of the elongated through-holes does not substantially affect the desired uniformity of the coupling between the turns of the wires. 
     The structure of the transformer  500  of  FIGS. 5-7  improves the operation of transformers at higher data communications rates by increasing the coupling between the turns of the wires in the windings and also reducing the parasitic elements in the transformer that are parallel with the winding (e.g., the distributed capacitance between the start of the winding and the finish of the winding, which are at opposite ends of the elongated bores as shown in  FIG. 7 . 
     The two interleaved three-wire cables  510 ,  512  of  FIGS. 5-7  of the transformer  500  provide coupling between the primary winding and the secondary winding for data communications at wide bandwidths up to approximately 1,800 MHz. However, winding the transformer with the two three-wire cables requires that the two cables be wound onto the transformer core  300  in two separate steps or by using a technique to allow the two cables to be wound at the same time while maintaining the perimeters of the two cables against the inner surfaces  356  of the core. 
     If the bandwidth provided by the two interleaved three-wire cables  510 ,  512  is not required, the transformer core  300  can be wound with a single multi-wire cable. For example,  FIG. 8  illustrates a segment of a multi-wire cable  800  that can be wound onto the transformer core in a single operation. As illustrated, the multi-wire cable includes six conductive magnet wires with a thin enameled insulator formed thereon. Such magnet wire is commercially available from many vendors. In the illustrated embodiment, the magnet wires comprise 38-gauge wires having outer diameters of approximately 0.0045 inch; however, the following description is readily adaptable to wires of a different gauge. For convenience in referring to the wires in the following discussion, the six wires are labeled as B 1 , B 2 , R, N 1 , N 2  and G. The selected labels B, R, N and G may refer to blue, red, natural and green colors, respectively; however, other colors or other techniques may also be used to identify the wires. In a particular implementation, the six wires may have corresponding colors for the insulation to allow each particular wire to be easily identified when interconnected as described below. 
     As shown in  FIG. 8 , the six conductive magnet wires B 1 , B 2 , R, N 1 , N 2 , G in the cable  800  are twisted around a central non-conductive core filament  830  having a diameter generally corresponding to the diameter of each of the six magnet wires. For example, the core filament diameter may be the same as the diameter of the magnet wires, or the core filament diameter may be slightly larger than the diameter of the magnet wire. Preferably, the core filament comprises a non-magnetic material. For example, in one embodiment, the non-conductive, non-magnetic core filament comprises a monofilament material such as nylon, fluorocarbon, polyethylene, polyester, or other suitable material. Such materials may be similar to materials used for fishing line. The six conductive wires may be twisted in a clockwise or counterclockwise direction around the central core filament. The clockwise twist direction is shown in  FIG. 8 . The twist density (or tightness) may be varied as required. In the illustrated embodiment, the twist density is selected to be in a range of 16 twists per inch (TPI) to 20 TPI. As illustrated, each of the six conductive wires is helically wound about the central non-conductive core filament with the start of the helical pattern of each conductive wire spaced apart angularly by 60 degrees with respect to the starts of the helical pattern of the two adjacent conductive wires. Accordingly, the centers of the six wires form a hexagonal pattern about the central non-conductive core filament as illustrated in the cross-sectional view of the six-wire cable in  FIG. 9 . 
     In the illustrated embodiment of the six-wire cable  800 , the R wire is positioned between the B 1  wire and the B 2  wire, and the three wires form a first group of wires. The G wire is positioned between the N 1  wire and the N 2  wire, and the three wires form a second group of wires. The B 1  wire is adjacent the N 2  wire, and the B 2  wire is adjacent to the N 1  wire. The numbering of the B wires and the numbering of the N wires is arbitrary in the embodiment described herein because each B wire performs the same function and each N wire performs the same function as will be apparent in the following description. The six conductive wires are wound tightly around the central core  830 . The inclusion of the central core prevents the six conductive wires from being forced inward during the twisting process. Thus, the six conductive wires retain the initial B 1 -R-B 2 -N 1 -G-N 2  configuration around the central core throughout the twisting process. The three wires in each group remain together over the length of the cable with the R wire positioned tightly between the B 1  and B 2  wires and with the G wire positioned tightly between the N 1  and N 2  wires. The six conductive wires also retain the desired configuration when wound about the transformer core  300  as described below. 
     The ease of winding the six-wire cable  800  is illustrated in  FIGS. 10 and 11  wherein the six-wire cable is wound onto the transformer core  300  in the form of a three-turn coil  1010  threaded through the first (upper) elongated through-bore  340  and the second (lower) through-bore  342  to form a transformer  1000  structure around the central core portion  360  of the core. For the purposes of the following discussion, the three-turn coil “starts” as it enters the second (lower) elongated through bore and “finishes” as it exits the first (upper) elongated through-bore. Accordingly, a respective first end segment of each of the six wires N 1 , N 2 , B 1 , B 2 , G, R of the six-wire cable at the start end of the cable is labeled with a suffix “S” (e.g., N 1 S, N 2 S, B 1 S, B 2 S, GS, RS). A respective second end segment of each of the six wires at the finish end of the cable is labeled with a suffix “F” (e.g., N 1 F, N 2 F, B 1 F, B 2 F, GF, RF). 
     The previously described transformer  500  required three turns each of two three-wire cables  510 ,  512  to be wound onto the transformer core, for a total of six winding turns. Unlike the transformer  500  of  FIG. 5 , the transformer  1000  of  FIG. 10  only requires the single three-turn single coil  1010  to be wound onto the transformer core. As shown in  FIGS. 10 and 11 , the three turns of the six-wire cable  800  in the single coil occupy substantially less longitudinal (e.g., left-to-right) space within the elongated through bores  340 ,  342  as compared to the six turns of the two three-wire cables described above. Thus, each of the three turns of the six-wire cable is positioned against the respective inner flat surfaces  356  of the through bores. 
     In addition to being easier to wind than the two three-wire cables  510 ,  512 , the single six-wire cable  800  may improve the balance or symmetry between the first and second groups of windings. As discussed above, the first group of windings comprises the B 1  wire and the B 2  wire along with the R wire. The R wire is positioned tightly between the B 1  wire and the B 2  wire. The second group of windings comprises the N 1  wire and the N 2  wire along with the G wire. The G wire is positioned tightly between the N 1  wire and the N 2  wire. The wiring positions of the two groups of wires achieve symmetrical coupling between the two groups of wires (e.g., the coupling from the B 1  and B 2  wires to the R wire is similar to the coupling from the N 1  and N 2  wires to the G wire). A further advantage is that the six wires of the six-wire cable twist in unison as the cable is threaded through the elongated through bores and around the front surface  318  and rear surface  320  of the transformer core. Thus, the six wires experience similar electromagnetic perturbations and other perturbations. 
     The advantages of the single six-wire cable  800  over the two three-wire cables  510 ,  512  provided by the common helical winding about the central non-conductive core  810  are offset in part by a reduced bandwidth. The first set of wires N 1 , G, N 2  are closely wound with respect to the second set of wires B 1 , R, B 2 . The close winding increases parasitic capacitive coupling between the two commonly wound sets of wires in comparison with the parasitic coupling between the two separately wound sets of wires in the two three-wire cables. The increased parasitic capacitive coupling may reduce the overall bandwidth of the transformer  1000  with respect to the transformer  500 . For example, the transformer  1000  wound with the six-wire cable may operate at a bandwidth up to approximately 1,200 MHz in comparison to the approximately 1,800 MHz bandwidth of the transformer  500  wound with the two three-wire cables. 
       FIG. 12  illustrates a basic schematic diagram of the transformer  1000  of  FIGS. 10 and 11 . As illustrated, the transformer comprises six windings wound onto the core  300 . A first winding  1200  comprises the N 1  wire between the start end segment N 1 S and the finish end segment N 1 F. A second winding  1210  comprises the N 2  wire between the start end segment N 2 S and the finish end segment N 2 F. A third winding  1220  comprises the B 1  wire between the start end segment B 1 S and the finish end segment B 1 F. A fourth winding  1230  comprises the B 2  wire between the start end segment B 2 S and the finish end segment B 2 F. A fifth winding  1240  comprises the R wire between the start end segment RS and the finish end segment RF. A sixth winding  1250  comprises the G wire between the start end segment GS and the finish end segment GF. 
     The six-wire cable  800  of  FIG. 8  can also be used with other transformer configurations. For example,  FIG. 13  illustrates a perspective view of a transformer  1300  in which the six-wire cable of  FIG. 8  is wound onto a toroidal core structure  1310 . The toroidal transformer configuration of  FIG. 13  includes the advantages of being able to wind all of the transformer windings in a single operation, as described above with respect to the transformer  1000  of  FIGS. 10 and 11 . 
       FIG. 14  illustrates an embodiment of a high data rate coupler system  1400  that incorporates the transformer  1000  of  FIGS. 10 and 11 . For example, the high data rate coupler may operate at bandwidths up to 1,200 MHz. 
     The coupler system  1400  of  FIG. 14  includes the transformer  1000  wound with the six-wire cable  800  of  FIGS. 8 and 9 , as described above. The coupler system further includes a toroidal choke  1410  comprising a toroidal core  1412  wound with a coil  1414  having a plurality of turns (e.g., three turns) of a three-wire cable. The toroidal choke is connected to the transformer as described below. Extended ends of the six-wire cable are selectively interconnected to interconnect the transformer and the choke and to form leads to the transformer. An enlarged view of a first set of interconnections is shown in  FIG. 15 . An enlarged view of a second set of interconnections is shown in  FIG. 16 . When interconnected as shown in  FIGS. 14-16 , the transformer and the toroidal choke form the electrical circuit illustrated schematically in  FIG. 17 . 
     In  FIG. 15 , the R wire and the G wire of the three-turn coil  1010  are truncated at the first (upper) through-bore  340  and at the second (lower) through bore  342  of the core  300  so that only the connections to the N 1  wire, the N 2  wire, the B 1  wire and the B 2  wire are shown. As shown in  FIG. 15  and as represented schematically in  FIG. 17 , the respective first end (start) segments N 1 S, N 2 S of the N 1  wire and the N 2  wire extending from the second (lower) elongated through-bore of the core  300  are twisted together to form a first two-wire cable  1420  with a twist density of between 16 and 20 twists per inch. The first two-wire cable formed by the first end segments N 1 S, N 2 S has a length extending from the three-turn coil of approximately 1 inch. The exposed distal ends (ends farthest from the three-turn coil) of the first end segments N 1 S, N 2 S are soldered or otherwise electrically connected together. As shown schematically in  FIG. 17 , the first two-wire cable forms a first outer lead  1432  of a primary winding  1430  of the center-tapped transformer  1000 . 
     As further shown in  FIG. 15  and as shown schematically in  FIG. 17 , the respective second end segments N 1 F, N 2 F of the N 1  wire and the N 2  wire extending from the first (upper) elongated through-bore  340  of the core  300  are twisted together with the respective first end segments B 1 S, B 2 S of the B 1  wire and the B 2  wire extending from the second (lower) elongated through-bore  342 . The four end segments N 1 F, N 2 F, B 1 S, B 2 S form a four-wire cable  1440  that is twisted with a twist density of between 16 and 20 TPI. The four end segments may have a length of approximately 1 inch. The exposed distal ends of the four end segments are soldered or otherwise electrically connected together. As shown schematically in  FIG. 17 , the four end segments form a center-tap lead  1442  of the primary winding  1430  of the transformer  1000 . 
     As further shown in  FIG. 15  and as shown schematically in  FIG. 17 , the respective second end segments B 1 , B 2 F of the B 1  wire and the B 2  wire extending from the first (upper) elongated through-bore  340  of the core  300  are twisted together to form a second two-wire cable  1450  with a twist density of between 16 and 20 twists per inch. The second two-wire cable formed by the second end segments B 1 F, B 2 F has a length extending from the three-turn coil of approximately 1 inch. The exposed distal ends of the second end segments B 1 F, B 2 F are soldered or otherwise electrically connected together. The second two-wire cable forms a second outer lead  1452  of the primary winding  1440  of the center-tapped transformer  1000 . 
     In  FIG. 16 , the extended portions of the R wire and the G wire of the three-turn coil  1010  are again shown. The extended portions of the N 1  wire, the N 2  wire, the B 1  wire and the B 2  wire are truncated at the first (upper) through-bore  340  and at the second (lower) through bore  342  of the core  300  so that the R wire and the G wire can be seen in  FIG. 16 . As shown in  FIG. 16  and as represented schematically in  FIG. 17 , the first end segment RS of the R wire extends from the second (lower) elongated through-bore  340  by a distance of approximately 0.15 inch to approximately 0.2 inch. Similarly, the second end segment GF of the G wire extends from the first (upper) elongated through-bore  342  by a distance of approximately 0.1 inch to approximately 0.15 inch. The distal ends of the end segment RS and the end segment GF are electrically connected to a first end of a third N wire. The third N wire (without a suffix) is not part of the six-wire cable  800  of the transformer  1000 . As shown in  FIG. 16 , the two end segments RF, GS and the end of the N wire form a center-tap  1462  of a secondary winding  1460  of the transformer. 
     As further shown in  FIG. 17 , the first end segment RS of the R wire forms a first outer lead  1464  of the center-tapped secondary winding  1460  of the transformer  1000 . The second end segment GF of the G wire forms a second outer lead  1466  of the secondary winding. The first end segment RS of the R wire and the second end segment GF of the G wire are twisted together with the third N wire to form a three-wire cable  1470  that extends from the transformer  1000  to the toroidal choke  1410 , which is spaced apart from the transformer by approximately 0.1 inch to 0.15 inch. In the illustrated embodiment, the three-wire cable is twisted together with a twist density of approximately 10 twists per inch. As illustrated in  FIG. 14 , the three-wire cable is wound around the toroidal core  1412  of the toroidal choke to form the three-turn toroidal coil  1414 . The three turns of the coil are distributed evenly over approximately 180 degrees of the circular core. As shown schematically in  FIG. 17 , the RS end segment of the R wire is wound into a first coil  1472  to form a first winding of the toroidal choke and the GF end segment of the G wire is wound into a second coil  1474  to form a second winding of the toroidal choke. The toroidal choke operates in a conventional manner to suppress common mode noise in the RS end segment of the R wire and the GF end segment of the G wire when the two wires form part of a data communications line. The N wire connected to the center-tap  1462  of the secondary winding  1460  of the transformer  1000  also passes through toroidal core as a third coil  1476  wound with the first and second coils. The N wire is electrically connectable to a source (or a destination) for a DC voltage that provides power over an Ethernet cable, as described, for example, in US Patent Application Publication No. 2016/0187951 A1 to Buckmeier et al., which published on Jun. 30, 2016, and which is incorporated by reference herein in its entirety. 
     In alternative embodiments, the N wire may be extracted from the three-wire cable  1470  prior to bypass the winding of the toroidal choke  1410  such that the toroidal core is wound with only two wires, the RS end segment of the R wire and the GF end segment of the G wire. In a further alternative configuration, if power over an Ethernet cable is not required, the N wire from the center tap of the secondary winding of the transformer can be eliminated such that the toroidal core is wound with only two wires, the RS end segment of the R wire and the GF end segment of the G wire and is only connected to the isolation transformer by the two end segments. 
     As illustrated in  FIGS. 14, 15 and 16 , the extended end segments of the six wires are continuous segments of the six-wire cable  800  forming the three-turn coil  1010 . The two outer leads  1432  and  1452  and the center-tap lead  1442  of the primary winding  1430  of the transformer  1000  only require electrical connections to other circuitry (not shown) into which the coupler system  1400  is incorporated. Similarly, the R wire and the G wire of the toroidal choke  1410  are uninterrupted continuations of the RS end segment of the R wire and the GF segment of G wire, respectively. The only electrical connection made within the immediate vicinity of the transformer is the electrical connection from the third N wire and the RF end segment of the R wire and the GS segment of the G wire. By eliminating the electrical interconnections between the wires within the transformer and the toroidal choke, the transformer is compact and simple to manufacture. Accordingly, the combination of the transformer core  300 , which has the elongated through-bores  340 ,  342 , and the six-wire cable  800 , which has all of the winding wires combined into a single compact cable provide substantial improvements in manufacturability and functionality. 
       FIGS. 18 and 19  illustrate a coupler system  1800 , which is similar to the coupler system  1400  of  FIGS. 14-17 , and which operates at a higher data rate. The coupler system of  FIGS. 18 and 19  is implemented with the transformer  500  of  FIGS. 5 and 6 , which incorporates the two three-wire cables  510 ,  512 . As described above, the N 1 S and N 2 S end segments of the two cables are connected together to form the first outer lead  704  of the primary winding  700 . The N 1 F, N 2 F, B 1 S and B 2 S end segments are connected together to form the center-tap  702  of the primary winding. The B 1 F and B 2 F end segments are connected together to form the second outer lead  706  of the primary winding. The RS end segment forms the first outer lead  714  of the secondary winding  710  of the transformer. The RF and GS end segments and an additional N wire form the center-tap  712  of the secondary winding. The GF end segment forms the second outer lead  716  of the secondary winding. The toroidal coil  1410  is implemented as described above by twisting the first outer lead, the second outer lead and the additional N wire together and winding the three wires onto the toroidal core  1412  to form the three coils of the toroidal choke. The coupler system of  FIGS. 18 and 19  may operate at bandwidths of 1,800 MHz in accordance with the requirements of the IEEE 802.3bq-2016 for a 40 GBaseT interface. 
     The multi-wire cable of  FIG. 8  can be configured to have additional conductive wires around the non-conductive core. For example,  FIG. 20  illustrates a cable  1900  comprising eight conductive wires  1920  helically around a non-conductive core  1910 . In the illustrated embodiment wherein the conductive wires are 38-gauge wires (e.g., approximately 0.0045 inch in diameter), the non-conductive core has a diameter of approximately 0.0073 inch, which is slightly larger than the diameter of a 34-gauge magnet wire. In  FIG. 20 , each helically wound wire is spaced apart angularly by 45 degrees from the two adjacent wires. As a further example,  FIG. 21  illustrates a cross-sectional view similar to the view of  FIG. 8  wherein the multi-wire cable comprises nine conductive wires  2020  around a non-conductive core  2010 . In the illustrated embodiment wherein the conductive wires are 38-gauge wires (e.g., approximately 0.0045 inch in diameter), the non-conductive core has a diameter of approximately 0.0087 inch, which is slightly larger than the diameter of a 32-gauge magnet wire. In  FIG. 21 , each helically wound wire is spaced apart angularly by 40 degrees from the two adjacent wires. 
     One skilled in art will appreciate that the foregoing embodiments are illustrative of the present invention. The present invention can be advantageously incorporated into alternative embodiments while remaining within the spirit and scope of the present invention, as defined by the appended claims.