METHOD AND DEVICE FOR TRANSMITTING DATA BY INDUCTIVE COUPLING WITH CONTROLLED SELF-OSCILLATION

The present invention relates to a method for transmitting data by inductive coupling, comprising a step of applying bursts of a periodic signal to a tuned inductive antenna circuit, to produce an antenna signal generating a magnetic field, in which the amplitude of each burst is delimited by an envelope signal having a rising edge and a falling edge.

The present invention relates in particular to an active load modulation method for NFC device ("Near Field Communication") of the type described in the patent EP 1 327 222. The method comprises applying the bursts of the periodic signal to the antenna circuit in the presence of an external magnetic field emitted by an NFC reader. The bursts of the periodic signal generate corresponding bursts of a magnetic field that is superimposed on the one emitted by the NFC reader, and are perceived by the latter as a passive load modulation. Compared to passive load modulation, active load modulation is advantageous in that it offers a greater communication distance.

Patent application EP 2 431 925 relates to an improvement of this active load modulation method and suggests resynchronizing the frequency of the periodic signal to that of the external magnetic field after each application of a burst to the antenna circuit, instead of resynchronizing this signal before each new data frame. The active load modulation method thusalternately comprises periods of emitting bursts and phases of ^synchronization to the external magnetic field.

However, patent application WO 2012/038664 indicates that after applying a burst of the periodic signal to the antenna circuit, the latter has a ringing that is superimposed on the "useful" antenna signal induced by the external magnetic field and is susceptible of distorting the ^synchronization process. The resynchronization process indeed uses the antenna signal induced by the external field as resynchronization signal. If this signal comprises oscillatory residues from the burst that has just been applied to the antenna circuit, they are superimposed on the useful antenna signal induced by the external field and may distort the resynchronization process on the external field.

For example, Figure 1 shows the shape of a burst Bl of a periodic signal Sim applied to an antenna circuit ACT, and the resulting antenna signal Vam. The antenna circuit ACT is a resonant circuit tuned to the frequency of the periodic signal Sim, and comprises for example an antenna coil AC, a series capacitor Ca and a parallel capacitor Cb. The burst Bl, or "incident burst", produces in the antenna circuit of a burst BT of antenna signal Vam, or "resulting burst", that itself generates a burst of magnetic field of the same shape.

The incident burst Bl is delimited in its duration and. amplitude by an envelope signal El that is square in shape, of duration Tl, having a rising edge and a falling edge. The rising edge extends between a low inflection point il and a high inflection point i2. The falling edge extends between a high inflection point i3 and a low inflection point i4. The amplitude of the periodic signal Sim is zero before the rising edge and after the falling edge, and is generally constant between the two edges. After applying the incident burst Bl, the antenna signal Vam has a transient oscillation 1 of not insignificant amplitude that may, in certain cases, have overshoots2 of amplitude greater than the maximum amplitude of the antenna signal Vam during the application of the incident burst Bl. As a result, the resulting burst Bl' has a duration Tl' that may be much greater than the duration Tl of the incident burst Bl. When the time Tl'-Tl is equal to or greater than the time separating the emission of two bursts Bl, the device that emits the bursts Bl cannot be resynchronized to an external field, at the risk of resynchronizingto the signal it has itself transmitted.

To overcome this drawback, application WO 2012/038664 teaches to short-circuit or de-tune the antenna circuit by means of a switch, immediately after applying the incident burst Bl. The active load modulation method then comprises, after applying the burst Bl and before the ^synchronization phase, a damping phase during which the antenna circuit is short-circuited or de-tuned by the switch, followed by a restoration phase during which the useful antenna signal induced by the external magnetic field is naturally restored without being "polluted" by the transient oscillation generated by the incident burst Bl.

This solution offers the advantage of being simple to implement and efficient. However, the damping switch must withstand antenna voltages which can reach 10 to 15V. Now, some areas of technology likely to be used to produce NFC devices integrated on semiconductor chips, such as the deep-submicron areas, do not allow transistors capable of withstanding such high voltages to be produced.

It could thus be desired to provide an active load modulation method not requiring such a damping switch.

More generally, it could be desired to.provide a method for transmitting data by emitting bursts of magnetic field, in which the self-oscillation phenomenon of the antenna circuit is controlled by a means other than a damping switch.

Some embodiments of the invention relate to a method for transmitting data by inductive coupling, comprising a step of applying bursts of a periodic signal to a tuned inductive antenna circuit, to produce an antenna signal generating a magnetic field, in which the amplitude of each burst is delimited by an envelope signal having a rising edge and a falling edge, and a step of shaping at least the falling edge of the envelope signal by means of a digital or analog shaping circuit so as to attenuate or remove transient oscillations of the antenna signal that would appear in the antenna circuit after each application of a burst if bursts a of periodic signal with a square wave envelope signal were applied thereto.

According to one embodiment, the falling edge of the envelope signal is shaped so that its first derivative does not exceed a limit value determined by taking into account the maximum amplitude of the transient oscillations that may be tolerated in the antenna circuit.

According to one embodiment, the falling edge of the envelope signal is shaped so that its first derivative has a maximum value equal to A0*7c*Fc/2, Fc being the frequency of the periodic signal, and AO the amplitude of the envelope signal.

According to one embodiment, the falling edge of the envelope signal is shaped so that immediately after applying a burst of the periodic signal, the antenna signal has, in the absence of an external magnetic field, an amplitude lower than a percentage of the maximum amplitude it has during the application of the burst, chosen between 5% and 50%.

According to one embodiment, the method comprises the step of giving the falling edge of the envelope signal a shape determined by a mathematical function the derivative of which is a continuous function.

According to one embodiment, the method comprises the step of giving the falling edge a shape determined by the raised cosine function calculated on a time scale giving it the value 1 at a high inflection point of the falling edge and the value 0 at a low inflection point of the falling edge.

According to one embodiment, the method comprises the steps of giving the falling edge of the envelope signal a shape determined by a set of points stored in a memory and defining by discrete values a burst of the periodic signal..

According to one embodiment, the method comprises a step of also shaping the rising edge of the envelope signal in such a way that the first derivative of the rising edge is continuous.

According to one embodiment, the periodic signal has a total harmonic distortion rate lower than 20%.

According to one embodiment, the method is applied to the transmittingof data by active load modulation, the bursts of the periodic signal are applied to the antenna circuit in the presence of an external alternating magnetic field, and the method comprises a step of synchronizing the frequency of the periodic signal with the frequency of the external magnetic field, between two applications of a burst of the periodic signal to the antenna circuit Some embodiments of the invention also relate to a device for transmitting data by inductive coupling, comprising a tuned inductive antenna circuit and an amplitude modulation circuit for applying bursts of a periodic signal to the antenna circuit, and produce an antenna signal generating a magnetic field, the amplitude of each burst being delimited by an envelope signal having a rising edge and a falling edge, the modulation circuit being configured to implement the method as described above.

Some embodiments of the invention also relate to a portable electronic object comprising such a device.

Some examples of embodiments of a method for transmitting data according to the present invention and of an NFC device implementing this method will be described below in relation with, but not limited to, the accompanying figures, in which:

- Figure 1 described above shows the shape of an incident burst of a periodic signal applied to an antenna circuit, and the shape of the resulting burst,

- Figure 2 shows the shape of an incident burst of a periodic signal according to a first embodiment of the method of the present invention, and the shape of the resulting burst,

- Figure 3 shows the shape of an incident burst of a periodic signal according to a second embodiment of the method of the present invention, and the shape of the resulting burst,
- Figure 4 is the block diagram of a first example of an embodiment of an NFC device implementing the method of the present invention,

- Figures 5A to 5E are timing diagrams showing various signals appearing in the device in Figure 4,

- Figure 6 shows examples of embodiments of a phase locked loop and of a modulation circuit represented in block form on Figure 4,

- Figure 7 is the curve of a digital signal supplied by the modulation circuit on Figure 6,

- Figure 8 shows a second example of an embodiment of the modulation circuit represented in block form on Figure 4,

- Figures 9, 10 and 11 show examples of portable electronic objects comprising an NFC device according to the present invention.

In connection with Figure 1, the response of a tuned inductive antenna circuit ACT to an incident burst Bl of a periodic signal Sim the amplitude of which is delimited by an envelope signal El that is square in shape was described above. This response is characterized by a resulting burst Bl' including a transient oscillation of the antenna signal Vam appearing after applying the incident burst B1.

Some embodiments of the present invention relate to a method whereby it is possible to control the response of the antenna circuit to an incident burst of a periodic signal, and thus to control the shape of the resulting burst, without requiring any damping switch. This method is initially intended to be applied to the active load modulation technique to enable a resynchronization to an external field after emitting a burst, but can find other applications that will be mentioned below.

This method is based on the theory of resonant circuits transposed to the field of the present invention, i.e. the application of bursts of a periodic signal to a tuned inductive antenna circuit. The theory of resonant circuits indicates that the application to a resonant circuit of an excitation signal, the shape of which is that of a step function, results in a transient oscillation of the resonant circuit due to the fact that such a function has a discontinuous derivative. A square signal is a specific case of step function, and has a derivative that tends towards plus infinity on its rising edge and towards minus infinity on its falling edge. A "real" square signal supplied by an electronic circuit, even if it comprises rising and falling edges that are not perfectly vertical due to spurious capacitances or inductances in the transmission lines, does however have significant jumps of its derivative at the low il and high i2 inflection points of the rising edge and at the low i3 and high i4 inflection points of the falling edge. These jumps, without necessarily tending towards infinity, are also considered in the framework of the present invention as discontinuities in that they are responsible for the oscillatory responseof the antenna circuit to which the excitation signal is applied.

This transient oscillatory response can be attenuated or even removed by using an excitation signal having a continuous derivative. In particular, the mathematical theories developed in the framework of the implementation of the Fast Fourier Transform (FFT) have defined so-called window functions, or "observation window functions", such as the Harm function, enabling a signal to be sampled with a view to its Fast Fourier analysis on a finite number of points, without generating any spurious harmonic components on the edge of the window that would distort the harmonic analysis of this signal. When they are used to shape an excitation signal applied to a resonant circuit, such functions do not cause any transient oscillatory response of the latter, or cause a greatly attenuated transient response.

Some embodiments of the present invention are based on the above by making the analogy between an excitation signal applied to a resonant circuit and the envelope signal of a burst of the periodic signal Sim applied to a tuned inductive antenna circuit. It is considered here that the envelope signal is responsible for the oscillatory response of the antenna circuit, in particular when the periodic signal Sim the amplitude of which it delimits has a low harmonic distortion rate and is not susceptible of generating such an oscillatory response.

According to one embodiment of the present invention, a burst B2(Slm) of the periodic signal Sim is shaped as shown on Figure 2, so as to have an envelope signal E2 with a continuous derivative on its falling edge. According to another embodiment, a burst B3(Slm) of the periodic signal Sim is shaped as shown on Figure 3, so as to have an envelope signal E3 having a continuous derivative on its rising edge and on its falling edge.

The notion of "continuous derivative" within the meaning of the present embodiments of the present invention, includes the case in which the derivative has value jumps that are acceptable for the intended application, i.e. value jumps that do not exceed a limit value determined by taking account of the maximum amplitude of the transient oscillations that may be tolerated in the antenna circuit, this maximum amplitude depending on the intended application. This notion can thus be implemented in two ways:

i) empirically, by searching, from an antenna circuit of known structure, for example by means of simulation tools, shapes of envelope signal having derivative jumps that are below a certain threshold and such that the oscillatory response of the antenna circuit is acceptable, for the intended application. For example, in an application to active load modulation, an oscillating period of a duration that is less than the time period available to resynchronize the modulating device to the external magnetic field will be sought,

ii) by defining a digital limit to the jumps of the derivative. It was indicated above that the real derivative of a square envelope signal applied to a transmission line having spurious capacitances or inductances has no jumps towards infinity and is thus stricto sensu "continuous" due to these spurious elements. However, this derivative has value jumps that generate an oscillatory response of the antenna circuit that may be unacceptable for the intended application. Furthermore, even if its theoretical derivative is not continuous, a trapezoidal envelope signal substantially improves the response of the antenna circuit compared to an envelope signal that is square in shape. Thus, a definition of the notion of "continuous derivative" within the meaning of some embodiments of the present invention

includes an envelope signal having a derivative that does not exceed a limit such that the falltime of the envelope signal is at least greater than or equal to the period of the periodic signal Sim. In this case, this limit is equal to A0*7c*Fc/2, Fc being the frequency of the signal Sim, and AO the amplitude of the envelope signal. In an application where Fc=13.56MHz, the maximum value of the derivative of the envelope signal is thus equal to A0*21.29*106 s"1.

On Figure 2, the duration of the incident burst B2 of the periodic signal Sim is noted Tm. The envelope signal E2 comprises a steep rising edge between the inflection point il and the inflection point i2, of a theoretically zero duration, a plateau of duration Tp between the inflection point i2 and the inflection point i3, and a gentle falling edge substantially in the form of a half bell, of duration Tf, between the inflection point i3 and the inflection point i4. This shape of falling edge with gentle contours is determined so as to have a continuous derivative.

The resulting burst B2' of antenna signal Vamproduced in the antenna circuit ACT has an overshoot 3 caused by the rising edge of the envelope signal E2 but has no transient ringing caused by the falling edge. The effective duration of the resulting burst B2' is here equal to the duration Tm of the incident burst B2.

On Figure 3, the duration of the incident burst B3 of the periodic signal Sim is also Tm. The envelope E3 of the incident burst has an axial symmetry and comprises a gentle rising edge in the form of a half bell between the inflection point il and the inflection point i2, of duration Tr, a plateau of duration Tp between the inflection point i2 and the inflection point i3, and a gentle falling edge in the form of a half bell, of duration Tf, from the inflection point i3 to the inflection point i4. The antenna signal Vamproduced in the circuit ACT here has no overshoot of transient oscillation and the effective duration of the resulting burst B3f is again equal to the duration Tm of the burst B3.

The shapes of the resulting bursts B2', B3' shown on Figures 2 and 3 are "ideal". However, in practice, the degree of attenuation of the ringing after applying the incident burst depends on the precision of the means implemented to shape the envelope of the incident burst and the choice of shaping of the burst, in particular:

1) the incident burst may be shaped on its falling edge only (burst of B2 type) or on its rising and falling edges (burst of B3 type). The use of a burst of B3 type may be advantageous on a burst of B2 type when the rising edge of the burst B2 generates transient oscillations of duration greater than the duration of the burst,

2) the shaping of edges so that their derivative is continuous may be achieved by means of a low-pass analog filter of order 1 or of order 2, by transforming by means of such a filter a square envelope signal into an envelope signal having softened edges,
3) the shaping of the edges so that their derivative is continuous may be achieved with high precision by means of a mathematical function of the type described above, by synthetizing the periodic signal Sim to digitally control its amplitude variations.

It results from what has just been described that the amplitude of the ringing of the antenna signal Vam after applying the incident burst (i.e. after the last inflection point i4) is not necessarily zero. It may merely be lower than the maximum amplitude of the antenna signal Vam during the application of the incident burst in a proportion of between 5% and 50%, or between 1% and 50%. An "imperfect" attenuation may indeed prove to be sufficient according to the required specifications.

For example, in an application of the method of the present invention to active load modulation with ^synchronization to an external magnetic field immediately after applying an incident burst, the amplitude of the ringing that may be tolerated at the end of the incident burst, i.e. at the time of the ^synchronization, depends on the amplitude of the signal induced by the external magnetic field, and more particularly on the ratio between the amplitude of the induced signal and that of the ringing. It has been shown, for example, that to obtain a phase error below 30°, the amplitude of the ringing must not exceed 57.7% of the amplitude of the induced signal. This ratio corresponds to one of the above-mentioned attenuations, that have been previously expressed by percentages referring to the maximum amplitude of the antenna signal Vam during the application of the incident burst, without referring to the amplitude of the signal induced by the external field.

If furthermore a restoration period is provided after the period of active load modulation, to be sure that the aforementioned amplitude ratio is below 57.7% at the time of the ^synchronization, the time remaining to proceed with this ^synchronization depends on the frequency of the data signal transmitted by means of the incident bursts, which determines the duration of the "silences" between two bursts, from which the duration of the restoration phase must be subtracted, which itself depends on the amplitude of the ringing at the end of burst and will be all the longer as the amplitude of the ringing is high, until the amplitude ratio below 57.7% is reached.

According to one embodiment of the method of the present invention, the amplitude A(t) of the falling edges of the incident bursts is digitally shaped by means of the raised cosine function formulated in the following manner:

• A(t) = A0*(l+cos(7c*t/Tf))/2,

the derivative of which is:

d(A(t)/dt=A0*7r/Tf^2*sin(7C*t/Tf)

AO being the maximum amplitude of the envelope signal (i.e. the maximum amplitude of the periodic signal Sim), Tf the duration of the falling edge, and t the time according to a time axis having as origin the inflection point i3 of the falling edge. This amplitude function A(t) then has a constant value AO before the inflection point i3 and a zero value after the second inflection point i4.

As shown on the table below, the derivative of the function A(t) is thus zero before and after the inflection points i3, i4. Between the inflection points i3 and i4, i.e. during the falling edge, the derivative of the function A(t) is the derivative of the raised cosine function. This derivative is also zero at the inflection point i3 and i4, and changes constantly according to the sine function between the two points. The function A(t) thus has no jump of its derivative between the inflection points i2 and i4.

The raised cosine function can also be used to shape the rising edge, and obtain a burst of B3 type having an envelope with an axial symmetry. In this case, the derivative of the envelope signal is perfectly continuous throughout the burst.

Any other mathematical function, in particular a window function for the Fast Fourier Transform, having a derivative offering the required properties, may be used by those skilled in the art instead of the raised cosine function.

Furthermore, it goes without saying that the periodic signal Sim must not itself have very steep edges that would generate an oscillatory response of the antenna circuit. In prior art, the signal Sim is generally filtered before being applied to the antenna circuit, and is thus not responsible for the technical problem solved in the present application by controlling the shape of the envelope signal. Generally speaking, if the signal Sim is obtained by filtering a signal that was initially square in shape, this filtering must preferably be such that the total harmonic distortion rate of the signal Sim is lower than 20% (i.e. the ratio between, on the one hand, the sum of the energies of the harmonics and, on the other, the energy of the fundamental of the signal Sim). When the signal Sim is digitally generated as is the case in an example of an embodiment of the present invention described below, the signal Sim may be generated as a perfect sine curve having a zero or almost zero harmonic distortion rate.

An example of an embodiment of an active load modulation NFC device implementing an embodiment of a method for transmitting data according to the present invention will be described below. This example of an embodiment is a specific case of application of the method of the present invention to active load modulation, avoiding the use of a damping switch after applying an incident burst, for the ^synchronization of the periodic signal between two load modulation bursts.

The device ND1 comprises:

- a contact communication interface circuit ICT,

- a tuned inductive antenna circuit ACT, comprising an antenna coil AC1 and which may comprise various tuning components such as the capacitors Ca, Cb described above,

- a demodulation circuit DMCT coupled to a decoding circuit DCCT, to receive data DTr via the antenna circuit,

- a coding circuit CCT coupled to a modulation circuit MCT, to transmit data DTx via the antenna circuit,

- a phase locked loop PLL, and

- a clock extracting circuit CKCT.

Figure 4 also shows a host processor HP1 of the device ND1, and an external NFC device EDV equipped with an antenna coil AC2 and operating in the NFC reader mode by emitting a periodic external magnetic field FLD1 that oscillates for example at a carrier frequency of 13.56 MHz (standards ISO 14443, ISO 13693, Sony Felica®). The antenna circuits of the two devices ND1 and EDV are assumed to be tuned to this frequency, possibly to within a few percent thereof.

The contact communication interface circuit ICT enables the device ND1 to be coupled to the host processor HP1. It more particularly enables the host processor HP1 to supply the device ND1 with data DTx intended for the external device EDV, and receives from the device ND1 data DTr sent by the external device EDV. The data DTx/DTr is for

example application data of an NFC application (transaction, payment, data exchanges, etc.). In one alternative, the device ND1 may comprise an internal processor configured to manage contactless applications, which then itself generates the data DTx and processes the data DTr without using any host processor.

The clock extracting circuit CKCT and the demodulation circuit DMCT receive, through an amplifier Al, an antenna signal Vai induced in the antenna circuit ACT by the external magnetic field FLD1. The clock extracting circuit CKCT supplies an external clock signal CKe the frequency of which is that of the external magnetic field, i.e. 13.56 MHz in the framework of the abovementioned standards. The phase locked loop PLL receives the external clock signal CKe and supplies an internal clock signal CKs as well as a sampling clock signal N*CKs the frequency of which is N times that of the clock signal CKs.

The phase locked loop PLL comprises a synchronous oscillation mode wherein it is bound to the external clock signal CKe, the signal CKs then being phase and frequency synchronized tothe clock signal CKe, and a free oscillation mode wherein the signal CKs is no longer set in phase and frequency to the signal CKe. The free oscillation mode is activated by a logic signal MSK supplied by the modulation circuit MCT.

To send data DTr to the device ND1, the external device EDV applies to the magnetic field FLD1 an amplitude modulation by means of a binary data-carrying modulation signal MS(DTr). The signal MS(DTr) is reflectedin the induced antenna signal Vai and is extracted therefrom by the demodulation circuit DMCT, after removal of the carrier at 13.56MHz. The circuit DMCT supplies the modulation signal MS(DTr) to the circuit DCCT, which extracts therefrom the data DTr and supplies it to the host processor HP1 via the communication interface circuit ICT.

The data DTx to be sent to the external device EDV is supplied to the coding circuit CCT by the host processor HP1 via the communication interface circuit ICT. The circuit CCT then supplies the modulation circuit MCT with a binary data-carrying modulation signal MS(DTx). The modulation signal MS(DTx) has a frequency derived from the frequency of the clock signal CKs, for example 848kHz (standard ISO 14443). To generate this signal, the circuit CCT receives the internal clock signal CKs supplied by the phase locked loop PLL.

The modulation circuit MCT is here an active load modulation circuit that receives, on one hand, the sampling signal N*CKs and, on the other hand, the modulation signal MS(DTx). The circuit MCT applies to the antenna circuit ACT bursts B3 of a periodic signal Sim of the type described above, separated by periods of non-modulation during which the signal Sim has a default value, generally 0. The signal Sim has a frequency equal to that of the clock signal CKs and the bursts B3 are emitted at the rate of the modulation signal MS(DTx).

The bursts B3 are shaped in the manner described above, so as to cancel out or at least limit the transient oscillation phenomenon of the antenna circuit ACT after each application of a burst. Therefore, when the signal MS(DTx) changes to 1, the antenna circuit ACT receives a burst B3 of the signal Sim and the antenna coil AC1 emits a corresponding burst of a magnetic field FLD2. The bursts of magnetic field FLD2 are detected by the external device EDV as a passive load modulation. The latter may thus extract, from its own antenna signal, the signal MS(DTx) to deduce therefrom the data DTx sent by the device ND1.

Figures 5A to 5E show the operation of the device ND1 when transmitting data DTx and respectively represent the clock signal CKs, the data signal MS(DTx), the logic signal MSK, the signal Sim supplied by the modulator circuit MCT, and the antenna signal Va present in the antenna circuit ACT. Active load modulation phases Pm of duration Tm and ^synchronization phases Psyn of duration Tsyn can be distinguished.

Each load modulation phase Pm is initiated when the signal MS(DTx) changes to 1 (Figure 5B). The modulation circuit MCT applies a burst B3 of the signal Sim to the antenna circuit ACT (Figure 5D) and sets the signal MSK to 0 throughout the burst (Figure 5C) so that the phase locked loop PLL operates in free oscillation mode. The antenna signal Va (Figure 5E) then comprises a component Vai induced by the external magnetic field FLD1 and a component Vam generated in the antenna circuit by the signal Sim. It is assumed here that the bursts B3 do not leave any transient oscillation of the component Vam remaining in the antenna circuit. Therefore, after a burst B3 has been applied to the antenna circuit, the antenna signal Va comprises only the component Vai induced by the external field FLD2 and the synchronization phase is immediately initiated. The signal MSK is put back to 1 by the circuit MCT and the phase locked loop PLL resynchronizes to the external clock signal CKe. .

According to one embodiment where the bursts B3 leave a trace of oscillation of the component Vam of the antenna signal, a relaxation time may be provided between the modulation phases Pm and the synchronization phases Psyn. Generally speaking, the damping of the transient oscillation of the component Vam thanks to the specific shape given to the bursts B3 is determined so as not to render necessary the use of the damping switch described by the application WO 2012/038664.

In other words, the damping phase conventionally carried out by means of such a switch is implicitly included in whole or in part in the falling edge of the bursts B3. In the non-restrictive embodiment represented, the frequency of the signals CKe, CKs, Sim is

13.56MHz and that of the data signal MS(DTx) is 848KHz. One period of the data signal SM(DTx) corresponds to 16 periods of the signal Sim. The duration of the bursts B3 is substantially greater than the duration Tl during which the signal MS(DTx) is on 1, which extends over 8 periods of the signal Sim. The bursts have a rising edge of duration Tr which extends over 4 periods of the signal Sim, a plateau of duration Tp which extends over 2 periods of the signal Sim, and a falling edge of duration Tf which extends over 4 periods of the signal Sim, such that Tr+Tp+Tf/2=T1. The duration of the bursts B3 is thus greater than the duration Tl of Tf/2, i.e. 2 periods of the signal Sim. The synchronization period extends over 6 remaining periods of the signal Sim.

Figure 6 represents one embodiment PLL1 of the phase locked loop PLL and one embodiment MCT1 of the modulation circuit MCT.

The phase locked loop comprises a phase comparator PFD, a charge pump CP, a loop filter LF, a voltage controlled oscillator VCO and a frequency divider DIV by N, for example a counter modulo N. The oscillator VCO supplies the modulation circuit MCT1 with the sampling signal N*CKs. This signal is also applied to the divider DIV that supplies the signal CKs at an input of the comparator PFD and to the circuits CCT and DCCT. Another input of the comparator PFD receives the external clock signal CKe and a control input of the comparator PDF receives the signal MSK. When the signal MSK is on 1, the comparator supplies the charge pump with two error signals U, D ("Up" and "Down") representing the phase or frequency error between the signals CKe and CKs. The charge pump supplies the filter LF with a voltage Vp that increases or decreases according to the signals U, D. The. voltage Vp is applied to the filter LF the output of which supplies a loop voltage VI applied to the oscillator VCO.

The change to 0 of the signal MSK stops the phase comparator. The signals U and D are then forced to 0 and the charge pump also stops, which results in freezing the loop voltage VI applied to the oscillator VCO. The phase locked loop thus continues to oscillate in free oscillation mode at the last synchronized frequency reached before the signal MSK changed toO.

The modulation circuit MCT1 synthetizes the bursts B3 of the periodic signal Sim by means of a set of digital values stored in lookup tables LUT1 to LUT9 located for example in a non-volatile memory MEM. As shown in Figure 7, these stored values are points Pi of a curve of a digital signal Simd that has the desired shape of the bursts B3 of the analog signal Sim, here a pure sine curve the amplitude of which is determined by the envelope signal E3 of the burst B3. In the embodiment chosen here, each of the tables LUT1, LUT2, LUT3 and

LUT4 is assigned to the storage of the points Pi of a period of the signal Slmd during the rising edge of the burst B3, between the inflection points il and i2. Each of the tables LUT6, LUT75 LUT8 and LUT9 is assigned to the storage of the points Pi of a period of the signal Slmd during the falling edge of the burst B3, between the inflection points i3 and i4. Finally, the table LUT5 is assigned to the storage of the points Pi corresponding to two periods of the signal CKs during the plateau phase of the burst B3, between the inflection points i2 and i3. The number of points Pi provided to synthetize each period of the signal Slmd is determined by the sampling frequency N*CKs supplied by the phase locked loop and is thus equal to N.

With reference again to Figure 6, the circuit MCT1 also comprises a state machine SM, an analog-digital converter DAC, and an amplifier Al. The state machine SM comprises an address counter ADCNT for addressing and reading the look-up tables LUT1-LUT9, in the memory MEM, and is paced by the signal N*CKs. The state machine also receives the data signal MS(DTx) and is configured to initiate the reading of tables LUT1-LUT9 when the signal MS(DTx) changes to 1, so that the emission of a burst B3 is synchronized to this signal. The output of the memory MEM supplies the points Pi to the converter DAC the output of which supplies the antenna circuit ACT, through the amplifier Al, with the bursts B3 of the analog periodic signal Sim.

According to one embodiment, the state machine SM also receives a phase control signal PCS and is configured to phase shift the periodic signal Slmd/Slm by an angle determined by the value of this signal, relative to the clock signal CKs, which is in phase with the external clock signal CKe. Some applications indeed require the bursts of magnetic field FLD2 to have a phase shift relative to the external magnetic field FLD1, for a better detection of the active load modulation by the external device EDV.

this embodiment MCT1 of the modulation circuit MCT offers full freedom in controlling the envelope signal E3 of the burst B3, each point Pi being mathematically calculated before being stored in the look-up tables LUT1 to LUT9. The envelope signal E3 may therefore strictly conform to the desired shaping function, for example the raised cosine function.

Figure 8 shows a low-cost analog embodiment MCT2 of the modulation circuit MCT. In this example of embodiment, the data-carrying binary signal MS(DTx) is used as an . envelope signal of the bursts B3, and has a duty cycle controlled by the coding circuit CCT.

As above, it is assumed that one period of the data signal SM(DTx) corresponds to 16 periods of the signal Sim.

The circuit MCT2 comprises low-pass filters FLT1, FLT2 of the first or of the second order, a mixer MIX, an amplifier A2, and an edge detection circuit EDT supplying the phase locked loop PLL with the signal MSK. The filter FLT1 receives the data-carrying logic signal MS(DTx) as envelope signal and supplies at an input of the mixer MIX an envelope signal E3 as previously described having rising and falling edges softened by the filtering of the first or second order, the derivative of which is continuous. The clock signal CKs supplied by the phase locked loop is here square in shape and is transformed by the filter FLT2 into a periodic signal Sim with low harmonic distortion before being applied to a second input of the mixer MIX. The output of the mixer supplies the antenna circuit ACT, through the amplifier A2, with the bursts B3 of the signal Sim. Finally, the detector EDT receives the data signal MS(DTx) and the clock signal CKs, and sets the signal MSK to 0 upon detecting a change to 1 of the signal MS(DTx).

According to one embodiment, the value 1 of the signal MS(DTx) is maintained during 6 cycles of the clock signal CKs, whereas the value 0 is maintained during 10 clock cycles, i.e. an initial duty cycle of 6/16 compared to 10/16 previously, to take account of a temporal spread of the rising and falling edges of the signal MS caused by the filter FLT1.

As above, a setting of the phase of the signal Sim could be provided, for example by inserting an adjustable phase shifter at the second input of the mixer MIX. In one alternative embodiment, the signal CKs is applied without being filtered at the second input of the mixer MIX, and the filter FLT2 is arranged at output of the amplifier A2.

As indicated above, a method for transmitting data according to the present invention, including a control of the shape of the burst envelope, is susceptible of various other alternatives and applications. It may in particular be applied to the transmitting of data in NFC reader mode, to more precisely control the duration of the amplitude modulation gaps of the magnetic field emitted, by removing the spurious oscillations appearing in the modulation gaps, for example to increase the flow rate of data transmitted by reducing the duration of the modulation gaps. .

A data transmitting/receiving device according to the present invention is also susceptible of various applications. In an example of application represented in Figure 9, the device ND1 is integrated into a portable device HD1 and is linked to one or more host processors, here two host processors HP1, HP2 that use the device ND1 as a contactless communication interface (NFC interface). The portable device HD1 is for example a mobile telephone, a digital music player, or a personal digital assistant (PDA). The processor HP1 may be the main processor of the device, or a secure processor such as a smart card processor.

The processor HP2 may for example be the baseband processor of a mobile telephone, also ensuring communications via a mobile telephony channel.

In another example of application respectively represented on Figures 10, 11 by a top view and a bottom view, the device ND1 is coupled to a host processor HP1 and the assembly is integrated into a plastic CD medium to form a smart card HD2. The antenna coil AC1 is for example a coplanar coil having one or more turns. On the back face (Figure 11) of the plastic CD medium, the card HD2 is equipped with a group CP of contacts. The card HD2 may for example form an NFC SIM card. The group of contacts may comprise contacts CI to C8 according to the standard ISO 7816. The card HD2 may also form a smart card intended to be inserted into any type of device (mobile telephone, personal computer, etc.) as an NFC communication interface.

CLAIMS

1. 1. A method for transmitting data by inductive coupling, comprising a step of applying bursts (B2, B3) of a periodic signal (Sim) to a tuned inductive antenna circuit (ACT) at the rate of a data-carrying signal (MS(DTx)), to produce an antenna signal (Vam) generating a magnetic field (FLD2), in which the amplitude of each burst is delimited by an envelope signal (E2, E3) having a rising edge (il-i2) and a falling edge (i3-i4),

characterized in that it comprises a step of shaping at least the falling edge of the envelope signal (E2, E3) by means of a digital (MCT1) or analog (FLT1) shaping circuit so as to flatten its slope to attenuate or remove transient oscillations of the antenna signal (Vam) that would appear in the antenna circuit after each application of a burst if bursts of a periodic signal with a square wave envelope signal were applied thereto.
2. The method according to claim 15 wherein the falling edge of the envelope signal is shaped so that its first derivative does not exceed a limit value determined by taking into account the maximum amplitude of the transient oscillations that may be tolerated in the antenna circuit.

3. The method according to one of claims 1 and 25 wherein the falling edge of the envelope signal is shaped so that its first derivative has a maximum value equal to A0*7t*Fc/2, Fc being the frequency of the periodic signal (Sim), and AO the amplitude of the envelope signal.

4. The method according to one of claims 1 to 3, wherein the falling edge of the envelope signal is shaped so that immediately after applying a burst of the periodic signal, the antenna signal (Vam) has, in the absence of an external magnetic field (FLD1), an amplitude lower than a percentage of the maximum amplitude it has during the application of the burst, chosen between 5% and 50%.

4. The method according to one of claims 1 to 4, comprising the step of giving the falling edge of the envelope signal (E2, E3) a shape determined by a mathematical function the derivative of which is a continuous function.

5. The method according to claim 5, comprising the step of giving the.falling edge a shape determined by the raised cosine function calculated on a time scale giving it the value 1 at a high inflection point (i3) of the falling edge and the value 0 at a low inflection point (i4) of the falling edge.

6. The method according to one of claims 1 to 6, comprising the steps of giving the falling edge of the envelope signal (E2, E3) a shape determined by a set of points (Pi) stored in a memory (MEM, LUT1-LUT9) and defining by discrete values a burst (B3) of the periodic signal (Sim).

8. The method according to one of claims 1 to 7, comprising a step of also shaping the rising edge of the envelope signal (E3) so that the first derivative of the rising edge is continuous.
9. The method according to one of claims 1 to 8, wherein the periodic signal (Sim) has a total harmonic distortion rate lower than 20%.

10. The method according to one of claims 1 to 9, for transmitting data by active load modulation, wherein the bursts (B2, B3) of the periodic signal (Sim) are applied to the antenna circuit (ACT) in the presence of an external alternating magnetic field (FLD1), and comprising a step of synchronizing the frequency of the periodic signal (Sim) with the frequency of the external magnetic field, between two applications of a burst of the periodic signal (Sim) to the antenna circuit (ACT).

11. A device (ND1) for transmitting data by inductive coupling, comprising a tuned inductive antenna circuit (ACT) and an amplitude modulation circuit (MCT1, MCT2) for apply bursts (B2, B3) of a periodic signal (Sim) to the antenna circuit (ACT), and produce an antenna signal (Vam) generating a magnetic field, the amplitude of each burst being delimited by an envelope signal (E2, E3) having a rising edge and a falling edge, characterized in that the modulation circuit (MCT1, MCT2) is configured to implement the method according to one of claims 1 to 10.

12. A portable electronic object (HD1, HD2) comprising a device according to claim 11.