Abstract:
A system for matching impedance in the circuitry of an xDSL communication device to improve noise cancellation resulting from signal leakage between transmit and receive signals. The system for matching impedance includes a transformer configured to couple an impedance, which is substantially equal to a line impedance, to a line coupling transformer, and is applicable to all known hybrid topologies. The transformer is ideally as closely matched to the line coupling transformer as possible. This technique allows greatly improved impedance matching in the hybrid, which directly benefits the performance of xDSL communication devices.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is related to, and claims the benefit of, U.S. Provisional Patent Application No. 60/336,283, entitled “Transformer-Coupled Matching Impedance,” filed on Oct. 25, 2001. The subject matter of the related application is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of xDSL (x Digital Subscriber Line) line interface circuitry, and more particularly to hybrid circuitry known as 2-to-4 wire converters. 
     2. Description of the Background Art 
     Typically, xDSL modems separate transmit and receive signals to cancel noise. However, when the transmit signal (one band) leaks into the receive signal (another band), the noise degrades xDSL performance. Thus, xDSL modems often use filters to prevent this leakage between bands. However, the filters cannot be efficiently sharpened to optimally cancel noise. 
     Instead of filters, hybrids (2-to-4 wire converters) are used to cancel noise in xDSL line interface circuitry. However, hybrids typically cannot achieve enough noise cancellation because it is difficult to construct a circuit of resistors, capacitors, and inductors with a total impedance that closely matches the transmission line characteristic impedance. It is relatively easy to match impedances at a single specific frequency, but maintaining an impedance match over a range of frequencies is difficult. 
     Further, noise cancellation is difficult to achieve with a hybrid because the transmission line can only be “seen” through a mandatory line-coupled transformer. The transformer has less than ideal properties that alter the perception of the line impedance. The most significant properties are the magnetizing and leakage inductances, which significantly degrade noise cancellation. In practice, it is not practical to produce transformers with sufficiently small leakage inductance or sufficiently large magnetizing inductance to achieve acceptable noise cancellation. 
     One method to improve noise cancellation is to use inductors to incorporate compensating inductances into the impedance of the hybrid. However, this method is limited by the problem of accurately matching the discreet inductor values to the inductance values intrinsic in the transformer. The inductance values intrinsic in the transformer cannot be precisely controlled and incorporating inductors does not yield optimum hybrid performance. 
     Therefore, what is needed is a technique that permits xDSL line interface circuitry in a communication system to cancel noise between transmit and receive signals while matching impedance on the xDSL line interface circuitry. 
     SUMMARY OF THE INVENTION 
     A system for matching impedance in the circuitry of an xDSL communication device to improve noise cancellation resulting from signal leakage between transmit and receive signals. In one embodiment, a line interface circuit includes a transformer configured to couple a matching impedance, which is substantially equal to a line impedance, to a line coupling transformer, and is applicable to all known hybrid topologies. The transformer is ideally as closely matched to the line coupling transformer as possible. This technique allows greatly improved impedance matching in the hybrid, which directly benefits the performance of xDSL communication devices. 
     The line coupling transformer and the other transformer are substantially identical, having substantially identical leakage inductances and substantially identical magnetizing inductances. Substantially identical transformers can be achieved using an identical manufacturing process at a single manufacturing facility. In one embodiment, the matching impedance is a complex impedance network configured to have an impedance substantially equal to the line impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of one embodiment of a communication system in accordance with the invention; 
     FIG. 2 is a diagram of one embodiment of an xDSL device in accordance with the invention; 
     FIG. 3 is a diagram of one embodiment of a circuit including a hybrid with transformer-coupled matching impedance in accordance with the invention; 
     FIG. 4 is a diagram of another embodiment of a circuit including a hybrid with transformer-coupled matching impedance in accordance with the invention; and 
     FIG. 5 is a diagram of another embodiment of a circuit including a hybrid with transformer-coupled matching impedance in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagram of one embodiment of a communication system in accordance with the invention. A premises  110 , including property with any type of structure, is coupled via a line  142  to a PSTN (Public Switched Telephone Network)  130  that provides copper wires as a telecommunications medium and can also include Cat  5  copper cables (not shown) and fiber optic cables (not shown). PSTN  130  is further coupled to a central office  120  that provides telecommunication services for a particular area. Central office  120 , operated by a service provider (not shown), provides switching technology for Plain Old Telephone Service (POTS), Integrated Services Digital Network (ISDN) service, and xDSL service. 
     In premises  110 , an xDSL device  140 , such as a DSL modem or router, communicates via line  142  with PSTN  130  and via a path  144  with multiple telecommunication devices. The telecommunication devices include, but are not limited to, computers  150  with network/telecommunication hardware and software (not shown) and other devices  170 , such as set-top boxes, home network gateways, PDAs (Personal Digital Assistants), and printers. A telephone  160  is coupled to line  142  and includes a low pass filter (not shown) for filtering out non-POTS band signals. Other POTS devices, such as a facsimile machine, may also be coupled to line  142 . 
     FIG. 2 is a diagram of one embodiment of xDSL device  140  of FIG. 1 in accordance with the invention. xDSL device  140  includes, but is not limited to, a hybrid  210  and other xDSL circuitry  220 . Hybrid  210  is a 2-to-4 wire converter that electromagnetically couples xDSL device  140  to line  142 . Other xDSL circuitry  220 , which includes filters and a transceiver, communicates via path  144  with computers  150  and other device  170 . 
     FIG. 3 is a diagram of one embodiment of a circuit including hybrid  210  (FIG. 2) in accordance with the invention. All node voltages discussed in FIG. 3 are with respect to ground. A Vtx+  310  transmit signal and a Vtx−  320  transmit signal are complimentary differential signals produced by other xDSL circuitry  220 . A Vrx signal  330  is a non-differential receive signal. Trans-hybrid loss is defined as Vrx  330  with respect to Vtx when there is no signal being received by hybrid  210  from line  142 . 
     The circuit of FIG. 3 shows impedance values Zsrc  360 , Zdrv  370 , Zout  390 , and Zline  380 . Zline  380  represents the impedance of line  142  as seen by a T 1  transformer  340 , which is coupled to line  142 . Zout  390 , seen at the other side of T 1  transformer  340 , represents Zline  380  in combination with the impedance of T 1  transformer  340 . Zsrc  360  represents the impedance of a complex impedance network (source impedance) coupled to a T 2  transformer  350 . Zsrc  360  is designed and built to be substantially equal to Zline  380 . Zdrv  370 , seen at the opposite side of T 2  transformer  350 , represents Zsrc  360  in combination with the impedance of T 2  transformer  350 . 
     T 1  transformer  340  and T 2  transformer  350  are substantially identical. Substantially identical transformers can be achieved by producing the transformers using the same manufacturing process at the same manufacturing facility. Substantially identical T 1  transformer  340  and T 2  transformer  350  have substantially identical impedances, including substantially identical leakage inductances and substantially identical magnetizing inductances. By coupling Zsrc  360  into hybrid  210  using T 2  transformer  350  instead of directly coupling Zsrc  360  in series with T 1  transformer  340 , there is no need to modify Zsrc  360  with discreet inductors to match intrinsic inductances of T 1  transformer  340 . Typically, it is difficult to manufacture inductors and other circuit elements to match intrinsic inductances present in T 1  transformer  340 . By coupling Zsrc  360  using T 2  transformer  350 , which is substantially identical to T 1  transformer  340 , Zdrv  370  and Zout  390  are substantially identical. 
     If T 1  transformer  340  and T 2  transformer  350  are non-ideal but identical, and if Zsrc  360  equals Zline  380 , then Zdrv  370  would equal Zout  390  and hybrid  210  would provide a perfect voltage divider resulting in ideal trans-hybrid loss. This perfect voltage divider permits the transformers to have significant leakage and magnetizing inductances without compromising hybrid  210  performance. By utilizing substantially identical T 1  transformer  340  and T 2  transformer  350 , and utilizing Zsrc  360  that is substantially equal to Zline  380 , Zdrv  370  is substantially equal to Zout  390  and an almost perfect voltage divider can be achieved in hybrid  210  that provides effective noise cancellation between transmit and receive signals. 
     FIG. 4 is a diagram of another embodiment of a circuit including hybrid  210  in accordance with the invention. Specifically, FIG. 4 shows hybrid  210  with a differential transmit signal (Vtx+  410  and Vtx−  420 ) and a differential receive signal (Vrx+  430  and Vrx−  440 ). In order to cancel the transmit signal in the receive path while maintaining a fully differential receive signal, the receive path includes voltage divider networks. In FIG. 4, a resistor  450  with a value of 2R and a resistor  460  with a value of R operate in conjunction as an R-2R voltage divider network for passive hybrid cancellation. A resistor  452  and a resistor  462  also operate in conjunction as an R-2R voltage divider network. Alternately, for active hybrid cancellation, hybrid  210  can include an operational amplifier circuit (not shown) with similar resistor ratios. Typically, R is much greater than a Zline  492  so as to cause negligible loading effects, where Zline  492  represents the impedance of line  142  (FIG.  1 ). 
     Further, a T 1  transformer  490  and a T 2  transformer  495  have split windings with a winding ratio of 1:1:1:1. T 1  transformer  490  and T 2  transformer  495  are substantially identical, and a Zsrc  497  represents a complex impedance network with an impedance that is substantially equal to Zline  492 . By using split winding T 2  transformer  495 , a single instance of Zsrc  497  operates as two equal impedances, each with a value of ½ Zsrc, that are isolated from one another. The FIG. 4 embodiment advantageously requires only one matching impedance network, Zsrc  497 , rather than two distinct matching impedance networks each having a value of ½ Zsrc as required by prior art embodiments that utilize a split winding line coupling transformer. 
     Other variations on the circuit topologies previously described are compatible with the transformer-coupled matching impedance of the invention. Exemplary circuit topologies include circuits with single ended or differential transmit or receive paths, circuits with single or split source impedance, circuits with transformer winding ratios other than 1:1, and circuits with a split winding transformer such that the line-side is wired in series while the driven side is wired in parallel. 
     FIG. 5 is a diagram of another embodiment of a circuit including hybrid  210  (FIG. 2) in accordance with the invention. In the FIG. 5 embodiment, the line side of a T 1  transformer  510  is wired in series and the driven side of T 1  transformer  510  is wired in parallel, which is an efficient way of driving line  142 . A T 2  transformer  520  is substantially identical to T 1  transformer  510  and a Zsrc  540  is substantially equal to a Zline  530 , which represents the impedance of line  142  (FIG.  1 ). Similar to the previous embodiments, matching impedance Zsrc  540  coupled to the hybrid  210  circuit via T 2  transformer  520  improves noise cancellation between the transmit and receive signals. 
     In addition to improving hybrid noise cancellation, another advantage of implementing transformer-coupled matching impedance in hybrid  210  involves linearity. All transformers are less than ideal in terms of linear parasitics, such as unwanted capacitance and unwanted inductance. Further, transformers are less than linear devices because of the magnetic properties of the materials used in making the transformers. Thus, all transformers will introduce some distortion. Typically, transformers used in xDSL applications, such as xDSL device  140 , must have low distortion. Otherwise, the non-linear transfer function modulates the signals passing through the transformer. This modulation causes harmonic distortion and creates inter-modulation products, which appear as noise, and degrade performance of the xDSL system. 
     In the FIG. 3 embodiment for example, T 2  transformer  350  is substantially identical to T 1  transformer  340 , and thus both transformers will have substantially the same non-linear effects. Since Zsrc  360  is substantially equal to Zline  380 , Zdrv  370  is substantially equal to Zout  390 , in spite of the non-linear effects of T 2  transformer  350  and T 1  transformer  340 . 
     Another advantage occurs in embodiments where xDSL device  140  is designed for optimal hybrid matching to more than one reference line impedance. Implementing such an embodiment requires incorporating relays or other switching elements in hybrid  210  to select among alternate matching impedance networks to find the best match to a current line impedance. In the most common topology of prior art hybrids having differential transmit and differential receive paths, there are typically two impedance networks each equal to one half the value of the impedance needed to match the line impedance. Both of the two impedance networks will need to be switched to match different line impedance values. Such a hybrid requires two instances of the switching relay in addition to the two matching impedance networks. 
     However, an embodiment of hybrid  210  in accordance with the invention having differential transmit and receive paths requires only one matching impedance network, since a second split winding transformer, for example T 2  transformer  495  of FIG. 4, allows a single instance of a matching impedance network, for example Zsrc  497 , to be used. Thus an embodiment of hybrid  210  having differential transmit and receive paths configured to include multiple matching impedance networks only requires one switching network to select among the multiple matching impedance networks, which results in significant cost and space savings. 
     The invention has been explained above with reference to specific embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. The present invention may readily be implemented using configurations other than those described in the embodiments above. Additionally, the present invention may effectively be used in conjunction with systems other than the one described above. Therefore, these and other variations upon the above embodiments are intended to be covered by the present invention, which is limited only by the appended claims.