Integrated tunable RF notch filter

• US 8,428,203 B1
• Filed: 01/06/2009
• Issued: 04/23/2013
• Est. Priority Date: 01/06/2009
• Status: Expired due to Fees

• Current Assignee: Maxlinear Asia Singapore Pte Ltd. (Maxlinear Incorporated)
• Original Assignee: PMC-Sierra Incorporated (Microchip Technology Incorporated)
• Inventors: Zortea, Anthony Eugene, Gagnon, Mathieu, Boyd, Graeme, Mok, Winston Ki-Cheong, McAdam, Mathew
• Primary Examiner(s): FILE, ERIN M
• Application Number:
  US12/349,328
• Time in Patent Office: 1,568 Days
• Field of Search: 375/346
• US Class Current: 375/346
• CPC Class Codes: H03H 11/10 using negative impedance converters H03H11/08 takes precedence
• CPC Class Codes: H04B 1/1036 with automatic suppression of narrow band noise or interference, e.g. by using tuneable notch filters H04B1/123 takes precedence; filter circuits H03H

Abstract

Large interfering signals (interferers) with spectra near a desired signal can cause distortion in a wireless receiver due to a non-linear signal path. It is typically a performance advantage to attenuate these interferers earlier in the signal path, rather than later in the signal path, because these interferers can cause saturation of amplifying stages. In certain situations, the frequency offset of an interfering signal, with respect to the desired signal, can be on the order of 10 megahertz (MHz), whereas the center frequencies can be on the order of several gigahertz (GHz). Thus, a filter with “baseband” precision would be needed at radio frequency to notch out the interferer, which is relatively difficult to do. Disclosed is a technique to estimate the relative strength and center frequency of the interferer and to place the center frequency of a notch filter adaptively and precisely at the interferer location.

Claims

1. An apparatus comprising:

a low noise amplifier (LNA) configured to receive a radio frequency input signal, wherein the LNA is configured to generate an amplified RF signal;

a downconverter operatively coupled to the LNA to receive the amplified RF signal, wherein the downconverter is configured to generate a baseband signal as an output;

a slicer operatively coupled to an output of the downconverter, wherein the slicer is configured to generate hard-decision samples of ones and zeroes as an output;

a processor operatively coupled to the slicer, wherein the processor is configured to analyze the hard-decision samples to generate a control signal for control of filtering of at least one frequency range of the amplified RF signal; and

a controllable notch filter operatively coupled to an output of the processor and to an output of the LNA, wherein the controllable notch filter is configured to receive the control signal and to filter out the at least one frequency range from the amplified RF signal.

2. The apparatus of claim 1, wherein the processor is further configured to compute Fourier Transforms of the hard-decision samples to generate an estimate of a frequency of an interferer or of the at least one filter frequency of the controllable notch filter.

3. The apparatus of claim 2, wherein the processor is further configured to compute a plurality of fast Fourier Transforms, and to generate an average of the plurality of fast Fourier Transforms to generate the estimate of the frequency of the interferer or the at least one filter frequency of the controllable notch filter.

4. The apparatus of claim 3, wherein the processor is further configured to compute 128 32-point fast Fourier Transforms, and to generate an average of the 128 32-point fast Fourier Transforms to generate the estimate of the frequency of the interferer or the at least one filter frequency of the controllable notch filter.

5. The apparatus of claim 2, further comprising a broadband noise source operatively coupled to an input of the LNA, wherein the broadband noise source is activated such that the processor can generate the estimate of the at least one filter frequency of the controllable notch filter.

6. The apparatus of claim 2, wherein the LNA is configurable to be able to switch to a high-noise state to generate broadband noise for spectral analysis of the controllable notch filter.

7. The apparatus of claim 1, wherein the processor further comprises a digital-to-analog converter configured to convert the control signal from digital form to analog form, and to provide the control signal to the controllable notch filter in analog form.

8. The apparatus of claim 1, wherein the processor is configured to identify one or more interferers from analysis of the hard decision samples, to store previously detected interferer levels in a list, and to determine the at least one frequency to filter out based at least partly on the previously detected interferer levels.

9. The apparatus of claim 1, wherein the processor is configured to identify one or more interferers from analysis of the hard decision samples, to store previously detected interferer levels in a list, to determine whether a currently detected interferer had been previously detected, and to combine current data with prior data to refine a center frequency estimate of the currently detected interferer.

10. The apparatus of claim 1, wherein the processor is configured to identify one or more interferers from analysis of the hard decision samples, is configured to temporarily disable the controllable notch filter to refresh a peak level of a previously detected interferer frequency, and wherein the processor is further configured to reevaluate whether to continue to filter out the previously identified and filtered interferer based at least partly on the refreshed peak level.

11. The apparatus of claim 1, wherein the processor is configured to identify one or more interferers from analysis of the hard decision samples, to gradually change a depth of filtering of the controllable notch filter to determine whether a previously identified and filtered interferer remains present, and wherein the processor is configured to reevaluate whether to continue to filter out the previously identified and filtered interferer based at least partly on the determination of presence.

12. The apparatus of claim 1, wherein a frequency and depth of the controllable notch filter is two-dimensionally controlled, wherein the processor is configured to generate a two-dimensional control for the control signal, wherein both a frequency and depth of filtering is two-dimensionally controlled such that both frequency and depth of filtering varies in two dimensions.

13. A method of filtering in a radio frequency front-end, the method comprising:

receiving a radio frequency input signal and amplifying the radio frequency input signal to generate an amplified RF signal;

downconverting the amplified RF signal to generate a baseband signal;

generating hard-decision samples of ones and zeroes from the baseband signal;

analyzing the hard-decision samples to detect at least one interferer and to generate a control signal for control of filtering of at least one frequency range of the amplified RF signal corresponding to the at least one interferer; and

notch filtering out the at least one frequency range from the amplified RF signal based at least partly on the control signal to filter out the at least one interferer.

14. The method of claim 13, further comprising maintaining a list of the one or more interferers, and determining the at least one frequency to filter out based at least partly on the list.

15. The method of claim 13, further comprising maintaining a list of the one or more interferers, temporarily disabling notch filtering to refresh a peak level of a previously detected interferer frequency, and reevaluating whether to continue to filter out the previously detected and filtered interferer based at least partly on the refreshed peak level.

16. The method of claim 13, further comprising:

maintaining a list of the one or more interferers;

determining whether a currently detected interferer had been previously detected;

combining current data with prior data to refine a center frequency estimate of the currently detected interferer.

17. The method of claim 13, further comprising gradually changing a depth of filtering to determine whether a previously detected and filtered interferer remains present, and reevaluating whether to continue to filter out the previously detected and filtered interferer based at least partly on the determination of presence.

23. The method of claim 13, further comprising generating a two-dimensional control for the control signal, wherein both a frequency and depth of the notch filtering is two-dimensionally controlled such that both frequency and depth of filtering varies in two dimensions.

18. A method of filtering an interferer in a radio frequency front-end, the method comprising:

receiving a radio frequency input signal and amplifying the radio frequency input signal to generate an amplified RF signal;

downconverting the amplified RF signal to generate a baseband signal;

generating hard-decision samples of ones and zeroes from the baseband signal;

analyzing the hard-decision samples to generate a control signal for control of filtering of at least one frequency range of the amplified RF signal;

filtering out the at least one frequency range from the amplified RF signal based at least partly on the control signal; and

computing Fourier Transforms of the hard-decision samples to generate an estimate of a frequency of the interferer or the at least one filter frequency of filtering.

19. The method of claim 18, further comprising computing a plurality of fast Fourier Transforms, and generating an average of the plurality of fast Fourier Transforms to generate the estimate of the frequency of the interferer or filtering.

20. The method of claim 19, further comprising computing 128 32-point fast Fourier Transforms, and generating an average from the 128 32-point fast Fourier Transforms to generate the estimate of the frequency of the interferer or filtering.

21. The method of claim 18, further comprising activating a broadband noise source for estimation of the at least one filter frequency.

22. The method of claim 18, further comprising configuring a low-noise amplifier (LNA) of the radio frequency front-end to switch to a high-noise state to generate broadband noise for estimation of the at least one filter frequency.

24. A method of locating a contour of optimal attenuation for a notch filter having a two-dimensional control characteristic so that both frequency and depth of filtering vary according to variation with a first control input and a second control input, the method comprising:

(a) activating broadband noise in a front-end of a wireless receiver for calibration;

(b) filtering the broadband noise in the front-end of the wireless receiver with the notch filter;

(c) selecting an initial settings for the first control input of the notch filter, and then performing (d)-(f);

(d) observing a frequency response of the notch filter for a first plurality of settings of the second control input, wherein the first plurality of settings cover a first range and are spaced apart at a first spacing;

(e) observing the frequency response of the notch filter for a second plurality of settings of the second control input, wherein the second plurality of settings cover a second range that is smaller than the first range and covers a frequency response observation from the first range that has the deepest notch characteristic, and wherein the second plurality of settings are spaced apart by a second spacing that is tighter than the first spacing;

(f) collecting the setting from within the second plurality of setting having the deepest notch characteristic for the particular settings of the first control input and the second control input;

(g) repeating (d)-(f) for other settings of the first control input; and

(h) determining a contour for control of the notch filter based on the collected setting for the first control input and the second control input having the deepest notch characteristic.

25. The method of claim 24, wherein observing the frequency response comprises estimating the frequency response by computing a fast Fourier transform of a sign of a downconverted signal associated with the broadband noise as filtered by the notch filter.

26. The method of claim 24, further comprising performing (d), (e), and (f) in sequence.

27. The method of claim 24, wherein the first control input corresponds to capacitance control and the second control input corresponds to resistance control.

28. The method of claim 24, wherein the second spacing is tighter than the first spacing by at least a factor of 4.

29. The method of claim 24, wherein determining the contour comprises using a least squares fit to the collected deepest notch characteristic settings to find coefficients that model the contour.

30. An apparatus comprising:

(a) a controllable source of broadband noise;

(b) a notch filter having a two-dimensional control characteristic so that both frequency and depth of filtering vary according to variation with a first control input and a second control input; and

(c) a processor configured to control activation of the controllable source of broadband noise and control filtering by the notch filter, the processor configured to select an initial settings for the first control input of the notch filter, and then is configured to (d)-(f);

(d) observe a frequency response of the notch filter for a first plurality of settings of the second control input, wherein the first plurality of settings cover a first range and are spaced apart at a first spacing;

(e) observe the frequency response of the notch filter for a second plurality of settings of the second control input, wherein the second plurality of settings cover a second range that is smaller than the first range and covers a frequency response observation from the first range that has the deepest notch characteristic, and wherein the second plurality of settings are spaced apart by a second spacing that is tighter than the first spacing;

(f) collect the setting from within the second plurality of setting having the deepest notch characteristic for the particular settings of the first control input and the second control input;

(g) wherein the processor is configured to repeat (d)-(f) for other settings of the first control input;

(h) wherein the processor is configured to determine a contour for control of the notch filter based on the collected setting for the first control input and the second control input having the deepest notch characteristic.

31. The apparatus of claim 30, wherein the processor is configured to observe the frequency response by estimation of the frequency response via computation of a fast Fourier transform of a sign of a downconverted signal associated with the broadband noise as filtered by the notch filter.

32. The apparatus of claim 30, wherein the processor is configured to perform (d), (e), and (f) in sequence.

33. The apparatus of claim 30, wherein the first control input corresponds to capacitance control and the second control input corresponds to resistance control.

34. The apparatus of claim 30, wherein the second spacing is tighter than the first spacing by at least a factor of 4.

35. The apparatus of claim 30, wherein the processor is configured to determine the contour via computation of a least squares fit to the collected deepest notch characteristic settings to find coefficients that model the contour.