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DSL COMPATIBLE LOAD COIL
CROSS REFERENCE TO RELATED
This application is related to and claims priority of U.S. 5 Provisional Patent Application No. 60/262,492 entitled "DSL Compatible Load Coil" filed Jan. 17, 2001 by Andrew Norrell and James Schley-May, the disclosure of which is hereby incorporated by reference. This application also relates to commonly assigned U.S. patent application Ser. No. 09/569,470 entitled "DSL Repeater" filed May 12, 2000 by Brian Hinman, Andrew Norrell and James Schley-May, and to U.S. patent application Ser. No. 09/670,475 entitled "Load Coil and DSL Repeater Including Same" filed on Sep. 26,2000 by Brian Hinman, Andrew Norrell, Carl Alelyunas, and James Schley-May the disclosures of which are also hereby incorporated by reference.
1. Technical Field
The present system and method generally relate to load coils, and more particularly to an xDSL (Digital Subscriber Line) compatible load coil for conditioning POTS (Plain Old Telephone Service)-band signals while permitting xDSL 25 signals to traverse the load coil with low attenuation.
2. Description of Background Art
Load coils, also referred to as "loading coils," are conventionally positioned along long local loops to improve POTS, or voice-grade, communications over the loop. Conventional load coils are inductive devices that are positioned along a local loop to compensate for, or counteract, the distributed parallel capacitance of the local loop. Such use of load coils generally conditions long local loops for POTSband communications by flattening out the POTS band up to 35 about 3.6 KHz. These load coils, however, also significantly limit, or prevent, the provision of digital services over a loaded loop due to the attenuation conventional load coils impart to higher frequency signals, such as ADSL (Asymmetric DSL) signals.
ADSL signals, for example, typically reside between about 26 KHz-1.1 MHz and are highly attenuated by conventional load coils. Indeed, in the past, load coils are routinely removed from local loops in order to provide ^ ADSL service over such loops. The removal of such load coils, in turn, impairs or prevents the provision of POTS service over long loops, such as over loops longer than about 18,000 feet.
A need exists, therefore for an improved load coil that 50 compensates for the distributed parallel capacitance of a local loop while permitting passage of higher frequency digital signals.
Additional background details regarding DSL technology more generally are described in Understanding Digitial 55 Subscriber Line Technology by Starr, CiofE, and Silverman, Prentice Hall 1999, ISBN 0137805454 and in DSL— Simulation Techniques and Standards Development for Digital Subscriber Line Systems by Walter Y. Chen, Macmillan Technical Publishing, ISBN 1578700175, the disclo- so sures of which are hereby incorporated by reference.
loop to improve transmission of POTS-band signals and capacitive elements for facilitating passage of the higher frequency signals, such as xDSL signals, over the local loop. The load coil improves POTS performance and passes signals above the POTS band, such as xDSL signals, with significantly less attenuation than conventional load coils.
In one embodiment, the load coil includes a coupled inductor having an inter-winding capacitance and capacitive elements for significantly increasing the effective interwinding capacitance of the coupled inductor. Pursuant to one particular embodiment, the capacitive elements may comprise a pair of capacitors each having a capacitance in the range of about 5 nF-82 nF, and preferably a value of about 39 nF. One capacitor is disposed between the input of a first inductor winding and the input of the second inductor winding; the other capacitor is disposed between the output of the second inductor winding and the output of the first inductor to increase the effective inter-winding capacitance of the coupled inductor for improving high frequency signal transmission across the load coil.
In another embodiment, the load coil includes a coupled inductor having an intra-winding capacitance and capacitive elements for increasing the effective intra-winding capacitance of the coupled inductor. Pursuant to one particular embodiment, the capacitive elements may comprise a pair of capacitors each having a capacitance in the range of about 5 nF-82 nF, and preferably a value of about 39 nF. Each capacitor is positioned in parallel with one of the windings of the coupled inductor to increase the effective intrawinding capacitance of the coupled inductor for improving high frequency signal transmission across the load coil.
According to another embodiment, a system for transmitting POTS and xDSL signals over a local loop includes an xDSL repeater and a load coil positioned in cascaded fashion along the local loop. The load coil includes inductive elements for conditioning the loop to improve POTS-band transmissions and capacitive elements to permit xDSL signals to traverse the load coil with low attenuation. The load coil included within the repeater may be configured differently from the load coil disposed in cascaded fashion with the repeater. The repeater amplifies the xDSL signals to compensate for the attenuation of the xDSL signals as they traverse the loop.
Additional details and features of the present system and method will be apparent to those skilled in the art from the following detailed description and the accompanying drawings.
FIG. 1 illustrates a communication system for providing POTS and xDSL service over local loops;
FIG. 2 illustrates details of one embodiment of a FIG. 1 repeater;
FIG. 3 illustrates details of one embodiment of a FIG. 1 load coil;
FIG. 4 illustrates details of another embodiment of a FIG. 1 load coil; and
FIG. 5 is a graph illustrating the frequency responses of different load coils.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a communication system 100 that includes a central office 102, a customer premises 104 and a customer premises 106. The customer premises 104 and 106 are respectively coupled to the central office 102 by
local loops 114 and 116. Each local loop comprises a twisted pair of copper wires; commonly know in the art as a "twisted pair." Typically, the copper wires are formed of 22,24, or 26 AWG wire.
Moreover, as those skilled in the art will appreciate, the central office 102 and each of the customer premises 104 and 106 includes DSL termination equipment, such as a DSL modem or the like, for transmitting and receiving DSL signals over the associated local loop.
A load coil 124 is disposed along the local loop 114 between the central office 102 and the customer premises 104 to condition the loop for transmission of POTS-band signals and includes inductive elements, such as a coupled inductor, for compensating for, or counteracting, the distributed capacitance, or parallel capacitance of the local loop 114. The inductive elements flatten out the frequency response of the local loop for signals below about 3.6 KHz. Importantly, the load coil 124 also includes capacitive elements for significantly increasing the effective interwinding or intra-winding capacitance of the load coil to permit higher frequency signals associated with the provision of digital services, such as xDSL service, to traverse the load coil 124 with significantly less attenuation than with conventional load coils. Thus, both POTS-band and higher frequency signals may traverse the local loop 114 simultaneously, with the POTS-band signals being conditioned by the load coil 124 and the higher frequency signals, such as xDSL signals, passing across the load coil 124 with little, if any, attenuation.
A loop 116 is illustrated as having a load coil 130, a repeater 132, a load coil 134, and a repeater 136 disposed in cascaded fashion along the loop 116 to provide POTS and digital services over the loop 116 to the customer premises 106. Additional details of the load coil 130, which may be identical to the load coils 124 and 134 are described below with reference to FIGS. 3, 4, and 5. The DSL repeaters 132 and 136 are coupled to the local loop 116 to amplify digital signals, such as ADSL or VDSL signals, passing over the loop 116 between the central office 102 and the customer premises 106.
As those skilled in the art are generally aware, DSL signals are attenuated as they travel along a local loop, such as the local loop 116. The repeaters 132 and 136 are disposed along the loop 116 between the central office 102 and the customer premises 204 to at least partially compensate for the DSL signal attenuation by amplifying the transmitted DSL signals. Additional details of the repeaters 132 and 136, which may be configured identically, are described below with reference to FIG. 2.
Further, FIG. 1 illustrates that multiple DSL repeaters, such as the repeaters 132 and 136, may be coupled in series, or in cascaded fashion, to a single loop, such as the loop 116, for amplifying transmitted DSL signals multiple times and in multiple locations between the customer premises and the central office to permit DSL signals to be transmitted over greater distances while still maintaining an acceptable DSL signal amplitude.
According to one embodiment, the loop 116 comprises a loop length of about 20,250 feet of 24 AWG twisted pair cabling with a distance of about 2,250 feet between the central office 102 and the load coil 130. The loop distance between the load coil 130 and the repeater 132 is about 4,500 feet. The loop distance between the repeater 132 and the load coil 134 is about 4,500 feet. The loop distance between the load coil 134 and the repeater 136 is about 4,500 feet. The loop distance between the repeater 136 and the customer
premises is about 4,500 feet. In this configuration, the repeaters 132 and 136 provide xDSL signal amplification and, preferably, some loop conditioning. The load coils 130 and 134 provide additional loop conditioning and permit
5 passage of xDSL signals with little or no significant attenuation. Thus, according to one embodiment, the loop 116 may provide POTS and xDSL services over a loop having a length of 20,250 feet.
Those skilled in the art will appreciate that other separa
1° tion loop distances may be employed. For example, in another embodiment using 26 AWG twisted pair cabling, the loop distance between the central office 102 and the load coil may be about 3,000 feet with the remaining repeaters and load coils being spaced apart by loop distances of 6,000 feet
15 for a total loop length of about 27,000 feet.
FIG. 2 illustrates details of one embodiment of the repeater 132 of FIG. 1. As shown, the repeater 132 is coupled to the local loop 116 between the central office 102 and the customer premises 106, and in particular is disposed
20 between load coils 130 and 134 (FIG. 1). The repeater 132 is depicted as including a downstream filter 202 and a downstream amplifying element or stage 204 and an upstream filter 212 and an upstream amplifying element or stage 214. The filters 202 and 212 and the amplifying
25 elements 204 and 214 are disposed between a pair of electromagnetic hybrid couplers 222 and 224. The amplifying elements 204 and 214 may comprise amplifiers or amplifying equalizers. Moreover, those skilled in the art will appreciate that the filter 202 and the amplifying element 204
30 may be embodied as a single circuit to filter and amplify downstream data signals. Similarly, the filter 212 and the amplifying element 214 may be embodied as a single circuit to filter and amplify upstream data signals.
35 In general, the hybrid coupler 222 receives downstream DSL signals from the central office 102 (FIG. 1) along the local loop 116 and outputs the downstream DSL signals to the downstream filter 202 along line 232. The hybrid coupler 222 also receives amplified upstream DSL signals from the
4Q upstream amplifying element 214 along line 234 and transmits the upstream DSL signals onto the local loop 116 for transmission to the central office 102.
Similarly, the hybrid coupler 224 receives upstream DSL signals from the customer premises 106 along the local loop
45 116 and outputs the upstream DSL signals to the upstream filter 212 along line 242. The hybrid coupler 224 also receives amplified downstream DSL signals from the downstream amplifying element 204 along line 244 and transmits the downstream DSL signals onto the local loop 116 for
50 transmission to the customer premises 106.
As those skilled in the art will appreciate, where the hybrid coupler 222 is imperfect, at least a portion of the upstream amplified DSL signal received via the line 234 will leak through the hybrid coupler 222 onto the line 232.
55 Likewise, where the hybrid coupler 224 is imperfect, at least a portion of the downstream amplified DSL signal received via the line 244 will leak through the hybrid coupler 224 onto the line 242. Without the presence of the filters 202 and 212, this DSL signal leakage could cause a phenomenon
go known in the art as "singing"—that is oscillations caused by introducing gain into a bi-directional system due to signal leakage.
The signal leakage problem is overcome, or substantially alleviated, through the use of the downstream filter 202 and 65 the upstream filter 212. One version of Category 1 ADSL upstream signals generally occupy the frequency spectrum between about 26-120 KHz and ADSL downstream signals