1. Field of the Invention
The present invention generally relates to electromagnetic transponder systems and, more specifically, to electromagnetic transponders that do not have their own power supply, but rather which extract the power required for the operation of the electronic circuits comprised therein from an electromagnetic field radiated by a read and/or read/write terminal.
An example of application of the present invention relates to electronic tags (TAG) comprising an electronic chip and a radio-frequency field reception antenna.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram illustrating an example of an electromagnetic transponder system of the type to which the present invention applies. An electronic tag 1 forming an electromagnetic transponder is based on an oscillating circuit 10 formed, for example, of an inductive element 11 in parallel with a capacitive element 12 between two terminals 13 and 14 of circuit 10. Terminals 13 and 14 are connected to an electric circuit 15 (IC), generally a single integrated circuit, intended to exploit the voltage sampled across oscillating circuit 10 when tag 1 is a radio-frequency field radiated by a terminal 2 (READER) or read or read/write terminal. Terminal 2 comprises an oscillating circuit 20 based on an inductive element 21 forming an antenna, for example, in series with a capacitive element 22 and a resistive element 26 between two terminals 23 and 24 of an electronic circuit 25 (ICS). Circuit 25 comprises one or several integrated circuits for exciting the oscillating circuit and interpreting possible transmissions coming from electronic tag 1.
The operation of such a system is based on the coupling of oscillating circuits 20 and 10 of terminal 2 and of transponder 1. On the side of terminal 2, circuit 25 generates a high-frequency excitation signal (typically with a carrier at a frequency on the order of 13.56 MHz or on the order of 125 kHz according to applications). This signal is applied to antenna 21 of generation of an electromagnetic field in the vicinity of the terminal. When a transponder 1 is in the field of the terminal, its antenna 11 collects the power radiated by the terminal and resonant circuit 10 develops between its terminals 13 and 14 a voltage exploitable by circuit 15. Oscillating circuits 10 and 20 are generally tuned to a same resonance frequency approximately corresponding to the carrier frequency of the signal transmitted by the terminal. Generally, circuit 15 has no autonomous power supply and samples the power necessary for its operation from the field radiated by the terminal. Circuit 15 integrates so-called back-modulation means for modifying the load formed by transponder 1 in the field of the terminal to enable a communication in the transponder-to-terminal direction. On the side of terminal 2, the voltage across capacitive element 22 is for example measured, the interconnection point between antenna 21 and capacitor 22 being connected (connection 27) to circuit 25 to enable demodulation of transponder-to-terminal transmissions. According to applications, the high-frequency carrier generated by terminal 2 may also be modulated to transmit information to the transponder.
FIG. 2 very schematically shows, in top view, an example of an electronic tag 1 of the type to which the present invention more specifically applies. Such a tag is formed of a plate 16 (flexible or rigid) on which is deposited a metal 11 in the form of a planar winding of concentric spirals to form the antenna, the two ends of track 11 being connected to terminals of circuit 15, here assumed to integrate capacitor 12.
A tag 1 such as illustrated in FIG. 2 is generally associated with an object or an element, for example, for identification purposes. These may be products (for example, products for sale in a store), smart cards in access-control applications, etc. More generally, an electronic tag may be associated with any object or system (for example, a vehicle) for identification, counting, or other purposes.
FIG. 3 shows an example of an object 30 on which (for example glued) a tag 1 of the type illustrated in FIG. 2 is placed. Product 30 is assumed to be made of an insulating material (DIEL), for example, cardboard, plastic matter, etc. When the planar antenna (not visible in FIG. 3) of tag 1 is close to a reader (represented in FIG. 3 by its antenna 21), the electric field of antenna 21 is likely to be sensed by product 1, field lines EF crossing plate 16 (FIG. 2) of tag 1 and object 30 by passing through the center of planar winding 11.
A problem is however posed in the case where tag 1 is placed on a metal object at least at the surface thereof. Indeed, the electromagnetic field is disturbed by this object that it cannot cross. Further, this causes a detuning of the oscillating circuits of the terminal and of the transponder, which adversely affects the remote supply of the tag and the information transmission.
FIG. 4 very schematically shows a first known example of a solution for placing a planar-antenna electronic tag 1 on a metal object 40 (METAL). This solution consists of interposing a spacer 41 formed of an insulating block between tag 1 and object 40. A disadvantage is the bulk of spacer 41, the thickness of which must in practice be greater than 5 millimeters to enable field lines EF to come out through the lateral surfaces of this spacer.
FIG. 5 very schematically shows a second conventional example of a solution for placing an electronic tag 1 on a metal object 40. In this solution, a ferrite spacer 43 is interposed between metal object 40 and electronic tag 1. The use of a ferrite spacer enables reducing the thickness of this spacer, the ferromagnetic material conducting the field to enable looping back of the field lines and avoid the metal disturbance. A disadvantage of ferrite or another ferromagnetic material is that such materials are expensive, in practice incompatible with the low costs desired for electronic tag systems.
The problem of the disturbance created by a metal object on the operation of a transponder system is all the more critical as the carrier frequency is high. Indeed, the higher the frequency, the smaller the number of turns of planar winding 11 of the antenna (typically from 1 to 3 turns for a 13.56-MHz frequency). Now, the smaller the number of turns, the lower the coupling and the more the system is sensitive to disturbances.