Improvement of the capacity of demodulation of an electromagnetic transponder

The present invention relates to an electromagnetic transponder, that is to say, a transmitter / receiver (usually mobile) capable of being interrogated contactless and wireless, by an entity (generally fixed), called the reading terminal and / or writing. The present invention relates, more particularly, transponders no independent power supply. these transponders extract the necessary power to the electronic circuits they comprise a high-frequency field radiated by an antenna of the terminal to read and write the information transmitted from the fixed entity to the transponder are transmitted by the high frequency field into amplitude modulation. The invention applies to such transponders, it either of read-only transponders, that is to say suitable for operate with a terminal merely read data transponder or transponders to be read-write containing data that can be modified by the terminal.

Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna and transponder side terminal side of read-write. These circuits are intended to be coupled by a close magnetic field when the transponder enters in the field of the terminal read-write.

1 shows, very schematically and Simplified, a classic example of a data exchange system between a terminal 1 and a read-write transponder 10 type to which the present invention applies.

Generally, terminal 1 is essentially a series resonant circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1 between a terminal 2 output of an amplifier or antenna coupler (not shown) and a terminal 3 of reference (usually ground). The antenna coupler is part of a control circuit 4 of the circuit oscillating and exploiting received data including, among others, a modulator / demodulator and a microprocessor order processing and data. In the example shown in Figure 1, item 5 of the capacitor C1 connection and the inductor L1 constitute a terminal for sampling a received data signal to the demodulator. The circuit 4 of the terminal generally communicates with different circuits input / output (keyboard, screen, exchange means with a server, etc.) and / or treatments not shown. the circuits terminal read-write generally draw the power necessary for their operation from a supply circuit (not shown) connected, for example, the distribution network electric or battery.

A transponder 10 for cooperating with a terminal 1, essentially comprises a parallel resonant circuit formed of an inductance L2 in parallel with a capacitor C2 between two terminals 11, 12 AC input of a circuit 13 recovery (for example, a wave rectifier bridge). The output voltage of the bridge 13, taken from the terminals 14, 15 of rectified output thereof, is intended to provide not only a supply energy to electronic circuits 16 (ELEC) processing of data but also data themselves, amplitude modulated to a demodulator 17 (DEM).

As transponder 10 draws its energy field radiated by the terminal 1, it is necessary to provide a circuit 20 limiting the input voltage of the rectifier system 13 which might otherwise be damaged by tension too high or too convey these high voltages downstream and thus damage electronic circuits. The circuit 20 protection is generally placed as early as possible, that is to say upstream of the bridge 13. It is, for example, consists of two associations diode series-opposition Zener 21, 22, 23, 24 of identical thresholds. As is upstream the rectifier bridge 13, a first series association-opposition Zener diodes 21 and 22 is connected between terminal 11 and 25 a mass terminal (e.g., coincident with the terminal 16 mass output of the bridge 13). A second association Zener diode series-opposition 23 and 24 is connected between the terminals 12 and 25.

In the example of Figure 1, the transponder includes, downstream of the bridge 13, a voltage regulating circuit 30 whose role is to provide a more regular supply possible to the electronic circuits 16. For example, the circuit 30 consists of a resistor 31 in series with a diode Zener 32, between terminals 14 and 15. The center point 33 of the series connection constitutes an output terminal delivering a supply approximately DC voltage to the circuit 16. This supply voltage is smoothed by a capacitor 34 in parallel with the Zener diode 32 whose anode is connected to the terminal 15 and whose cathode is connected to terminal 33. Note that another capacitor 18 is usually connected directly between terminals 14 and 15 for smoothing the voltage destination of the demodulator 17 as discussed later.

The transponder transmitting information to 10 Terminal 1 is generally performed by changing the charge constituted by transponder on the field of the terminal. For this, a means way is to connect between terminals 14 and 15, a circuit 40 said retromodulation. This circuit is a simplified way, consisting of a resistor 41 in series with a switch 42 (E.g., a MOS transistor), whose control terminal is connected to the electronic circuit 16, more precisely, the output of a modulator (not shown).

The oscillating circuit of terminal 1 is excited by a high frequency signal, for example, 13.56 MHz. the circuits oscillating from terminal 1 and of transponder 10 are generally tuned to the frequency of the corresponding transmission carrier to this high frequency signal, that is to say that their frequencies respective resonance is set to a frequency of, for example, 13.56 MHz. This agreement aims to maximize the distribution of energy to the transponder, generally a credit card shaped card or tag (TAG) format preferably less incorporating the various components of the transponder. The high-frequency remote supply carrier emitted by the terminal 1 also serves as a data transmission carrier. This carrier is usually amplitude modulated by the terminal according to different coding techniques to transmit the data to the transponder. In response, the back modulation performed by the transponder is generally at a frequency significantly less (e.g., 847 kHz), which allows the terminal to detect changes in load (either by a amplitude demodulation or phase).

Figure 2 illustrates a classic example of transmission from terminal 1 data to a transponder 10. This Figure shows an example of shape of the excitation signal L1 antenna for transmission of a code 0101. Modulation commonly used is an amplitude modulation with a rate 106 kbits per second (1 bit is transmitted in approximately 9.4 microseconds) much smaller than the frequency of the carrier from the transmission oscillator (periods of about 74 nanoseconds for a frequency of 13.56 MHz). modulation amplitude is generally carried out with a modulation rate, defined as the difference of the peak amplitudes (a, b) between two states (0 and 1), divided by the sum of these magnitudes, well below the unit due to the need to diet the transponder 10. For example, the modulation rate is of the order of 10%. Note that regardless of the type of coding data used (NRZ, NRZI, BPSK, Manchester, ASK, etc.), transmission is by jumping between two levels binary on the remote supply carrier.

A disadvantage of conventional transponders is that the using means of capping (20, Figure 1) of the voltage recovered across the oscillating circuit (L2, C2, Figure 1) night upon correct receipt of data in a transmission amplitude jumps that may not be all for nothing. While the transponder is relatively close to the terminal, the voltage is likely to be clipped by the circuit 20 in such a way the transponder of the demodulator is then unable to distinguish a state 0 to state 1 because of the modulation rate used. Moreover, this loss of information can occur without capping the level is below the state level 0 (B, Figure 2). It suffices that the level to 1 or clipped so that there is a risk of misinterpretation by the demodulator of the transponder.

This drawback is illustrated in Figure 3, shows a simplified example of the shape of the voltage V13 between terminals 14 and 15 (Figure 1) of output of the rectifier bridge 13 function of the inverse of the distance d separating the transponder of the terminal. Since the curves of Figure 3 that signals will be described below show an amplitude modulation the remote supply carrier, we can also consider the voltage V13 is expressed as a function of time while the transponder gradually approaches the terminal.

A first curve 26 in dotted lines illustrates the operation the transponder in the absence of a control circuit 30. In such a configuration, the voltage across the Capacitor C2 is clipped when the threshold V20 Zener diodes of circuit 20 is reached. Therefore, we can consider that from a distance d1 from the transponder is no longer able demodulate the data carried by the signal 26 in the Since this signal has turned continuous level and substantially corresponding to the constant voltage V20 (neglecting the series voltage drop in the rectifier bridge 13).

The operation of the system is improved by the presence of the control circuit 30. This operation is illustrated by the curve 36 in Figure 3 where consumption is neglected circuit 16. A first difference with the curve 26 is that the presence of the resistor 31 in series with the diode Zener 32 (or the capacitor 34 smoothing) causes voltage drop from the previous case in that where the energy of the radiated field is not editable. It results the distance d2 which appears capping at V20 voltage downstream of the bridge rectifier, is much closer the distance d1. Therefore, the operation is maintained for greater distance range. By against the presence of this resistance that distributes power between the one destination of food and that to the demodulator reduces the amplitude available for the demodulator. This attenuation is even more important from the moment the Zener diode 32 is in avalanche. In Figure 3, it is assumed that the V32 voltage level corresponding to the threshold of the Zener diode 32 carried between terminals 14 and 15 (taking into account the resistance 31) is reached at a distance d0. Until this distance is not reached, that is to say as long as the transponder is further from the terminal that this threshold, the attenuation amplitude modulation effected by the resistor 31 is relatively small and can be neglected. The curves 26 and 36 are combined for greater than d0 distances (left of Figure 3). Distances between d0 and d2, the diode 32 is avalanche and the amplitude of the modulation of the signal 36 is attenuated. From the distance d2, the diodes of the circuit 20 are avalanche and modulation can not be detected.

It is clear that the use of a control circuit 30 as described in FIG 1 is already an improvement compared to single use limit circuit upstream the rectifier bridge. However, it is necessary to carry out a compromise between the value given to the resistance 31 and the area called "glare", that is to say the range of distances (distances less than d2) wherein the transponder can detecting more data. More resistance value is 31 , the more the shape of the curve will be close to the pace 26 without regulator. The higher the resistance value, the lower removing the glare area but the modulation of data is of low amplitude between the distances d0 and d2.

Another known solution to the problem voltage varying depending on the distance is to limit the transmission power of the terminal. A disadvantage of such a However, this solution is that then limits the scope of the system transponder. In addition, the magnetic fields that are supposed bear the transponders are usually imposed by standards and the application of existing standards results in that the magnetic field received by the transponder, when the clipping means to implement, is significantly less than the maximum magnetic field that the transponder must be able to support by the standards. Therefore, the transponder is often supplied by a clipped signal by the circuit 20 and information is lost.

The above problems are even more critical to low-power transponders. Indeed, in this case, the transponder internal circuits designed to consume low are not able to withstand high voltages, so that the capping means must be sized relatively weakly.

An object of the present invention is to overcome the disadvantages of the known electromagnetic transponders terms of adverse effects means capping the amplitude demodulation.

The invention aims more particularly to provide a new electromagnetic transponder that can withstand high magnetic fields near a read-write terminal, without impairing the recovery of data transmitted by this terminal.

The present invention also aims at providing a solution which requires no modification of the terminal read-write, and which is therefore compatible with systems existing read-write.

The invention also aims at providing a solution that is compatible with the search for a minimum consumption of transponder.

The invention further aims at providing a solution that requires no modification of conventional electronic circuits (Demodulator and data processing circuit) of transponder.

To achieve these objects, the present invention provides an electromagnetic transponder comprising circuitry oscillating own extracting a radiation field a high signal frequency amplitude modulated, means for extracting from said high frequency signal an approximately DC voltage power, a data demodulator conveyed by the high frequency signal, and means for separately regulating the supply voltage and a working voltage conveying the data. The regulation means of the voltage of the useful signal has a time constant greater by the voltage regulation power.

According to one embodiment of the present invention, the transponder comprises a voltage rectifying means taken across the oscillating circuit and, in series between two rectified output terminals of the rectifying means, a first transistor and a second transistor, the midpoint This series connection constituting a pickup terminal signal to the demodulator.

According to one embodiment of the present invention, the second transistor control terminal is connected to the midpoint of a resistive dividing bridge between said terminal delivering the signal to be demodulated and ground, a capacitor being connected between said control terminal and ground.

According to one embodiment of the present invention, delay in consideration of the voltage variation by the second controller is set by the value of said capacitor.

According to one embodiment of the present invention, said first transistor is connected in parallel with a resistance and is controlled by measuring the voltage across outlet rectifying means.

According to one embodiment of the present invention, said transistors are MOS transistors.

These objects, features and advantages, as well as others of the present invention will be described in detail in the following description of specific embodiments made in non-limiting in connection with the accompanying drawings among :<sl><li>Figures 1 to 3 which have been described previously are meant to show the state of the art and the problem asked;</li><li>4 shows schematically and partially, one embodiment of an electromagnetic transponder according to the present invention; and</li><li>Figure 5 illustrates the operation of a transponder according to the present invention.</li></sl>

The same elements have been designated with the same references the different drawings. For reasons of clarity, only the elements that are necessary for the understanding of the invention have been shown in the drawings and will be described after. In particular, the various processing circuits and operating the signals received and transmitted by the transponder have not been detailed and are not the subject of the invention. We simply noted that the receiving circuits are intended to receiving signals modulated with amplitude jump, of Preferably, a modulation rate less than 30%.

A feature of the present invention is to provide a separate control of the supply voltage electronic circuits of the transponder and the input voltage the demodulator. Another feature of the invention is that these voltage regulators are provided with constants different time. In other words, for regulating the voltage Attack of the demodulator, we will choose a relatively regulator slow compared to the modulation frequency conveyed by the remote supply carrier (e.g., an equivalent frequency to 106 kbits per second). Side power supply, it preferably choose a fast feedback controller for smooth as possible to the supply voltage of the transponder.

One would think that recourse to a regulator voltage for the signal to be demodulated in amplitude interfere with the change detection therefore amplitude demodulation. However, thanks to the delay in the response of this regulator, the invention overcomes this problem.

An advantage is to provide two affected regulators each with a different function (feed control and recovery of the amplitude modulation) is that it is now possible to dimension the regulator for the supply voltage for this need. Therefore, it is no longer necessary to make compromises on the choice a resistance value as it was the case in the prior art (Resistor 31, Figure 1). The regulator for the voltage Power can be provided without resistive drop series.

Preferably, the voltage value provided by the demodulator of the regulator is less than the voltage value provided by the power supply regulator. This ensures proper operation of the demodulator ensuring Power always greater than the signal to be demodulated.

4 shows one embodiment of a electromagnetic transponder according to the present invention. The representation of Figure 4 is similar to that of 1 considering that the parties intended the transponder for demodulation and processing of the signals obtained does not have been shown (blocks 16 and 17 of Figure 1). Similarly, stage 50 retromodulation consisting of transistor 42 and the resistor 41 has been illustrated in dotted lines in Figure 4 for emphasize its ancillary within the meaning of the invention.

As previously, an electromagnetic transponder is based on the use of a parallel resonant circuit consisting of an inductance L2 in parallel with a capacitor C2 between two terminals 11 and 12 AC input of reorganization entity 13 (for example a diode bridge). Also as before, the input of the diode bridge is associated with a circuit 20 consisting of protection, for example, two associations Zener diode opposition-series 21, 22 and 23, 24 between each terminals 11 and 12 and a 25 mass terminal.

The present invention operates downstream of the bridge rectifier 13 to separately regulate a supply voltage Go, issued between terminals 14 and 15 of the rectified output bridge 13 and for electronic processing circuits transponder, and a Vd voltage supplied between a terminal 60 and terminal 15 and carrying the useful data signal to the demodulator (17, Figure 1) of the transponder.

The controller 61 for the useful signals is essentially consisting of a transistor 62 (e.g., a transistor MOS) connected between the terminals 60 and 15, the gate of this transistor being connected to the midpoint 63 of a divider bridge voltage formed, for example, two resistors 64 and 65 in series between terminals 60 and 15. A delay element of the regulation of the voltage Vd is constituted by a capacitor 66 connected in parallel with the resistor 65, that is to say between the gate of transistor 62 and ground. The terminal 60 is further connected to the terminal 14 by means of a resistive element 67.

In the absence of other components in the circuit, the transistor 62 acts as a regulator of the voltage level Vd. Indeed, any increase in the voltage at the terminals 14 and 15 of rectified output of the bridge 13 translates into an increase in the Vd voltage that causes a proportional increase in transistor gate voltage 62. This has the effect of increasing of the transistor 62, thus changing the ratio voltage set by the voltage divider consisting of resistance 67 and the equivalent resistance of the components 62, 64 and 63. However, the effect of the transistor 62 is delayed by means of capacitor 66 which differs slightly increasing level grid voltage 63. Assuming now a decrease instant of the voltage V13 at the terminals 14 and 15 of the rectifier bridge 13, corresponding to a transition from the state 1 to state 0 (Figure 2) of the remote supply signal, the transistor 62 will become passing less to compensate for this decline but the tension Vd decrease over a period set by the capacitor value 66 before this reduction was offset by a decrease in of transistor 62.

This shows that the regulatory effect works to rise or fall but the fronts of the modulation signal are transmitted on the voltage Vd and thus interpretable by demodulator. Indeed, in the case of demodulation amplitude between two voltage levels, an amplitude demodulator base will be capable of interpreting the changes in levels. Any edge corresponds to a transition to a state 1 while any falling edge corresponds to a transition to a 0 state.

The controller 70, for generating the supply voltage Va, is based, in the example of Figure 4, the use a first transistor 71 (for example, MOS) connected between terminals 14 and 60. The gate of this transistor is connected to the midpoint of a series connection of a second transistor 72 (eg MOS) with a resistor 73, the transistor 72 being rendered more or less passing accordance with the magnitude of the difference between the voltage V13 and reference voltage Vref. By example, there is provided a resistive divider bridge consisting of two resistors 74 and 75 in series between terminals 14 and 15. Point middle 76 of the bridge is connected to a power terminal of the transistor 72 whose gate receives the reference voltage Vref. Vref reference voltage is, for example, provided by electronics transponder processing or so simpler, by a Zener diode (not shown) associated with series with a resistor between the terminals 14 and 15, the voltage Vref being levied on the midpoint of that association series. The voltage Vref is of course chosen according to the minimum voltage of the circuit operation. The divider bridge resistive 74-75 provides a voltage proportional to the voltage V13. Therefore, the gate voltage of the transistor 71 increase or decrease depending on whether the voltage will increase or V13 respectively decrease. This increase or decrease is, in the embodiment of Figure 4, based versus the reference value Vref. The transistor 71 will lead to much more than its gate voltage will increase, so that the voltage V13 increase. Therefore, any increase of the voltage V13 results in an increase in conduction of transistor 71 to compensate for this effect on blood pressure Go to diet.

The use of a reference voltage rather than a bridge resistive divider directly attacking the transistor gate 71 allows to increase the loop gain of the regulator. However, in a simplified embodiment, it is possible of course provide directly attack the gate of the transistor 71 by the the midpoint of a resistive dividing bridge between terminals 14 and 15. This respects the principle of the invention is to have two voltage regulating elements in series between the terminals 14 and 15, the midpoint of these regulatory elements providing voltage Vd with a different time constant for the lower controller. This allows operation in dynamic, this controller is transparent.

The operation of the transponder of Figure 4 is illustrated in Figure 5 which shows an example of shape of voltages Va and Vd as a function of the inverse of the distance. The representation of Figure 5 is similar to that of Figure 3. The shape of the supply voltage Va is shown 80 by the dotted line while the shape of the voltage Vd useful signal is illustrated by the line 81 in solid lines. so the voltage Vd has not reached a threshold set by the sizing respective resistors 64, 65, 67 and the transistor 62, the latter is blocked. The transistor 71 is in turn saturation so that short-circuits the resistor 67, the V13 voltage is too low to cause a change in the conduction of the transistors 71 and 72. Therefore, in this greater distance position at the distance d10 (that is to say in the left part of Figure 5), the voltages Va and Vd are approximately similar neglecting the voltage drops series in the transistor 71. When the voltage Vd becomes sufficient to turn on the regulator 61, it is then regulated at a predetermined average level Vr, letting through fronts corresponding to state changes of the modulation signal. The voltage Va (dotted line 80) continues on it increases with decreasing distance to reach a distance d11 where the voltage of point 76 becomes enough to make the transistor 72. The level of the voltage Va is then regulated to a predetermined value V2.

resistance 67 Note that can transmit the data to the terminal 60 outside the operating range of the voltage regulator, that is to say when the transistor 71 is completely blocked, the voltage V13 being clipped by the circuit 20.

Note also that the values ​​V1 and V2 voltages Vr and Va depend on the respective sizings resistances of the circuit and the transistors used.

It is further noted that the voltage levels Vr and V2 are below the activation threshold of the clipping means 20. Through regulation effected by the invention, this threshold may now only be chosen to meet the constraints safety circuit.

Although this has not been shown in Figure 4, a smoothing capacitor (18, figure 1) will generally be provided between terminals 14 and 15 for smoothing the rectified voltage.

An advantage of the present invention is that it makes voltage independent demodulation of the supply voltage.

Another advantage of the present invention is that it increases transponders operating range.

Another advantage of the present invention is that it preserves the complete security by providing a transponder clipping circuit 20 upstream of the rectifying bridge. Of Moreover, this protection is valid for both brief surge that lasting surge, unlike that give a solution consisting of not regulate the voltage power.

Another advantage of the present invention is that it improves safety transponder vis-à-vis certain fraud attempts. Indeed, one of the classic opportunities fraud involves measuring the consumption of variation the transponder supply from external terminals the integrated circuit. The invention eliminates this possibility implementing a control system that makes these invisible variation at the antenna terminals, thus the external terminals of the integrated circuit.

Of course, the present invention is capable of variants and modifications which will appear to man art. In particular, components other than those listed in the embodiment taken as an example may be used. In this regard, it is noted that, for example, one may use bipolar transistors instead of the MOS transistors and the resistive elements may be formed of different components than simple resistance (eg, be formed transistors). In addition, the dimensioning of a putting transponder out the present invention is to man's reach the art based on the functional indications given above and desired operating characteristics for voltage levels.