Method and apparatus for RF common-mode noise rejection in a DSL receiver

Abstract

A receiver for high-speed data communications, which receives a differential signal through a pair of signal lines. The receiver includes a common-mode choke, which has first and second signal windings, which are respectively coupled in series to the pair of signal lines so as to attenuate common-mode interference in the differential signal. The choke also has a sampling winding, which is inductively coupled to the signal windings so as to generate a sampled signal responsive to current flowing in the signal windings. Signal processing circuitry is coupled to receive the sampled signal from the sampling winding and to receive the differential signal from the signal windings and to process the differential signal responsive to the sampled signal.

Description

FIELD THE INVENTION[0001]The present invention relates generally to high-speed data communications over telephone cables, and specifically to methods and systems for rejection of radio frequency (RF) noise in Digital Subscriber Line (DSL) modems.BACKGROUND OF THE INVENTION[0002]Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. This is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure. DSL systems use special line signals that are well-adapted to the characteristics of twisted...

AI Technical Summary

Benefits of technology

[0008]It is an object of some aspects of the present invention to provide improved devices and methods for sampling common-mode signals.

[0011]The three-winding choke of the present invention thus provides two stages of interference suppression in a single device: a first stage of attenuation by the high common-mode impedance of the choke itself, and a second stage of RF noise suppression by the noise cancellation circuit. It thus provides superior attenuation of RF interference, without the need for a resistive tap off the input lines to the receiver, as is used in noise cancellation circuits known in the art, such as that described in the above-mentioned PCT patent application. Therefore, it also eliminates the requirement that the sampled common-mode interference signal be referred to ground, and protects the noise cancellation circuit from high-voltage surges.

[0026]processing the differential signal responsive to the sampled signal so as to enhance a signal-to-noise characteristic of the differential signal following the inductive elements.

Problems solved by technology

One problem of VDSL implementation is that the frequency band of VDSL (up to 20 MHz) overlaps with several RF bands used for amateur radio and radio broadcast transmissions.

Twisted-pair wire is particularly prone to picking up interference signals from external RF sources.

This problem becomes more severe the higher the transmission frequency and can cause significant degradation of VDSL signals.

The common mode choke attenuates RF noise by about 30 dB, but this attenuation is not enough for long cables, wherein the VDSL line signal is typically very small and the RF noise may be large.

The technical solution proposed by PCT / US97 / 06381 has several disadvantages which make its practical realization difficult.

Therefore, a common mode reference noise signal from the center tap of the primary winding cannot be connected directly to the RF noise canceller, and an additional high-voltage transformer is required between the center tap of the line transformer and the RF noise canceller.

A further difficulty with this solution is that it allows RF common mode noise to reach the primary winding of the line transformer without any attenuation.

Because of inter-winding capacitance of the transformer, a strong noise signal arises on the secondary winding of transformer, as well.

But if a digital noise canceller is to be used (generally the most practical solution), the high input noise level makes it necessary to use a costly analog / digital (A / D) converter with high dynamic range.

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