Patent Publication Number: US-8987591-B2

Title: Communication channels with crosstalk-mitigating material

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to communication cables and more specifically directed to communication cables having layers of crosstalk-mitigating materials. 
     BACKGROUND OF THE INVENTION 
     Communication cables comprised of multiple twisted pairs of conductors are common, with four-pair cables being widely used. In high-speed data networks, crosstalk can result within communication cables and between nearby communication cables. Crosstalk occurring within a cable includes near-end crosstalk (NEXT) and far-end crosstalk (FEXT), and alien crosstalk occurring between cables includes alien near-end crosstalk (ANEXT) and alien far-end crosstalk (AFEXT). Suppression of alien crosstalk in communication channels is important, because alien crosstalk can reduce the signal-to-noise ratio in a communication channel and increase the channel&#39;s bit error rate. As communication bandwidth increases, the reduction of noise such as alien crosstalk in communication cables becomes increasingly important. 
     In high-bandwidth communication applications, communication cables are commonly installed alongside one another, and ANEXT and AFEXT can result between adjacent or nearby communication cables. ANEXT and AFEXT become more problematic at frequencies above 10 MHz, and ANEXT and AFEXT noise at high frequencies are present in high-speed data transmission systems such as 10 Gigabit Ethernet signaling. 
     Alien crosstalk includes the following: 
     1. Differential mode crosstalk produced by differential signals propagating in a twisted pair in one cable coupling to another twisted pair in another cable; 
     2. Common mode crosstalk produced by common mode signals propagating in one cable or external sources coupling to all wires in another cable. This coupled common mode signal can then convert to differential mode alien crosstalk. The conversion is typically produced by a wire pair imbalance or a connecting hardware imbalance. 
     3. Differential mode crosstalk produced by a differential signal propagating between two twisted pairs in one cable coupling to the wires in another cable either differentially or in common mode. This differential signal propagating via two twisted pairs in a cable is also called a “super pair mode,” which can be produced in connecting hardware due to a “split pair” (wires  3  and  6 ) coupling to wire pairs  1 - 2  and  7 - 8  forming a “super” twisted pair. 
     ANEXT and AFEXT arise due to electrical and magnetic couplings between conductors in different cables. The magnitude of ANEXT in twisted pair systems is proportional to the difference between the magnitude of the electrical coupling and the magnitude of the magnetic coupling (in the following formulas, “C” refers to coupling):
 
|ANEXT|=| C   (electric)   −C   (magnetic) |.
 
     In order to decrease ANEXT, the electrical and magnetic couplings can both be decreased. For example, assume that the C (electric) =Ce=0.25 and C (magnetic) =Cm=0.15, then the difference, Cd=0.1. If both couplings are reduced by an order of magnitude, then Ce=0.025 and Cm=0.015 and the difference would be Cd=0.01. NEXT can also be reduced by reducing the coupling giving rise to the larger of the two magnitudes. For example, again assuming that Ce=0.25 and Cm=0.15 corresponding to a Cd=0.1. If Ce is reduced by 20% or Ce=0.2 then Cd will be reduced to Cd=0.05. 
     AFEXT in twisted pair systems is found by determining the sum of the electrical coupling and the magnetic coupling:
 
|AFEXT|=| C   (electric)   +C   (magnetic) |.
 
In order to decrease AFEXT, either or both of the electrical coupling and the magnetic coupling should be reduced.
 
     It is desirable to reduce alien crosstalk. It is particularly desirable to achieve this reduction in a way that addresses the electrical and magnetic couplings that give rise to alien crosstalk. 
     SUMMARY OF THE INVENTION 
     Improved communication cables are provided with a layer of crosstalk-mitigating material having discrete conductive areas. 
     According to one embodiment of the present invention, a cable core comprising four twisted pairs of conductors is surrounded with a layer of crosstalk-mitigating material having discrete conductive areas. 
     According to some embodiments of the present invention, the layer of crosstalk-mitigating material having discrete conductive areas comprises a semiconductive foil having discrete conductive areas placed thereon. 
     According to some embodiments of the present invention, the layer of crosstalk-mitigating material having discrete conductive areas comprises a highly electrically resistive layer having discrete conductive areas placed thereon. 
     According to some embodiments of the present invention, a crosstalk-mitigating material comprises a thin resistive layer of metal. 
     According to another embodiment of the present invention, a crosstalk-mitigating material comprises a thin resistive layer of metal having discrete conductive areas placed thereon. 
     According to different embodiments of the present invention, crosstalk-mitigating materials are used to surround: (a) an entire cable core; (b) each of the twisted pairs within the cable; or (c) a subset of twisted pairs within the cable. According to some embodiments of the present invention, crosstalk-mitigating material surrounds both the entire cable core and either each of the twisted pairs within the cable, or a subset of twisted pairs within the cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing two adjacent communication cables according to the present invention; 
         FIG. 2  is a plan view of a crosstalk-mitigating material having discrete conductive areas according to one embodiment of the present invention; 
         FIG. 3  is a plan view of a crosstalk-mitigating material having discrete conductive areas according to another embodiment of the present invention; 
         FIG. 4  is a cross-sectional side view of a segment of crosstalk-mitigating material according to another embodiment of the present invention; 
         FIG. 5  is a perspective view of a crosstalk-mitigating material according to one embodiment of the present invention; 
         FIG. 6  is a cross-sectional side view of a segment of crosstalk-mitigating material according to one embodiment of the present invention; 
         FIG. 7  is a cross-sectional side view of a segment of crosstalk-mitigating material according to another embodiment of the present invention; 
         FIG. 8  is a cross-sectional side view of a segment of crosstalk-mitigating material having a protective layer; 
         FIG. 9  is a cross-sectional side view of a segment of crosstalk-mitigating material according to another embodiment of the present invention; and 
         FIG. 10  is an illustration showing the assembly of a crosstalk-mitigating material according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     ANEXT and AFEXT can result from unbalanced coupling from conductive pairs in one cable to another cable or from balanced couplings that get converted to differential signals within the cabling. 
     The present invention is directed to a cable construction that reduces ANEXT and AFEXT between adjacent cables by addressing the electrical and magnetic interactions that give rise to ANEXT and AFEXT.  FIG. 1  is a cross-sectional view of first and second cables  10  and  12  according to one embodiment of the present invention. The first cable  10  has four twisted wire pairs  14   a ,  14   b ,  14   c , and  14   d . The second cable has four twisted wire pairs  16   a ,  16   b ,  16   c , and  16   d . In the embodiment shown, the twisted pairs of each cable are separated by a crossweb  18 . It is to be understood that in other embodiments of the present invention, other types of separators—or no separator at all—may be employed. 
     The twisted pairs in each cable  10  and  12  comprise cable cores, and are surrounded by a layer  20  of a crosstalk-mitigating material. The layer  20  of crosstalk-mitigating material may be placed inside of the cable jacket (not shown). One embodiment of a crosstalk-mitigating material  21  according to the present invention is shown in  FIG. 2 . In the embodiment of  FIG. 2 , the crosstalk-mitigating material  21  consists of a substrate  22  having conductive areas  24  overlaid thereon. 
     According to one embodiment of the present invention, the substrate  22  is made of a highly electrically resistive material such as a plastic, and the conductive areas  24  are made of a highly electrically conductive material. This combination of materials primarily reduces magnetic coupling that gives rise to alien crosstalk, but also to a lesser extent reduces capacitive coupling. The crosstalk-mitigating material  21  has beneficial effects on the magnetic coupling because of the loss due to eddy currents  26  (as shown in  FIG. 2 ) formed within the conductive areas  24  by the magnetic fields B of the twisted wire pairs. The conductivity of the material used in the conductive areas  24  can determine the level of the reduction in magnetic coupling. 
     Crosstalk-mitigating materials similar to the crosstalk-mitigating material  21  shown in  FIG. 2  can be made using a variety of different dimensions and shapes for the conductive areas. For example, according to one embodiment of the present invention, conductive areas may be 0.2 inch×0.3 inch rectangles, with 0.005 inches between rectangles. According to other embodiments of the present invention, the conductive areas may be made of different shapes such as regular or irregular polygons, other irregular shapes, curved closed shapes, isolated regions formed by conductive material cracks, and/or combinations of the above.  FIG. 3  shows an alternative crosstalk-mitigating material  28  in which a substrate  22  is overlaid with hexagonal conductive areas  30 . Similarly to the crosstalk-mitigating material  21 , the hexagonal conductive areas  30  result in eddy currents  26  when acted upon by a magnetic field B. 
     In the embodiments of  FIGS. 2 and 3 , the material for the conductive areas  24  and  30  may be selected from a range of metals, including such metals as copper, aluminum, and silver. The material for the substrate  22 , and for other substrates according to other embodiments, may be a plastic. Examples of plastics according to some embodiments include polyimide, polyester, polypropylene, polyethylene, PVC (polyvinyl chloride), PTFE (polytrifluoroethylene), and foamed variances of these materials. The thicknesses of the conductive areas  24  and  30  may range from about 0.2 μm to about 0.8 μm. The thickness of the substrate  22  may range from about 0.5 mils to about 15 mils. Other thicknesses for both the conductive areas  24  and the substrate  22  may be selected based on desired physical and electromagnetic characteristics for particular implementations. According to some embodiments, the materials and thickness of the conductive areas  24  may be chosen to provide a sheet resistance ranging from about 1 mΩ/sq. to about 10 mΩ/sq. 
     Other types of crosstalk-mitigating material may be used in different embodiments of the present invention.  FIG. 4  is a cross-sectional view of a segment of a crosstalk-mitigating material  32  comprising a dielectric layer  34  and a thin metal layer  36 . Similarly to the crosstalk-mitigating material  21 , described above, the dielectric layer may comprise a plastic. The thin metal layer  36  may comprise a metal such as aluminum, copper, silver, chromium, or other metals. According to some embodiments, the thin metal layer  36  has a thickness of between about 1 nm and about 5 nm. The thickness of the dielectric layer  34  may be between about 1 mil and about 15 mils, with thicknesses from about 10 mils to about 15 mils being useful in some embodiments. Other thicknesses for both the thin metal layer  36  and the dielectric layer  34  may be selected based on desired physical and electromagnetic characteristics for particular implementations. The materials and thickness of the thin metal layer  36  may be chosen to provide a sheet resistance ranging from about 1 kΩ/sq. to about 20 kΩ/sq. 
     The twisted pairs of a cable assembly couple both magnetically and electrically (i.e., capacitively) to neighboring cable assemblies via the resistive cross-talk mitigating material surrounding each of the cable assemblies.  FIG. 1  illustrates an electrical effect of a crosstalk-mitigating layer  20  using capacitive indicators to show capacitive coupling. The embodiment of  FIG. 1  will now be described, in which the layer  20  is the crosstalk-mitigating material  32  of  FIG. 4 . Since the sheet resistance of the crosstalk-mitigating material  32  is large, the magnetic coupling between the cables will be minimally affected. However, the electrical capacitive coupling between the cables will be reduced. This reduction occurs due the charge buildup on the resistive material  32  due to the electric field resulting from the twisted pairs. This induced charge is distributed longitudinally along the length of the cable assembly due to the propagating electromagnetic waves within the twisted pairs. This induced charge also moves according to the charge difference that occurs longitudinally along the crosstalk mitigating material along the cable as well as around its circumference. As this induced charge re-distributes itself, its charge density is reduced which reduces the capacitive coupling between the cables  10  and  12 . The crosstalk-mitigating material  32  primarily reduces the capacitive (or “electrical”) coupling, but also to a lesser extent reduces the magnetic coupling between twisted pairs in different cables. Additionally, the crosstalk-mitigating material  32  increases the attenuation of the signal that is propagating within the cable containing the “super pair.” 
       FIG. 5  is a perspective view of a segment of crosstalk-mitigating material  40  according to another embodiment of the present invention. The crosstalk-mitigating material  40  comprises a substrate  42 , a thin metal layer  44 , and conductive areas  46 . The substrate  42  is overlain with the thin metal layer  44 , and the conductive areas  46  are placed atop the thin metal layer  44 . As with other embodiments of crosstalk-mitigating materials according to the present invention, the crosstalk-mitigating material  40  is designed to be wrapped around: (a) a cable core comprising a plurality of twisted wire pairs; (b) one or more twisted wire pairs within a cable core; or (c) both a cable core and one or more twisted pairs within the core. According to some embodiments, the conductive areas  46  may comprise a metal selected from a variety of metals such as aluminum, copper, and silver. The thin metal layer  44  may comprise a metal selected from a variety of metals such as aluminum, copper, silver, and chromium. In other embodiments, different metals or combinations of metals may be selected for the thin metal layer  44  and the conductive areas  46 . Similarly to the embodiments of  FIGS. 2 and 3 , above, the conductive areas  46  may be sized and shaped in a variety of ways in order to achieve particular structural, electrical, and magnetic characteristics. 
       FIG. 6  is a cross-sectional view of a segment of the crosstalk-mitigating material  40 , showing the substrate  42 , the thin metal layer  44 , and the conductive areas  46 . The thin metal layer  44  has a thickness, t m  of from about 1 nm to about 5 nm. The conductive areas  46  have a total depth, d c , from about 0.2 μm to about 0.8 μm. 
       FIG. 7  is a cross-sectional view of a segment of crosstalk-mitigating material  50 . The specifications of the crosstalk-mitigating material  50  are similar to those of crosstalk-mitigating material  40  of  FIG. 6 , except that the conductive areas  48  have rounded corners. 
     In some embodiments of the present invention, if foil-shielded twisted pairs are being implemented, and if a thin substrate is used for a crosstalk-mitigating material, a “substrate-metal layer-substrate” construction should be used for the crosstalk-mitigating material in order to keep the crosstalk-mitigating material away from the twisted pairs. If foil-shielded twisted pairs are being implemented, and if a thicker substrate is used for the crosstalk-mitigating material, a “metal layer-substrate” construction should be used in which the metal layer of the crosstalk-mitigating material is farther than the substrate layer from the twisted pairs. 
       FIG. 8  is a cross-sectional view of a crosstalk-mitigating material  52  in which a protective covering  54  is used to prevent the metal surfaces from corroding or oxidizing. Techniques for providing the protective covering  54  may include tin or silver plating of the top surface, or placing a plastic film on top of the metal. 
       FIG. 9  shows a crosstalk-mitigating material  56  according to another embodiment of the present invention. The embodiment of  FIG. 9  features a semiconductive substrate  58  with conductive areas  60  placed thereon. According to one embodiment, the sheet resistance of the semiconductive substrate  58  may be selected from a range of from about 1 kΩ/sq. to about 20 kΩ/sq. Similarly to the other embodiments described herein, the conductive areas  60  may be provided in a variety of sizes and shapes. 
       FIG. 10  shows a process for manufacturing an alternative crosstalk-mitigating material  60 . The crosstalk-mitigating material  60  comprises first and second outer substrate layers  62  and  64 , a thin metal layer  66 , and conductive areas  68 . The depth of the conductive areas is shown as d c ′. Before assembly of the crosstalk-mitigating material  60 , the thin metal layer  66  is on the first outer substrate layer  62 , and the conductive areas  68  are on the second outer substrate layer  64 . The two sub-assemblies are combined as shown into the crosstalk-mitigating material  60 . 
     According to different embodiments of the present invention, crosstalk-mitigating materials are used to surround: (a) an entire cable core; (b) each of the twisted pairs within the cable; or (c) a subset of twisted pairs within the cable. According to some embodiments of the present invention, crosstalk-mitigating material surrounds both the entire cable core and either each of the twisted pairs within the cable, or a subset of twisted pairs within the cable. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein, and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention.