RFID reader systems with double conversion and methods

• US 8,390,425 B1
• Filed: 05/01/2007
• Issued: 03/05/2013
• Est. Priority Date: 07/21/2006
• Status: Active Grant

• Current Assignee: Impinj, Inc.
• Original Assignee: Impinj, Inc.
• Inventors: Cooper, Scott Anthony, Sundstrom, Kurt Eugene
• Primary Examiner(s): Mehmood, Jennifer
• Assistant Examiner(s): Mahase, Pameshanand
• Application Number:
  US11/742,650
• Time in Patent Office: 2,135 Days
• Field of Search: 342/357.25
• US Class Current: 340/3.2
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer

Abstract

RFID readers, reader systems, and methods are provided that utilize double conversion for received tag response signals. A digitized signal is derived from the tag response signal by first shifting the response signal to about DC. The components of the digitized signal are then up converted and down converted and filtered such that only components around DC remain. The up converted and down converted signals may then be compared and one selected or the two combined after weighting to enhance demodulation and reduce circuit complexity.

Claims

1. A method for an RFID reader system, the method comprising:

transmitting a carrier signal at a first frequency;

receiving a backscattered tag-response signal modulated on the carrier signal which includes a first component offset above the carrier signal by approximately a subcarrier link frequency, and a second component offset below the carrier signal by approximately the subcarrier link frequency;

frequency shifting the tag-response signal to form an intermediate signal centered at approximately zero Hz;

frequency shifting the intermediate signal such that one of the first and second components of the tag-response signal is centered at approximately zero Hz; and

performing an operation on at least one of the first and second components of the twice frequency-shifted signal.

2. The method of claim 1, further comprising:

filtering one of the first and second components from one of the intermediate signal and the twice frequency-shifted signal prior to performing the operation.

3. The method of claim 1, further comprising:

detecting at least one of a pilot tone and a preamble in the tag-response signal;

determining a residual frequency error based on the detection; and

employing the determined residual frequency error to initialize at least one demodulator.

4. The method of claim 1, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform the operation based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

5. The method of claim 1, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform the operation based on detecting at least one of a pilot tone and a preamble in one of the first and second components.

6. The method of claim 1, further comprising:

frequency shifting the intermediate signal such that both of the first and second components of the tag-response signal are centered at approximately zero Hz.

7. The method of claim 6, further comprising:

weighting the first component by a first weighting factor;

weighting the second component by a second weighting factor;

summing the weighted components; and

performing the operation on the weighted sum.

8. The method of claim 7, wherein the weighting factors are based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

9. The method of claim 7, wherein the weighting factors are based on at least one of a detected pilot tone and a detected preamble in the first and second components.

10. The method of claim 1, further comprising:

filtering the intermediate signal by using a series of decimation and finite-impulse-response filters.

11. The method of claim 10, in which

at least one of the decimation filters is a Cascade Integrator-Comb (CIC) filter.

12. The method of claim 1, in which

instructing the tag to provide a tag response at a subcarrier link frequency includes adjusting the subcarrier link frequency.

13. The method of claim 12, in which

the subcarrier link frequency is adjusted based on a result of the performed operation.

14. The method of claim 1, in which

an amount of frequency shifting to form the intermediate signal is the instructed subcarrier link frequency.

15. The method of claim 1, further comprising:

employing a bank of matched filters to filter the twice frequency-shifted signal.

16. The method of claim 15, further comprising:

detecting at least one of a pilot tone and a preamble in the twice frequency-shifted signal;

determining a residual frequency error from the detection; and

adjusting the matched filter banks based on the residual frequency error.

17. A method for an RFID reader system, the method comprising:

instructing the tag to provide a tag-response signal at a subcarrier link frequency;

instructing the tag to include a pilot tone in the tag-response signal;

transmitting a carrier signal at a first frequency;

receiving the tag-response signal which includes a first component offset above the carrier signal by approximately the subcarrier link frequency, and a second component offset below the carrier signal by approximately the subcarrier link frequency;

frequency shifting the tag-response signal to form an intermediate signal centered at approximately zero Hz;

frequency shifting the intermediate signal such that one of the first and second components of the tag-response signal is centered at approximately zero Hz;

detecting the pilot tone in the twice frequency-shifted signal; and

determining a residual frequency error from the detected pilot tone.

18. The method of claim 17, further comprising:

filtering one of the first and second components from one of the intermediate signal and the twice frequency-shifted signal prior to performing the operation.

19. The method of claim 17, further comprising:

employing the determined residual frequency error to initialize at least one demodulator.

20. The method of claim 17, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform an operation based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

21. The method of claim 17, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform an operation based on detecting a preamble in one of the first and second components.

22. The method of claim 17, further comprising:

frequency shifting the intermediate signal such that both of the first and second components of the tag-response signal are centered at approximately zero Hz.

23. The method of claim 22, further comprising:

weighting the first component by a first weighting factor;

weighting the second component by a second weighting factor;

summing the weighted components; and

performing an operation on the weighted sum.

24. The method of claim 23, wherein the weighting factors are based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

25. The method of claim 23, wherein the weighting factors are based on the detected pilot tone.

26. The method of claim 17, further comprising:

filtering the intermediate signal by using a series of decimation and finite-impulse-response filters.

27. The method of claim 26, in which

at least one of the decimation filters is a Cascade Integrator-Comb (CIC) filter.

28. The method of claim 17, in which

instructing the tag to provide a tag response at a subcarrier link frequency includes adjusting the subcarrier link frequency.

29. The method of claim 28, in which

the subcarrier link frequency is adjusted based on a result of a performed operation.

30. The method of claim 17, in which

an amount of frequency shifting to form the intermediate signal is the instructed subcarrier link frequency.

31. The method of claim 17, further comprising:

employing a bank of matched filters to filter the twice frequency-shifted signal.

32. The method of claim 31, further comprising:

adjusting the matched filter banks based on the residual frequency error.

33. A method for an RFID reader system, the method comprising:

instructing the tag to provide a tag-response signal at a subcarrier link frequency;

transmitting a carrier signal at a first frequency;

receiving the tag-response signal which includes a first component offset above the carrier signal by approximately the subcarrier link frequency, and a second component offset below the carrier signal by approximately the subcarrier link frequency;

frequency shifting the tag-response signal to form an intermediate signal centered at approximately zero Hz;

frequency shifting the intermediate signal such that one of the first and second components of the tag-response signal is centered at approximately zero Hz;

performing an operation on at least one of the first and second components of the twice frequency-shifted signal; and

adjusting the subcarrier link frequency based on a result of the performed operation.

34. The method of claim 33, further comprising:

filtering one of the first and second components from one of the intermediate signal and the twice frequency-shifted signal prior to performing the operation.

35. The method of claim 33, further comprising:

detecting at least one of a pilot tone and a preamble in the tag-response signal;

determining a residual frequency error based on the detection; and

employing the determined residual frequency error to initialize at least one demodulator.

36. The method of claim 33, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform the operation based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

37. The method of claim 33, further comprising:

selecting one of the first and second components of the twice frequency-shifted signal on which to perform the operation based on detecting at least one of a pilot tone and a preamble in one of the first and second components.

38. The method of claim 33, further comprising:

frequency shifting the intermediate signal such that both of the first and second components of the tag-response signal are centered at approximately zero Hz.

39. The method of claim 38, further comprising:

weighting the first component by a first weighting factor;

weighting the second component by a second weighting factor;

summing the weighted components; and

performing the operation on the weighted sum.

40. The method of claim 39, wherein the weighting factors are based on at least one of the respective Signal-to-Noise Ratios (SNRs) and power levels of the first and second components.

41. The method of claim 38, wherein the weighting factors are based on at least one of a detected pilot tone and a detected preamble in the first and second components.

42. The method of claim 33, further comprising:

filtering the intermediate signal by using a series of decimation and finite-impulse-response filters.

43. The method of claim 42, in which

at least one of the decimation filters is a Cascade Integrator-Comb (CIC) filter.

44. The method of claim 33, in which

instructing the tag to provide a tag response at a subcarrier link frequency includes adjusting the subcarrier link frequency.

45. The method of claim 44, further comprising

reinstructing the tag to provide another tag response signal at the adjusted subcarrier link frequency.

46. The method of claim 33, in which

an amount of frequency shifting to form the intermediate signal is the instructed subcarrier link frequency.

47. The method of claim 33, further comprising:

employing a bank of matched filters to filter the twice frequency-shifted signal.

48. The method of claim 47, further comprising:

detecting at least one of a pilot tone and a preamble in the twice frequency-shifted signal;

determining a residual frequency error from the detection; and

adjusting the matched filter banks based on the residual frequency error.