Abstract:
A communications system including an over-voltage and over-current surge protection circuit is provided that passes high frequency signals between transmitting and receiving devices with low attenuation. The protection circuit includes a current limiter and over-voltage protection device. The over-voltage protection device includes a diode device comprising a set of anti-parallel diodes, which is connected in series with a shunt device. The over-voltage protection device is coupled between a signal transmission line and ground. The current limiter is coupled between the transmitting and receiving devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of electronic surge protection circuits. More specifically, the invention relates to a surge protection circuit for communication networks that carry high-speed digital signals. 
     2. Description of the Related Art 
     In a typical communication network, telephone lines carry voice and data signals from a remote unit to a local unit. For example, in the context of a digital loop carrier (“DLC”) system for extending fiber optic cable into the local-loop between a central office location and a plurality of subscribers, the remote unit could be a remote digital terminal (“RDT”) or an optical network unit (“ONU”), and the local unit could be a network interface device (“NID”). The NID further couples the signals to the subscriber&#39;s telephony and data devices, which are referred to as customer premises equipment (“CPE”). 
     The connections between the local units and the CPE usually include unshielded twisted pair (“UTP”) wire. There is a first twisted pair used for incoming signals that travel from the network towards the CPE and a second twisted pair used for outgoing signals that travel away from the CPE towards the network. These UTP wires are susceptible to voltage and current surges often caused by lightning strikes or AC power (60 Hz) crosses. Therefore, surge protection devices or circuits are typically coupled to the UTP wires to protect the remote and local units, and the CPE, from being damaged by over-voltage and over-current conditions. For signal lines that carry high speed signals, such as 10Base-T Ethernet signals, such surge protection devices must have a low insertion-loss in order to avoid attenuation of the signals at high frequencies. 
     SUMMARY OF THE INVENTION 
     A communications system including an over-voltage and over-current surge protection circuit is provided that passes high frequency signals between transmitting and receiving devices with low attenuation. The protection circuit includes a current limiter and over-voltage protection device. The over-voltage protection device includes a diode device comprising a set of anti-parallel diodes, which is connected in series with a shunt device. The over-voltage protection device is coupled between a signal transmission line and ground. The current limiter is coupled between the transmitting and receiving devices. 
     As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description of the preferred embodiments are to regarded as illustrative in nature and not restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be apparent to those skilled in the art upon reading the following description in view of the accompanying drawings, wherein: 
     FIG. 1 is a block diagram illustrating a signal transmission system utilizing a protection device according to one embodiment of the present invention; 
     FIG. 2 is a schematic diagram of a preferred surge protection device and an isolation transformer incorporated into a Network Interface Device as shown in FIG. 1; 
     FIG. 3A illustrates a protection device circuit architecture according to a first embodiment of the present invention; and 
     FIG. 3B illustrates a protection device circuit architecture according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a system  10  according to a preferred embodiment of the present invention. This system is preferably a fiber-to-the-curb (“FTTC”) DLC system in which the fiber optic connections are pushed deep into the local loop to within  500  feet of the subscriber&#39;s CPE  28 . The system  10  includes a remote unit  12 , which is preferably an RDT that transmits and receives voice and/or data signals on one or more fiber optic connections  14 . These fiber optic connections  14  couple the RDT to a plurality of ONUs  20 . The ONUs are, in turn, coupled to a plurality of NIDs  24  via a pair of UTP wires  16 ,  18 . Alternatively, the RDT  12  could connect directly to the NIDs  24  via UTP wires coupled to the RDT  12  and the NIDs  24 . (Note that in this case the protection device  22  would be installed within the RDT  12 , or in close proximity thereto.) 
     The NIDs  24  are preferably mounted on the exterior of a subscriber&#39;s house or building  26 , although they could be mounted internally to this structure. From the NID  24 , a second pair of wires  30 ,  32  couple the NID to the CPE  28 . The protection circuit of the present invention  22  is preferably mounted within the ONU  20  and the NID  24 , although it could be installed in just one of these two devices, or it could be installed external to these devices. Moreover, in the alternative embodiment in which the RDT  12  is connected directly to the NIDs  24 , the protection circuit  22  would be mounted within or nearby the RDT  12 . 
     In this system  10 , a signal is transmitted from the remote unit  12  along the fiber optic connection  14  to the ONU  20 . At the ONU  20 , the signal is coupled through the protection device  22  onto one of the UTP wires  16 , and is then coupled to the NID  24 . At the NID  24 , the signal is preferably coupled through a second protection device  22 , onto the internal wiring  30  and then to the CPE  28 . Similarly, signals from the CPE  28  are coupled to the RDT  12  through the NID  24 , UTP wires  18 , ONU  20 , and the two protection devices  22 . The protection circuit  22  provides over-voltage and over-current protection for the ONU  20  and the CPE  28 , while also permitting high-frequency signals to pass through the circuit  22  with low signal attenuation. 
     A more detailed view of a protection circuit  22  according to the preferred embodiment of the present invention is shown in FIG.  2 . As seen here, the protection circuit  22  includes current limiters  22 A and  22 C, i.e., over-current protection devices, and over-voltage protection devices  22 B and  22 D. The current limiters  22 A and  22 C are connected in series with first and second lines  16 A and  16 B of the incoming twisted pair, and the over-voltage protection devices  22 B and  22 D are connected between the lines  16 A and  16 B and ground. Each of the current limiters  22 A and  22 C is preferably a bi-directional device that limits the amount of current that can pass through the device, e.g., resistance or inductance based devices. Various commercially available solid-state devices can be used for current limiters  22 A and  22 C. 
     FIG. 2 also shows a more detailed view of the NID  24 . The NID  24  includes a pair of isolation transformers  34  and  36 , which share the same core. These transformers  34  and  36  preferably have a 1:1 ratio, and they are inserted between the twisted pairs  16  and  30 , and the twisted pairs  18  and  32 , respectively. Various commercially available solid state devices can be used for the isolation transformers  34  and  36 . 
     Due to the long distance signals must travel along lines  16 A,  16 B,  18 A, and  18 B, a ground voltage potential can build up that may affect the function of the system  10 . Also this ground potential can vary along a wire of this length. The transformers  34  and  36  isolate this ground potential and decouple the CPE from the loop. 
     As seen in FIG. 2, the system  10  further includes a second protection device  38 , which is identical to the first protection device  22 . The second protection device performs the same function as the first device, but for the outgoing twisted pair  18 . The second protection device  38  includes first and second current limiters  38 A and  38 C, and first and second over-voltage devices  38 B and  38 D. 
     FIGS. 3A and 3B illustrate first and second preferred embodiments, respectively, of the over-voltage protection device  22 B, which is identical to the over-voltage protection devices  22 D,  38 B, and  38 D. In the first embodiment shown in FIG. 3A, the over-voltage protection device  22 B includes a diode device  40  and a shunt device  46 . The diode device  40  preferably includes a set of anti-parallel diodes  42  and  44 , which are coupled between line  16 A and the shunt device  46 . The shunt device  46  is coupled between the diode device  40  and ground. 
     In contrast, in the second embodiment shown in FIG. 3B, the protection device  22 B′ includes the same serially-connected elements as FIG. 3A, but connected in reverse order. Thus, the over-voltage device  22 B′ includes a diode device  40 ′, which preferably includes a set of anti-parallel diodes  42 ′ and  44 ′, coupled between a shunt device  46 ′ and ground, where the shunt device  46 ′ is coupled between the line  16 A and the diode device  40 ′. 
     Each of the solid-state shunt devices  46  and  46 ′ is a bi-directional device designed to limit the voltage across the device to a particular threshold voltage. Preferably, these devices utilize transient voltage supressor (“TVS”) clamping or thyristor “crow bar” devices. Other devices could also be used in place of the TVS clamp or crow bar device. Various commercially available solid state devices can be used for the over-voltage shunting devices. 
     The sets of anti-parallel diodes  42  and  44 , and  42 ′ and  44 ′ are connected in series with the shunt devices  46  and  46 ′ between line  16 A and ground to reduce the overall capacitance of the protection device. High capacitance in such a protection device causes attenuation of signals at high frequencies. 
     In operation, the protection circuits  22  and  38  shunt any voltage surge or over-voltage signal to ground. The diode devices  40  and  40 ′ connected in series with the shunt device  46  and  46 ′ between line  16 A and ground reduce the overall capacitance C eq  of the over-voltage protection device  22 B and  22 B′. The capacitance value C eq  is reduced by placing the intrinsic capacitances of the solid-state devices in series, e.g., the shunt devices  46  and  46 ′ and the diode devices  40  and  40 ′. When capacitors are connected in series the overall capacitance C eq  is reduced according to the formula [1]:                1   Ceq     =       1   C1     +     1   C2     +   …   +     1   Cn               [   1   ]                                
     Thus, with a lower capacitance (C eq ,) high frequency signals pass through the protection devices  22  and  38  with low signal loss, i.e., low attenuation. 
     The invention has been described with reference to preferred embodiments. Those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes and modifications are intended to be covered by the appended claims.