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.

1. A receiver for high-speed data communications, which is adapted to receive a differential signal through a pair of signal lines, the receiver comprising:
 a common-mode choke, comprising: 
 first and second signal windings, which are configured to be respectively coupled in series to the pair of signal lines so as to attenuate common mode interference in the differential signal; and  
 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; and  
 
 signal processing circuitry, which 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.  
 
     
     
       2. A receiver according to  claim 1 , wherein the sampled signal is generated responsive to the common-mode interference, and wherein the circuitry processes the differential signal to cancel the attenuated common-mode interference out of the differential signal. 
     
     
       3. A receiver according to  claim 2 , wherein the circuitry processes the sampled signal to derive an estimate of the common-mode interference, which is subtracted from the differential signal. 
     
     
       4. A receiver according to  claim 2 , wherein the sampled signal is conveyed to the processing circuitry substantially without reference to a ground. 
     
     
       5. A receiver according to  claim 1 , wherein the sampling winding is electrically isolated from the signal windings. 
     
     
       6. A receiver according to  claim 1 , and comprising a transformer, which comprises a primary winding coupled to receive the differential signal from the signal windings of the choke and a secondary winding coupled to convey the differential signal to the signal processing circuitry, wherein the primary winding comprises a center tap which is grounded. 
     
     
       7. A receiver according to  claim 1 , wherein the pair of signal lines comprises a twisted pair of lines. 
     
     
       8. A receiver according to  claim 7 , wherein the differential signal comprises a Digital Subscriber Line (DSL) signal, and wherein the signal processing circuitry comprises modem circuitry for decoding the DSL signal. 
     
     
       9. A receiver according to  claim 8 , wherein the Digital Subscriber Line signal comprises a Very High Rate Digital Subscriber Line (VDSL) signal. 
     
     
       10. A receiver according to  claim 1 , wherein the first and second signal windings are wound together in a common winding direction and have substantially equal respective inductances. 
     
     
       11. A receiver according to  claim 1 , wherein the common-mode choke comprises a core upon which the signal windings and the sampling winding are commonly wound. 
     
     
       12. A receiver according to  claim 1 , wherein the sampling winding is electrically isolated from the signal windings. 
     
     
       13. A method for processing a differential signal received on a pair of signal lines, comprising:
 coupling a pair of inductive elements in series with the respective signal lines so as to attenuate common-mode interference in the lines relative to the differential signal;  
 inductively sampling electrical currents in the inductive elements so as to generate a sampled signal; and  
 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.  
 
     
     
       14. A method according to  claim 13 , wherein inductively sampling the currents comprises sampling the common-mode interference, and wherein processing the differential signal comprises reducing the common-mode interference in the differential signal. 
     
     
       15. A method according to  claim 14 , wherein reducing the common-mode interference comprises deriving an estimate of the interference and subtracting the estimated interference from the differential signal. 
     
     
       16. A method according to  claim 13 , wherein the differential signal comprises a Digital Subscriber Line signal. 
     
     
       17. A differential receiver for high- speed data communications comprising:      first and second signal lines receiving a differential signal;        an analog - to - digital converter;        a common - mode filter connected in series between the first and second signal lines and the analog - to - digital converter, the common - mode filter attenuating common - mode interference in the differential signal;        a sampling unit generating a sampled signal responsive to current through the common - mode filter; and        signal processing circuitry receiving a filtered signal representative of the differential signal and the sampled signal and processing the filtered signal based upon the sampled signal.     
     
     
       18. The differential receiver according to  claim 17 , wherein the common- mode filter comprises first and second coils respectively connected to the first and second signal lines.   
     
     
       19. The differential receiver according to  claim 18 , wherein the first and second coils present a higher overall inductance to a common mode portion of the differential signal than to a differential portion of the differential signal. 
     
     
       20. The differential receiver according to  claim 19 , further comprising:
   a high pass filter connected in series between the common - mode filter and the analog - to - digital converter.     
     
     
       21. The differential receiver according to  claim 20 , wherein the high pass filter comprises first and second capacitors respectively connected to the first and second coils. 
     
     
       22. The differential receiver according to  claim 21 , further comprising:
   a transformer and a differential amplifier connected in series between the first and second capacitors and the analog - to - digital converter.     
     
     
       23. The differential receiver according to  claim 17 , wherein the common- mode filter comprises a sampling coil, the sampling unit receiving a signal representative of the current through the common - mode filter from the sampling unit.   
     
     
       24. The differential receiver according to  claim 23 , further comprising:
   an interference filter connected in series between the sampling coil and the signal processing circuitry, the filter providing a signal representative of noise within the signal received by the signal processing circuitry from the analog - to - digital converter.     
     
     
       25. The differential receiver according to  claim 24 , further comprising:
   an interference filter connected in series between the sampling coil and the signal processing circuitry, the filter wherein the filter comprises a sampling coil, the sampling unit receiving a signal representative of the current through the filter from the sampling unit.     
     
     
       26. A differential receiver for high- speed data communications comprising:      first and second signal lines receiving a differential signal;        an analog - to - digital converter; and        a common - mode filter connected in series between the first and second signal lines and the analog - to - digital converter, the common - mode filter attenuating common - mode interference in a differential signal.     
     
     
       27. A differential receiver for high- speed data communications comprising:      first and second signal lines receiving a differential signal;        first and second coils respectively connected to the first and second signal lines, the first and second coils attenuating common - mode interference in the differential signal;        a sampling coil generating a sampled signal representative of current through the first and second coils; and        signal processing circuitry receiving a filtered signal representative of the differential signal and the sampled signal and processing the filtered signal based upon the sampled signal.