Apparatus and methods for eliminating DC offset in a wireless communication device

• US 7,155,185 B2
• Filed: 06/09/2004
• Issued: 12/26/2006
• Est. Priority Date: 06/09/2004
• Status: Active Grant

• Current Assignee: Theta IP, LLC
• Original Assignee: Theta Microelectronics, Inc.
• Inventors: Pipilos, Spyros
• Primary Examiner(s): LE, LANA N
• Application Number:
  US10/864,139
• Publication Number:
  US 20050277396A1
• Time in Patent Office: 930 Days
• Field of Search: 455/232.1, 455/235.1, 455/245.1, 455/245.2, 455/251.1, 455/341, 455/253.1, 455/240.1, 455/253.2, 455/307, 455/313, 455/296, 455/306, 455/337
• US Class Current: 455/232.1
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22
• CPC Class Codes: H04L 25/061 providing hard decisions only; arrangements for tracking or suppressing unwanted low frequency components, e.g. removal of dc offset removal of dc offset in coupling arrangements H04L25/029, H04L25/0296

Abstract

Apparatus and methods for eliminating DC offset in a wireless communication device operable on a continuous basis or on a sampled basis. In a receive channel, the output of a forward variable gain amplifier is fed back to an RC circuit to charge the capacitor (C) to a voltage dependent on the DC offset in the variable gain amplifier output. The voltage on the capacitor is amplified and summed with the input to the variable gain amplifier. The RC circuit is configured to provide a high gain feedback at DC and very low frequencies, but very low gain at signal frequencies. Preferably the output of the forward variable gain amplifier is fed back to the RC circuit with a gain that is inversely proportional to the forward gain. Disconnection of the capacitor and feedback of the capacitor voltage provides sampled operation. Various embellishments and sample applications are disclosed.

Claims

1. An amplifier system for amplifying signals having frequencies above a second frequency, the amplifier system having a frequency dependent direct current (DC) offset canceling system, comprising:

a forward amplifier having a forward amplifier gain and providing a system output;

a feedback amplifier having an input coupled to the system output and having a feedback amplifier gain, wherein the product of said forward amplifier gain and said feedback amplifier gain is larger than one below a first frequency and substantially less than one above the second frequency, the second frequency being higher than the first frequency; and

,

a summing unit coupled to combine an input signal and a feedback signal from said feedback amplifier, and providing the combined signal as an input to said forward amplifier.

2. The system of claim 1 wherein the magnitude of the product of said forward amplifier gain and said feedback amplifier gain above the second frequency is 0.1 or less.

3. The system of claim 1 wherein said system is at least partially realized in integrated circuit form.

4. The system of claim 1 wherein said feedback amplifier comprises:

a first transconductor amplifier having a first transconducting gain, the output of said first transconductor amplifier being connected to a first node of a resistor and a first node of a capacitor, a second node of said resistor and a second node of said capacitor being connected together and to a reference voltage; and

,

a second transconductor amplifier having a second inverting transconducting gain, the input of said second transconductor amplifier being connected to said output of said first transconductor amplifier.

5. The system of claim 4, wherein said capacitor and said resistor cause said first frequency to be less than 50 Hz.

6. The system of claim 4 wherein said forward amplifier further comprises:

a baseband filter having a transresistance gain; and

,

a baseband amplifier having a baseband gain;

wherein said forward amplifier gain is equal to the product of said transresistance gain and said baseband gain.

7. The system of claim 6 wherein said baseband gain is variable, and the product of said baseband gain and said first transconducting gain is constant.

8. The system of claim 4 wherein said first node of said resistor is connected to said first node of said capacitor through a switch, said first node of said resistor being connected to said output of said first transconductor amplifier and said first node of said capacitor being connected to said input of said second transconductor amplifier.

9. The system of claim 8 wherein said switch is closed during at least an initial portion of a preamble period of an orthogonal frequency division multiplexing (OFDM) input signal.

10. The system of claim 9 wherein when said switch is closed, said capacitor is charged to a voltage canceling the DC offset at the output of said system.

11. The system of claim 10 wherein upon opening of said switch, the voltage stored on said capacitor causes the continued cancellation of the DC offset at the output of said system.

12. The system of claim 4, wherein a filtering circuit is added to said feedback amplifier path for the purpose of filtering out high frequency ripple.

13. The system of claim 4, wherein sensing means is connected to said output of said system and further compared against a threshold value.

14. The system of claim 13, wherein upon exceeding of said threshold value a speed-up circuit is activated to charge said capacitor.

15. The system of claim 1, wherein during at least an initial portion of the preamble period of an OFDM signal, said first frequency is higher than said first frequency following said preamble period.

16. The system of claim 1, wherein during at least an initial portion of the preamble period of an OFDM signal said second frequency is higher than said second frequency following said preamble period.

17. A wireless receiver comprising:

a low-noise amplifier having an input coupled to an antenna output;

a mixer coupled to receive a signal from said low-noise amplifier, said mixer being capable of mixing said signal from said low-noise amplifier with a local oscillator frequency and providing a mixer output signal;

a forward amplifier coupled to receive said mixer output signal and providing a receiver output signal, the forward amplifier having a forward amplifier gain;

a feedback amplifier coupled to receive an output from said forward amplifier and having a feedback amplifier gain, the magnitude of the product of said forward amplifier gain and said feedback amplifier gain being larger than one below a first frequency and substantially less than one above a second frequency higher than the first frequency; and

,

a summing unit for combining said mixer output signal and a feedback signal from said feedback amplifier, and providing the combined signal as an input to said forward amplifier.

18. The system of claim 17 wherein the magnitude of the product of said forward amplifier gain and said feedback amplifier gain above the second frequency is 0.1 or less.

19. The wireless receiver of claim 17, wherein said wireless receiver is at least partially realized in integrated circuit form.

20. The wireless receiver of claim 17, wherein said feedback amplifier further comprises:

a first transconductor amplifier having a first transconducting gain, the output of said first transconductor amplifier being connected to a first node of a resistor and a first node of a capacitor, a second node of said resistor and a second node of said capacitor being connected together and to a reference voltage; and

,

a second transconductor amplifier having a second inverting transconducting gain, the input of said second transconductor amplifier being connected to said output of said first transconductor amplifier.

21. The wireless receiver of claim 20, wherein said capacitor and said resistor cause said first frequency to be less than 50 Hz.

22. The wireless receiver of claim 20, wherein said forward amplifier further comprises:

a baseband filter having a transresistance gain; and

,

a baseband amplifier having a baseband gain;

wherein said forward amplifier gain is equal to the product of said transresistance gain and said baseband gain.

23. The wireless receiver of claim 22 wherein said baseband gain is variable, and the product of said baseband gain and said first transconducting gain is constant.

24. The wireless receiver of claim 20, wherein said first node of said resistor is connected to said first node of said capacitor through a switch, said first node of said resistor being connected to said output of said first transconductor amplifier and said first node of said capacitor being connected to said input of said second transconductor amplifier.

25. The wireless receiver of claim 24 wherein said switch is closed during at least an initial portion of the preamble period of an orthogonal frequency division multiplexing (OFDM) input signal.

26. The wireless receiver of claim 25 wherein when said switch is closed, said capacitor is charged to a voltage canceling the DC offset at said output of said wireless receiver.

27. The wireless receiver of claim 26 wherein upon opening of said switch, the voltage stored on said capacitor causes the continued cancellation of the DC offset at the output of said system.

28. The wireless receiver of claim 20, wherein a filtering circuit is added to said feedback amplifier path for the purpose of filtering a high frequency ripple.

29. The wireless receiver of claim 20, wherein sensing means is connected to said output of said system and further compared against a threshold value.

30. The wireless receiver of claim 29, wherein upon exceeding of said threshold value a speed-up circuit is activated to charge said capacitor.

31. The wireless receiver of claim 17 wherein during at least an initial portion of the preamble period of an OFDM signal said first frequency is higher than said first frequency after the preamble period.

32. The wireless receiver of claim 17 wherein during at least an initial portion of the preamble period of an OFDM signal said second frequency value is higher than said second frequency after the preamble period.