Abstract:
A cable connector assembly for receiving a shielded cable assembly includes a conductive connector shield and an impedance operable to couple the connector shield to a shield of the shielded cable assembly. In an embodiment, the connector shield is positioned such that the connector shield does not directly contact the cable assembly shield when the cable assembly is received by the connector assembly.

Description:
BACKGROUND 
   In the Local-Area-Networking (LAN) industry, Unshielded-Twisted-Pair (UTP) cables are predominant. UTP cables are used because the twisting provides high immunity to electromagnetic interference (EMI) and electromagnetic compatibility (EMC). Further, because they are unshielded, UTP cables provide isolation between stations that might have unequal ground potentials and thus prevent ground loops between such stations. 
   For 10-Gigabit-Ethernet-on-UTP solutions being considered by the IEEE, there is a problem associated with UTP that may limit feasibility and increase cost of the silicon being considered. This problem is typically referred to as Alien-Near-End Crosstalk (ANEXT) which is noise/interference that comes from electrical signals on adjacent UTP cables and typically cannot be eliminated by a transceiver using correlated noise cancellation circuits. 
   In addition, Near-End Crosstalk (NEXT) caused by adjacent transmitters also causes interference, but is typically less of a problem as associated electrical signals are correlated to a reference source that a transceiver can use to cancel much of the interference. 
   There are types of shielded cables that would substantially reduce the effects of this interference, but because the shielding is connected to ground at both ends of a cable, these cables may generate ground loops. 
   A ground loop exists when two pieces of equipment, which are on different power circuits and are referenced to different ground potentials, are connected together with a cable having a shield that connects the equipment shields&#39; grounds together with low DC impedance. 
   Because of this problem with ground loops, the US LAN industry has traditionally supported UTP. In Europe, where Shielded-Twisted-Pair (STP) cables are more common, extensive management of power grids (to maintain equal ground potential from one location to another) is typically required to suppress ground looping. Europe has also adopted a 100-ohm UTP look-alike cable that contains a light foil shield (FTP) and that utilizes a common RJ-45 connector and is field terminable. However, because it is shielded, the UTP look-alike cable typically has the same problems as STP cables with respect to ground loops. 
   Referring to  FIGS. 1 and 2 , a conventional shielded modular plug  10  for terminating a shielded multi-pair communication cable  14  is illustrated. Cable  14  comprises an insulating sheath  16  enclosing a conductive cable shield  15  that, in turn, encloses four pairs of conductors or wires  18 , each wire pair or signal pair twisted together (not shown) and forming a respective signal path during use. The construction of plug  10  is well known and generally comprises a dielectric housing  19  having a closed forward free end  22 , a cable-receiving rearward end  24 , a terminal receiving side  26  and a cable-receiving cavity (not shown) extending longitudinally from the rearward end  24  of the housing  19  to the terminal receiving side  26 . 
   The plug  10  further includes a conductive shield portion  20  that electrically contacts the cable shield  15  when, as seen in  FIG. 2 , the plug  10  receives the cable  14 . Eight parallel slots  28  defined by corresponding fins  29  open on to the terminal-receiving side  26  of housing  19  for receiving flat contact terminals  30 . The eight slots  28  are aligned over a planar array of respective longitudinally extending wire-receiving parallel passages (not shown) which communicate with the cable-receiving cavity and which receive the ends of respective cable wires  18 . Each flat contact terminal  30  is inserted into and fixed within an associated terminal-receiving slot  28  to terminate a respective wire  18  located in a respective wire-receiving passage. 
   SUMMARY 
   According to an embodiment of the present invention, a cable connector assembly for receiving a shielded cable assembly comprises a conductive connector shield and an impedance operable to couple the connector shield to a shield of the shielded cable assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a conventional modular plug and multi-pair cable prior to termination according to the prior art; 
       FIG. 2  is a top partial-cutaway view of the conventional modular plug and multi-pair cable of  FIG. 1  in terminal combination; 
       FIG. 3  is a top partial-cutaway view of a modular plug and multi-pair cable in combination according to an embodiment of the present invention; and 
       FIG. 4  is a schematic diagram of an electronic system according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  illustrates a combination of the shielded multi-pair communication cable  14  of  FIG. 2  and a shielded modular plug  40  according to an embodiment of the present invention. As is the case with the prior-art plug-cable combination illustrated in  FIGS. 1 and 2 , the plug  40  includes contact terminals  30  that terminate respective wires  18  associated with the cable  14 . As can be seen in  FIG. 3 , the plug  40  similarly has disposed thereon a conductive shield portion  50  that is arranged on a dielectric housing  55  such that the conductive shield portion  50  does not contact the cable shield  15  when the cable  14  mates with the plug  40 . Disposed within the housing  55  and contacting the cable shield  15  is a conductive element  60  which, in one embodiment, is annular. In alternative embodiments, the conductive element  60  may be of any other shape or configuration suitable for providing an electrical contact with the cable shield. For example, the element  60  may comprise multiple conductive portions. 
   Attached to the conductive element  60  by respective contact terminals  62  and  64  are a capacitor  70  and a resistor  80 . The capacitor  70  and the resistor  80  also contact the shield portion  50  by terminals  82  and  84 , respectively. As such, the capacitor  70  and resistor  80  are positioned electrically in parallel between the cable shield  15  and the shield portion  50 . In one embodiment, the capacitor  70  and resistor  80  are embedded in the housing  55 , the capacitor  70  has a value C of approximately 0.01 μF, and the resistor has a value R of approximately 2 MΩ. The contact terminals  62 ,  82 ,  64 ,  84  are, in one embodiment, made from an elastic conductive material, such as stainless steel, so as to allow relative movement between the shield portion  50  and conductive element  60  without compromising contact with each. 
   In operation, the resistor  80  enables a relatively small discharge current to flow between the cable shield  15  and ground (via the shield portion  50 ) when the plug  40  is coupled to an electronic device, such as a computer (not shown). Such a small discharge current prevents static-charge buildup on the cable shield  15 . Moreover, by positioning the capacitor  70  between the cable shield  15  and ground, a low-AC impedance connection is created thereby allowing the shield  15  to provide optimal shielding from EMI and EMC. Put another way, the resistor  80  limits to a safe level a DC current that flows between two grounds (at the ends of the cable  14 ) that are at unequal potentials, but the capacitor  70  grounds the shield  15  for AC signals, particularly for signals that contain AC frequencies that may cause interference. 
   In an alternative embodiment illustrated in  FIG. 3  that excludes the capacitor  70 , a capacitance  85  is formed from a combination of the conductive element  60  and a flange  86  (shown in broken lines) of the shield portion  50 . That is, the element  60  and flange  86  form respective plates of a capacitor having a capacitance  85 . The region between these plates may be filled with air or another dielectric, as is known. The capacitance  85  functions in a manner similar to that described above in connection with the capacitor  70 . 
     FIG. 4  illustrates an electronic system  87  according to an embodiment of the present invention. The electronic system  87  may, for example, be a LAN, or any other system utilizing electrical signals. The electronic system  87  comprises devices  90  and  91  that communicate via a transmission medium  95 , which includes the cable  14  and plugs  40  and  40 ′. At least the device  90  includes a processor  92 , and the devices  90  and  91  may be, e.g., personal computers or computer workstations, testing devices, or set-top boxes configured to deliver media to a display device. Alternatively, the device  90  may be an oscilloscope, in which case the device  91  may be a probe assembly as known in the art. 
   The electronic system  87  further comprises the signal-transmission medium  95  coupled to the devices  90  and  91  as described above. The signal-transmission medium  95  comprises the combination of the cable  14  with, at one end of the cable  14 , the plug  40  illustrated in and discussed with reference to  FIG. 3 , and, at the other end of the cable  14 , a plug  40 ′ similar to the plug  40 . 
   By employing the plugs  40  and  40 ′, the electronic system  87  is minimally susceptible to problems associated with ground loops. For example, suppose, as illustrated in  FIG. 4 , the difference in ground potential between devices  90  and  91  is 2V and the resistance of the cable  14  is negligible. Because the resistor  80  of the plug  40  has, in an embodiment described above, a resistance of 2 MΩ, and the sum of the resistors from plug  40  and  40 ′ is 4 MΩ, the DC current in the cable  14 , when joined with both devices  90  and  91 , is a mere 0.5 μA. 
   The preceding discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.