PROGRAMMABLE IF FREQUENCY FILTER FOR ENABLING A COMPROMISE BETWEEN DC OFFSET REJECTION AND IMAGE REJECTION

• US 20100015939A1
• Filed: 08/20/2009
• Published: 01/21/2010
• Est. Priority Date: 03/31/2004
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: PAN, Meng-An
• Granted Patent: US 8,675,777 B2
• Time in Patent Office: Days
• US Class Current: 455/302
• CPC Class Codes: H03D 3/007 by converting the oscillations into two quadrature related signals H03D3/245 takes precedence

Abstract

A center frequency of an adjustable filter is controlled to achieve a compromise between DC offset rejection and image rejection.

Claims

1. An apparatus, comprising:

an adjustable filter configured to filter a communication signal to provide a filtered communication signal, the adjustable filter being characterized by a center frequency; and

a digital processing circuit configured to measure an image rejection and an DC offset rejection of the filtered communication signal and to cause the adjustable filter to adjust the center frequency from a first center frequency to a second center frequency to optimize the image rejection and the DC offset rejection of the filtered communication signal.

2. The apparatus of claim 1, wherein the adjustable filter is configured such that the image rejection and the DC offset rejection are at respective compromise positions to thereby optimize the filtered communication signal.

3. The apparatus of claim 2, wherein the compromise is reached when the DC offset rejection is within acceptable tolerances and image rejection meets minimum pre-specified requirements.

4. The apparatus of claim 1, wherein the first center frequency is less than the second center frequency, an adjustment of the center frequency from the first center frequency to the second center frequency causing an increase in the DC offset rejection and a decrease in the image rejection.

5. The apparatus of claim 1, wherein the first center frequency is greater than the second center frequency, an adjustment of the center frequency from the first center frequency to the second center frequency causing a decrease in the DC offset rejection and an increase in the image rejection.

6. The apparatus of claim 1, wherein the digital processing circuit is configured to change a control word from a first value to a second value to cause the adjustable filter to adjust the center frequency from the first center frequency to the second center frequency.

7. The apparatus of claim 1, further comprising:

a mixer configured to downconvert the communication signal to the first center frequency based upon a first local oscillator (LO) signal, the image rejection and the DC offset rejection being unoptimized when the center frequency is at the first center frequency.

8. The apparatus of claim 7, wherein the first LO signal is set to a second LO signal to cause the mixer to downconvert the communication signal to the second center frequency, the image rejection and the DC offset rejection being optimized when the center frequency is at the second center frequency.

9. The apparatus of claim 1, wherein the adjustable filter comprises:

a first cross coupled low pass filter including a first cross coupled variable resistor; and

a second cross coupled low pass filter including a second cross coupled variable resistor, wherein the digital processing circuit causes the adjustable filter to adjust the first cross coupled variable resistor and the second cross coupled variable resistor to adjust the center frequency from the first center frequency to the second center frequency.

10. The apparatus of claim 1, wherein at least one of a group consisting of the first cross coupled low pass filter and the second cross coupled low pass filter comprises:

an active low pass filter.

11. The apparatus of claim 1, wherein the adjustable filter is further characterized by a gain, and wherein the digital processing circuit is configured to measure a signal to noise ratio of the filtered communication signal and to cause the adjustable filter to adjust its respective gain until the signal to noise ratio is sufficient to allow for measurement of the image rejection and the DC offset rejection of the filtered communication signal.

12. The apparatus of claim 11, wherein the adjustable filter comprises:

a first cross coupled low pass filter including a first variable feedback resistor; and

a second cross coupled low pass filter including a second variable feedback resistor, wherein the digital processing circuit causes the adjustable filter to adjust the first variable feedback resistor and the second variable feedback resistor to adjust the gain.

13. The apparatus of claim 12, wherein at least one of a group consisting of the first cross coupled low pass filter and the second cross coupled low pass filter comprises:

an active low pass filter.

14. The apparatus of claim 1, wherein the adjustable filter is configured to receive the communication signal is received from a wireless communication channel.

24. The method of claim 13, wherein step (C) comprises:

(C)(i) causing the adjustable filter to adjust the first cross coupled variable resistor and the second cross coupled variable resistor to adjust the center frequency from the first center frequency to the second center frequency.

15. A method, comprising:

(A) filtering a communication signal to provide a filtered communication signal using an adjustable filter;

(B) measuring an image rejection and an DC offset rejection of the filtered communication signal; and

(C) causing a center frequency of the adjustable filter to adjust from a first center frequency to a second center frequency to optimize the image rejection and the DC offset rejection of the filtered communication signal.

16. The method of claim 15, wherein step (C) comprises:

(C)(i) causing the adjustable filter to adjust the center frequency from a first center frequency to a second center frequency to reach a compromise between the DC offset rejection and the image rejection.

17. The method of claim 16, wherein step (C)(i) comprises:

(C)(i)(A) causing the adjustable filter to adjust the center frequency from a first center frequency to a second center frequency such that the DC offset rejection is within acceptable tolerances and image rejection meets minimum pre-specified requirements.

18. The method of claim 15, wherein step (C) comprises:

(C)(i) causing the adjustable filter to adjust the center frequency from the first center frequency to the second center frequency, the first center frequency being less than the second center frequency, wherein the adjustment of the center frequency from the first center frequency to the second center frequency causing an increase in the DC offset rejection and a decrease in the image rejection.

19. The method of claim 15, wherein step (C) comprises:

(C)(i) causing the adjustable filter to adjust the center frequency from the first center frequency to the second center frequency, the first center frequency being greater than the second center frequency, wherein the adjustment of the center frequency from the first center frequency to the second center frequency causing a decrease in the DC offset rejection and an increase in the image rejection.

20. The method of claim 15, wherein step (C) comprises:

(C)(i) changing a control word from a first value to a second value to cause the adjustable filter to adjust the center frequency from the first center frequency to the second center frequency.

21. The method of claim 15, further comprising:

(D) downconverting the communication signal to the first center frequency based upon a first local oscillator (LO) signal, the image rejection and the DC offset rejection being unoptimized when the center frequency is at the first center frequency.

22. The method of claim 21, further comprising:

(E) adjusting the first LO signal to a second LO signal; and

(F) downconverting the communication signal to the second center frequency using the second LO signal, the image rejection and the DC offset rejection being optimized when the center frequency is at the second center frequency.

23. The method of claim 15, wherein step (A) comprises:

(A)(i) filtering the communication signal using a first cross coupled low pass filter and a second cross coupled low pass filter, the first cross coupled low pass filter including a first cross coupled variable resistor and the second cross coupled low pass filter including a second cross coupled variable resistor.

25. The method of claim 15, wherein the adjustable filter is further characterized by a gain and further comprising:

(D) measuring a signal to noise ratio of the filtered communication signal; and

(E) causing the adjustable filter to adjust the gain until the signal to noise ratio is sufficient to allow for measurement of the image rejection and the DC offset rejection of the filtered communication signal.

26. The method of claim 25, wherein step (A) comprises:

(A)(i) filtering the communication signal using a first cross coupled low pass filter and a second cross coupled low pass filter, the first cross coupled low pass filter including a first variable feedback resistor and the second cross coupled low pass filter including a second variable feedback resistor.

27. The method of claim 25, wherein step (E) comprises:

(E)(i) causing the adjustable filter to adjust the first variable feedback resistor and the second variable feedback resistor until the signal to noise ratio is sufficient to allow for measurement of the image rejection and the DC offset rejection of the filtered communication signal.

28. The method of claim 15, further comprising:

(D) receiving the communication signal from a wireless communication channel.