Method and apparatus for reducing DC offsets in a communication system

• US 7,072,427 B2
• Filed: 11/07/2002
• Issued: 07/04/2006
• Est. Priority Date: 11/09/2001
• Status: Expired due to Term

• Current Assignee: ParkerVision, Inc.
• Original Assignee: ParkerVision, Inc.
• Inventors: Rawlins, Gregory S., Sorrells, David F., Brown, Kevin, Rawlins, Michael W.
• Primary Examiner(s): VO, DON NGUYEN
• Application Number:
  US10/289,377
• Publication Number:
  US 20030128776A1
• Time in Patent Office: 1,335 Days
• Field of Search: 375/222, 375/285, 375/346, 375/351, 375/319, 455/343, 455/209, 327/307, 330/85, 330/86, 330/83, 330/191, 330/198, 330/9
• US Class Current: 375/346
• CPC Class Codes: H03G 3/20 Automatic control H03G3/005 takes precedence; combined with volume compression or expansion H03G7/00
• 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. The stored DC offset is subtracted from the receiver channel. First and second automatic gain control (AGC) amplifiers are coupled in respective portions of the receiver channel. In a third aspect, a feedback loop circuit reduces DC offset in a WLAN receiver channel. The feedback loop circuit includes a storage element that samples and stores receiver channel DC offset. The loop is opened, and the DC offset stored in the storage element is subtracted from 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. An apparatus for reducing a DC offset voltage in a receiver channel of a communication channel, comprising:

a summer that receives a receiver channel signal;

a storage element coupled to said summer; and

a switch coupled in series between a node of the receiver channel and said storage element;

wherein said switch receives a control signal, wherein said storage element stores an offset voltage during a time period when said control signal causes said switch to close; and

wherein said receiver channel signal is substantially nulled during said time period.

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, wherein said storage element includes a capacitor.

4. The apparatus of claim 1, wherein the DC offset voltage is present in said receiver channel signal, wherein said DC offset voltage is stored in said storage element, wherein said stored DC offset voltage is subtracted from said receiver channel signal at said summer.

5. The apparatus of claim 1, further comprising:

at least one amplifier coupled in the receiver channel between said summer and said node.

6. The apparatus of claim 5, wherein a first amplifier of said at least one amplifier comprises an automatic gain control (AGC) amplifier.

7. The apparatus of claim 3, wherein an amplifier is coupled in series with said switch between said node and said storage element, wherein said amplifier is configured in an inverting configuration.

8. The apparatus of claim 1, wherein said receiver channel signal is substantially nulled at least in part by reducing a gain setting of an AGC amplifier that precedes the summer in the receiver channel during said time period.

9. The apparatus of claim 8, wherein said gain setting is reduced to be substantially equal to zero during said time period.

10. The apparatus of claim 1, wherein a second control signal coupled to a down-converter module is set to inactive during said time period.

11. The apparatus of claim 10, wherein said down-converter module includes a frequency down-conversion module, wherein said frequency down-conversion module includes a second switch and a second storage element, wherein said second control signal is coupled to said second switch.

12. The apparatus of claim 1, wherein a path from said summer, to said node, to said switch, to said storage element, and back to said summer, does not include an adjustable baseband amplifier.

13. The apparatus of claim 12, further comprising a baseband amplifier downstream from said path.

14. The apparatus of claim 1, further comprising:

an adjustable RF amplifier coupled to said communication channel; and

an adjustable baseband amplifier coupled to said communication channel;

wherein said adjustable RF amplifier has a rate of adjustment that is greater than a rate of adjustment of said adjustable baseband amplifier.

15. The apparatus of claim 1, wherein said node of the receiver channel is downstream from said summer in the receiver channel.

16. An apparatus for reducing DC offset in a communication channel, comprising:

a differential output amplifier that has an inverting output and a non-inverting output;

a first storage element that has a first terminal coupled to a non-inverting input of said differential output amplifier, wherein said first storage element has a second terminal that receives a first receiver channel signal;

a first switch coupled between said non-inverting input and said inverting output;

a second storage element that has a first terminal coupled to an inverting input of said differential output amplifier, wherein said second storage element has a second terminal that receives a second receiver channel signal; and

a second switch coupled between said inverting input and said non-inverting output.

17. The apparatus of claim 16, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

18. The apparatus of claim 16, wherein said amplifier is an automatic gain control amplifier.

19. The apparatus of claim 16, wherein said first and second storage elements each include a capacitor.

20. The apparatus of claim 16, wherein said first and second switches receive at least one control signal, wherein said first and second storage elements each store an offset voltage during a time period when said at least one control signal causes said first and second switches to close.

21. The apparatus of claim 20, wherein said first and second receiver channel signals form a differential receiver channel signal, wherein said differential receiver channel signal is substantially nulled during said time period.

22. The apparatus of claim 21, wherein a gain setting of an AGC amplifier that precedes said differential output amplifier in a receiver channel is reduced during said time period.

23. The apparatus of claim 22, wherein said gain setting is reduced to be substantially equal to zero during said time period.

24. The apparatus of claim 21, wherein a second control signal coupled to a down-converter module that precedes said differential output amplifier in a receiver channel is set to inactive during said time period.

25. The apparatus of claim 24, wherein said down-converter module includes a differential frequency down-conversion module.

26. The apparatus of claim 25, wherein said differential module includes

a third storage element;

a fourth storage element; and

a third switch coupled between said third and fourth storage elements;

wherein said second control signal is coupled to said third switch.

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

(1a) substantially nulling a receiver channel signal;

(1) receiving a charge from a first node of a receiver channel;

(2) storing the charge;

(3) de-coupling the stored charge from the first node; and

(4) summing at a second node in the receiver channel a voltage that corresponds to the stored charge with the receiver channel signal, wherein the first node is downstream from the second node in the receiver channel.

28. The method of claim 27, wherein the communication channel is a wireless local area network (WLAN) receiver channel.

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

storing the charge in a capacitor.

30. The method of claim 29, further comprising the step of:

coupling a switch between the first node and the capacitor.

31. The method of claim 27, further comprising the step of:

(5) coupling at least one amplifier in the receiver channel between the first and second nodes.

32. The method of claim 31, wherein step (5) comprises the step of:

coupling an automatic gain control (AGC) amplifier in the receiver channel between the first and second nodes.

33. The method of claim 27, wherein step (1a) comprises the step of:

(a) reducing a gain setting of an AGC amplifier that precedes the second node in the receiver channel.

34. The method of claim 33, wherein step (a) comprises the step of:

reducing the gain setting to be substantially equal to zero.

35. The method of claim 34, wherein step (2) comprises the step of:

storing a charge proportional to a measured DC offset voltage in a storage element.

36. The method of claim 35, wherein step (4) comprises the step of:

subtracting a voltage signal corresponding to the stored charge from the receiver channel signal at the second node.

37. The method of claim 35, wherein step (3) comprises:

decoupling the storage element from the first node after step (1) is substantially complete.

38. The method of claim 27, wherein the second node is preceded by a down-converter module, wherein step (1a) comprises the step of:

(i) setting a control signal coupled to a down-converter module to inactive.

39. The method of claim 38, wherein the down-converter module includes a switch and a storage element, wherein the control signal is coupled to the switch, wherein step (i) comprises the step of:

setting the control signal coupled to the switch to inactive.

40. An apparatus for reducing a DC offset voltage in a communication channel, comprising:

a summing node in a receiver channel that receives as a first input a receiver channel signal;

a storage element coupled as a second input to said summing node;

a switch coupled between an output node of the receiver channel and said storage element;

an adjustable RF amplifier coupled to said communication channel; and

an adjustable baseband amplifier coupled to said communication channel;

wherein said adjustable RF amplifier has a rate of adjustment that is greater than a rate of adjustment of said adjustable baseband amplifier.