Adaptable detection threshold for RFID tags and chips

• US 8,258,955 B1
• Filed: 01/24/2011
• Issued: 09/04/2012
• Est. Priority Date: 04/13/2004
• Status: Active Grant

• Current Assignee: Impinj, Inc.
• Original Assignee: Impinj, Inc.
• Inventors: Hyde, John D., Sundstrom, Kurt E.
• Primary Examiner(s): Nguyen, Simon
• Application Number:
  US13/012,413
• Time in Patent Office: 589 Days
• Field of Search: 455 411- 413, 455/296, 455/309, 455/312, 455336-337, 340/10.1, 340/10.31, 340/12.34, 340/12.51, 340/13.26, 3405721-5724, 375/260, 375/340, 375/343
• US Class Current: 340/572.1
• CPC Class Codes: H04B 5/0062 in RFID [Radio Frequency Identification] Systems
• CPC Class Codes: H04B 5/02 using transceiver
• CPC Class Codes: H04B 5/48 Transceivers
• CPC Class Codes: H04B 5/77 for interrogation

Abstract

RFID tags, tag circuits, and methods are provided that reduce at least in part the distortion to received wireless signals, which is caused by interference in the environment. Two or more thresholds are used to digitize the received signal implemented by two or more demodulators. Multiple low pass and digital filters may be implemented with the demodulators, allowing removal of narrow pulses caused by the interference and reduction of beat tone amplitude.

Claims

1. A dual threshold circuit of a Radio Frequency Identification (RFID) tag, comprising:

an envelope detector configured to receive a modulated wireless RF input signal and further configured to derive an analog output signal responsive to the modulated wireless RF input signal;

a first slicer circuit configured to derive a first unfiltered digital output signal responsive to the analog output signal and to a first analog decision threshold;

a second slicer circuit configured to derive a second unfiltered digital output signal responsive to the analog output signal and to a second analog decision threshold;

a selection circuit configured to derive a filtered digital output signal by selecting one of the first and second unfiltered digital outputs;

a multiplexer of the selection circuit configured to select one of the first and second unfiltered digital outputs; and

a digital decision circuit for configuring the multiplexer to select one of the first and second unfiltered digital outputs based on a performance criterion.

2. The dual threshold circuit of claim 1, wherein at least a portion of the circuitry for the first and second slicer circuits is common to both.

3. The dual threshold circuit of claim 1, further comprising:

at least one additional slicer circuit configured to derive an additional unfiltered digital output signal responsive to the analog output signal and to an additional analog decision threshold.

4. The dual threshold circuit of claim 1, wherein the selection circuit includes a digital filter responsive to at least one of the unfiltered digital outputs.

5. The dual threshold circuit of claim 4, wherein the digital filter is a narrow pulse digital filter.

6. The dual threshold circuit of claim 1, wherein

the performance criterion includes at least one from a set of:

a valid frame synch, a valid preamble, and a valid command.

7. The dual threshold circuit of claim 1, wherein

the envelope detector contains a low pass analog filter.

8. The dual threshold circuit of claim 1, wherein the digital filtered output signal includes a number.

9. The dual threshold circuit of claim 1, wherein the digital filtered output signal includes a signal.

10. The dual threshold circuit of claim 1, wherein the selection circuit includes a plurality of digital filters responsive to at least one of the unfiltered digital outputs and is further configured to:

set one of the digital filters to detect all data rates; and

set at least one of the remaining filters to detect a subset of the data rates.

11. The dual threshold circuit of claim 10, in which the filter set to detect all data rates includes an aperture set to a minimum width.

12. A dual threshold circuit for a circuit of a Radio Frequency Identification (RFID) tag, comprising:

an envelope detector configured to receive a modulated wireless RF input signal and further configured to derive an analog output signal responsive to the modulated wireless RF input signal;

a first and a second slicer circuits configured to receive power until a performance criterion is met, the performance criterion signifying that a frame synch has been detected;

a selection circuit configured to;

select one of the first and second slicer circuits to generate an unfiltered digital output;

discontinue applying power to the unselected one of the first and second slicer circuits; and

derive a filtered digital output signal.

13. The dual threshold circuit of claim 12, in which the selection circuit is configured to select one of the first and second slicer circuits based on:

detection of a falling edge of a symbol following an RTcal signal; and

if a higher threshold slicer circuit has detected the falling edge, using the higher threshold slicer circuit to generate the unfiltered digital output,

else if a lower threshold slicer circuit has detected the falling edge, using the lower threshold slicer circuit to generate the unfiltered digital output.

14. The dual threshold circuit of claim 12, in which the selection circuit is further configured to:

determine a data rate of the analog output signal; and

adjust a clock frequency of the selection circuit based on the determined data rate.

15. The dual threshold circuit of claim 14, in which

the selection circuit clock frequency is adjusted by reducing the clock frequency if the data rate is below a predefined value.

16. The dual threshold circuit of claim 14, in which the selection circuit clock frequency is adjusted based on which slicer circuit is selected to generate the unfiltered digital output.