Patent Publication Number: US-7897872-B2

Title: Spirally wound electrical cable for enhanced magnetic field cancellation and controlled impedance

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electrical cable, and in particular, to a spirally wound cable that enhances the cancellation of magnetic fields and better controls differential mode impedance and common mode impedance. 
     2. Background Information 
     Twisted pair cable is a form of wiring in which two signal conductor wires are wound together for the purpose of canceling out electromagnetic interference (EMI) from external sources, electromagnetic radiation from the cable, and/or crosstalk between signal conductor pairs. The typical twisted pair cable operates in a differential mode where the two wires carry equal and opposite signals, and in a common mode where the two wires carry equal signals in the same direction. 
     The twisted pair cable generates magnetic fields around each of the two conductor wires. So as to produce equal and opposite magnetic fields that cancel each other out, the two wires should be uniformly wound. If the two wires in the given twisted pair cable are not identically wound, generated magnetic fields will not be exactly equal and opposite and so will not exactly cancel. However, it is difficult to twist the two wires in the twisted pair cable uniformly or precisely throughout the cable. 
     When the two wires are driven differentially, i.e., in the differential mode, signals propagating along the wires will encounter differential mode impedance. The differential mode impedance is determined by the size of the wires, the spacing between the two wires, and type of dielectric used between them. However, the twisted pair cable includes individual loosely paired wires that are hard to wind uniformly. Due to the non-uniform winding of the two wires, the distance between the two wires may not be identical along the twisted pair cable. Accordingly, the twisted pair cable may have imprecise differential mode impedance per unit length of the cable. 
     The typical balanced twisted pair cable does not have a return wire. When the two balanced twisted pair wires are unbalanced and carry a common mode current, the wiring experiences common mode impedance to the earth, commonly known as displacement capacitance. Also, in the twisted pair cable, the two signal conductor wires are twisted about each other. Further, due to the non-uniform twisting of the two wires, some portions may be tightly twisted, while other portions loosely twisted, and the number of twists per unit distance may not be uniform throughout its total length, resulting in unequal cancellation of the magnetic fields. 
     Further, in the twisted pair cable, an insulative jacket surrounds each twisted signal conductor pair. For example, a first insulative jacket may surround a first signal wire, while a second insulative jacket may surround a second signal wire. Therefore, between the first signal wire and the earth, the twisted pair cable may have the first insulative jacket on the first wire and the second insulative jacket on the second signal wire, with varying dielectric thickness. The non-uniform distance between the pairs, inconsistent twisting, and varying dielectric thickness make it difficult to control the common mode impedance per unit length of the cable. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an electrical cable is provided, which incorporates two spirally wound signal wires. The electrical cable includes a pair of signal conductor wires, a first dielectric layer, a second dielectric layer, a third dielectric layer, and a central conductor core. The two signal conductor wires are spirally disposed on an inner surface of the first dielectric layer, are separated from each other by the second dielectric layer. The signal conductor wires and the second dielectric layer between the wires may surround the third dielectric layer that surrounds the central conductor core. The electrical cable may include multiple pairs of signal conductor pairs. 
     In another exemplary aspect of the invention, the two wires are spaced apart from each other and aligned parallel to each other, allowing a specified distance between the two wires. The two wires may be formed on an inner surface of the first dielectric layer by a photolithography process. Then, the first dielectric layer and the two wires thereon may be wound together to form a spiral collectively. Such a spiral configuration allows precise spiral winding and precise location of the two signal conductor wires within the electrical cable, cancelling differential magnetic fields generated by each signal conductor wire. 
     In a further exemplary aspect of the invention, the second dielectric layer disposed between the two wires may be made of a flexible dielectric material, air or gas. The second dielectric layer is added between the two spiral wires, so that it fills spaces between the two spiral wires. Alternatively, the first dielectric layer, the two wires on the first dielectric layer, and the second dielectric layer between the two wires can be wound together to collectively form a spiral. 
     In a further exemplary aspect of the invention, the third dielectric layer surrounds the central conductor core and disposed between the central conductor core and the signal conductor wires. The third dielectric layer may be made of a plastic material, such as polyethylene. The central conductor core functions as a ground return path, and is made of copper, tin-plated copper, or other metal. 
     Impedances between the signal conductor wires are controllable by setting spacing between the signal wires, spacing of the signal wires from the central conductor core, and dielectric characteristics of the second and third dielectric layers. Further, the spiral configuration of the signal conductor wires allows a uniform winding rate of the wires per unit length of the cable. 
     In a further exemplary aspect of the invention, the electric cable further includes a fourth dielectric layer surrounding the first dielectric layer, a shield layer surrounding the fourth dielectric layer, and a jacket surrounding the shield layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a spirally wound cable, according to an exemplary embodiment of the present invention. 
         FIG. 2  is a schematic perspective view, illustrating a configuration of two signal conductor wires and a first dielectric layer in the cable shown in  FIG. 1 . 
         FIG. 3  is a schematic perspective view, illustrating a configuration of a second dielectric layer in the cable shown in  FIG. 1 . 
         FIG. 4  is a schematic perspective view, illustrating a configuration of a third dielectric layer in the cable shown in  FIG. 1 . 
         FIG. 5  is a schematic perspective view, illustrating a configuration of a central conductor core in the cable shown in  FIG. 1 . 
         FIG. 6  is a schematic perspective view, illustrating a configuration of a fourth dielectric layer, a shield layer and a jacket in the cable shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described in more detail by way of example with reference to the embodiments shown in the accompanying figures. It should be kept in mind that the following described embodiments are only presented by way of example and should not be construed as limiting the inventive concept to any particular physical configuration. 
     Further, if used and unless otherwise stated, the terms “upper”, “lower”, “front”, “back”, “over”, “under”, and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis. 
       FIG. 1  is a cross-sectional view of a spirally wound cable  10 , according to an exemplary embodiment of the present invention. The cable  10  includes a pair of signal conductor wires  12   a  and  12   b , a first dielectric layer  14 , a second dielectric layer  16 , a third dielectric layer  18 , a central conductor core  20 , a fourth dielectric layer  22 , a shield layer  24  and an outer jacket  26 . 
     The two signal conductor wires  12   a  and  12   b  are disposed on an inner surface of the first dielectric layer  14 , are separated from each other by the second dielectric layer  16 , and surround the third dielectric layer  18  that surrounds the central conductor core  20 . The signal conductor wires  12   a  and  12   b  may be made of copper, other metal, or other conductive materials. The cable  10  is not limited to a single pair of signal conductor wires, but may include multiple pairs of signal conductor pairs. 
     The first dielectric layer  14  is a base on the inner surface of which the signal conductor wires  12   a  and  12   b  are disposed. The first dielectric layer  14  may concentrically surround the central conductor core  20 . The first dielectric layer  14  may be made of a plastic material, such as polyethylene, or any other dielectric material, and insulates the signal conductor wires  12   a  and  12   b  from the shield layer  24 . 
     As shown in  FIG. 2 , the two wires  12   a  and  12   b  are spirally disposed on the inner surface of the first dielectric layer  14 , and are spaced apart from each other and aligned parallel with each other, allowing a specified distance between the two wires  12   a  and  12   b . According to an exemplary embodiment of the invention, the two wires  12   a  and  12   b  may be formed by a photolithography process which allows precise location of the wires  12   a  and  12   b.    
     The photolithography process may include a step of providing a dielectric layer for forming the first dielectric layer  14 , and a step of precisely removing upper portions of the dielectric layer to form regions in which the two wires  12   a  and  12   b  are to be placed, by etching the dielectric layer in the areas that are not protected by a photoresist. Then, the wires  12   a  and  12   b  may be placed in the regions of the dielectric layer. In this way, the precise location of the wires  12   a  and  12   b  may be achieved. Then, the dielectric layer may be further etched to form the first dielectric layer  14 . Then, the first dielectric layer  14  and the signal conductor wires  12   a  and  12   b  may be collectively wound to form a spiral. The spiral configuration of the signal wires allows the manufacturing of the cable through the photolithography process. The photolithograph process, combined with a subsequent winding, allows precise spiral location and precise spiral winding of the two signal conductor wires  12   a  and  12   b  within the cable  10 . Due to the precise spiral location and winding, the differential magnetic field will cancel. 
     The second dielectric layer  16  is disposed between the two wires  12   a  and  12   b , as shown in  FIGS. 1 and 3 . The second dielectric layer  16  may be made of any dielectric material, such as a flexible dielectric material, air or gas. Air or gas may be appropriate for maximizing propagation velocity within the cable  10 . The second dielectric layer  16  can be formed in various ways. 
     According to an exemplary embodiment of the invention, the second dielectric layer  16  can be added between the two spiral wires  12   a  and  12   b , after providing a spiral including the first dielectric layer  14  and the two spiral wires  12   a  and  12   b . The second dielectric layer  16  may be provided by filling the space between the two spiral wires  12   a  and  12   b.    
     According to another exemplary embodiment of the invention, the second dielectric layer  16  can be wound together with the first dielectric layer  14  and the signal conductor wires  12   a  and  12   b , so as collectively to form a spiral. In detail, a flat dielectric layer may be prepared, followed by a precise removing of upper portions of the dielectric layer in areas to form regions to receive the two wires  12   a  and  12   b . The removing process may be performed by etching the dielectric layer in the areas that are not protected by a photoresist. Then, the two wires  12   a  and  12   b  may be placed in the receiving regions of the dielectric layer. Further etching may be performed to remove rest upper portions of the dielectric layer between the two wires, forming the first dielectric layer  14 . Then, the second dielectric layer  16  may be deposited on the first dielectric layer  14  between the two wires  12   a  and  12   b . The etching and deposition of layers may be performed through the photolithography process. Then, the second dielectric layer  16 , the two wires  12   a  and  12   b , and the first dielectric layer  14  may be wound together to collectively form a spiral. 
     According to a further exemplary embodiment of the invention, the second dielectric layer  16  can be added on the first dielectric layer  14 , before the signal conductor wirings  12   a  and  12   b  are placed on the first dielectric layer  14 . The first and second dielectric layers  14  and  16  may be flat in shape until a subsequent winding process is performed. Then, a portion of the second dielectric layer  16  is removed, by etching the second dielectric layer  16 . Through such etching, regions are formed to receive the pair of signal conductor wirings  12   a  and  12   b . Then, the two wires  12   a  and  12   b  are disposed in the receiving regions. The etching and deposition of layers may be performed through the photolithography process. Then, the second dielectric layer  16 , the two wires  12   a  and  12   b , and the first dielectric layer  14  may be wound together to collectively form a spiral. 
     According to a further embodiment of the invention, the cable  10  may be filled with air, gas or other dielectric materials that are capable of flowing within the confines of the cable  10 . 
     When the cable  10  includes multiple pairs of signal conductor wires, each pair of the signal conductor wires may be spaced from an adjacent pair with a dielectric layer that may be made of the same material as that of the second dielectric layer or any other dielectric material, such as plastics, air and/or gas. The cable  10  may further include guard traces (not shown) between each pair of the signal conductor wires, so as to avoid interferences between the pairs. 
     The third dielectric layer  18  surrounds the central conductor core  20 , and is disposed between the central conductor core  20  and the signal conductor wires  12   a  and  12   b . As shown  FIGS. 4 and 5 , the third dielectric layer  18  may be formed as a cylinder, and may be inserted inside a layer formed by the combination of the signal conductor wires  12   a  and  12   b  and the second dielectric layer  16 . However, the third dielectric layer  18  may be formed first, and, then, the combination of the signal conductor wires  12   a  and  12   b  and the second dielectric layer  16  may be wound on the third dielectric layer  18 . 
     The central conductor core  20  may be inserted into a hollow of the third dielectric layer  18 . Alternatively, the central conductor  20  may be initially provided, and, then, the third dielectric layer  18  may be formed to surround the central conductor core  20 . The third dielectric layer  18  may be made of a plastic material, such as polyethylene, or any other dielectric material. The central conductor core  20  may function as a ground return path, and may be made of copper, tin-plated copper, other metal, or any other conductive material. 
     Impedances between the signal conductor wires  12   a  and  12   b , including a differential mode impedance and a common mode impedance, are set by, e.g., spacing between the signal wires  12   a  and  12   b , a dielectric material between the wires  12   a  and  12   b , spacing of the wires  12   a  and  12   b  with respect to the central conductor core  20 , and a dielectric material between the core  20  and the signal conductor wires  12   a  and  12   b . The cable  10  with the spirally wound signal wires  12   a  and  12   b  allows precise spacing between the signal wires  12   a  and  12   b . In addition, the second dielectric layer  16  between the two wires  12   a  and  12   b  may be made of a material with appropriate dielectric characteristics, e.g., a desired dielectric constant, so as to control the impedances. The third dielectric layer  18  between the core  20  and the signal conductor wires  12   a  and  12   b  may also be made of a material with appropriate dielectric characteristics, e.g., a desired dielectric constant. The spacing of the signal wires  12   a  and  12   b  from the core  20  is easily controllable by a thickness of the third dielectric layer  18 . Accordingly, the common mode impedance of the wires  12   a  and  12   b  with regard to the ground return core  20  and the differential mode impedance of the wires  12   a  and  12   b  with regard to each other are both easily controllable. 
     Further, the two wires  12   a  and  12   b  may be formed using, e.g., the photolithography process, which allows a uniform winding rate of the wires  12   a  and  12   b  per unit length of the cable  10  as well as precise spacing between the wires  12   a  and  12   b , thereby ensuring uniformity of the impedances per unit length of the cable  10 . 
     The fourth dielectric layer  22  surrounds the first dielectric layer  16 , the shield layer  24  surrounds the fourth dielectric layer  22 , and the jacket  26  surrounds the shield layer  24 , as shown in  FIGS. 1 and 6 . The fourth dielectric layer  22  may be made of a plastic material, such as polyethylene, or any other dielectric material. In an exemplary embodiment of the invention, the material forming the fourth dielectric layer  22  may have such dielectric characteristics that the fourth dielectric layer  22  combined with the first dielectric layer  16  gives desired dielectric and insulating characteristics between the shield layer  24  and the signal conductor wires  12   a  and  12   b . In another exemplary embodiment of the invention, the cable  10  may not include the fourth dielectric layer  22 , which achieves a simple structure and may save on manufacturing costs. 
     The shield layer  24  may be made of copper, other metal, or other shielding material, and may be formed as a foil or a braid. The shield layer  24  shields the cable  10  and prevents EMI from emitting out of the cable  10 . However, for the purpose of providing a high degree of flexibility or other purposes, the cable may be unshielded. Thus, in another exemplary embodiment of the invention, the cable  10  may not include the shield layer  24 . The jacket  26  may be made of a plastic material such as polyethylene, or any other insulative material. 
     It should be understood, however, that the invention is not necessarily limited to the specific arrangement and components shown and described above, but may be susceptible to numerous variations within the scope of the invention. 
     It will be apparent to one skilled in the art that the manner of making and using the claimed invention has been adequately disclosed in the above-written description of the preferred embodiments taken together with the drawings. It will be understood that the above description of the preferred embodiments of the present invention are susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.