Sizing of an electromagnetic transponder system for an operation in extreme proximity

• US 7,058,357 B1
• Filed: 07/13/2000
• Issued: 06/06/2006
• Est. Priority Date: 07/20/1999
• Status: Expired due to Term

• Current Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Inventors: Enguent, Jean-Pierre, Wuidart, Luc
• Primary Examiner(s): Banks-Harold, Marsha D.
• Assistant Examiner(s): Ly, Nghi H.
• Application Number:
  US09/615,430
• Time in Patent Office: 2,154 Days
• Field of Search: 455/41, 455/151.2, 455/575, 455/95, 455/66, 455/274, 455/277.1, 455/73, 455/41.1, 455/575.1, 455/66.1, 342/51, 340/10.1, 340/10.4, 340/10.51, 340/572.1
• US Class Current: 455/41.1
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• CPC Class Codes: G06K 19/07777 the antenna being of the inductive type
• CPC Class Codes: G06K 7/10336 the antenna being of the near field type, inductive coil

Abstract

A system of contactless electromagnetic transmission between a terminal including a series oscillating circuit for generating an electromagnetic field and a transponder including a parallel oscillating circuit, these oscillating circuits being sized so that the coupling coefficient between them strongly decreases when the distance separating the transponder from the terminal becomes greater than a predetermined value.

Claims

1. An electromagnetic transponder including a parallel oscillating circuit adapted to being excited by a series oscillating circuit of a read/write terminal when the electromagnetic transponder enters the field of the read/write terminal, wherein components of the parallel oscillating circuit of the transponder are sized based on a predetermined distance so that a coupling coefficient between respective oscillating circuits of the read/write terminal and of the electromagnetic transponder rapidly decreases when a distance separating the electromagnetic transponder from the read/write terminal becomes greater than the predetermined distance,

wherein an inductance of the parallel oscillating circuit is chosen in accordance with the following relation:

$\mathrm{kopt}=\sqrt{\frac{\mathrm{R1L2}}{\mathrm{R2L1}}},$

where kopt is a coupling coefficient providing a maximum voltage across the parallel oscillating circuit, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the transponder parallel to an inductance L2 of the transponder, and L1 is an inductance of the series oscillating circuit.

2. The electromagnetic transponder of claim 1, wherein the predetermined distance corresponds to 1 centimeter.

3. The electromagnetic transponder of claim 1, wherein a capacitive element of the parallel oscillating circuit is provided by a stray capacitance of an inductance of the parallel oscillating circuit.

4. The electromagnetic transponder of claim 1, wherein inductance of the parallel oscillating circuit is maximized, a capacitance of this oscillating circuit being minimized.

5. The electromagnetic transponder of claim 1, wherein the components of the parallel oscillating circuit of the transponder are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation:

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the parallel oscillating circuit for optimal coupling between the parallel and series oscillating circuits and Vg is an excitation voltage of the series oscillating circuit.

6. The electromagnetic transponder of claim 1, wherein a number of turns of an inductance of the parallel oscillating circuit of the transponder is in a range of between 5 and 15.

7. The electromagnetic transponder of claim 1, wherein respective values of a capacitance and of an inductance of the parallel oscillating circuit range between 5 and 100 pf and between 2 and 25 μ

H.

8. A system of contactless electromagnetic transmission between a terminal and a transponder, wherein the transponder is that of claim 1.

9. A terminal for generating an electromagnetic field adapted to cooperate with at least one transponder when said at least one transponder enters the electromagnetic field, including a series oscillating circuit for generating the electromagnetic field, the series oscillating circuit being sized based on a predetermined distance so that a coupling coefficient between the series oscillating circuit of the terminal and an oscillating circuit of the at least one transponder strongly decreases when a distance separating the at least one transponder from the terminal becomes greater than the predetermined distance,

wherein an inductance of the series oscillating circuit is chosen in accordance with the following relation:

$\mathrm{kopt}=\sqrt{\frac{\mathrm{R1L2}}{\mathrm{R2L1}}},$

where kopt is a coupling coefficient providing a maximum voltage across the oscillating circuit of the at least one transponder, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the at least one transponder parallel to an inductance L2 of the at least one transponder, and L1 is an inductance of the series oscillating circuit.

10. The terminal of claim 9, wherein the oscillating circuit of the transponder is a parallel oscillating circuit adapted to being excited by the series oscillating circuit of the terminal when the transponder enters the electromagnetic field, and

wherein components of the series oscillating circuit are sized to fulfill operating conditions of the transponder.

11. The terminal of claim 10, wherein an inductance of the series oscillating circuit includes a single turn.

12. A system of contactless electromagnetic transmission between a terminal and a transponder, wherein the terminal is that of claim 9.

13. The terminal of claim 9, wherein one or more components of the series oscillating circuit of the transponder are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation:

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the oscillating circuit of the at least one transponder for optimal coupling between the series oscillating circuit and the oscillating circuit of the at least one transponder and Vg is an excitation voltage of the series oscillating circuit.

14. A transponder comprising:

an oscillating circuit adapted to be excited by an external electromagnetic field when the transponder enters the electromagnetic field, the oscillating circuit including an inductance, and wherein a stray capacitance of the inductance acts as a capacitive element for the oscillating circuit,

wherein components of the oscillating circuit are sized based on a particular distance, which serves as an operating point between the transponder and the terminal, to produce an operating condition in which a coupling coefficient between the transponder and a read/write terminal that generates the electromagnetic field rapidly decreases when a distance separating the transponder from the read/write terminal becomes greater than the particular distance,

wherein the components of the oscillating circuit are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation;

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the oscillating circuit for optimal coupling between the oscillating circuit and an oscillating circuit of the read/write terminal, R1 is a series resistance of the oscillating circuit of the read/write terminal, R2 is an equivalent resistance of the transponder parallel to the oscillating circuit of the transponder, and Vg is an excitation voltage of the oscillating circuit of the read/write terminal.

15. The transponder of claim 14, wherein the particular distance corresponds to approximately 1 centimeter.

16. A system for data transfer comprising:

a terminal including a series oscillating circuit having a first inductive element and a first capacitive element; and

a transponder including a parallel oscillating circuit having a second inductive element and a second capacitive element;

wherein the first and second inductive elements and first and second capacitive elements are sized based on a particular distance, which serves as an operating point between the transponder and the terminal, to produce an operating condition in which a coupling coefficient between the series oscillating circuit and the parallel oscillating circuit decreases rapidly when a distance between the terminal and the transponder is greater than the particular distance,

wherein the first inductive element and/or the second inductive element is chosen in accordance with the following relation;

$\mathrm{kopt}=\sqrt{\frac{\mathrm{R1L2}}{\mathrm{R2L1}}},$

where kopt is a coupling coefficient providing a maximum voltage across the parallel oscillating circuit, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the transponder parallel to the second inductive element having an inductance L2, and L1 is an inductance of the first inductive element.

17. The system for data transfer of claim 16, wherein the second capacitive element is provided by a stray capacitance of the second inductive element.

18. The system for data transfer of claim 16, wherein the particular distance is approximately 1 centimeter.

19. The system for data transfer of claim 16, wherein the first inductive element comprises a single turn.

20. A system for data transfer comprising:

a terminal including a series oscillating circuit having a first inductive element and a first capacitive element; and

a transponder including a parallel oscillating circuit having a second inductive element and a second capacitive element;

wherein the first and second inductive elements and first and second capacitive elements are sized based on a particular distance, which serves as an operating point between the transponder and the terminal, to produce an operating condition in which a coupling coefficient between the series oscillating circuit and the parallel oscillating circuit decreases rapidly when a distance between the terminal and the transponder is greater than the particular distance,

wherein at least one of the following elements;

the first and second inductive elements and first and second capacitive elements, are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation;

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the parallel oscillating circuit for optimal coupling between the parallel and series oscillating circuits, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the transponder parallel to the parallel oscillating circuit, and Vg is an excitation voltage of the series oscillating circuit.

21. An electromagnetic transponder including a parallel oscillating circuit adapted to being excited by a series oscillating circuit of a read/write terminal when the electromagnetic transponder enters the field of the read/write terminal, wherein one or more components of the parallel oscillating circuit of the transponder are sized based on a particular distance so that a coupling coefficient between respective oscillating circuits of the read/write terminal and of the electromagnetic transponder rapidly decreases when a distance separating the electromagnetic transponder from the read/write terminal becomes greater than the particular distance,

wherein the components of the parallel oscillating circuit of the transponder are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation:

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the parallel oscillating circuit for optimal coupling between the parallel and series oscillating circuits, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the transponder parallel to the parallel oscillating circuit, and Vg is an excitation voltage of the series oscillating circuit.

22. The electromagnetic transponder of claim 21, wherein the particular distance corresponds to 1 centimeter.

23. The electromagnetic transponder of claim 21, wherein a capacitive element of the parallel oscillating circuit is provided by a stray capacitance of an inductance of the parallel oscillating circuit.

24. The electromagnetic transponder of claim 21, wherein inductance of the parallel oscillating circuit is maximized, a capacitance of this oscillating circuit being minimized.

25. The electromagnetic transponder of claim 21, wherein a number of turns of an inductance of the parallel oscillating circuit of the transponder is in a range of between 5 and 15.

26. The electromagnetic transponder of claim 21, wherein respective values of a capacitance and of an inductance of the parallel oscillating circuit range between 5 and 100 pf and between 2 and 25 μ

H.

27. A terminal for generating an electromagnetic field adapted to cooperate with at least one transponder when said transponder enters the electromagnetic field, including a series oscillating circuit for generating the electromagnetic field, one or more components of the series oscillating circuit being sized based on a particular distance so that a coupling coefficient between the series oscillating circuit and an oscillating circuit of the at least one transponder strongly decreases when a distance separating the at least one transponder from the terminal becomes greater than the particular distance,

wherein one or more components of the series oscillating circuit of the transponder are sized based on an operating point at a zero distance, chosen to correspond to a coupling coefficient smaller than an optimal coupling coefficient in accordance with the following relation:

$\mathrm{V2}<br><br><br><br>\max <br><br>\left(\mathrm{kopt}\right)=\sqrt{\frac{\mathrm{R2}}{\mathrm{R1}}}<br><br>\frac{\mathrm{Vg}}{2},$

where V2max is a voltage across the oscillating circuit of the at least one transponder for optimal coupling between the and series oscillating circuits, R1 is a series resistance of the series oscillating circuit, R2 is an equivalent resistance of the transponder parallel to the oscillating circuit of the at least one transponder, and Vg is an excitation voltage of the series oscillating circuit.

28. The terminal of claim 27, wherein the oscillating circuit of the transponder is a parallel oscillating circuit adapted to being excited by the series oscillating circuit of the terminal when the transponder enters the electromagnetic field, and

wherein components of the series oscillating circuit are sized to fulfill operating conditions of the transponder.

29. The terminal of claim 28, wherein an inductance of the series oscillating circuit includes a single turn.

30. A transponder comprising:

an oscillating circuit adapted to be excited by an external electromagnetic field when the transponder enters the electromagnetic field, the oscillating circuit including an inductance, and wherein a stray capacitance of the inductance acts as a capacitive element for the oscillating circuit,

wherein components of the oscillating circuit are sized based on a particular distance, which serves as an operating point between the transponder and the terminal, to produce an operating condition in which a coupling coefficient between the transponder and a read/write terminal that generates the electromagnetic field rapidly decreases when a distance separating the transponder from the read/write terminal becomes greater than the particular distance,

wherein the inductance of the oscillating circuit is chosen in accordance with the following relation;

$\mathrm{kopt}=\sqrt{\frac{\mathrm{R1L2}}{\mathrm{R2L1}}},$

where kopt is a coupling coefficient providing a maximum voltage across the oscillating circuit, R1 is a series resistance of an oscillating circuit of the read/write terminal, R2 is an equivalent resistance of the transponder parallel to an inductance L2 of the oscillating circuit, and L1 is an inductance of an oscillating circuit of the read/write terminal.