FINGER-CONTROLLED CONTACTLESS CHIP CARD

Abstract

The invention relates to a contactless chip card intended to communicate with a chip card reader operating at a resonant frequency F.sub.0. The chip card includes a booster antenna provided with an inductive main antenna U and with an inductive concentrator antenna L.sub.2, which antennae are connected in series or parallel with a capacitor C, wherein the inductances of L.sub.1 and L.sub.2 and the capacitance of C are chosen on the one hand to obtain a resonant frequency F.sub.0 that is far enough from the frequency of the signal emitted by the reader to limit radiofrequency communication with the reader, and on the other hand so that the connection in parallel of a predetermined capacitance C.sub.d brings the resonant frequency of the card into the vicinity of said frequency F.sub.0 of the signal emitted by said reader.

Claims

1. A chip card with contactless operation, configured to communicate with a chip card reader operating at a resonant frequency F.sub.0, said chip card being provided with a card body incorporating at least one antenna connected in series or in parallel with a capacitor, wherein inductances of the antennae and the capacitance of the capacitor have values that achieve a resonant frequency F.sub.0 remote enough from the frequency of a signal emitted by the reader to limit radiofrequency communication with the reader, such that the chip card is detuned by default with respect to the reader, and the values of said inductances and capacitor furthermore being such that the connection, in parallel, of a predetermined additional capacitance C.sub.d brings the resonant frequency of the card close to the frequency F.sub.0 of the signal emitted by said reader, such that the chip card is retuned with the reader, wherein the chip card includes a booster antenna provided with a main antenna with inductance L.sub.1 and with a concentrator antenna with inductance L.sub.2 that are connected in series or in parallel with a capacitor C, and wherein the values of L.sub.1, L.sub.2 and C are chosen firstly so as to achieve said resonant frequency F.sub.0 and secondly so that the connection, in parallel, of said predetermined capacitance C.sub.d brings the resonant frequency of the card close to said frequency F.sub.0 of the signal emitted by said reader.

2. The chip card as claimed in claim 1, wherein said predetermined capacitance C.sub.d for bringing the resonant frequency of the card close to the frequency F.sub.0 of the signal emitted by said reader corresponds to the capacitance of a finger placed between two metal contacts (23, 25) of the card.

3. The chip card as claimed in claim 2, wherein the predetermined capacitance C.sub.d has a value of the order of 7 picofarads.

4. The chip card as claimed in claim 1, in that wherein the values of L.sub.1, L.sub.2 and C are chosen such that L.sub.1+L.sub.2 is greater than or equal to 4 H, and C is of the order of 20 picofarads.

5. The chip card as claimed in any one of the preceding claims claim 1, wherein the capacitor C.sub.d is produced in the form of a metal wire embedded in an antenna carrier and arranged in the form of a coil, located by a marker on a face of the chip card.

6. The chip card as claimed in claim 1, wherein the capacitor C.sub.d is produced in the form of at least two metal half-dots arranged facing one another such that a finger placed straddling the half-dots creates an adjusting capacitance C.sub.d of the order of 7 pF that is connected in parallel with the capacitor C of the antenna.

7. The chip card as claimed in claim 6, wherein the capacitor C.sub.d is produced in the form of several pairs of metal half-dots arranged facing one another such that a combination of several fingers placed straddling several pairs of half-dots creates an adjusting capacitance C.sub.d of the order of 7 pF that is connected in parallel with the capacitor C of the antenna.

Description

[0017] The invention will be better understood upon reading the following description and the drawings, in which:

[0018] FIG. 1 shows the equivalent circuit diagram of a contactless chip card according to the invention;

[0019] FIG. 2 shows a first embodiment of the antenna of the chip card according to the invention;

[0020] FIG. 3 shows a second embodiment of the antenna of the chip card according to the invention.

DETAILED DESCRIPTION OF THE INVENTION:

[0021] FIG. 1 shows the equivalent circuit diagram of a chip card, comprising an electronic module 10 provided with an electronic chip (not shown) and with an antenna, and a booster antenna 20 arranged in the card body.

[0022] The electronic module 10 is equivalent to an RLC circuit, with a resistor R.sub.m, a capacitor C.sub.m and an inductor L.sub.m connected in parallel.

[0023] The booster antenna 20 includes a concentrator antenna with inductance L.sub.2 (and denoted L.sub.2) coupled to the antenna L.sub.m of the module, a resistor R, an antenna with inductance L.sub.1 and denoted L.sub.1, and a capacitor C. The antenna L.sub.1 has large turns that are generally arranged on the perimeter of the card body, in what is known as an ID1 format, in accordance with the ISO/CEI 7810 standard. This ID1 format corresponds to cards with dimensions of 85.6053.98 mm.

[0024] The equivalent circuit diagram also shows a variable capacitor C.sub.d across the terminals of the booster antenna 20. This is the capacitance formed by the finger(s) of the user when this finger or these fingers is/are placed on the card.

[0025] According to the invention, the finger(s) will be placed on visual markers that are printed on the card body opposite one or more electric dipoles formed by two metal pads placed side by side, such that, when the user's finger is absent, there is an open circuit (this corresponding to a zero capacitance C.sub.d) and that, when the user's finger is present, there is a nonzero capacitance C.sub.d of a few picofarads.

[0026] Specifically, the invention makes provision to use the capacitance of the user's finger as a switch that allows operation of the card in contactless mode.

[0027] Specifically, it has been found that the capacitance of a finger may be relatively stable from one user to another and depending on moisture conditions or other conditions. Tests have shown that the capacitance of a finger varies substantially within a range of 6.5 to 7.5 pF for a given dipole surface.

[0028] The invention exploits this feature by choosing values for the components L.sub.1, L.sub.2 and C of the booster antenna 20 of the card that bring about a resonant frequency that is greatly offset with respect to the resonant frequency of the reader, which means that the card is by default detuned with respect to the reader.

[0029] Thus, the resonant frequency of the booster antenna 20 is of the following type:

F.sub.0=1/(2**(L.sub.1+L.sub.2)*C)

[0030] Therefore, if the operating frequency of the chip card reader is of the order of 13.56 MHz, as is often the case in the field of contactless cards and their readers, the values of L.sub.1, L.sub.2 and C will be chosen so that, when the user's finger is absent (that is to say C.sub.d=0), a resonant frequency of the booster antenna 20 is achieved that is more than 1 MHz away from the target frequency of 13.56 MHz.

[0031] With such an offset resonant frequency, the chip card will be detuned with respect to the reader and will not be able to communicate with the chip card reader without intentional modification of the resonant circuit that is able to bring the resonant frequency of the booster antenna close to 13.56 MHz.

[0032] This modification will be made by adding the capacitance C.sub.d introduced by one or more of the user's fingers.

[0033] Specifically, the resonant frequency of the booster antenna, when a finger is positioned on the card, will be expressed using the following formula:

F.sub.0=1/(2**(L.sub.1+L.sub.2)*(C+C.sub.d))

with the knowledge that the electric dipoles where the finger(s) will be placed may be configured so that the additional capacitance C.sub.d is of the order of 7 pF.

[0034] It is therefore possible to determine a set of values of L.sub.1, L.sub.2 and C that bring about a resonant frequency F.sub.0 of greater than or equal to 14.7 MHz, and a value F.sub.0 (taking into account a value C.sub.d of the order of 7 picofarads) equal to 13.56 MHz.

[0035] Pairs of values that have been tested and that work are for example as follows:

[0036] L.sub.1+L.sub.2 greater than or equal to 4 H

[0037] C approximately equal to 20 pF.

[0038] Of course, other sets of values are possible and will easily be determined by those skilled in the art. It is simply necessary for the inductance L.sub.1+L.sub.2 to be very high (at least 4 H) in order for a small variation in the capacitance C to cause a significant variation in the resonant frequency F.sub.0.

[0039] FIG. 2 shows a first embodiment of a booster antenna 20 for a chip card incorporating a capacitor C.sub.d produced in wired form by very thin metal wires 21 embedded in the antenna carrier film using ultrasound and arranged in a coil or a very tight sinusoid.

[0040] To materialize the position of this wired capacitor, a visual marker (not shown) will be placed on an outer face of the chip card, opposite the capacitor C.sub.d. The user will thus know where to place his finger in order to retune the chip card to the target frequency, for example 13.56 MHz in this case.

[0041] FIG. 3 shows a booster antenna 20 with a second embodiment of the capacitor C.sub.d, in the form of a metal circuit etched at the same time as the metal tracks of the antennae L.sub.1, L.sub.2 and connected in parallel between L.sub.1 and L.sub.2.

[0042] This etched metal circuit includes at least one dipole formed by a first metal half-dot 23 connected to the antenna L.sub.2 by a track 24, and a second half-dot 25 connected to the antenna L.sub.1 by a track 26. The two half-dots 23, 25 are separated by a space 27, which means that their capacitance is zero when there is no finger placed straddling the two half-dots, and that their capacitance C.sub.d is equal to around 7 pF when a finger is placed straddling the two half-dots 23, 25.

[0043] Again, it will be useful to mark the location of the two half-dots 23, 25 using an appropriate marker on an outer face of the chip card so as to guide the user.

[0044] In the two embodiments of the capacitor C.sub.d according to FIGS. 2 and 3, it would be possible (as outlined in FIG. 3 by the half-dots 28, 29) to divide the capacitor C.sub.d into several portions produced in wired form or in etched form, into several subsets that are connected in parallel, so as to have to use a combination of several fingers placed at various locations of the card to create an equivalent capacitance of the order of 7 pF. Furthermore, this would make it possible to further strengthen the security of the chip card, by ensuring that only an intentional placement of several of the user's fingers facing the various antenna portions creates the tuning capacitance C.sub.d.

Advantages of the Invention

[0045] The invention achieves the aims that were set. The choice of the components for making the resonant frequency of the booster antenna remote from the resonant frequency of the reader makes it possible to ensure that the card is not able to operate and therefore perform transactions as it is. On the other hand, placing a finger on the card to create an additional adjusting capacitance C.sub.d brings the resonant frequency into a range that allows the card to operate with the reader, with the communication quality and distance performance that is normally required.

[0046] Furthermore, the invention is particularly easy to implement as it does not require any modification of the structure of the card, but only a particular choice of the values of the components L, C of its booster antenna.