WO2014191638A1

WO2014191638A1 - Radio frequency device provided with an optimized antenna

Info

Publication number: WO2014191638A1
Application number: PCT/FR2014/000118
Authority: WO
Inventors: Elias Sabbah
Original assignee: Ask S.A.
Priority date: 2013-05-27
Filing date: 2014-05-27
Publication date: 2014-12-04
Also published as: FR3006085A1

Abstract

The invention relates to a radio frequency device intended for contactless communication with an associated reader, made up of a planar support layer (11) and an electrical circuit, the electrical circuit comprising an antenna (12) formed by a planar winding of at least two turns (10) that intersect without short-circuiting owing to a dielectric layer (9) and a chip or an electronic module (14) connected to the antenna. According to the main feature of the invention, the relative electric permittivity of the dielectric layer (9) has a value above 20 and creates a crossover capacity Ce whereof the value increases the resultant capacity C of the electric circuit so as to decrease the inductance L of the antenna by decreasing its length by a whole number of turns, the electric circuit thus forming a resonant circuit allowing communication with the associated reader.

Description

Radio frequency device with an optimized antenna

Technical area

The present invention relates to the manufacture of an antenna for radiofrequency device equipping smart cards or semi-finished products for the realization of electronic passports and concerns in particular a radiofrequency device with an optimized antenna.

State of the art

Radio frequency devices embedded in flat and flexible media such as smart cards, contactless tickets, security or valuable documents have a chip or an electronic module and an antenna connected together. These devices allow the exchange of information with the outside by electromagnetic coupling at a distance and therefore without physical contact, between its antenna and a second antenna located in an associated reading device. These devices are used today in a large number of applications and in particular for the identification of people traveling in areas with controlled access or transit from one area to another.

It is known that a circuit formed by a chip and an antenna is a resonant circuit whose optimal operation is obtained when the resonance law of the following circuit is respected:

LCco ₀ ² = 1 (1)

where L represents the inductance of the antenna, C represents the total capacity connected to the antenna and ω the pulsation equal to 2πί, in which f represents the desired resonance frequency (for example at 13.56 MHz). Therefore it follows that the resonance frequency of the circuit f ₀ is equal to:

f ₀ = 1/2 LC (2) In order to reduce the cost price of the chips, the founders are led more and more to reduce the size of the capacitor integrated therein and thus reduce the input capacity of the circuit. They can thus produce chips of smaller dimensions.

In order to respect the law LCco ² = 1 and to obtain an optimal coupling, it was sought to increase the inductance L of the antenna to compensate for the decrease in the value of the input capacitance of the chip. In the case of antennas made by chemical etching of copper or aluminum, in the form of turns on a plastic dielectric support, the inductance has been increased by increasing for example the number of turns. This solution, however, causes several disadvantages. Indeed, any electrical circuit having a resistance, the increase in the number of turns, which in fact corresponds to an elongation of the circuit, causes an increase in the value of this resistance. This affects the performance of the antenna and therefore the card. Indeed, the reading distance of the card is decreased.

The unit cost of the engraved antenna increases significantly. Thus, the cost reduction obtained by the smelters with chips having a lower input capacity is canceled by this extra cost of the antennas. The production and use of cards is therefore not more profitable.

The document FR 2 803 439 describes a solution to the aforementioned problems. This document describes a coupling antenna connected to a chip containing one or more integrated capacitors. This coupling antenna contains at least one screen-printed turn on a support consisting of an insulating dielectric substrate and also comprises a capacitor screen-printed on the support, connected in parallel, thus making it possible to reduce the capacitance provided by the integrated capacitor (s). (s) in the device, so that the resulting capacitance forms a resonant circuit with the coil. This solution also does not save on the price of the screen-printed material which then includes the capacitor in addition to the antenna. Presentation of the invention

This is why the object of the invention is to overcome the aforementioned drawbacks by providing an antenna with a parallel capacitance obtained at a lower cost, making it possible to obtain a chip card whose cost price is greatly reduced compared to that of the smart cards currently on the market, thanks to the decrease in the number of turns of the antenna.

The object of the invention is therefore a radiofrequency device intended to communicate without contact with an associated reader, composed of a plane support layer and an electrical circuit, the electrical circuit comprising an antenna formed of a planar winding. at least two turns that intersect without being short-circuited by a dielectric layer and a chip or an electronic module connected to the antenna. According to the main characteristic of the invention, the relative electrical permittivity of the dielectric layer has a value greater than 20 and creates a crossing capacitance Ce whose value increases the resulting capacitance C of the electrical circuit so as to reduce the inductance L of the antenna by decreasing its length by an integer number of turns, the electric circuit thereby forming a resonant circuit for communication with the associated reader.

Brief description of the figures

The objects, objects and features of the invention will appear more clearly on reading the following description given with reference to the drawings in which:

FIG. 1 represents a general diagram of the radiofrequency device according to the invention, FIG. 2 represents the electrical diagram of the radio frequency device according to the invention.

Detailed description of the invention

The radio frequency device according to the invention is intended to communicate without contact with an associated reader. According to a preferred embodiment of the invention, the device is composed mainly of a plane support layer 11 on which an antenna 12 to which a chip or an electronic module 14 is connected is connected. The device according to the invention advantageously applies to a card or a contactless ticket in the standard format of a smart card or to an identity booklet such as a passport. It can also be applied to a semifinished product intended for the manufacture of a card, a contactless ticket, or an identity book, it is then commonly called "inlay".

The support ^* 11 is made of paper, synthetic paper such as teslin or plastic such as polyvinyl chloride (PVC), polyester (PET, PETG), polycarbonate (PC) or acrylonitrile-butadiene- styrene (ABS).

The antenna is produced by offset, serigraphic, heliographic or flexographic printing of a conducting ink of the polymer conductive ink type, loaded with conducting elements such as silver, copper or carbon. The antenna is formed of a planar winding of at least two turns 10 which intersect without being short-circuited by passing on and under an electrically insulating layer such as a dielectric layer 9. The antenna being cut off it comprises two connection ends on which the chip is connected.

FIG. 2 represents the equivalent electrical diagram of the device according to the invention shown in FIG. 1. According to the diagram, the radio frequency device according to the invention is equivalent to a resonant electrical circuit realized by an antenna 22 tuned by a chip 24. The chip 24 has an internal capacitance Cp obtained via the capacitors contained in the chip and a load resistor Rp. The chip also comprises an electronic part corresponding to the memory zone and the processor. The antenna 22 is equivalent to an inductance L, a resistance r, an interspire capacitance Cf and a crossover capacitance Ce. The inductance L is obtained by the equivalent length of the windings of the antenna so L is directly related to the number of turns of the antenna. The inter-turn capacitance Cf is created in the space between the turns 10 and represents the capacitance due to the winding of the turns; the value of this capacity is negligible.

For a target resonance frequency, the inductance must reach a nominal value according to the relation f ₀ = 1/2 TC. (3)

In radio frequency devices of the prior art, the value of the capacitor C is set exclusively by the value of the capacitance. Cp of chip 22, the other capacities represented by the interspire capacity and the crossover capacity being considered negligible.

On the other hand, in radio frequency devices of the prior art using the printed antenna technology, the dielectric used to make the electrical bridge of the antenna at the point of intersection of the turns is made of a material said to "Transparent" that is to say not influencing its near environment so as not to disturb the adjustment of the antenna to the chip. It is considered as air and its electrical permittivity is low. Therefore, the value of the crossing capacity is negligible. So in order to obtain the target frequency, and according to the type of chip used, the antenna is adjusted to the chip by the value of its inductance. According to the preferred embodiment of the invention, the capacitance C is reevaluated according to the value of the crossing capacitance Ce of the antenna 22 and consequently the value of the inductance L decreases. It is considered that the capacitance C is equal to the sum of the capacitance of the chip Cp and the crossover capacitance Ce. Equation (3) becomes: fo = 1 / 2TT L. (Cp + Cc + Cf). (4)

In our case, crossover capability is determined to reduce the length of the antenna winding by an integer number of turns so that the resulting capacitance of the antenna and chip / module composite circuit electronic 14 forms a resonant circuit, that is to say able to communicate properly with the associated reader. The crossing capacity is determined according to the following formula: C = ε 0 * ar * - (5)

d

Or :

ε0 = 8.854187 10 ^"12 Fm ^{" 1} (Permittivity of the air)

εχ relative electrical permittivity of the dielectric 9

A: Crossing surface of the conductive turns opposite and on either side of the dielectric layer 9 d: thickness of the dielectric forming the bridge

Thus, for radio frequency devices operating at the frequency of 13.56 MHz, the relative electrical permittivity of the dielectric according to the invention is greater than 20 and preferably equal to 40. The thickness of the dielectric is between 10 and 40 μπι. The dielectric layer 9 with a relative electrical permittivity εζ greater than 20 may be based on barium titanate, strontium titanate or titanium dioxide. Polyethylene or polypropylene layers can also be used. The device according to the invention can be used as a laminate for the manufacture of an identity document such as a passport or for the manufacture of a security document or of value such as an identity card or a permit To drive.

It can also be used in the manufacture of a contactless smart card or a contactless ticket.

One of the advantages of the device according to the invention is that it allows the uniformization of the antennas. Indeed, according to the techniques usually used, the antenna is tuned to the chip and it is still necessary to modify the antenna design for each type of chip. Thanks to the device according to the invention, it becomes possible to use a single antenna pattern for several chips by modifying the electrical bridge made with the dielectric described above. With a dielectric of a relative electrical permittivity determined between 20 and 40, it is sufficient to modify the surface A which corresponds to the crossing surface of the conductive turns facing and on either side of the dielectric layer.

Claims

1. Radio frequency device intended to communicate without contact with an associated reader, composed of a plane support layer (11) and an electric circuit, said electrical circuit comprising an antenna (12) formed of a planar winding of at least two turns (10) which intersect without being short-circuited by means of a dielectric layer (9) and a chip or an electronic module (14) connected to said antenna,

Characterized in that the relative electrical permittivity of said dielectric layer (9) has a value greater than 20 and creates a crossover capacitance Ce whose value increases the resulting capacitance C of said electric circuit so as to reduce the inductance L of the antenna by decreasing its length by an integer number of turns, said electric circuit thus forming a resonant circuit for communication with the associated reader.

2. Device according to claim 1 wherein the antenna is made by printing a conductive ink.

3. Device according to claim 1 or 2, wherein said support layer (11) is made of synthetic paper such as teslin.

4. Device according to claim 1 or 2, wherein said support layer (11) is polycarbonate.

5. Device according to one of the preceding claims wherein said dielectric layer has a thickness between 10 and 40 μτη.

6. Device according to the invention according to one of the preceding claims wherein the dielectric layer (9) is based on barium titanate.

7. Use of the radiofrequency device according to one of the preceding claims as laminated for the manufacture of an identity document such as a passport or for the manufacture of a security document or value such as a card identity or a driving license.

8. Use of the radiofrequency device according to one of claims 1 to 6 in the manufacture of a contactless smart card.

9. Use of the radiofrequency device according to one of claims 1 to 6 in the manufacture of a contactless smart ticket.