LOOP DRIVER FOR POTS, XDSL, OR INTEGRATED POTS/XDSL INTERFACE

Abstract: A driver circuit (180) comprising a transformer having a primary winding (106, 108) for coupling to a subscriber loop and a secondary winding (104) for coupling to an interface circuit, a feedback circuit for coupling said primary winding to said secondary winding to provide a predetermined impedance match between said interface circuit and said loop over a predetermined frequency band of said driver circuit. In a preferred embodiment, the interface circuit is integrated line card for supporting both POTS and ADSL.

Description: Loop Driver For POTS, XDSL, Or Integrated POTS/XDSL Interface The present invention relates to the field of voice and data communications system and more particularly to interface circuits for coupling a combination of telephony and high-rate data communications functions to a 2- wire telephone loop. BACKGROUND OF THE INVENTION With the increasing popularity of the Internet, there has been a corresponding requirement for high rate digital transmission over the local subscriber loops of telephone companies. A loop is a twisted-pair copper telephone line. Loops can differ in distance, diameter, age, and transmission characteristics, depending on the network. These loops are a natural conduit for provision of high speed digital communications services due to the large installation base that already exists. Digital transmission systems on these loops include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL, and VDSL respectively. Normally these and other similar protocols are known as xDSL. Of these flavours of xDSL, ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. In the future, it is possible that multiple different transmission schemes may be employed in different frequency bands on the same loop, and that these transmission schemes may include traditional analog voice services as well as current and new forms of xDSL. In today's ADSL system, the plain old telephone services (POTS) uses the frequency spectrum between 0 and 4kHz and the ADSL uses the frequency spectrum between 30kHz and 1.1MHz for data over the telephone line. This is shown schematically in figure la. ADSL also partitions its frequency spectrum with upstream (subscriber to CO) transmission in a lower frequency band, typically 30kHz to 138kHz, and with downstream transmission in a higher frequency band , typically 138kHz to 550kHz or 1.1MHz. ADSL uses a discrete multi- tone (DMT) multi-carrier technique that divides the available bandwidth into approximately 4kHz sub-channels. The architecture, interfaces and protocols for telecommunications networks incorporating ADSL modems is shown in figure lb. The elements consist of one of more ATU-C's 2 or ADSL modems at a central office end 4. The ATU-C 2 can be integrated within an access node 6 which is the concentration point for broadband data 8 and narrowband data 10. Broadband and narrowband in this context is meant switching systems for data rates above 1Mbps and switching systems for data rates at or below 1Mbps respectively. The access node 6 can be located at a central office, or a remote site. Also a remote access node can subtend from a central access node. The ATU-C's 2 are coupled via a splitter 12 to the telephone loop 14. The loop 14 at the customer end is also coupled via a splitter 12 to the telephone loop 14. The loop 14 at the customer end is also coupled via a splitter 12 to an ATU-R 16 or an ADSL modem at the customer end that can be integrated within an SM (Service Module) which are devices that perform terminal-adaptation functions. Examples are set top boxes, PC interfaces, or LAN routers. The PSTN (Public Switched Telephone Network) 18 to the subscriber phones 20 shares the loop 14 via the splitter 12 which isolate the POTS form the ADSL modems. As shown in figure 2, an analog splitter 24 provides the filtering required to separate the POTS and ADSL bands before being input to their respective transceivers. Generally, the splitter 34 consists of a low pass filter 36 between the telephone and the loop and a highpass filter between the ADSL transceiver and the loop. The low frequency components output form the LPF are sent to the conventional telephone line card. These analog signals are converted to digital signals and encoded as PCM signals by and AID CODEC. These signals may then be combined with other PCM signals and transmitted to other central offices or switching networks. The splitter is generally a very bulky and expensive component. A number of solutions have been proposed to eliminate the splitter. For example, US Patent No. 5,757,803 describes an improved splitter, while US Patent No. 5,889,856 describes an integrated ADSL line card with a digital splitter. It is generally assumed that the function of the POTS splitter is only to separate the different frequency bands and send them to their respective transceiver. The actual function is more complex and deals with the need to provide the correct impedance in different frequency bands in order to allow the signals to properly propagate and meet the relevant specifications. While the conventional POTS splitter does eliminate or reduce the effects of interference from the POTS and ADSL equipment, a properly designed interface may eliminate these problems with adding the low or high pass filters in the signal path. Matching a line interface card to the loop is relatively less complicated in a narrow frequency band. Various circuits for achieving such a matching are described U.S. Patent 5,515,433. However, these circuits are limited when a broadband matching is required. The present invention thus seeks to mitigate some of the above disadvantages. SUMMARY OF THE INVENTION The invention seeks to provide a solution to the general problem of requiring a POTS splitter in an integrated line interface module and, in particular, at the central office or "head end". An advantage of the present invention is that it may be used for a combined telephony and digital subscriber loop ("DSL") service or other communications systems where the telephony and data, or different types of data, occupy different frequency spectra on a 2-wire communication line. A further advantage of the invention is to eliminate the traditional POTS splitter and to provide a broadband loop driver and termination that allows frequency dependent impedance synthesis to be implemented using highly integrated circuits using either digital, analog or a combination of digital and analog means. A further advantage of the invention is to minimize power, cost and size for delivering integrated voice and data. In accordance with this invention there is provided a drive circuit comprising a transformer having a primary winding for coupling to a subscriber loop and a secondary winding for coupling to an interface circuit; a feedback circuit for coupling the primary winding to the secondary winding to provide a predetermined impedance match between the interface circuit and the loop over a predetermined frequency band of the circuit. BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: Figure 1(a) is a diagram showing the frequency spectrum of an ADSL system; Figure 1(b) is a schematic diagram of an ADSL system architecture; Figure 2 is a schematic diagram of a prior art ADSL modem; Figure 3 is a schematic diagram of a prior art DSL transceiver; Figure 4 is a schematic diagram of an ADSL modem according to an embodiment of the present invention; Figure 5 is a schematic block diagram of the an ADSL modem according to an embodiment of the present invention; Figures 7(a)-(f) are schematic diagrams of drive circuits according to embodiments of the present invention; and Figures 8 (a)-(f) are detailed schematic circuit diagrams of the circuits in figures 7(a)-(f). DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description, like numeral refer to like structures in the drawings. Referring to figure 2, a DSL central office system, according to the prior art, is shown generally by numeral 20. The system comprises an ADSL transceiver 21 for processing digital data, a POTS splitter 34, and a POTS transceiver incorporating a SLIC (subscriber line interface circuit) 26 and a CODEC 22 for processing voice data. The POTS splitter 24 is coupled to a subscriber loop 28 which then splits off the voice signal and provides it to the SLIC and CODEC 22 and a high frequency data signal to the ADSL transceiver 21. The ADSL transceiver includes a DSL baseband circuit 31 coupled to the broadband data path, a DSL analog front-end (AFE) 33 which is fed from the DSL baseband circuit couples to the splitter 24 via a loop driver. The ADSL transceiver provides bi-directional communication between the broadband data path and the loop. The digital processing circuits provide for the digital signal processing function such a modulation, echo cancellation and equalization. Typically the AFE includes both transmit and receive channels. The transmitter channel consists of a digital to analog converter (D/A) coupled to the broadband data bus, transmit filters, followed by a line driver coupled through a hybrid to the splitter. The receive channel includes a receive filter coupled to the hybrid, a programmable gain amplifier driving an analog to digital converter, which in turn outputs digital signal to the digital processing unit onto the broadband data path. Both the transmit and receive channels are couple to the splitter which is in turn coupled by a transformer to the loop. As described in the background of the invention, there are many disadvantages with this arrangement. The present invention involves modification to this conventional arrangement by integrating the voice/data analog front end (AFE) 42 and the voice/data baseband 44 with a common loop driver 46 as shown schematically in figure 4. The integrated unit thus provides bi-directional communication between the loop and a system bus or busses carrying ADSL and PCM data. If this integrated approach is taken, the frequency dependent termination in each frequency band must be synthesized by the AFE 42, the baseband 44, or a combination of the two. Further, the loop driver must provide relatively flat response over the full range of frequency bands, which in an integrated POTS and ADSL application would be from 200Hz to l .lMHz. Referring to figure 5, a top level block diagram of an integrated line interface module 40 of figure 4, for voice and data requirements according to an embodiment of the present invention is shown in greater detail. The AFE 42 includes a loop driver 52 coupled to the loop, a wide band AFE 54 possibly including impedance synthesis and coupled to the loop driver 52, and A/D and D/A converters 56. The AFE thus provides bi-directional communication between the loop and the digital processing section 44 and may provide full or partial frequency dependent impedance synthesis. The digital processing section 44 may include a digital impedance synthesis unit 58 which is coupled to the AFE 42. The unit performs either full or partial impedance synthesis required for the composite system and provides frequency dependent filtering and equalization. The composite signal is coupled to a baseband processing unit comprising one or more DSP based processing elements which implement the voice CODEC 60 and ADSL modem 62 functions. The baseband processing unit is coupled to one or more system busses carrying a high speed ADSL data and PCM voice signals. A feature of this architecture is a broadband loop drive circuit which allows the integration of the complex impedance synthesis function in highly integrated digital or analog circuit. In a preferred embodiment, the loop driver provides DC feed capability of up to 100mA; flat or nearly flat frequency response between 200Hz and l.lMHz or beyond; balanced loop drive; AC signal swing of up to 22V peak; and low power dissipation. One approach in the prior art is to utilize a broadband SLIC implemented in a high voltage integrated circuit technology. In this approach, the SLIC needs to carry both DC loop currents, low frequency voice signals, and high frequency data signals. This approach suffers from a variety of problems including excessive power dissipation because of the large signal currents and signal voltage headroom required on the driver amplifiers; difficulty of implementation in silicon technologies; and limited loop range with 48V battery feed. A preferred approach is to use a transformer based drive circuit for coupling the integrated voice/xDSL unit to the loop. The primary difficulties in using a transformer- based solution are caused by transformer roll-off at low and high frequencies because of a combination of primary inductance, leakage inductance and interwinding capacitance and core limitations. Referring to figures 7(a)-(f), various transformer based drive circuits for coupling the integrated voice/xDSL unit to the loop are shown. All seek to avoid the problems caused by transformer limitations by using feedback techniques to linearize the performance of the circuits in this application. In figure 7(b) a transformer configuration is shown having a feedback loop for overdriving the input. In figure 7(c) a pair of transformers is provided, with a high frequency feed forward on the second transformer. Figure 7(d) is a variation of the configuration of figure 7(c) wherein a high frequency feed forward with a capacitor is provided. Finally in figure 7(e) two transformers are provided with a separate drive to the second transformer. In figure 8(a) - 8(f) a broadband transformer has a split primary with the first winding and a second winding having polarities in the manner shown. The transformer has a single secondary winding 104. As used herein, the primary refers to the loop side of the line feed transformer and the secondary refers to the voice/xDSL modem side of the transformer. The transformer's primary to secondary turns ratio is preferably about 1 :1 but can be other ratios. Typically, the line transformer is a 1 : 1 which must maintain a magnetizing inductance of lOOmH to 2H while carrying 20mA to 100mA in the primary winding. The typical AC voltage swing is up to 22 volts peak, while the impedance looking onto the loop is in the order of 900 ohms in the voice band and 100 ohms in the ADSL band. Referring to figure 8(a) and an interface circuit (not shown) provides an input V, to a driver circuit 180 which is coupled to a twisted pair telephone loop having TIP and RING leads which generally extend between the central office and customer premises. The drive circuit includes a transformer having a pair of primary windings coupled to the respective TIP and RING lines by a respective line feed elements R feed which are typically 50 ohms. The other ends of the primary windings are connected to respective battery return or TIP DC and a talk battery or RING DC. On the secondary side of the transformer, the input voltage V, is provided to each terminal of the secondary via respective inverting opamps. In figure 8(a) the first primary winding 106 is coupled at one end to a feed resistor 107 coupled one of the loop conductors TIP, while the second primary winding 108 is coupled to a feed resistor 110 coupled to the second of the loop conductors RING. In figure 8(b) the differential voltage at the primary side is sensed and fed back to the secondary side. The feedback circuit senses the differential output voltage Vo on the primary side of the transformer, compares that differential output voltage Vo to the desired output voltage Vi, and multiplies the error signal by a high gain to generate a feedback signal Vf and drives the secondary of the with the signal Vf to provide increased drive to the transformer. In figure 8(c) the differential voltage at the primary side is sensed and fed back to the secondary side. The feedback circuit senses the differential output voltage Vo on the primary side of the transformer, compares that differential output voltage Vo to the desired output voltage Vi, and multiplies the error signal by a high gain to generate a feedback signal Vf and drives the primary side of the transformer with Vf through capacitors. The secondary side of the transformer is driven by the desired input voltage Vi. In figure 8(d) the differential voltage at the primary side is sensed and fed back to the secondary side. The feedback circuit senses the differential output voltage Vo on the primary side of the transformer, compares that differential output voltage Vo to the desired output voltage V„ and multiplies the error signal by a high gain to generate a feedback signal V f and drives the primary side of the transformer with complimentary signals V f through respective capacitors. The secondary side of the transformer also driven by the feedback signal V f . In figure 8(e) the differential voltage at the primary side is sensed and fed back to the secondary side. The feedback circuit senses the differential output voltage Vo on the primary side of the transformer, compares that differential output voltage Vo to the desired output voltage Vi, and multiplies the error signal by a high gain to generate a feedback signal Vf and drives the primary side of the transformer a second transformer coupled through capacitors. The signal Vf drives the secondary of the second transformer and the primary windings of the second transformer are capacitively coupled to the output voltage Vo. The secondary side of the first transformer is driven by the desired input signal Vi. In figure 8(f) the differential voltage at the primary side is sensed and fed back to the secondary side. The feedback circuit senses the differential output voltage Vo on the primary side of the transformer, compares that differential output voltage Vo to the desired output voltage Vi, and multiplies the error signal by a high gain to generate a feedback signal Vf and drives the primary side of the transformer a second transformer coupled through capacitors. The signal Vf drives the secondary of the second transformer and the primary windings of the second transformer are capacitively coupled to the output voltage Vo. The secondary side of the first transformer driven by the output signal Vf. Figure 8(g) is similar to figure 8(f) except that an impedance is also added between the primary windings of the first transformer and the output voltage Vo. In this configuration, the two transformers can be configured to provide drive in different frequency bands, with the crossover frequency determined by the impedances in series with the two transformers. Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims: WHAT IS CLAIMED IS: 1. A driver circuit comprising: (a) a transformer having a primary winding for coupling to a subscriber loop and a secondary winding for coupling to an interface circuit; (b) a feedback circuit for coupling said primary winding to said secondary winding to provide a predetermined impedance match between said interface circuit and said loop over a predetermined frequency band of said circuit. 2. A driver as defined in claim 1, said interface circuit for supporting a POTS protocol and an xDSL protocol. 3. A driver as defined in claim 1, said feedback circuit including a comparator for comparing a differential output voltage Vo on said loop to an input voltage N from said interface circuit to generate an error signal V f for driving said primary winding of said transformer. 4. A driver as defined in claim 1 , said predetermined frequency band including a high frequency band for data communication and a low frequency band for voice communication. 5. A driver circuit as defined in claim 1, said transformer including a first and second transformer each for driving a respective first and second type of signal on to said loop from a common feed point. 6. A driver as defined in claim 5, said first type of signal including a POTS signal and said second type of signal including a DSL signal. 7. A driver as defined in claim 1, said feedback circuit providing compensation for transformer roll off. 8. A driver as defined in claim 1, said feedback circuit including a comparator for comparing a differential output voltage on said loop to a input voltage Vi to generate a feedback signal Vf; and DC isolation circuit for coupling said feedback signal to the loop. 9. A driver as defined in claim 8, said DC isolation circuit being a capacitor. 10. A driver as defined in claim 8, said DC isolation circuit for coupling said including coupling said feedback signal V f to said secondary of said transformer. 11. A driver as defined in claim 8, said DC isolation circuit being a third transformer. 12. A driver as defined in claim 11, said third transformer being capacitavely coupled to said loop. 13. A driver as defined in claim 11, including a line impedance element coupled between said primary and said loop. 14. A driver as defined in claim 11, said third transformer for coupling said feedback signal V f to said secondary of said first transformer. 15. A driver as defined in claim 14, said third transformer also being capacitively coupled to said loop. 16. A driver as defined in claim 14, including a line impedance element coupled between said primary and said line. 17. A driver as defined in claim 1, said interface circuit being an integrated line card for providing telephony and DSL. 18. A driver as defined in claim 17, said integrated line card including an impedance synthesis circuit for generating a desired termination impedance in each frequency band of operation of said card.

Inventors: FEELEY MARK, FEELEY, MARK
Applicants: CATENA NETWORKS INC, FEELEY MARK, CATENA NETWORKS, INC., FEELEY, MARK
Application year: 2000
Main classification: H04M1
IPC codes: H03F1, H04L27, H04M11, H04Q3