Abstract:
An electromagnetic transponder including an oscillating circuit adapted to extracting from a radiating field a high-frequency amplitude-modulated signal, circuitry for extracting from said high-frequency signal an approximately D.C. supply voltage, a demodulator of data carried by the high-frequency signal, and circuitry for separately regulating the supply voltage and a useful voltage carrying the data.

Description:
This application is a continuation of prior application Ser. No. 09/847,531, filed on May 2, 2001, entitled Improvement of the Demodulation Capacity of an Electromagnetic Transponder. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electromagnetic transponder, that is, a transceiver (generally mobile) capable of being interrogated in a contactless and wireless manner by an entity (generally fixed), called a read and/or write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal, and the data transmitted from the fixed entity to the transponder are transmitted by this high-frequency field in amplitude modulation. The present invention applies to such transponders, be they read-only transponders, that is, transponders adapted to operating with a terminal which only reads the transponder data, or read/write transponders, which contain data that can be modified by the terminal. 
   2. Discussion of the Related Art 
   Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal. 
     FIG. 1  very schematically shows a conventional example of a data exchange system between a read/write terminal  1  and a transponder  10  of the type to which the present invention applies. 
   Generally, terminal  1  is essentially formed of a series oscillating circuit formed of an inductance L 1  in series with a capacitor C 1  and a resistor R 1 , between an output terminal  2  of an amplifier or antenna coupler (not shown) and a reference terminal  3  (generally the ground). The antenna coupler belongs to a circuit  4  for controlling the oscillating circuit and for exploiting the received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. In the example shown in  FIG. 1 , node  5  of connection of capacitor C 1  to inductance L 1  forms a terminal for sampling a data signal received for the demodulator. Circuit  4  of the terminal generally communicates with different input/output circuits (keyboard, screen, means of exchange with a provider, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system or to batteries. 
   A transponder  10 , intended for cooperating with a terminal  1 , essentially includes a parallel oscillating circuit formed of an inductance L 2 , in parallel with a capacitor C 2  between two A.C. input terminals  11 ,  12  of a rectifying circuit  13  (for example, a fullwave rectifying bridge). The output voltage of bridge  13 , sampled across the rectified output terminals  14 ,  15  thereof, is intended for providing, not only a power supply to electronic data processing circuits  16  (ELEC), but also the very data, modulated in amplitude for a demodulator  17  (DEM). 
   Since transponder  10  draws its power from the field radiated by terminal  1 , it is necessary to provide a circuit  20  for limiting the input voltage of rectifying system  13  that would otherwise risk being damaged by voltages that are too high or carrying these excessively high voltages downstream and thus damaging the electronic circuits. Protection circuit  20  is generally placed as high upstream as possible, that is, upstream of bridge  13 . It is, for example, formed of two series-opposition associations of zener diodes  21 ,  22 ,  23 ,  24  with identical thresholds. Being upstream of rectifying bridge  13 , a first series-opposition association of zener diodes  21  and  22  is connected between terminal  11  and a ground terminal  25  (for example, confounded with ground terminal  16  at the output of bridge  13 ). A second series-opposition association of zener diodes  23  and  24  is connected between terminals  12  and  25 . 
   In the example of  FIG. 1 , the transponder includes, upstream of bridge  13 , a voltage regulation circuit  30 , the function of which is to provide as regular a power supply as possible to electronic circuits  16 . For example, circuit  30  is formed of a resistor  31  in series with a zener diode  32 , between terminals  14  and  15 . The midpoint  33  of this series connection forms an output terminal providing an approximately D.C. supply voltage to circuit  16 . This supply voltage is smoothed by a capacitor  34  in parallel with zener diode  32 , the anode of which is connected to terminal  15  and the cathode of which is connected to terminal  33 . It should be noted that another capacitor  18  is generally directly connected between terminals  14  and  15  to smooth the voltage transmitted to demodulator  17 , as will be seen hereafter. 
   The transmission of information from transponder  10  to terminal  1  is generally performed by modifying the load formed by this transponder on the terminal&#39;s field. A simple way to achieve this is to connect, between terminals  14  and  15 , a so-called back-modulation circuit  40 . This circuit is, to simplify, formed of a resistor  41  in series with a switch  42  (for example, a MOS transistor), the control terminal of which is connected to electronic circuit  16 , and more precisely to the output of a modulator (not shown). 
   The oscillating circuit of terminal  1  is excited by a high-frequency signal, for example, at 13.56 MHz. The oscillating circuits of terminal  1  and of transponder  10  are generally tuned on the frequency of a transmission carrier corresponding to this high-frequency signal, that is, their respective resonance frequencies are set to a frequency of, for example, 13.56 MHz. This tuning aims at maximizing the power diffusion to the transponder, generally, a card of credit card format or a tag of still smaller format, integrating the different transponder components. The high-frequency remote supply carrier transmitted by terminal  1  is also used as a data transmission carrier. This carrier is generally modulated in amplitude by the terminal according to different coding techniques to transmit the data to the transponder. In response, the back modulation performed by the transponder generally is at a much lower frequency (for example, 847 kHz), which enables the terminal to detect the load variations (be it by amplitude or phase modulation). 
     FIG. 2  illustrates a conventional example of a data transmission from terminal  1  to a transponder  10 . This drawing shows an example of shape of the excitation signal of antenna L 1  for the transmission of a code 0101. The modulation currently used is an amplitude modulation with a 106-kbit per second rate (1 bit is transmitted in approximately 9.4 microseconds) much smaller than the frequency of the carrier coming from the transmission oscillator (period of approximately 74 nanoseconds for a 13.56 MHz frequency). The amplitude modulation is generally performed with a modulation rate, defined as being the difference of the peak amplitudes (a, b) between two states ( 1  and  0 ) divided by the sum of these amplitudes, much smaller than one due to the need for supply of transponder  10 . For example, the modulation rate is on the order of 10%. It should be noted that, whatever the type of modulation used (for example, amplitude, phase, or frequency modulation) and whatever the type of data coding (NRZ, NRZI, BPSK, Manchester, ASK, etc.), the transmission is performed by shifts between two binary levels on the remote supply carrier. 
   A disadvantage of conventional transponders is that the use of means ( 20 ,  FIG. 1 ) for clipping the voltage recovered across the oscillating circuit (L 2 , C 2 ,  FIG. 1 ) adversely affects the correct data reception in a transmission by amplitude shifts that is not in all or nothing. Indeed, if the transponder is relatively close to the terminal, the voltage is likely to be clipped by circuit  20  in such a way that the transponder demodulator is then incapable of making out a state  0  from a state  1  due to the modulation rate used. Further, this loss of information can occur without having a clipping level lower than the level of state  0  (b, FIG.  2 ). It is indeed sufficient for the level at state  1  to be clipped to have a risk of interpretation error by the transponder demodulator. 
   This disadvantage is illustrated by  FIG. 3 , which shows a simplified example of the shape of voltage V 13  across output terminals  14  and  15  ( FIG. 1 ) of rectifying bridge  13  as a function of the inverse of distance d separating the transponder from the terminal. Since the signal shapes of  FIG. 3  which will be described hereafter will show an amplitude modulation of the remote supply carrier, it may also be considered that voltage V 13  is expressed as a function of time as the transponder progressively comes close to the terminal. 
   A first curve  26  in dotted lines illustrates the transponder operation in the absence of a regulation circuit  30 . In such a configuration, the voltage across capacitor C 2  is clipped as soon as threshold V 20  of the zener diodes of circuit  20  is reached. Accordingly, it can be considered that from a distance d 1  on, the transponder is no longer capable of demodulating the data carried by signal  26  since this signal has turned into a continuous and constant level substantially corresponding to voltage V 20  (neglecting the series voltage drop in rectifying bridge  13 ). 
   The system operation is improved by the presence of regulation circuit  30 . This operation is illustrated by curve  36  in  FIG. 3  where the power consumption of circuit  16  is neglected. A first difference with curve  26  is that the presence of resistor  31  in series with zener diode  32  (or with smoothing capacitor  34 ) causes a voltage drop with respect to the preceding case since the power of the radiated field cannot be modified. As a result, distance d 2  at which the clipping appears on the level of voltage V 20 , downstream of the rectifying bridge, is much closer than distance d 1 . Accordingly, the operation is maintained for a wider range. However, the presence of this resistor that distributes the power between that for the supply and that for the demodulator attenuates the amplitude available for the demodulator. This attenuation is even greater once zener diode  32  is in avalanche. In  FIG. 3 , it has been assumed that voltage level V 32 , corresponding to the threshold of zener diode  32  between terminals  14  and  15  (taking account of resistance  31 ) is reached for a distance d 0 . As long as this distance has not been reached, that is, as long as the transponder is further away from this terminal than this threshold, the amplitude attenuation of the modulation performed by resistor  31  is relatively low and can be neglected. Curves  26  and  36  are confounded for distances greater than d 0  (left-hand portion of FIG.  3 ). Between distances d 0  and d 2 , diode  32  is in avalanche and the amplitude of the modulation of signal  36  is attenuated. From distance d 2  on, the diodes of circuit  20  are in avalanche and the modulation can no longer be detected. 
   The use of a regulation circuit  30  such as described in  FIG. 1  clearly already is an improvement as compared to the simple use of the limiting circuit upstream of the rectifying bridge. However, it is necessary to perform a compromise between the value given to resistance  31  and the so-called “blinding” area, that is, the distance range (distances smaller than d 2 ) in which the transponder can no longer detect data. The greater the value of resistance  31 , the closer the shape of the curve will be to shape  26  with no regulator. The smaller the resistance, the more the blinding area is reduced, but the smaller the amplitude of the data modulation between distances d 0  and d 2 . 
   Another known solution to solve the problem of a voltage varying according to distance consists of limiting the transmission power of the terminal. A disadvantage of such a solution however is that this then limits the transponder system range. Further, the magnetic fields that the transponders are supposed to withstand are most often imposed by standards and the application of the standards currently in force results in that the magnetic field received by the transponder, when its clipping means starts operating, is much smaller than the maximum magnetic field that the transponder must be able to withstand according to standards. Accordingly, the transponder is often supplied by a signal clipped by circuit  20  and the data are then lost. 
   The above problems are more critical still for low power consumption transponders. Indeed, in this case, the circuits internal to the transponder provided to have a low power consumption are not able to withstand high voltages, so that the clipping means must be sized relatively low. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to overcome the disadvantages of known electromagnetic transponders as concerns the unwanted effects of the clipping means on the amplitude demodulation. 
   The present invention more specifically aims at providing a novel electromagnetic transponder that can withstand high magnetic fields in the vicinity of a read/write terminal without for all this adversely affecting the recovery of the data transmitted by this terminal. 
   The present invention also aims at providing a solution that requires no modification of the read/write terminals and that is thus compatible with existing read/write systems. 
   The present invention also aims at providing a solution that is compatible with the search for a minimum transponder consumption. 
   The present invention further aims at providing a solution that requires no modification of the conventional electronic circuits (demodulator and data processing circuit) of the transponder. 
   To achieve these and other objects, the present invention provides an electromagnetic transponder including an oscillating circuit adapted to extracting from a radiating field a high-frequency amplitude-modulated signal, a means for extracting from said high-frequency signal an approximately D.C. supply voltage, a demodulator of data carried by the high-frequency signal, and a means for separately regulating the supply voltage and a useful voltage carrying the data. The means for regulating the voltage of the useful signal has a time constant greater than that of the supply voltage regulation means. 
   According to an embodiment of the present invention, the transponder includes a means for rectifying the voltage sampled across the oscillating circuit and, in series between two rectified output terminals of this rectifying means, a first transistor and a second transistor, the midpoint of this series connection forming a terminal for sampling the signal transmitted to the demodulator. 
   According to an embodiment of the present invention, the control terminal of the second transistor is connected to the midpoint of a resistive dividing bridge between said terminal providing the signal to be demodulated and the ground, a capacitor being connected between said control terminal and the ground. 
   According to an embodiment of the present invention, the delay of taking account of the voltage variation by the second regulator is determined by the value of said capacitor. 
   According to an embodiment of the present invention, said first transistor is connected in parallel with a resistor and is controlled by a measurement of the voltage across the output terminals of the rectifying means. 
   According to an embodiment of the present invention, said transistors are MOS transistors. 
   The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1  to  3 , previously described, are intended to show the state of the art and the problem to solve; 
       FIG. 4  schematically and partially shows an embodiment of an electromagnetic transponder according to the present invention; and 
       FIG. 5  illustrates the operation of a transponder according to the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been referred to with the same references in the different drawings. For clarity, only those elements necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the different circuits of processing and exploitation of the signals received and transmitted by the transponder have not been detailed. It should only be noted that the reception circuits are intended for receiving signals in amplitude shift keying with, preferably, a modulation rate under 30%. 
   A feature of the present invention is to provide separate regulation of the supply voltage of the electronic circuits of the transponder and of the demodulator input voltage. Another feature of the present invention is that these voltage regulators are provided with different time constants. In other words, to regulate the demodulator drive voltage, a relatively slow regulator with respect to the modulation frequency carried by the remote supply carrier (for example, a frequency equivalent to 106 kbits per second) will be chosen. On the supply voltage side, a fast response regulator will preferably be chosen to smooth as much as possible the transponder supply voltage. 
   It could have been thought that the use of a voltage regulator for the signal to be demodulated in amplitude would adversely affect the amplitude variation detection, and thus the demodulation. However, due to the delay introduced in the response of this regulator, the present invention overcomes this problem. 
   An advantage of providing two regulators each assigned to a different function (power supply regulation and amplitude modulation recovery) is that it is now possible to size the regulator intended for the supply voltage only for this need. Accordingly, it is no longer necessary to perform a compromise on the choice of a resistance value as used to be the case in prior art (resistor  31 , FIG.  1 ). The regulator intended for the supply voltage can thus be provided with no series resistive voltage drop. 
   Preferably, the value of the voltage provided by the demodulator regulator is smaller than the value of the voltage provided by the supply regulator. Thus, an adequate operation of the demodulator is guaranteed by guaranteeing a power supply always greater than the signal to be demodulated. 
     FIG. 4  shows an embodiment of an electromagnetic transponder according to the present invention. The representation of  FIG. 4  is to be compared to that of  FIG. 1 , considering that the transponder portions intended for the demodulation and for the processing of the obtained signals have not been shown (blocks  16  and  17  of FIG.  1 ). Similarly, back-modulation stage  50  formed of transistor  42  and of resistor  41  has been illustrated in dotted lines in  FIG. 4  to insist on its incidental character in the sense of the present invention. 
   As previously, an electromagnetic transponder is based on the use of a parallel oscillating circuit formed of an inductance L 2  in parallel with a capacitor C 2  across two A.C. input terminals  11 ,  12  of a rectifying entity  13  (for example, a diode bridge). As previously still, the input of the diode bridge is associated with a protection circuit  20  formed, for example, of two series-opposition associations of zener diodes  21 ,  22  and  23 ,  24  between each of terminals  11  and  12  and a ground terminal  25 . 
   The present invention intervenes downstream of rectifying bridge  13  to separately regulate a supply voltage Va, provided between rectified output terminals  14  and  15  of bridge  13  and intended for the transponder electronic processing circuits, and a voltage Vd provided between a terminal  60  and terminal  15  and carrying the useful data signal to the demodulator ( 17 ,  FIG. 1 ) of the transponder. 
   Regulator  61  intended for the useful signals is essentially formed of a transistor  62  (for example, a MOS transistor) connected between terminals  60  and  15 , the gate of this transistor being connected to midpoint  63  of a voltage dividing bridge formed, for example, of two resistors  64  and  65  in series between terminals  60  and  15 . An element delaying the regulation of voltage Vd is formed of a capacitor  66  connected in parallel with resistor  65 , that is, between the gate of transistor  62  and the ground. Terminal  60  is further connected to terminal  14  by a resistive component  67 . 
   In the absence of other components in the circuit, transistor  62  acts as a regulator of the level of voltage Vd. Indeed, any increase of the voltage across rectified output terminals  14  and  15  of bridge  13  translates as an increase of voltage Vd which causes a proportional increase of the gate voltage of transistor  62 . This results in increasing the conduction of transistor  62 , and thus in modifying the voltage ratio determined by the dividing bridge formed of resistor  67  and of the equivalent resistor of components  62 ,  64 , and  63 . However, the effect of transistor  62  is delayed by means of capacitor  66 , which slightly delays the voltage level increase of gate  63 . Now assuming an instantaneous decrease of voltage V 13  across terminals  14  and  15  of rectifying bridge  13 , corresponding to a switching from state  1  to state  0  ( FIG. 2 ) of the remote supply signal, transistor  62  will become less conductive to compensate this decrease but voltage Vd will decrease during a time determined by the value of capacitor  66  before this decrease is compensated for by a decrease of the conduction of transistor  62 . 
   It can thus be seen that the regulation effect operates for increases as well as for decreases but that the edges of the modulation signal are transmitted on voltage Vd and are thus interpretable by the demodulator. Indeed, it being an amplitude demodulation between two voltage levels, a basic amplitude demodulator will be able to interpret the level variations. Any rising edge corresponds to a switching to a state  1  while any falling edge corresponds to a switching to a state  0 . 
   A regulator  70 , intended for generating supply voltage Va, is based, in the example of  FIG. 4 , on the use of a first transistor  71  (for example, a MOS transistor) connected between terminals  14  and  60 . The gate of this transistor is connected to the junction point of a second transistor  72  (for example, a MOS transistor) with a resistor  73 , transistor  72  being made more or less conductive according to the amplitude of the difference between voltage V 13  and a reference voltage Vref. For example, a resistive dividing bridge formed of two resistors  74  and  75  in series between terminals  14  and  15  is provided. The midpoint  76  of this bridge is connected to one of the power terminals of transistor  72 , the gate of which receives reference voltage Vref. Reference voltage Vref is, for example, provided by the transponder electronic processing circuits or, more simply, by a zener diode (not shown) connected in series with a resistor between terminals  14  and  15 , voltage Vref being sampled from the midpoint of this series connection. Voltage Vref is of course chosen according to the minimum operating voltage of the circuit. Resistive dividing bridge  74 - 75  provides a voltage proportional to voltage V 13 . Accordingly, the gate voltage of transistor  71  will increase or decrease according to whether voltage V 13  respectively increases or decreases. This increase or decrease is, in the embodiment of  FIG. 4 , based with respect to reference value Vref. Transistor  71  will be all the more conductive as its gate voltage increases, and thus as voltage V 13  increases. Accordingly, any increase of voltage V 13  translates as an increase of the conduction of transistor  71  to compensate for this effect on supply voltage Va. 
   The use of a reference voltage rather than a resistive dividing bridge directly driving the gate of transistor  71  enables increasing the loop gain of the regulator. However, in a simplified embodiment, it may of course be provided to directly drive the gate of transistor  71  with the midpoint of a resistive dividing bridge between terminals  14  and  15 . This respects the principle of the present invention, which is to have two voltage regulation components in series between terminals  14  and  15 , the junction point of these regulation components providing voltage Vd with a different time constant for the lower regulator. This enables this regulator to be transparent in dynamic operation. 
   The operation of the transponder of  FIG. 4  is illustrated by  FIG. 5  which shows an example of the shape of voltages Va and Vd as a function of the inverse of distance. The representation of  FIG. 5  is to be compared with that of FIG.  3 . The shape of supply voltage Va is illustrated by dotted line  80  while the shape of voltage Vd of the useful signal is illustrated by plot  81  in full line. As long as voltage Vd has not reached a threshold determined by the respective sizings of resistors  64 ,  65 ,  67  and of transistor  62 , said transistor is off. Transistor  71  is saturated, so that it short-circuits resistor  67 , voltage V 13  being too low to generate a variation of the conduction of transistors  71  and  72 . Accordingly, in this situation where the distance is greater than distance d 10  (that is, in the left-hand portion of FIG.  5 ), voltages Va and Vd are approximately similar, neglecting the series voltage drops in transistor  71 . When voltage Vd becomes sufficient to operate regulator  61 , it is then regulated to a mean predetermined level Vr, letting through the edges corresponding to the state switchings of the modulation signal. Voltage Va (plot in dotted lines  80 ) continues increasing with the distance increase until reaching a distance d 11  where the voltage of point  76  becomes sufficient to turn transistor  72  on. The level of voltage Va is then regulated on a predetermined value V 2 . 
   It should be noted that resistor  67  enables transmitting the data to terminal  60  outside of the operating range of the supply voltage regulator, that is, when transistor  71  is completely off, voltage V 13  being clipped by circuit  20 . 
   It should also be noted that values V 1  and V 2  of voltage Vr and Va depend on the respective sizings of the circuit resistors and on the transistors used. 
   It should further be noted that voltage levels Vr and V 2  are lower than the activation threshold of clipping means  20 . Due to the regulation performed by the present invention, this threshold can now be chosen only to respect the circuit security constraints. 
   Although this has not been shown in  FIG. 4 , a smoothing capacitor ( 18 ,  FIG. 1 ) will generally be provided across terminals  14  and  15  to smooth the rectified voltage. 
   An advantage of the present invention is that it makes the demodulation voltage independent from the supply voltage. 
   Another advantage of the present invention is that it increases the transponder operating range. 
   Another advantage of the present invention is that it provides the complete security of the transponder by providing the use of a clipping circuit  20  upstream of the rectifying bridge. Further, this protection applies for brief overvoltages as well as for lasting overvoltages, conversely to what would be provided by a solution consisting of only regulating the supply voltage. 
   Another advantage of the present invention is that it improves the security of transponders against some fraud attempts. Indeed, one of the conventional fraud possibilities is to measure the consumption variation of the transponder supply from the external terminals of its integrated circuit. The present invention eliminates this possibility by implementing a regulation system that makes these variations invisible across the antenna, and thus across the external terminals of the integrated circuit. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, other components than those indicated in the embodiment taken as an example may be used. On this regard, it should be noted that, for example, bipolar transistors may be used instead of the MOS transistors and the resistive elements may be formed of components different from simple resistors (they may for example be formed of transistors). Further, the sizing of a transponder implementing the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove and on the operating characteristics desired for the voltage levels. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.