Method and apparatus for reducing DC offsets in a communication system

• US 7,085,335 B2
• Filed: 11/09/2001
• Issued: 08/01/2006
• Est. Priority Date: 11/09/2001
• Status: Expired due to Fees

• Current Assignee: ParkerVision, Inc.
• Original Assignee: ParkerVision, Inc.
• Inventors: Rawlins, Gregory S., Rawlins, Michael W.
• Primary Examiner(s): VO, DON NGUYEN
• Application Number:
  US09/986,764
• Publication Number:
  US 20030103581A1
• Time in Patent Office: 1,726 Days
• Field of Search: 375/316, 375/319, 375/346
• US Class Current: 375/346
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

Methods and apparatuses for reducing DC offsets in a communication system are described. In a first aspect, a feedback loop circuit reduces DC offset in a wireless local area network (WLAN) receiver channel. The frequency response of the feedback loop circuit can be variable. In a second aspect, a circuit provides gain control in a WLAN receiver channel. First and second automatic gain control (AGC) amplifiers are coupled in respective portions of the receiver channel. Circuits for monitoring DC offset, and for providing control signals for controlling the frequency response of the DC offset reducing circuits are also provided.

Claims

1. A feedback loop circuit apparatus for reducing DC offset in a communication channel, comprising:

a summer coupled to a receiver channel, wherein a receiver channel signal is coupled as an first input to said summer; and

an integrator comprising an input coupled to said receiver channel, wherein an output of said integrator is coupled as a second input to said summer, wherein said integrator includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

wherein said integrator has a time constant that is variable according to at least one control signal to vary a frequency response of the integrator.

2. The apparatus of claim 1, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

3. The apparatus of claim 1, further comprising means for controlling said integrator frequency response to vary a frequency response of the feedback loop circuit to a first frequency response, a second frequency response, and a third frequency response each having a corresponding lower 3 dB frequency, wherein said first frequency response has a relatively low lower 3 dB frequency, said second frequency response has a relatively medium lower 3 dB frequency, and said third frequency response has a relatively greater lower 3 dB frequency.

4. The apparatus of claim 1, further comprising means for varying said variable resistor to alter the frequency response of said integrator.

5. The apparatus of claim 4, wherein said variable resistor includes:

at least one resistor; and

at least one switch across said at least one resistor.

6. The apparatus of claim 4, wherein said variable resistor includes:

a first resistor;

a second resistor coupled in series with said first resistor;

a first switch coupled in parallel across said second resistor;

a third resistor coupled in series with said second resistor; and

a second switch coupled in parallel across said third resistor.

7. The apparatus of claim 6, wherein said first switch receives a first control signal, and said second switch receives a second control signal, and further comprising means for sequencing said first and second control signals in three consecutive time periods according to the following table:

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

8. The apparatus of claim 7, wherein said third resistor has a larger resistance value than said second resistor, and wherein said second resistor has a larger resistance value than said first resistor.

9. The apparatus of claim 7, wherein said first time period is within the range of 5 to 6 microseconds, and wherein said second time period is within the range of 55 to 128 microseconds.

10. The apparatus of claim 1, wherein said integrator is configured in an inverting integrator configuration.

11. The apparatus of claim 1, further comprising:

at least one amplifier that couples said receiver channel to said input of said integrator.

12. The apparatus of claim 11, wherein said at least one amplifier is configured as a differential amplifier.

13. The apparatus of claim 1, wherein said receiver channel signal is a radio frequency signal.

14. The apparatus of claim 1, wherein said receiver channel signal is an intermediate frequency signal.

15. The apparatus of claim 1, wherein said summer is a node.

16. The apparatus of claim 1, wherein said summer is a summing circuit.

17. A method for reducing DC offset in a communication channel, comprising the steps of:

(1) integrating an output signal available at a first node to generate an integrated signal using an integrator circuit, wherein the integrator circuit includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

(2) summing the integrated signal with a receiver channel signal at a second node, wherein the first node is downstream from the second node in a receiver channel; and

(3) varying a time constant associated with step (1) in response to at least one control signal to vary a frequency response of said integration.

18. The method of claim 17, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

19. The method of claim 17, wherein step (1) includes the step of:

generating the integrated signal as an integrated and inverted version of the output signal.

20. The method of claim 17, wherein step (3) comprises the step of:

varying the value of the variable resistor to alter the frequency response of the integrator circuit.

21. The method of claim 17, further comprising the step of:

(i) configuring the variable resistor.

22. The method of claim 21, wherein the variable resistor includes at least one resistor and at least one switch, step (i) comprising the step of:

coupling said at least one switch in parallel across said at least one resistor.

23. The method of claim 21, wherein the variable resistor includes a first resistor, a first switch, a second resistor, a second switch, and a third resistor, step (i) comprising the steps of:

coupling the first switch in parallel across the second resistor;

coupling the second resistor in series with the first resistor;

coupling the second switch in parallel across the third resistor; and

coupling the third resistor in series with the second resistor.

24. The method of claim 23, wherein the first switch receives a first control signal and said second switch receives a second control signal, further comprising the step of:

(I) sequencing the first and second control signals according to the following table;

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

25. The method of claim 24, wherein step (I) comprises the step of:

sequencing the first and second control signals, wherein the first time period is in the range of 4 to 6 microseconds, and wherein the second time period is in the range of 55 to 128 microseconds.

26. The method of claim 24, further comprising the steps of:

receiving a data frame preamble during the first and second time periods; and

receiving a data portion of the data frame corresponding to the preamble during the third time period.

27. The method of claim 17, wherein step (2) comprises the step of:

receiving the receiver channel signal, wherein the receiver channel signal is a radio frequency signal.

28. The method of claim 17, wherein step (2) comprises the step of:

receiving the receiver channel signal, wherein the receiver channel signal is an intermediate frequency signal.

30. The method of claim 20, wherein step (2) comprises the step of:

subtracting the integrated signal from the receiver channel signal at the second node.

29. A method for reducing DC offset in a communication channel, comprising the steps of:

(1) measuring a DC offset voltage present in an output signal at a first node, including the step of integrating the output signal available at the first node to generate an integrated signal that includes the DC offset voltage, wherein said integrating is performed by an integrator circuit that includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

(2) removing the measured DC offset voltage from a receiver channel signal at a second node, wherein the first node is downstream from the second node in a receiver channel; and

(3) varying a time constant of the integrator circuit in response to at least one control signal to vary a frequency response of the integrator circuit.

31. The method of claim 29, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

32. A feedback loop circuit apparatus for reducing DC offset in a communication channel, comprising:

a summer coupled to a receiver channel, wherein a receiver channel signal is coupled as an first input to said summer; and

an integrator comprising an input coupled to said receiver channel, wherein an output of said integrator is coupled as a second input to said summer;

wherein said integrator includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

wherein said variable resistor is varied to alter a frequency response of said integrator;

wherein said variable resistor includes;

a first resistor,

a second resistor coupled in series with said first resistor,

a first switch coupled in parallel to said second resistor,

a third resistor coupled in series with said second resistor, and

a second switch coupled in parallel to said third resistor.

33. The apparatus of claim 32, wherein said first switch receives a first control signal, and said second switch receives a second control signal, further comprising means for sequencing said first and second control signals in three consecutive time periods according to the following table:

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

34. The apparatus of claim 33, wherein said first time period is within the range of 5 to 6 microseconds, and wherein said second time period is within the range of 55 to 128 microseconds.

35. The apparatus of claim 32, wherein said third resistor has a larger resistance value than said second resistor, and wherein said second resistor has a larger resistance value than said first resistor.

36. The apparatus of claim 32, wherein said summer is a node.

37. The apparatus of claim 32, wherein said summer is a summing circuit.

38. A method for reducing DC offset in a communication channel, comprising the steps of:

(1) arranging an amplifier, capacitor, and variable resistor in an integrating amplifier configuration, wherein the variable resistor includes a first resistor, a first switch, a second resistor, a second switch, and a third resistor;

(2) configuring the variable resistor, including the steps of;

coupling the first switch in parallel across the second resistor,

coupling the second resistor in series with the first resistor,

coupling the second switch in parallel across the third resistor, and

coupling the third resistor in series with the second resistor;

(3) integrating an output signal available at a first node to generate an integrated signal, wherein said integrating is performed by an integrator circuit, wherein the integrator circuit includes the amplifier, capacitor, and variable resistor;

(4) summing the integrated signal with a receiver channel signal at a second node, wherein the first node is downstream from the second node in a receiver channel; and

(5) varying the frequency response of the integrator circuit in response to a control signal, including the step of varying the value of the variable resistor to alter the frequency response of the integrator circuit.

39. The method of claim 38, wherein the first switch receives a first control signal and the second switch receives a second control signal, further comprising the step of:

(I) sequencing the first and second control signals according to the following table;

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

40. The method of claim 39, wherein step (I) comprises the step of:

sequencing the first and second control signals, wherein the first time period is in the range of 4 to 6 microseconds, and wherein the second time period is in the range of 55 to 128 microseconds.

41. The method of claim 39, further comprising the steps of:

receiving a data frame preamble during the first and second time periods; and

receiving a data portion of the data frame corresponding to the preamble during the third time period.

42. A communications system that includes a feedback loop circuit for reducing DC offset in a communication channel, comprising:

a summer coupled to a receiver channel, wherein a receiver channel signal is coupled as an first input to said summer; and

an integrator comprising an input coupled to said receiver channel, wherein an output of said integrator is coupled as a second input to said summer, wherein said integrator includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

wherein said integrator has a time constant that is variable according to at least one control signal to vary a frequency response of the integrator.

43. The system of claim 42, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

44. The system of claim 42, wherein said integrator frequency response is controlled to vary the frequency response of the feedback loop circuit to a first frequency response, a second frequency response, and a third frequency response each having a corresponding lower 3 dB frequency, wherein said first frequency response has a relatively low lower 3 dB frequency, said second frequency response has a relatively medium lower 3 dB frequency, and said third frequency response has a relatively greater lower 3 dB frequency.

45. The system of claim 42, wherein said variable resistor is varied to alter the frequency response of said integrator.

46. The system of claim 45, wherein said variable resistor includes:

at least one resistor; and

at least one switch coupled in parallel across said at least one resistor.

47. The system of claim 45, wherein said variable resistor includes:

a first resistor;

a second resistor coupled in series with said first resistor;

a first switch coupled in parallel across said second resistor;

a third resistor coupled in series with said second resistor; and

a second switch coupled in parallel across said third resistor.

48. The system of claim 47, wherein said first switch receives a first control signal, and said second switch receives a second control signal, further comprising means for sequencing said first and second control signals in three consecutive time periods according to the following table:

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

49. The system of claim 48, wherein said third resistor has a larger resistance value than said second resistor, and wherein said second resistor has a larger resistance value than said first resistor.

50. The system of claim 48, wherein said first time period is within the range of 5 to 6 microseconds, and wherein said second time period is within the range of 55 to 128 microseconds.

51. The system of claim 42, wherein said integrator is configured in an inverting integrator configuration.

52. The system of claim 42, further comprising:

at least one amplifier that couples said second node to said input of said integrator.

53. The system of claim 52, wherein said amplifier is configured as a differential amplifier.

54. The system of claim 42, wherein said receiver channel signal is a radio frequency signal.

55. The system of claim 42, wherein said receiver channel signal is an intermediate frequency signal.

56. The system of claim 42, wherein said summer is a node.

57. The system of claim 42, wherein said summer is a summing circuit.

58. A communications system that includes a feedback loop circuit for reducing DC offset in a communication channel, comprising:

a summer coupled to a receiver channel, wherein a receiver channel signal is coupled as an first input to said summer; and

an integrator comprising an output coupled to said receiver channel, wherein an output of said integrator is coupled as a second input to said summer;

wherein said integrator has a time constant that is variable according to at least one control signal to vary a frequency response of the integrator;

wherein said integrator includes an amplifier, a capacitor, and a variable resistor arranged in an integrating amplifier configuration;

wherein said variable resistor is varied to alter the frequency response of said integrator;

wherein said variable resistor includes;

a first resistor,

a second resistor coupled in series with said first resistor;

a first switch coupled in parallel across said second resistor;

a third resistor coupled in series with said second resistor; and

a second switch coupled in parallel across said third sensor.

59. The system of claim 58, wherein said first switch receives a first control signal, and said second switch receives a second control signal, further comprising means for sequencing said first and second control signals in three consecutive time periods according to the following table:

first control second control signal signal first time period 1 1 second time period 0 1 third time period 0 0.

60. The system of claim 59, wherein said third resistor has a larger resistance value than said second resistor, and wherein said second resistor has a larger resistance value than said first resistor.

61. The system of claim 59, wherein said first time period is within the range of 5 to 6 microseconds, and wherein said second time period is within the range of 55 to 128 microseconds.