Feed-forward DC-offset canceller for direct conversion receiver

• US 20030174079A1
• Filed: 03/14/2002
• Published: 09/18/2003
• Est. Priority Date: 03/14/2002
• Status: Active Grant

• Current Assignee: NEC Corporation
• Original Assignee: NEC USA Inc (NEC Corporation)
• Inventors: Soltanian, Babak, Madihian, Mohammad
• Granted Patent: US 6,700,514 B2
• Time in Patent Office: Days
• US Class Current: 341/118
• CPC Class Codes: H03M 1/0607 Offset or drift compensation removal of offset already present on the analogue input signal H03M1/1295
• CPC Class Codes: H03M 1/0626 by filtering
• CPC Class Codes: H03M 1/12 Analogue/digital converters H03M1/001 – H03M1/10 take precedence
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A feed-forward dc-offset canceller, for use in direct conversion receiver (DCR) architecture wireless systems, in which dc-offset is estimated from the down-converted signal at the mixer output, and the offset is cancelled before applying the signal to the analog baseband section. A linear digital filter estimates and tracks the dc-offset, which is subtracted from the down-converted signal before the analog baseband input. The response of the filter can be adjusted by its coefficients according to different environments and standard requirements. Although higher order filters can be used depending on the requirements, preferably, a first order recursive filter is used as it has the advantages of being a very simple and efficient structure.

Claims

1. A direct conversion receiver utilizing feed-forward dc-offset cancellation, comprising:

a dc-offset tracking unit, sampling a down-converted signal, and outputting a dc-offset signal based upon the dc-offset in the sampled down-converted signal, said dc-offset tracking unit comprising a linear digital filter, estimating and tracking dc-offset in the sampled down-converted signal;

a subtraction circuit, receiving the down-converted signal after the down-converted signal is sampled by the dc-offset tracking unit, and subtracting the dc-offset signal from the received down-converted signal; and

an analog baseband circuit, comprising a variable gain amplifier, receiving the down-converted signal after the dc-offset is subtracted, and adjusting a gain of the signal.

2. A direct conversion receiver according to claim 1, further comprising:

a mixer, receiving an RF signal, down-converting the RF signal to a zero intermediate frequency, and outputting the down-converted signal;

an analog-to-digital converter, receiving the down-converted signal after the dc-offset is subtracted by said subtraction circuit and after the gain is adjusted by said analog baseband circuit, and converting the down-converted signal to digital for processing by a digital baseband receiver;

a low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, but before the down-converted signal is converted to digital by said analog-to-digital converter; and

said digital baseband receiver, processing the digital signal.

3. A direct conversion receiver according to claim 2, said dc-offset offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said linear digital filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

4. A direct conversion receiver according to claim 3, said dc-offset tracking unit further comprising a sampling rate converter, connected between said linear digital filter and said digital-to-analog circuit.

5. A direct conversion receiver according to claim 3, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

6. A direct conversion receiver according to claim 2, further comprising another low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, but before the down-converted signal is converted to digital by said analog-to-digital converter, low pass filters being both before and after said subtraction circuit.

7. A direct conversion receiver according to claim 1, said dc-offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said linear digital filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

8. A direct conversion receiver according to claim 7, said dc-offset tracking unit further comprising a sampling rate converter, connected between said linear digital filter and said digital-to-analog circuit.

9. A direct conversion receiver according to claim 7, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

10. A direct conversion receiver according to claim 1, wherein said linear digital filter is an N-order digital filter having a transfer function:

11. A direct conversion receiver according to 10, wherein at least one of a_l and b_l are adjusted adaptively.

12. A direct conversion receiver according to 11, wherein both of a_l and b_l are adjusted adaptively.

13. A direct conversion receiver according to 10, wherein both of a_l and b_l are fixed.

14. A direct conversion receiver according to 10, wherein said linear digital filter is an infinite impulse response filter, at least one a_l not being zero for l=1 to N.

15. A direct conversion receiver according to 10, wherein said linear digital filter is a finite impulse response filter, wherein a_l=0 for l=1 to N.

16. A direct conversion receiver according to claim 10, further comprising:

a mixer, receiving an RF signal, down-converting the RF signal to a zero intermediate frequency, and outputting the down-converted signal;

an analog-to-digital converter, receiving the down-converted signal after the dc-offset is subtracted by said subtraction circuit and after the gain is adjusted by said analog baseband circuit, and converting the down-converted signal to digital for processing by a digital baseband receiver;

a low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, but before the down-converted signal is converted to digital by said analog-to-digital converter; and

said digital baseband receiver, processing the digital signal.

17. A direct conversion receiver according to claim 16, said dc-offset offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said linear digital filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

18. A direct conversion receiver according to claim 17, said dc-offset tracking unit further comprising a sampling rate converter, connected between said linear digital filter and said digital-to-analog circuit.

19. A direct conversion receiver according to claim 17, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

20. A direct conversion receiver according to claim 16, further comprising another low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, but before the down-converted signal is converted to digital by said analog-to-digital converter, low pass filters being both before and after said subtraction circuit.

21. A direct conversion receiver according to claim 10, said dc-offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said linear digital filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

22. A direct conversion receiver according to claim 21, said dc-offset tracking unit further comprising a sampling rate converter, connected between said linear digital filter and said digital-to-analog circuit.

23. A direct conversion receiver according to claim 21, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

24. A direct conversion receiver according to claim 1, wherein said linear digital filter is a first order recursive filter, having a transfer function:

25. A direct conversion receiver according to claim 24, further comprising:

a mixer, receiving an RF signal, down-converting the RF signal to a zero intermediate frequency, and outputting the down-converted signal;

an analog-to-digital converter, receiving the down-converted signal after the dc-offset is subtracted by said subtraction circuit and after the gain is adjusted by said analog baseband circuit, and converting the down-converted signal to digital for processing by a digital baseband receiver;

a low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, before the down-converted signal is converted to digital by said analog-to-digital converter; and

said digital baseband receiver, processing the digital signal.

26. A direct conversion receiver according to claim 25, said dc-offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said first order recursive filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

27. A direct conversion receiver according to claim 26, said dc-offset tracking unit further comprising a sampling rate converter, connected between said first order recursive filter and said digital-to-analog circuit.

28. A direct conversion receiver according to claim 26, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

29. A direct conversion receiver according to claim 25, further comprising another low pass filter, filtering the down-converted signal after the down-converted signal is output by said mixer, but before the down-converted signal is converted to digital by said analog-to-digital converter, low pass filters being both before and after said subtraction circuit.

30. A direct conversion receiver according to claim 24, said dc-offset tracking unit further comprising:

an analog-to-digital circuit, comprising an analog-to-digital converter sampling the down-converted signal; and

a digital-to-analog circuit, comprising a digital-to-analog converter or a digitally programmable dc-voltage source, outputting the dc-offset signal,

wherein said first order recursive filter is connected between said analog-to-digital circuit and said digital-to-analog circuit.

31. A direct conversion receiver according to claim 30, said dc-offset tracking unit further comprising a sampling rate converter, connected between said first order recursive filter and said digital-to-analog circuit.

32. A direct conversion receiver according to claim 30, said direct conversion receiver further comprising an anti-aliasing filter, filtering the down-converted signal prior to said analog-to-digital circuit of said dc-offset tracking unit.

33. A feed-forward method of canceling dc-offset from a signal in a circuit, comprising:

monitoring the signal;

detecting the dc-offset in the monitored signal by applying a linear filtering function; and

subtracting the detected dc-offset from the signal at a point in the circuit after where the signal was monitored.

34. A method according to claim 33, wherein said linear filtering function is an N-order function having a transfer function of:

35. A method according to 34, wherein at least one of a_l and b_l are adjusted adaptively.

36. A method according to 35, wherein both of a_l and b_l are adjusted adaptively.

37. A method according to 34, wherein both a_l and b_l are fixed.

38. A method according to 34, wherein said linear filtering function is an infinite impulse response function, at least one a_l not being zero for l=1 to N.

39. A method according to 34, wherein the linear filtering function is a finite impulse response function, wherein a_l=0 for l=1 to N.

40. A method according to claim 33, wherein the linear filtering function is a first order recursive function, having a transfer function of:

41. A method operating a direct conversion receiver, comprising:

receiving an RF signal;

down-converting the RF signal to a zero intermediate frequency by mixing the RF signal with a signal from a local oscillator;

canceling a dc-offset in the down-converted signal, comprising;

monitoring the down-converted signal,

detecting the dc-offset in the monitored signal by applying a linear filtering function, and

subtracting the detected dc-offset from the down-converted signal at a point in the direct conversion receiver after where the down-converted signal was monitored;

adjusting gain of the down-converted signal after the dc-offset is cancelled;

converting the gain-adjusted signal from analog to digital;

low pass filtering the down-converted signal, in-between down-converting the RF signal and converting the gain-adjusted signal from analog to digital; and

processing the digitally-converted signal.

42. A method according to claim 41, wherein detecting the dc-offset further comprises:

converting the monitored signal from analog to digital before applying said linear filtering function; and

converting a result from said linear filtering function from digital to analog,

wherein the linear filtering function is applied digitally.

43. A method according to claim 42, wherein converting the result from digital to analog includes adjusting a digital sampling rate.

44. A method according to claim 42, further comprising applying an anti-alias filtering function to the down-converted signal prior to converting the monitored signal from analog to digital.

45. A method according to claim 41, wherein said linear filtering function is an N-order function having a transfer function of:

46. A method according to 45, wherein at least one of a_l and b_l are adjusted adaptively.

47. A method according to 46, wherein both of a_l and b_l are adjusted adaptively.

48. A method according to 45, wherein both a_l and b_l are fixed.

49. A method according to 45, wherein said linear filtering function is an infinite impulse response function, at least one a_l not being zero for l=1 to N.

50. A method according to 45, wherein the linear filtering function is a finite impulse response function, wherein a_l=0 for l=1 to N.

51. A method according to claim 45, wherein detecting the dc-offset further comprises:

converting the monitored signal from analog to digital before applying said linear filtering function; and

converting a result from said linear filtering function from digital to analog,

wherein the linear filtering function is applied digitally.

52. A method according to claim 51, wherein converting the result from digital to analog includes adjusting a digital sampling rate.

53. A method according to claim 51, further comprising applying an anti-alias filtering function to the down-converted signal prior to converting the monitored signal from analog to digital.

54. A method according to claim 41, wherein the linear filtering function is a first order recursive function, having a transfer function of:

55. A method according to claim 54, wherein detecting the dc-offset further comprises:

converting the monitored signal from analog to digital before applying the first order recursive filtering function; and

converting a result from the first order recursive filtering function from digital to analog,

wherein the first order recursive filtering function is applied digitally.

56. A method according to claim 55, wherein converting the result from digital to analog includes adjusting a digital sampling rate.

57. A method according to claim 55, further comprising applying an anti-alias filtering function to the down-converted signal prior to converting the monitored signal from analog to digital.