RFID tag containing two tuned circuits

1. An RFID tag adapted to be read by an interrogator including a reader antenna having a carrier frequency, said tag including:

a first circuit including a first inductor and a first capacitor, wherein said first circuit is tuned to a frequency above said carrier frequency to compensate for detuning in the presence of a plurality of tags;

a second circuit including a second inductor and a second capacitor; and

wherein said first and second inductors are arranged to be coupled electromagnetically with a coupling factor less than unity and said second circuit is tuned to increase coupling between said first circuit and said reader antenna.

2. A tag according to claim 1 wherein said first circuit includes a chip.

3. A tag according to claim 2 wherein said chip includes said first capacitor.

4. A tag according to claim 1 wherein said coupling factor is substantially 0.7.

5. A tag according to claim 1 wherein said first inductor includes a first loop and said second inductor includes a second loop larger than said first loop.

6. A tag according to claim 5 wherein an edge of said first loop is aligned with an edge of said second loop.

7. A tag according to claim 5 wherein said first and second loops are concentric.

8. A tag according to claim 5 wherein said first and second loops are eccentric.

9. A tag according to claim 5 wherein said second capacitor includes a parasitic capacitor formed by capacitance between windings of said second loop.

10. A tag according to claim 1 wherein said first and second inductors are connected at a common tapping point.

11. A tag according to claim 1 wherein said first and second inductors are formed on a common substrate.

12. A tag according to claim 11 wherein said first and second inductors are formed on opposite sides of said common substrate.

13. A tag according to claim 11 wherein said first and second inductors are formed on the same side of said common substrate and are separated by an insulator.

14. A tag according to claim 1 wherein said first and second inductors are formed on separate substrates.

15. A tag according to claim 1 wherein said first and second inductors are wound in the same sense.

16. A tag according to claim 1 wherein said second circuit is tuned above the frequency of said first circuit.

17. A tag according to claim 16 wherein said second circuit is tuned between 80-160% above the frequency of said carrier.

18. A tag according to claim 1 wherein said first circuit is tuned between 10-50% above the frequency of said carrier.

19. A tag according claim 1 wherein said carrier is 13.56 MHz, said first circuit is tuned to between 15-20 MHz and said second circuit is tuned to between 25-35 MHz.

20. An optical data carrier including a tag according to claim 1 .

21. A method of reading an RFID tag in the presence of a plurality of tags by an interrogator including a reader antenna having a carrier frequency, said tag including a first circuit having a first inductor and a first capacitor, said method including the steps of:

tuning said first circuit to a frequency above said carrier frequency to compensate for detuning in the presence of said tags;

providing a second circuit having a second inductor and a second capacitor;

electromagnetically coupling said first and second indicators with a coupling factor less than unity; and

tuning said second circuit to increase coupling between said first circuit and said reader antenna.

22. A method according to claim 21 wherein said first circuit includes a chip.

23. A method according to claim 22 wherein said chip includes said first capacitor.

24. A method according to claim 21 wherein said coupling factor is substantially 0.7.

25. A method according to claim 21 wherein said first inductor includes a first loop and said second inductor includes a second loop larger than said first loop.

26. A method according to claim 25 wherein an edge of said first loop is aligned with an edge of said second loop.

27. A method according to claim 25 wherein said first and second loops are concentric.

28. A method according to claim 25 wherein said first and second loops are eccentric.

29. A method according to claim 25 wherein said second capacitor includes a parasitic capacitor formed by capacitance between windings of said second loop.

30. A method according to claim 21 wherein said first and second inductors are connected at a common tapping point.

31. A method according to claim 21 wherein said first and second inductors are formed on a common substrate.

32. A method according to claim 31 wherein said first and second inductors are formed on opposite sides of said common substrate.

33. A method according to claim 31 wherein said first and second inductors are formed on the same side of said common substrate and are separated by an insulator.

34. A method according to claim 21 wherein said first and second inductors are formed on separate substrates.

35. A method according to claim 21 wherein said first and second inductors are wound in the same sense.

36. A method according to claim 21 including tuning said second circuit above the frequency of said first circuit.

37. A method according to claim 36 wherein said second circuit is tuned between 80-160% above the frequency of said carrier.

38. A method according to claim 21 wherein said first circuit is tuned between 10-50% above the frequency of said carrier.

39. A method according to claim 21 wherein said carrier is 13.56 MHz, said first circuit is tuned to between 15-20 MHz and said second circuit is tuned to between 25-35 MHz.