Patent Application
20050202724
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' 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
The plug 10 further includes a conductive shield portion 20 that electrically contacts the cable shield 15 when, as seen in
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.
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
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
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
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.
