Auto-tuned RFID reader antenna

• US 20070222605A1
• Filed: 03/22/2006
• Published: 09/27/2007
• Est. Priority Date: 03/22/2006
• Status: Active Grant

• Current Assignee: Assa Abloy AB
• Original Assignee: Assa Abloy AB
• Inventors: Andresky, David
• Granted Patent: US 7,439,860 B2
• Time in Patent Office: Days
• US Class Current: 340/572.700
• 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
• CPC Class Codes: G06K 7/10316 using at least one antenna particularly designed for interrogating the wireless record carriers antennas in general H01Q1/22

Abstract

A reader for an RFID system includes an antenna assembly, a signal driver and a controller. The antenna assembly has an antenna coupled to an antenna tuning circuit which includes a variable capacitance circuit. The signal driver is coupled to the antenna assembly to apply a drive signal to the antenna assembly. The controller is coupled to the antenna assembly to determine a difference between an antenna impedance and a signal driver impedance and to set the variable capacitance at a set capacitance value which reduces the impedance difference.

Claims

1. A reader for an RFID system comprising:

an antenna assembly including an antenna tuning circuit and an antenna, said antenna tuning circuit coupled to said antenna and said antenna having an antenna impedance;

a variable capacitance circuit included in said antenna tuning circuit, said variable capacitance circuit having a variable capacitance;

a signal driver coupled to said antenna assembly to apply a drive signal to said antenna, said signal driver having a signal driver impedance; and

a controller coupled to said antenna assembly to determine a difference between said antenna impedance and said signal driver impedance and to set said variable capacitance to a set capacitance value to reduce said difference.

2. The reader of claim 1 further comprising a variable voltage source coupled between said controller and said variable capacitance circuit to apply a tuning bias signal to said variable capacitance circuit, said tuning bias signal having a biasing voltage value selected to reduce said difference.

3. The reader of claim 1, wherein said antenna tuning circuit further includes a fixed value capacitance circuit having one or more fixed value capacitors coupled in parallel to said antenna.

4. The reader of claim 1, wherein said antenna tuning circuit further includes a fixed value capacitance circuit having one or more fixed value capacitors coupled in series to said antenna.

5. The reader of claim 1, wherein said variable capacitance circuit comprises a one or a plurality of parallely connected variable capacitance elements.

6. The reader of claim 1, wherein said variable capacitance circuit comprises a fixed value capacitor connected in series to one or a plurality of parallely connected variable capacitance elements.

7. The reader of claim 1 further comprising a radio frequency transmissive housing at least partially enclosing said antenna assembly and a radio frequency reflective baseplate coupled to said housing to reduce susceptibility of said antenna to variations caused by objects in an operating environment of a reader.

8. The reader of claim 7, wherein said housing is fabricated from a plastic.

9. The reader of claim 7, wherein said baseplate is fabricated from a metal.

10. A method comprising:

conveying a drive signal from a signal driver to an antenna assembly, wherein said antenna assembly includes an antenna tuning circuit coupled to an antenna, said antenna having an antenna impedance, said signal driver having a signal driver impedance, and said antenna tuning circuit having a variable capacitance;

determining a difference between said antenna impedance and said signal driver impedance; and

setting said variable capacitance at a set capacitance value to reduce said difference.

11. The method of claim 10 further comprising:

generating a tuning bias signal having a biasing voltage value; and

applying said tuning bias signal to said antenna tuning circuit to set said variable capacitance at said set capacitance value.

12. The method of 10 further comprising at least partially enclosing said antenna assembly in a radio frequency transmissive housing and coupling a radio frequency reflective baseplate to said housing to reduce susceptibility of said antenna to variations caused by objects in an operating environment of said antenna.

13. A method comprising:

selecting a set capacitance value of an antenna tuning circuit having variable capacitance, wherein said set capacitance value is selected by ranging an antenna operating parameter correlated to said variable capacitance through a progression of antenna operating parameter values; and

setting said set capacitance value of said antenna tuning circuit to reduce an imbalance between an impedance of an antenna coupled to said antenna tuning circuit and an impedance of a signal driver a applying drive signal to said antenna.

14. The method of claim 13, wherein said imbalance is due to variations in an operating environment of said antenna.

15. The method of claim 13, wherein said imbalance is due to variations in values of an antenna fabrication parameter of said antenna.

16. The method of claim 13, further comprising determining values of a circuit operating parameter of a circuit coupled to said antenna while ranging said values of said antenna operating parameter.

17. The method of claim 16, further comprising correlating said antenna operating parameter values to said values of said circuit operating parameter.

18. The method of claim 17, further comprising setting said antenna operating parameter to an antenna value correlated to said set capacitance value using a correlation between said antenna operating parameter values and said circuit operating parameter values.

19. The method of claim 13, wherein said antenna operating parameter is a biasing voltage of a tuning bias signal applied to said antenna tuning circuit.

20. The method of claim 16, wherein said circuit operating parameter is a magnitude of a drive signal applied to said antenna.

21. The method of claim 16, wherein said circuit operating parameter is a phase across an inductor in said circuit.

22. The method of claim 16, wherein said circuit operating parameter is a voltage magnitude on a terminal of an inductor in said circuit.