Abstract:
A surge protector for highspeed data networks is provided wherein a circuit board having a ground plane formed on one side of the circuit board, has respective pairs of conductive traces forming impedancematched transmission lines on the other side of the circuit board. Surge protection devices are connected in series with the conductors and with the ground plane. A compensating reactance may be provided along the transmission lines to correct for parasitic capacitance introduced by the threshold voltage conductor devices. Shielded and isolated connectors may be provided at either end of the circuit board to minimize insertion loss and provide isolation between respective transmission lines.

Description:
CROSSREFERENCE TO RELATED APPLICATION 
     Priority is claimed herein to U.S. application Ser. No. 60/179,859, filed Feb. 2, 2000, which is incorporated in its entirety by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a surge protector for high speed data networks. In particular, the invention relates to a surge protector apparatus providing a matched impedance to twistedpair electrical transmission media. 
     BACKGROUND 
     Conductive wire interconnections continue to be widely used for high speed computer networks, even though they were once believed to be in jeopardy of imminent obsolescence by fiber optic cables. Because many installations have a legacy investment in computer networks based on conductive wires, and due to other economic and easeofuse considerations, it has been found desirable to develop ways of increasing data transmission speeds on existing conductivebased computer network installations, rather than to replace them. Conductive computer networks, based on twistedpair cable technology, continue to be maintained and installed to meet higher operating bandwidth specifications. 
     Conductive wirebased computer networks are vulnerable to transients or surges due to, for example, lightning strikes in the vicinity of the network. A nearby lightning strike can induce voltage levels in the conductors constituting the network that are sufficiently high to damage terminal equipment connected to the network. The typical solution to this problem is to connect transient voltage suppression devices between the wires of the network and ground, in order to provide a conduction path for high voltage surges to discharge. Adding such devices, however, places a parasitic reactive load on the conductors. With the increasing bandwidth demands required of computer networks, the ability to conduct high speed data transmission can be impaired by radiative losses and crosstalk which occur whenever the pairs of the transmission cable are connected to a surge suppression device. Such impairments include radiative and coupling losses from the connectors employed to attach the wires to the surge suppression device, losses due to impedance mismatch with the suppression device itself, radiative loss due to the necessity to split the individual pairs for connection with voltage suppressors, as well as the parasitic impedance of the voltage suppressors. 
     In U.S. Pat. No. 5,706,160 there is shown a proposed surge arrester arrangement in which numerous banks of diodes are connected with conductive traces of a circuit board corresponding to the pairs of a twistedpair based signal transmission system. Additionally, the patent proposes the use of a verticallypositioned shield upon the circuit board in order to reduce crosstalk between conductors of the surge arrester. It would be desirable to provide a surge arrester for high speed computer networks which employed fewer parts and was simpler in overall assembly. 
     SUMMARY 
     According to one aspect of the invention a surge arrester for twistedpair transmission systems is provided in which each pair of a transmission cable is connected to a circuit board on which an impedancematching stripline is formed on one side of the circuit board. Corresponding isolated ground plane areas are provided on the other side of the circuit board in order to confine radiated signals to the respective vicinities of the striplines. A threshold voltage conduction circuit is connected along each stripline. The connection to the threshold voltage conduction circuit may be geometrically formed as a microstrip to provide a compensating reactance in series with the threshold voltage conduction circuit. Additionally, the threshold conduction circuit may comprise a series combination of threshold voltage conduction devices in order to present a selected capacitive load to the microstrip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a printed circuit board for a surge arrester in accordance with the invention; 
     FIG. 2 is a side view of a printed circuit board of the surge arrester of FIG. 1 as seen along the line  2 — 2 ; 
     FIG. 3 is a bottom plan view of the circuit board of FIG.  1 . 
     FIG. 4 is an elevational cutaway view of an electrical connector employed in the surge arrestor of FIG. 1; and 
     FIG. 5 is a perspective cutaway view of the connector of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown a surge arrester  10 . The surge arrester  10  comprises a circuit board  12 , such as a glassepoxy circuit board, upon which are mounted several connectors  14   a-d  for receiving respective pairs of a twisted pair transmission cable (not shown). The connectors  14   a-d  are preferably of the type referred to as “punch down” connectors, having compressive contact members formed in a plastic housing for simultaneously penetrating the insulation of a wire and for obtaining electrical contact with the conductor therein. Each of the connectors  14   a-d  is arranged to receive a single pair of wires within respective compressive contact members, of which  16  and  18  are typical. The connectors  14   a-d  are spaced apart from one another in order to reduce crosstalk between the exposed pairs of wires connected therein. The connectors may be of a conventional type, as shown in FIG. 2, or may comprise shielded punchdown connectors or impedancematched RJ45 connectors such as are described in U.S. application Ser. No. 60/179,859. In alternative embodiments, various other types of connectors may be provided. 
     Referring now to FIG. 4, there is shown a cutaway view of connector  14   a . The connector block  14   a  comprises an insulating body  70  within which is positioned a linear array of connector members  16 ,  18 . Each of the connector members  16 ,  18  comprises a forked planar conductor forming a compressive jaw for receiving a conductor such as a wire therein. The interior surfaces of the jaws are sufficiently thin or sharp so that the interior surfaces of the jaws will penetrate the insulator of an insulated wire inserted therein and establish a compressive contact with respective sides of the conductor. The bottoms of the connector members have mounting pins formed to extend out of the bottom of the insulating body  70  for mounting the connector  14   a  upon the circuit board and for establishing electrical connection with the conductive traces of the circuit board. The insulating body  58  of the connector block  54   a  is formed to provide verticallyextending guiding channels having a Vshaped wirereceiving top portion, and aligned with the compressive jaws of the forked conductors, for receiving and aligning wires to be inserted into the jaws. 
     Referring now to FIG. 5, a shielding member  21  is also housed within the insulating body of the connector block  14   a . Shielding member  21  comprises a planar conductor held vertically within the connector block  14   a  to extend horizontally and in parallel to the connector members  16  and  18 . The dielectric constant of the insulating material forming the housing  58 , and the separation distance between the shielding member  21  and the respective connector members  16 ,  18  are selected to provide an independence substantially matched to a standard transmission cable, e.g., 100 Ω. The bottom of shielding member is formed to have at least one connector pin, such as pin  23  extending out of the bottom of the insulating body  70  for mounting the connector block  14   a  upon the circuit board and for establishing electrical connection between the shielding member and the ground plane of the circuit board. The shielding member  21  serves to confine fringing fields from the connector members to the space between the connector members and the shielding member, in order to provide impedance matching and to reduce crosstalk and radiative loss relative to conventional compressive a connector blocks. 
     A ground plane  20  is formed on the top surface of the circuit board  12 . The ground plane area  20  is formed to provide respective ground plane areas  22   a-d , which are separated along their length by isolation channels  24 ,  26 , and  28 . The isolation channels  24 ,  26 , and  28  provide localized isolation of the ground plane areas, as shall be discussed further below. In operation, the ground plane areas  22   a-d  are connected to ground. A plated throughhole  30  provides a ground connection to the other side of the circuit board  12 . 
     Also mounted on the circuit board  12  are respective electrical connectors  32   a-d , which are mounted at opposite ends of respective ground plane areas  22   a-d  from the connectors  14   a-d . The connectors  14   a  and  32   a  provide input and output terminals for one of four pairs of conductors that can be connected to the surge arrester. The circuit components associated with providing surge protection for the corresponding pair of conductors shall now be described, and it will be understood that similar components are provided for each pair of the four pairs. 
     As shown in FIG. 2, the compressive contact members  16  and  18  penetrate the board  12 . Referring to FIG. 3, the compressive contact members  16  and  18  emerge from the board and are connected with one end of each of parallel stripline conductors  42  and  44 . The stripline conductors  42  and  44  are sized and spaced apart in accordance with the electrical permitivity of the circuit board to provide a transmission line along the length thereof that is impedancematched to the corresponding pair of the transmission cable. At the other end of the parallel stripline conductors  42  and  44 , a second pair of compressive contact members are connected to provide electrical connection with the connector  32   a.    
     The stripline conductors  42  and  44  are located on the bottom of the board and aligned with ground plane area  22  to provide confinement of electrical fields between the two stripline conductors, and between the respective conductors and the ground plane area  22 . Adjacent pairs of stripline conductors are arranged in parallel such that each pair is spaced apart to reduce crosstalk. Additional crosstalk isolation is provided by the isolation gaps between adjacent ground plane areas, so that mirror or eddy currents induced in one ground plane area do not inductively couple into adjacent stripline conductors. The ground plane area  22   a  extends along the circuit board  12  along the entire length of the stripline conductors from the input connector  14   a  to the output connector  32   a , so that shielding is provided along the entire length of the data communication path through the device. The use of such ground planes provides impedance matching and reduces radiative emissions along the length of the stripline conductors. 
     Threshold voltage conduction devices are connected between ground and a connection point on the stripline conductors. As shown in FIG. 3, one such connection arrangement is a conductive trace  46  leading from one end of the stripline conductor  42  as an input to one or more threshold voltage conduction devices  48 , preferably a pair of transient protection diodes such as a pair of SEMTECH LCDA12 surface mounted diodes. Diodes of this type are provided in a quad surfacemountable package, and include integrated lowcapacitance compensation diodes in series with TVS (transient voltage suppression) breakdown diodes. 
     A second threshold conduction device  52  is connected in series with the device  48  via an interconnection  50  formed on the circuit board  12 . In a preferred embodiment, the device  52  comprises a VISHAY SMBJ20(c)A bidirectional transient voltage suppressor. 
     The second threshold conduction device  52  is connected with a ground trace  56  which is connected to the other grounded elements on the board  12 . It should be noted that providing multiple threshold voltage conduction devices in series provides a reduced capacitance for the overall surge conduction path. Additionally, the interconnection traces  46  between the striplines  42  and  44  and the threshold voltage conduction devices can be sized relative to the striplines to provide a geometric discontinuity that produces an effective compensating reactance to negate the parasitic capacitance presented by the threshold voltage conduction devices. In an alternative embodiment, a discrete inductive device, or an additional geometric discontinuity, can be connected or formed along the striplines in order to provide a sufficient compensating reactance for other arrangements of threshold voltage conduction devices. 
     It will be appreciated that the provision of an impedancematched stripline arrangement along the entire length of the signal path through the surge arrester provides for surge protection without the load penalty associated with known surge protection devices. It shall also be appreciated that the provision of isolated ground plane confinement of radiative fields contributes to the impedance matching of the striplines and reduces crosstalk between adjacent pairs of conductors without requiring additional shielding structures to be attached to the board during assembly. Hence, there is hereby provided a surge protection arrangement that is compatible with traditional circuit board manufacturing techniques, which is operable at substantially higher bandwidths than have been provided in the past in such devices. 
     The terms and expressions employed herein are terms of description and not of limitation. There is no intention to limit the scope of the invention by the use of such terms, but to provide a exemplary description of a preferred embodiment. The invention is to be determined with reference to the appended claims, and to all equivalents thereof.