Energy transfer in an electronic identification system

• US 6,621,467 B1
• Filed: 11/02/2001
• Issued: 09/16/2003
• Est. Priority Date: 05/03/1999
• Status: Expired due to Fees

• Current Assignee: Trolley SCAN Limited
• Original Assignee: Trolley SCAN Limited
• Inventors: Marsh, Michael John Camille
• Primary Examiner(s): Ho, Tan
• Application Number:
  US09/980,700
• Time in Patent Office: 683 Days
• Field of Search: 343/850, 343/860, 343/793, 343/795, 343/803, 343/861, 343/862, 340/572.1, 340/572.7, 333/17.3, 333/32
• US Class Current: 343/850
• CPC Class Codes: G06K 19/0726 the arrangement including a circuit for tuning the resonance frequency of an antenna on the record carrier
• CPC Class Codes: H03H 7/38 Impedance-matching networks

Abstract

A radio frequency transponder assembly comprises a passive transponder and an antenna with a matching circuit comprising an inductor and a capacitor which are formed integrally with the antenna components. The antenna is a dipole with legs formed from wire or foil strips, in which case the inductor and capacitor are formed from same material as the antenna legs. The values of the inductor and capacitor are calculated to match the antenna impedance to the equivalent load of the transponder circuit as seen via the power rectification circuits of the transponder.

Claims

1. A method of matching an electronic circuit of a radio frequency transponder having a complex input impedance to an antenna, the method comprising:

measuring the complex input impedance of the electronic circuit;

calculating a first matching element value to transform the complex input impedance to an intermediate impedance value;

calculating a second matching element value to transform the intermediate impedance value to a real value corresponding to the antenna impedance; and

constructing a matching network with inductive and capacitive matching elements corresponding to the calculated first and second matching element values, including forming at least one of the inductive and capacitive elements of the matching network integrally with the antenna.

2. A method according to claim 1, wherein the complex input impedance has a negative reactance component, and wherein the first matching element value is inductive and the second matching element value is capacitive.

3. A method according to claim 2, including constructing a matching network comprising a capacitor in parallel with the antenna and an inductor in series with the antenna and the electronic circuit.

4. A passive radio frequency transponder assembly of the kind that extracts energy for its operation from an energizing field, the transponder assembly comprising an antenna;

a transponder circuit, the transponder circuit including a power supply circuit arranged to be fed with energy received by the antenna, a logic circuit, and a modulator circuit for generating data signals for transmission by the antenna; and

an impedance matching circuit effectively disposed between the antenna and the transponder circuit, wherein the transponder assembly is secured to a substrate, the antenna comprises at least one limb of conductive material supported by the substrate, the impedance matching network includes an inductive component formed from the same material as that of the antenna and attached to a limb of the antenna, and the matching circuit is optimized to enhance the transfer of energy collected by the antenna to the power supply circuit of the transponder circuit.

5. A radio frequency transponder assembly according to claim 4, wherein the transponder circuit has a complex input impedance, and wherein the impedance matching elements comprise a capacitor and an inductor.

6. A radio frequency transponder assembly according to claim 5, wherein the complex input impedance of the transponder circuit has a negative reactance component and the antenna is a dipole or folded dipole.

7. A radio frequency transponder assembly according to claim 6, wherein the matching circuit effectively comprises a capacitor connected in parallel with the antenna and an inductor connected in series between the antenna and the transponder circuit.

8. A radio frequency transponder assembly according to claim 7, wherein the capacitor is effectively connected between first and second terminals of the antenna in parallel with input terminals of the transponder circuit.

9. A radio frequency transponder assembly according to claim 8, wherein the inductor is effectively connected in series with a first terminal of the antenna and an input terminal of the transponder circuit.

10. A radio frequency transponder assembly according to claim 9, wherein the antenna has at least one element comprising a first length of conductive material, and the inductor comprises a second length of conductive material connected between the antenna and the transponder circuit, the inductance thereof being determined by said second length.

11. A radio frequency transponder assembly according to claim 10, wherein said at least one antenna element and said inductor comprise a conductive metallic foil.

12. A radio frequency transponder assembly according to claim 11, wherein the antenna has at least two elements and the capacitor is formed by overlapping adjacent elements of the antenna separated by a dielectric layer.

13. A radio frequency transponder assembly according to claim 10, wherein said at least one antenna element and said inductor comprises a metallic wire.

14. A radio frequency transponder assembly according to claim 13, wherein the antenna has at least two elements and the capacitor is formed from a length of wire connected to one element of the antenna and wound around another element thereof.

15. A radio frequency transponder assembly according to claim 4, having an operating frequency in the range of 440 to 930 MHZ and an operating range of at least 1 m for an effective radiated power of a reader energizing the transponder of no more than 500 mW.