WO2015104479A1 - Antenna with adjustable resonant frequency, in particular for chip card - Google Patents

Abstract

The invention relates to an antenna, in particular for chip card with contactless operation, composed of a series or parallel RLC type resonant circuit comprising a total capacitance C, a total inductance L, and a series resistance R, and having a resonant frequency F0 given by the formula F0 = 1/2TW0-C), characterized in that it comprises means of selective adjustment of the capacitance C, or of the inductance L, or for the combined adjustment of the capacitance C and the inductance L.

Description

 Antenna with adjustable resonance frequency, in particular for smart cards

The invention relates to the adjustment of the resonance frequency of antennas which are used for contactless smart cards and electronic passports, and more specifically amplification antennas known as "booster" in English terminology and which interface electromagnetically between the antenna of the electronic module of the smart card and the antenna for transferring the power and data of the smart card reader.

State of the art

There already exists in the state of the art, a standard concentrator antenna, made by an inductor arranged in the body of the smart card.

 We then have a system of two antennas in the smart card body, namely a standard format antenna, called ID1 antenna, intended to be electromagnetically coupled with an external reader, and a concentrator antenna (also referred to more simply as the concentrator) close to the format of an electronic module for a smart card, arranged in the card body or on a cover page of an electronic passport booklet. This concentrator antenna is intended to be electromagnetically coupled with the antenna of the module, and is connected directly with the antenna ID1.

 A network of adjustable parallel capacitors connected in series or in parallel with the two antennas mentioned above makes it possible to resonate the entire antenna system at a determined operating frequency.

The above-mentioned antenna system is electromagnetically coupled to an electronic module, which also includes an antenna, which is disposed at the periphery of the electronic module and is called the module antenna, for example the electronic module of a module. contactless smart card, or that of an electronic passport. The antenna of the module is limited in terms of number of turns by the reduced size of the module, the latter must also receive a chip electronic and possibly galvanic contacts type ISO 7816-2, for example for so-called dual cards, contact and contactless.

 The performance of the antenna system is sensitive to its resonance frequency which depends on its inductance and the adjustable capacitance network used. It may therefore be necessary to adjust the resonant frequency of the antenna system according to the intended application, the chip used on the electronic module, or to solve problems of industrialization.

 There is therefore a strong interest, in the case of antennas for contactless and contactless contactless smart cards, to find ways of finely adjusting the resonance frequency of the body's antenna system. Map. But this question of the fine tuning of the resonance frequency of an antenna may also have an interest in other areas than that of smart cards, provided that antennas are used.

However, the resonance frequency of an antenna is of the type F ₀ = 1 / 2Tb / LC), where

L represents the inductance of the antenna, and C its capacitance.

 In practice, the resonance frequency of the antennas used for contactless smart cards is fixed for a given value of inductance and capacity.

Therefore, if due to the aforementioned problems, a change of resonant frequency is necessary, the capacitance C of the antenna must be recalculated. It is therefore necessary to redo the design of the antenna to reach again the resonance frequency targeted by the application. However, this antenna design involves completion times and additional costs. It would be much more efficient to have an antenna system that has means for adjusting the resonance frequency F ₀ .

Goals of the invention

A general object of the invention is therefore to provide an antenna structure, particularly for the body of a contactless smart card or for an electronic passport, which has no aforementioned drawbacks. Another more specific object of the invention is to propose an antenna system which makes it possible to easily adjust the resonant frequency of the antenna to a wider range of use.

 Another object of the invention is to propose, in association with the new antenna system, methods for adjusting the resonance frequency making it possible to very precisely reach a target resonance frequency, with a minimum of capacitance and / or inductance adjustment operations.

Summary of the invention

According to the principle of the invention, the antenna consisting of a series or parallel RLC circuit whose resonant frequency is given by the formula F ₀ = 1 / 2U.V (LC) _f is adjusted capacity C and / or the inductance L to get as close as possible to a target resonant frequency.

 The capacitor C is configured in the form of a main capacitor connected to secondary capacitors, this connection being optimized according to the invention to allow a simple adjustment, also called "trimming" in English terminology, from a value of maximum capacity, by reducing this maximum capacity by removing unit capacities, preferably by means of operations of unique cuts of connections between the main capacity and one or more secondary capacities.

 The inductance L is configured as a main inductance connected in series with an inductance that can be adjusted upwards or downwards. According to the invention, this adjustable inductance is composed of elementary inductances which can be switched off selectively to increase or decrease the total inductance.

 The invention allows fine tuning of the antenna capacitance, or its inductance, or both in combination, which makes it possible to approach much more closely than before a target resonant frequency required within the scope of the invention. a given use of the antenna.

The invention therefore more precisely relates to an antenna, in particular for a smart card, as defined in the claims. Thus, the subject of the invention is an antenna, in particular for a non-contact operation smart card, composed of a series or parallel RLC type resonant circuit comprising a total capacitance C, a total inductance L, and a series resistance R, and a resonant frequency F ₀ given by the formula F ₀ = 1 / 2Tbi (LC), characterized in that it comprises means for selectively adjusting the capacitance C, or the inductance L, or for the combined adjustment of the capacitance C and inductance L said means for adjusting the total capacitance C are configured to decrease said capacitor C from an initial value, and said inductance adjusting means L are configured to increase or decrease the total inductance L from an initial value. In this way, it is possible to finely adjust the resonance frequency of the antenna, both upward and downward.

According to an advantageous embodiment of the invention, the total capacitance C comprises a main capacitance C _p connected in parallel on the one hand with a first secondary capacitance C _s using a first conductive line and on the other hand with a plurality of elementary secondary capacitors C _S i connected together by a second conductive line, and said first and second conductive lines are arranged to allow either simultaneous disconnection of certain secondary elementary capacitors C _S i, or simultaneous disconnection of said secondary capacitance Cs and some of said elementary secondary capacitances 0 ". In this way, it is possible to adjust the total capacitance C according to a first coarse adjustment with the capacitance C _s , and according to a finer complementary adjustment by means of the elementary secondary capacitors Csi.

The main capacitance C _p is formed between a first armature and a main metal cupboard, and said secondary capacitance Cs is formed between said first armature and a secondary metal cupboard smaller than the main metal cupboard and disposed next to it, and said capacitances elementary secondary members C _si are formed between said first frame and a plurality of smaller elementary metal cupboards disposed in the vicinity of the main and secondary metal cupboards.

Preferably, said main metal cabinet and said smaller elementary metal cabinets are connected by a conductive line series having alternately segments oriented in a first direction and segments oriented in a second direction, preferably orthogonal to the first direction.

In addition, said secondary metal cabinet and said main metal cabinet are connected by a parallel conductive line having segments which at least partially follow the associated segments of said series conductive line, so that the disconnection of one of the metal cabinets of the one of the secondary elementary capacitances C _S i by cutting said series conductive line also intersects the parallel conductive line, and therefore also disconnects the metal cabinet from the secondary capacitor Cs.

 In this way, a compact capacitance having a reduced surface is obtained, while optimizing the possibilities of disconnecting certain adjustment capacities with the aid of a single cutting line.

According to another embodiment of the invention, the total capacitance C comprises a main capacitance C _p , and a plurality of substantially identical secondary capacitors C _S i, as well as a capacitance C _a value of half less than said secondary capacitors Cs ,, said secondary capacitors and said auxiliary capacitance having one of their armatures connected together by a first conductive line configured to allow the disconnection of one or more of them by means of a single cutting (O _s ) of said conductive line and said booster capacity being connected to said first conductive line by a second conductive line so as to allow the disconnection of said booster capacity with a second line of cut (O _p ).

 According to another embodiment of the invention, the total capacitance C comprises means for analog adjustment of the antenna capacitance, consisting in the selective disconnection of a predetermined part of the capacitive surface of said capacitor, so as to reduce the value of the capacity by a predetermined value.

According to another embodiment of the invention, the tuning of the resonance frequency of the antenna is done by inductance adjustment. The antenna comprises for this purpose a main inductance (L _p ) fixed and a secondary inductance (U) variable up or down, connected in series. For example, said inductor secondary variable (U) is composed of the superposition of two inductances (Lsi _/ 2) of opposite directions comprising a main strand (U, L ₂ ) and one end of which has a plurality of secondary strands (1 ^) corresponding to elementary inductances, secondary strands being arranged to allow their selective disconnection.

 Other features and advantages of the invention will appear on reading the detailed description and the accompanying drawings in which:

 FIGS. 1A and 1B respectively recall the equivalent diagram of a conventional serial or parallel antenna without means for adjusting the resonance frequency;

 FIGS. 2A and 2B respectively show the equivalent diagram of a known serial and parallel antenna provided with adjustment capacitors connected in parallel to allow adjustment of the resonance frequency of the antenna;

 FIGS. 3A and 3B respectively illustrate the equivalent diagram and a known embodiment of a variable adjustment capacity to allow a so-called series adjustment;

 FIG. 4A illustrates the equivalent diagram of a known antenna provided with connected adjustment capabilities to allow a so-called parallel or binary adjustment;

 FIG. 4B illustrates a known embodiment of capabilities in accordance with the equivalent diagram of FIG. 4A to allow a so-called parallel adjustment;

 FIG. 4C illustrates a known embodiment of capacities in accordance with the equivalent diagram of FIG. 4A to allow a so-called binary adjustment;

 FIG. 5A illustrates the equivalent diagram of an adjustable antenna capacitor according to the invention, and FIGS. 5B to 5D illustrate possibilities of adjustment of this capacitance;

 FIG. 6 represents another adjustable antenna capacity design in accordance with the invention, allowing a so-called series-parallel or hybrid adjustment;

FIG. 7 illustrates the principle of an analogally adjustable capacitor according to the invention, by removal of material in a frame of the capacitor; - Figure 8 illustrates the equivalent electrical diagram of an antenna according to the invention, provided with an adjustable inductor;

 FIG. 9 represents the electrical diagram of an inductance that can be adjusted upwards or downwards in accordance with the invention;

 FIG. 10 schematically represents an embodiment of an adjustable inductor according to the principle of FIG. 9.

detailed description

Referring to Figure 1A. This figure shows the typical electrical diagram of an antenna 1 constituted by an RLC circuit consisting of a resistor 2 of value R, a capacitance 4 of value C and one or more inductors 3 of equivalent value L , connected in series. As is well known, this antenna has a fixed resonant frequency F ₀ = 1 / 2lb (LC). FIG. 1B represents a known equivalent diagram of an antenna comprising a capacitor 4 and an inductor 3 connected in parallel.

For the reasons mentioned above concerning the need in some cases to be able to adjust the resonant frequency, antennas whose capacitance C is designed to be variable in stages, such as that represented in FIGS. 2A and 2B, are known. For this, the capacitor C consists of a main capacitance C _p and a set of secondary capacitors C _s i, C _S i all connected in parallel. The initial total capacity before adjustment is then worth C = C _p + C _s i + ... + C _S i. The disconnection of one or more secondary capacitors then makes it possible to adjust the total capacitance as needed to bring the resonant frequency F ₀ closer to a target value. This disconnection can be done in several ways.

 FIG. 3A shows the principle of a known adjustment of the series type.

The initial total capacity is C _p + C _s i + ... + Cs7. By opening one of the connections between the capacitors C _s i, ... C "connected in parallel, one disconnects by means of a single connection break, denoted O _s , a given capacity, and all those which are connected in parallel behind it, that is to say the opposite side with respect to the main capacitance C _p . In the example shown, the opening O _s between the capacitors Cs3 and Q4 makes it possible to disconnect the capacitors C _S 4, C _S 5 and C _S 7, so that the residual capacitance after adjustment is C = C _p + C _s i + C _S 2 + C _S 3. In practice, in the field of antennas for smart cards which are typically made by conductive etchings arranged on an insulating substrate, the series cut is performed accurately and at a high rate by a laser cut line parameterized to perform the opening O _s successively on all the antennas of a production batch.

Figure 3B shows a typical embodiment of a capacitor C as shown schematically in Figure 3A. It consists of two armatures 6, 8 placed opposite one another and separated by a dielectric (not shown) so as to form a capacitor. The upper frame 6 comprises a main metal cupboard a _p and a plurality of secondary metal cupboards ai, a _{2 /} etc .. connected to each other by a conductive line 7. The main cupboard has _p form with the lower frame 8 a main capacity C _p . The secondary cabinets ai, a ₂ etc form with the frame 8 secondary capacitors C _s i, C _S 2 etc., which are all connected in parallel to the terminals A, B of the main capacitance C _p . As can be seen, the laser cut-off O _s in the conductive line 7 for connecting the metal cupboards makes it possible to disconnect all the elementary capacitors located behind it (that is to say those to the right of O _s in the figure), and the choice of the location of the cutoff O _s thus makes it possible to choose the number of secondary capacitors which remain connected and consequently to adjust the residual capacity.

As can be seen, the various secondary compartments a, have substantially the same size, which means that the different secondary capacitors Cs, (i = 1, 2, etc.) correspondingly have substantially the same value. The disadvantage of this series cut adjustment method therefore lies in the fact that the resolution of the adjustment of the total capacitance C is limited to the individual value of a capacitance C _S i, and the adjustment of the value the total capacitance C can only be reduced by constant steps of value Qj, starting from the initial value before breaking, and the adjustment range will be limited to the number of secondary capacitors, by decreasing from the initial capacitance maximum which consists of all the capacities in parallel. This limits the possibilities of adjusting the resonant frequency of the antenna integrating the capacitance.

Another known adjustment method, called parallel adjustment, is shown in relation to FIGS. 4A, 4B. As shown in FIG. 4A, selectively cutting certain parallel connections 9 of certain secondary capacitors (for example the connections 9 of capacitors C _S 2, C _S 5 and Cs ₇ ), by means of corresponding cuts O ₂ , 0 ₅ and 0 ₇ . In this example, the residual capacity will be worth C = Cp + Csi + C _S 3 + C _S 4 + C _S 6. The secondary capacitances C _S i may be identical or different.

In particular, as represented in FIG. 4C, in the case where the capacitor C is formed by a main capacitance C _p and several, in this case six, secondary capacitors Ci, ... C ₆ which are variable and obtained at the Using metal cupboards a, of non-uniform size, the ratios between the different secondary capacitors can correspond to a binary code. The following relation is then obtained for a unitary secondary capacity of rank n, denoted by C _n : C _n = 2 ^nl x Ci, the represented example corresponding to n not exceeding 6. Typically, the size of the conductive cupboards a, of so that the corresponding capacities correspond to the desired range of values. The adjustment step in the case of a binary parallel adjustment is given by the adjustment capacity of smaller value, which means in the example shown the closet ai smaller area. Then, the next adjustment capacity will be twice the value of the previous one and so on. In addition, in this configuration, the capacity adjustment typically means the parallel removal of several secondary capabilities. As a result, several O openings are required (O ₁ , O ₄ , 0e in Fig. 4C), which requires several laser cutting lines, and slows down the adjustment process.

 To overcome the aforementioned drawbacks, the invention notably proposes an improved capacitance configuration in order to increase both the capacitance adjustment resolution and its frequency adjustment range.

 Reference is now made to FIGS. 5A to 5D to describe a first embodiment of the invention.

FIG. 5A represents the equivalent diagram of a network of secondary capacitors connected to the terminals A, B of a main capacitance C _p , to allow the adjustment of the global capacitance C by disconnection of some of the secondary capacitors. In parallel with the main capacitance C _p , is connected a first secondary capacitance denoted C _s allowing a first level of adjustment of the overall capacity. In addition, at the terminals A, B of the main capacitance C _p , is also connected a network of second capacitances denoted C _if , for example 7 capacitances denoted by C _s i to C _S 7, connected in parallel. Of course, this is an example, since secondary capacity networks can be more complex in practice.

The global capacitance C is then equal to C _p + C _s + Qi + ... + Cs7- It can thus be seen that to vary the capacitance C to aim at a given resonance frequency F ₀ , it is sufficient to disconnect some of the secondary capacitors. C _s or Cs ,, depending on the values thereof, which allows a number of disconnect combinations. It is also necessary that the size of the metal cupboards forming the capacitors is reduced to a minimum, so that the gain on the number of adjustment combinations is not penalized by a much larger overall bulk. To this end, it is necessary on the one hand to have the metal cabinets forming the different capacities in a specific and optimized manner, and on the other hand to have the connection lines between the different secondary capacities also specifically and optimized .

As can be seen in FIG. 5B, the main capacitance C _p and the secondary capacitors C _s , Csi have a first common reinforcement, the armature 8, and the second armature, namely the upper armature 6 (in dark gray) is structured by metal cupboards that allow to realize between the two frames 6.8 the different capacitances C _p , and C *, as well as the connections between them.

As can be seen in FIG. 5A, serial openings O _s can be made selectively on the connection 7 which interconnects the secondary capacitors C _S i, in order to disconnect all the capacitors C _s situated downstream of the cutoff O _s with respect to the main capacitance C _p , according to the principle of series disconnection explained in connection with Figure 3A. Similarly, an opening O _p located on the connection line 30 selectively disconnects the secondary capacitance C _s , according to the parallel disconnection mode explained in relation to Figure 4A. And finally, a cutoff noted O ' _s located on both the connections 7 and 30 allows in a single operation to disconnect both the secondary capacity C _s and the set of capacitances C _s , located downstream of the cutoff O' _s . In order to be able to achieve all these combinations of disconnections of adjustment capabilities with a minimum of cuts or even a single cut, while maintaining a bulk of the overall capacitance C substantially constant, the invention provides a particular design of one of the reinforcements different capacities, namely the upper armature 6, as shown in Figure 5B.

As seen in this Figure 5B, the main capacitance C _p is formed between the frame 8 and a substantially rectangular metal cabinet. To the right of the capacitance C _p , another smaller closet makes it possible to form the secondary capacitor C _s . Below the two capacities C _p and C _s , a plurality of smaller metal cabinets form the secondary adjustment capacitors Csi to C _s7 .

According to the invention, a conducting line 7 formed of successive segments oriented in turn in a direction X and a direction Y connects a frame of the main capacitance C _p to the corresponding frames of the secondary capacitors Cs ,. In addition, another conductive line 30 connects the cabinet forming the secondary capacitance C _s to the cabinet forming the main capacitance C _p . According to the invention, this second conductive line 30 runs partly along the segments of the conductive line 7, so as to present segments oriented along at least one of the X or Y directions thereof, while being sufficiently close to the associated segments of the line 7 to allow simultaneous cutting of the associated segments of the lines 7 and 30, using a single laser cutting line. Thus, in the embodiment shown in FIG. 5B, segments 30 _y of line 30 are oriented in the Y direction and are parallel to and close to associated segments 7 _y of line 7. This description corresponds to an arrangement of the lines 7 and 30 in the same plane, but the reasoning can be extended to the case where the lines 7 and 30 would be arranged in different planes, for example on either side of the dielectric of the capacitor C. In this case certain segments of line 7 would be arranged in a Z direction (not shown) and would be associated with corresponding segments of the line 30, also arranged in the Z direction and located close enough to be cut together in a single operation.

As a result of this arrangement, capacitors C _p , C _s , C _S i and their connection lines 7, 30 have a set of breaking possibilities, each of which disconnects a certain combination of capacitors, while leaving the others open. connected capabilities. The possible cuts are shown schematically by horizontal lines (X direction) or vertical lines (Y direction) in Figure 5B. In the example shown, there are 7 vertical outage possibilities O _s , made on line 7 and corresponding to series interruptions of all the capacitors C _S i located downstream of a given break. Thus, as shown in FIG. 5C, the series cutoff _O.sub.4 disconnects all four capacitors corresponding to the four metal cabinets marked with a cross.

In the example shown in FIG. 5B, 8 horizontal switching possibilities O ' _s can also be distinguished, each being arranged simultaneously with the aid of a single laser cut on a segment directed along Y of the connection line 7 and on a associated segment of the connection line 30, or only on the line 30 in the case of the capacitance C _s . Thus, as represented in FIG. 5D, the horizontal cutoff O ' _S 3 cuts at this point at the same time the lines 7 and 30, and makes it possible to disconnect both the three secondary capacitors C _S i marked with a cross, and the secondary capacity C _s marked with a cross.

 With the aid of the possible cuts shown in FIG. 5B, 16 capacitance values C (ie, the value corresponding to the absence of a cutoff) are thus obtained, whereas a purely serial or parallel adjustment as described in FIG. relationship with FIGS. 3 and 4 would have allowed only 8 values of capacities to be obtained at most.

 As a result, for a similar capacity congestion C, the invention makes it possible to double the capacity adjustment resolution, and consequently, to double the resolution resolution of the resonance frequency of the antenna.

Another embodiment of the invention implementing a capacitive-type resonance frequency adjustment is that represented in FIG. 6. In this case, the capacitance C of the resonant circuit is composed of a main capacitance C _p and a set of secondary capacitors Cs, connected in parallel with the main capacitance, and substantially equivalent to each other, except one of them denoted C _a which is of value substantially equal to half the value of the capacitors C _s i at C _S i. In this case, it is possible to practice a corresponding hybrid capacity adjustment two adjustments of different types, namely a serial type adjustment realized using a series cut O _s to disconnect all the secondary capacitors (C _s i to C _S 4) downstream of the cutoff O _s , and a parallel type adjustment realized by means of a parallel cutoff O _p to disconnect the capacitance C _{has a} lower value. This makes it possible to double the number of combinations of capacities, and the resolution of the adjustments, for a given number of secondary capacities, thanks to the capacitance C _a of low value and disconnectable in parallel mode.

 Another embodiment of the invention implementing a capacitive-type resonance frequency adjustment is that represented in FIG. 7. The principle consists in removing from one of the armatures of the capacitor C a predetermined quantity of matter, the capacity then being decreased in the same proportions as the disconnected material surface. The upper part of the figure corresponds to a cross-sectional view of the two plates 8.6 and the dielectric 11, and the lower part is a plan view of the capacitor C. On the left, capacitance C is shown before adjustment. in the center, material is removed, in particular with the aid of a laser beam 13, on one of the armatures, namely the upper armature 6. If, as on the right, the laser beam travels a circle or a Another closed figure, the whole surface 14 of armature 6 inside this figure is electrically disconnected, and the capacitance C is reduced in proportion to the disconnected surface. The decrease in capacitance C will cause a correlative increase in the resonance frequency of the antenna circuit that uses this capacitance. In the case of an analog adjustment, the resolution of the capacity adjustment is in this case limited by a given step corresponding to the disconnection of a certain elementary capacity value, as in the previous cases.

In addition to or in replacement of the resonant frequency adjustments affecting the value of the resonant capacitance C (which only allows the capacitive zones to be removed and therefore the resonance frequency to be increased), the invention provides for inductive type adjustments. acting on the inductance L of the antenna circuit. The principle is that of the circuit of FIG. 8, where a variable secondary inductance U is connected between the terminals C and D, in series with a fixed main inductance L _p . It can be seen that when increasing L _S the total inductance L = L _p + increases, and the resonant frequency of the circuit (which is proportional to the inverse of the square root of the inductance L) decreases. And conversely when decreasing the inductance U-

An exemplary embodiment of the variable inductance L _s is shown in Figure 9. As shown, the variable inductance L _s is formed of two secondary inductors Ui and ₂ of opposite signs, obtained by coils wound in the opposite direction. Suppose in this example that i is of the same sign as the main inductance L _p , but the reasoning is the same in the opposite case.

The secondary inductance Ui is produced in this example by an inductance Li in series with a set of four adjustment inductances read at _{1- (} at least some (but not all) of which can be disconnected by means of strokes. laser cut (or other cutoff means) acting as On, O21 switches.

In the same way, the secondary inductance ₂ is produced in this example by an inductance L2 in series with a set of four adjustment inductances l ₂ i to I24 of which at least some (but not all) can be disconnected at the same time. using laser cutting lines acting as switches O2i, 0 ₂₂ .

With this arrangement, if one or more inductances In to I14 are disconnected with the same sign as Lp, the inductance L _s i increases and adds to Lp, which causes the resonance frequency F _{0 to} fall. If, on the other hand, one or more inductances I ₂ i to I 24 are disconnected, the inductance ₂ increases. Since it has a sign opposite to that of Lp, the secondary inductance L _s decreases, the overall inductance L decreases, and the resonance frequency Fo of the antenna increases.

Therefore, thanks to this system, we obtain an inductance that can be variable with the degree of fineness desired, and that up or down. _{Those skilled} in the art will of course be able to choose the number and the appropriate values for the inductances Ι _ΰ , depending on the fineness of adjustment desired for the resonance frequency.

 FIG. 10 shows an example of a practical embodiment of the variable inductance U of FIG. 9. The elementary inductances ly are produced by strands of conducting wire or conductive track which bear the same references as the inductors. corresponding in Figure 9.

Thus, the secondary inductance L _s i is embodied in the form of a conductive strand or of a conductive track wound in a first direction, denoted by SI, with inductance L _lf, and ends with a series of elementary strands of adjustment. read at 14, which are connected in parallel to a terminal B1.

Similarly, the secondary inductance ₂ is in the form of a conductor strand or conductor of inductance L ₂ , wound in a second direction noted S2, and ends with a series of strands of adjustment the _2i to ₂₄ which are connected in parallel to a terminal B2.

Then the two secondary inductors L _S i and Ls ₂ are assembled by superposition and connection of the terminals B1, B2. Advantageously, each secondary antenna Lsi and I_s2 can be made on a respective face of the body of a smart card, and the interconnection of terminals B1, B2 is then via a metal via.

 The read adjustment strands of each series can be disconnected selectively, for example by making laser connection cuts, in order to increase or decrease the resonant frequency and ultimately to bring the resonant frequency of the set of the antenna of a target value determined by the application, including the resonance frequency of a contactless smart card reader.

The resolution of the inductance adjustment corresponds to the inductance value of a unit strand, and the adjustment range depends on the inductance and the number of unitary adjustment strands. Depending on the values of the secondary inductances Ui, U ₂ and as a function of the cuts made, a resultant inductance is obtained which may be smaller or greater than the value of the main inductance L _p . The invention finally provides the possibility of combining the capacitance adjustments described above, and the inductance adjustments, which allows both to increase the adjustment range of the product LC, and the resolution of this adjustment, and definitive to fine-tune the resonance frequency of the antenna, and this over a wider range of adjustment.

Advantages of the invention

 Ultimately, the invention makes it possible to achieve the goals set. In addition, it makes it possible to increase both the resolution of the adjustment of the resonant frequency, while increasing the possible frequency adjustment range. In addition, it is possible to reconcile these advantages with a minimal number of laser cutting lines, which makes it possible not to slow down the manufacturing process of the antennas.

Claims

1 - Antenna for a contactless smart card, composed of a series or parallel RLC type resonant circuit comprising a total capacitance C, a total inductance L, and a series resistor R, and a resonance frequency F ₀ given by the formula F ₀ = i / 2UV (LQ, characterized in that it comprises means (C _s , C _si , 7, 30; L _s ) for selective adjustment of the capacitance C, or of the inductance L, or for the combined adjustment of capacitance C and inductance L. 2 - Antenna according to claim 1, characterized in that said means for adjusting the total capacitance C are configured to decrease said capacitor C from an initial value (C _p ), and in that said means of adjustment (L _s ) of the inductance L are configured to increase or decrease the total inductance L from an initial value (L _p ). 3 - Antenna according to claim 2, characterized in that the total capacity C comprises a main capacitance (C _p ) connected in parallel on the one hand with a first secondary capacitance (C _s ) using a first conductive line (30) and secondly with a plurality of elementary secondary capacitors (C _si ) connected with a second conductive line (7), and in that said first and second conductive lines (30, 7) are arranged to allow either simultaneous disconnection of some elementary secondary capacitances (C "), or simultaneous disconnection of said secondary capacitance (Cs) and some of said elementary secondary capacitances (C _S i). 4 - Antenna according to claim 3, characterized in that said main capacitance (C _p ) is formed between a first armature (8) and a main metal cupboard, in that said secondary capacitance (C _s ) is formed between said first armature (8) and a secondary metal cabinet smaller than the main metal cabinet and disposed adjacent thereto, and in that said elementary secondary capacities (Cs,) are formed between said first frame (8) and a plurality of smaller elementary metal cabinets arranged in the vicinity of the main and secondary metal cabinets. 5 - Antenna according to claim 4, characterized in that said main metal cabinet and said smaller elementary metal closets are connected by a series conductive line (7) having alternately segments (7 _X ) oriented in a first direction (X ) and segments (7 _y ) oriented in a second direction (Y), preferably orthogonal to said first direction. 6 - Antenna according to claim 5, characterized in that said secondary metal cabinet and said main metal cabinet are connected by a parallel conductive line (30) having segments (30 _y ) which at least partially along the associated segments (7 _y ) of said series conductive line (7), so that the disconnection of one of the metal cabinets of one of the secondary elementary capacitances (C _S i) by cutting off said series conductive line (7) causes said line to be cut off parallel conductor and also disconnects the metal cabinet of the secondary capacitance (Cs). 7 - Antenna according to claim 2, characterized in that the total capacity C comprises a main capacitance (C _p ), and a plurality of substantially identical secondary capacitors (C _S i), as well as a capacitance (C _a ) of value less than half said secondary capacitances (Csi), said secondary capacitances and said surge capacitance having one of their armatures connected together by a first conductive line (7) configured to allow the disconnection of one or more of them by means of a single cut (O _s ) of said conductive line (7) and said booster capacity being connected to said first conductive line (7) by a second conductive line (9) so as to to allow the disconnection of said makeup capacity with a second cutting line (Op). 8 - Antenna according to claim 2, characterized in that it comprises means for analog adjustment of the antenna capacity, consisting of the selective disconnection of a predetermined portion of the capacitive surface of said capacitor, so as to reduce the value of the capacity of a predetermined value. 9 - Antenna according to any one of the preceding claims, characterized in that it comprises a main inductance (L _p ) fixed and a secondary inductance (L _s ) variable rising or falling connected in series. 10 - Antenna according to claim 9, characterized in that said variable secondary inductance (U) is composed of the superposition of two inductances (L _s i, L _s2 ) of opposite directions comprising a main strand (U, L ₂ ) and of which one end comprises several secondary strands (ly) corresponding to elementary inductances, said secondary strands being arranged to allow their selective disconnection.