System and method for adapting an analog echo canceller in a transceiver front end

• US 6,980,644 B1
• Filed: 05/12/2000
• Issued: 12/27/2005
• Est. Priority Date: 05/12/2000
• Status: Expired due to Term

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Sallaway, Peter J., Raghavan, Sreen, Manickam, Thulasinath G.
• Primary Examiner(s): Isen, Forester W.
• Assistant Examiner(s): SINGH, RAMNANDAN P
• Application Number:
  US09/570,078
• Time in Patent Office: 2,055 Days
• Field of Search: 379/391, 379/392
• US Class Current: 379/391
• CPC Class Codes: H04M 9/082 using echo cancellers echo cancellers per se H04B3/23

Abstract

There is disclosed an echo canceller circuit for use in a full duplex transceiver of the type comprising a line driver capable of sending analog transmit signals through a cable and comprising a line receiver capable of receiving analog receive signals from the cable. An echo canceller impedance model circuit is coupled to an output of the line driver and is coupled to an input of the line receiver. The echo canceller impedance model circuit generates an echo canceller current that is equal in magnitude and opposite in phase to a current that represents signal echoes that are present in the analog receive signals. The echo canceller impedance model circuit has a variable impedance for generating the echo canceller current. The variable impedance has at least one variable resistor and at least one variable capacitor. The values of resistance and capacitance in the echo canceller impedance model circuit are varied in response to control signals from a echo canceller control circuit to compensate for and cancel signal echoes.

Claims

1. For use in a transceiver of the type comprising a line driver capable of sending analog transmit signals through a cable, and a line receiver capable of receiving analog receive signals from said cable, and an analog echo canceller capable of cancelling echoes of said analog transmit signals that are present in said analog receive signals by generating an echo canceller current that is equal in magnitude and opposite in phase to a current that represents said signal echoes, a system for adapting said analog echo canceller comprising:

an analog echo canceller control circuit capable of changing a first value of the resistance of at least one variable resistor in said analog echo canceller by subtracting from said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is positive.

2. The system as claimed in claim 1 wherein said analog echo canceller control circuit is capable of changing a first value of the resistance of at least one variable resistor in said analog echo canceller by adding to said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is negative.

3. The system as claimed in claim 1 wherein said system is capable of determining the value of p, the residual uncancelled echo at the output of said analog echo canceller, from at least one error signal at the output of a slicer of said transceiver.

4. The system as claimed in claim 3 wherein said system is capable of determining the value of p by subtracting an estimate of the magnitude of a receive signal at the output of said slicer from a signal that represents the sum of said receive signal and said residual uncancelled echo signal at the output of said slicer, and by filtering said residual uncancelled echo signal at the output of said slicer to reverse equalization and gain applied to said residual uncancelled echo signal between said analog echo canceller and said slicer.

5. The system as claimed in claim 4 wherein said system is capable of filtering said residual uncancelled echo signal at the output of said slicer with a filter that represents the channel response of a channel between said transceiver and a remote end transceiver.

6. The system as claimed in claim 5 wherein said system is capable of transferring said filtered uncancelled echo signal to an echo canceller update loop in said analog echo canceller control circuit to determine a new value of resistance for said at least one variable resistor in said analog echo canceller.

7. The system as claimed in claim 5 wherein said analog echo canceller control circuit is capable of changing a first value of the resistance of at least one variable resistor in said analog echo canceller by subtracting from said first value of resistance a second value of resistance equal to the product of the terms μ

, (p*F′

*1/C), and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where C represents the channel response of said transceiver, and where F′

represents the response of a filter applied to said residual uncancelled echo signal at the output of said slicer, and where F′

represents a response that is approximately equal to the channel response C of said transceiver, and where the symbol * represents the mathematical operation of convolution, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term (p*F′

*1/C) and the term tx to a quantity expressed in ohms, when the correlation of the term (p*F′

*1/C) and the term tx is positive.

8. The system as claimed in claim 7 wherein said filter response F′

equals the expression

1+2Z<sup>−

1</sup>+4Z<sup>−

2</sup>+8Z<sup>−

3</sup>+4Z<sup>−

4</sup>+2Z<sup>−

5</sup>+Z<sup>−

6</sup>

where Z represents the mathematical operation known as the Z transform.

9. The system as claimed in claim 5 wherein said analog echo canceller control circuit is capable of changing a first value of the resistance of at least one variable resistor in said analog echo canceller by adding to said first value of resistance a second value of resistance equal to the product of the terms μ

, (p*F′

*1/C), and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where C represents the channel response of said transceiver, and where F′

represents the response of a filter applied to said residual uncancelled echo signal at the output of said slicer, and where F′

represents a response that is approximately equal to the channel response C of said transceiver, and where the symbol * represents the mathematical operation of convolution, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term (p*F′

*1/C) and the term tx to a quantity expressed in ohms, when the correlation of the term (p*F′

*1/C) and the term tx is negative.

10. The system as claimed in claim 9 wherein said filter response F′

equals the expression

1+2Z<sup>−

1</sup>+4Z<sup>−

2</sup>+8Z<sup>−

3</sup>+4Z<sup>−

4</sup>+2Z<sup>−

5</sup>+Z<sup>−

6</sup>

where Z represents the mathematical operation known as the Z transform.

11. A transceiver comprising:

a line driver capable of sending analog transmit signals through a cable;

a line receiver capable of receiving analog receive signals from said cable;

an analog echo canceller capable of cancelling echoes of said analog transmit signals that are present in said analog receive signals by generating an echo canceller current that is equal in magnitude and opposite in phase to a current that represents said signal echoes; and

an analog echo canceller control circuit capable of;

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by subtracting from said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is positive; and

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by adding to said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is negative.

12. The transceiver as claimed in claim 11 wherein said transceiver comprises a full duplex transceiver.

13. For use in a transceiver of the type comprising a line driver capable of sending analog transmit signals through a cable, and a line receiver capable of receiving analog receive signals from said cable, and an analog echo canceller capable of cancelling echoes of said analog transmit signals that are present in said analog receive signals by generating an echo canceller current that is equal in magnitude and opposite in phase to a current that represents said signal echoes, a method for adapting said analog echo canceller comprising the steps of:

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by subtracting from said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is positive.

14. The method as claimed in claim 13 further comprising the step of:

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by adding to said first value of resistance a second value of resistance equal to the product of the terms μ

, p, and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term p and the term tx to a quantity expressed in ohms, when the correlation of the term p and the term tx is negative.

15. The method as claimed in claim 13 further comprising the step of:

determining the value of p, the residual uncancelled echo at the output of said analog echo canceller, from at least one error signal at the output of a slicer of said transceiver.

16. The method as claimed in claim 15 wherein the step of determining the value of p, the residual uncancelled echo at the output of said analog echo canceller, from at least one error signal at the output of a slicer of said transceiver comprises the steps of:

subtracting an estimate of the magnitude of a receive signal at the output of said slicer from a signal that represents the sum of said receive signal and said residual uncancelled echo signal at the output of said slicer; and

filtering said residual uncancelled echo signal at the output of said slicer to reverse equalization and gain applied to said residual uncancelled echo signal between said analog echo canceller and said slicer.

17. The method as claimed in claim 16 wherein the step of filtering said residual uncancelled echo signal at the output of said slicer to reverse equalization and gain applied to said residual uncancelled echo signal between said analog echo canceller and said slicer comprises:

filtering said residual uncancelled echo signal at the output of said slicer with a filter represents the channel response of a channel between said transceiver and a remote end transceiver.

18. The method as claimed in claim 17 further comprising the step of:

transferring said filtered uncancelled echo signal to an echo canceller update loop in an analog echo canceller control circuit to determine a new value of resistance for said at least one variable resistor in said analog echo canceller.

19. The method as claimed in claim 17 further comprising the step of:

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by subtracting from said first value of resistance a second value of resistance equal to the product of the terms μ

, (p*F′

*1/C), and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where C represents the channel response of said transceiver, and where F′

represents the response of a filter applied to said residual uncancelled echo signal at the output of said slicer, and where F′

represents a response that is approximately equal to the channel response C of said transceiver, and where the symbol * represents the mathematical operation of convolution, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term (p*F′

*1/C) and the term tx to a quantity expressed in ohms, when the correlation of the term (p*F′

*1/C) and the term tx is positive.

20. The method as claimed in claim 17 further comprising the step of:

changing a first value of the resistance of at least one variable resistor in said analog echo canceller by adding to said first value of resistance a second value of resistance equal to the product of the terms μ

, (p*F′

*1/C), and tx, where p represents residual uncancelled echo at the output of said analog echo canceller, and where C represents the channel response of said transceiver, and where F′

represents the response of a filter applied to said residual uncancelled echo signal at the output of said slicer, and where F′

represents a response that is approximately equal to the channel response C of said transceiver, and where the symbol * represents the mathematical operation of convolution, and where tx represents a transmit signal, and where μ

represents a scale factor for converting the product of the term (p*F′

*1/C) and the term tx to a quantity expressed in ohms, when the correlation of the term (p*F′

*1/C) and the term tx is negative.