Abstract:
The invention concerns a terminal generating a high-frequency electromagnetic field by means of an oscillating circuit, adapted to co-operate with at least a transponder when the latter enters said field, comprising means ( 37 ) for regulating the phase of the signal in the oscillating circuit with respect to a reference value, and means for detecting, on the basis of a current measurement in the oscillating circuit and a voltage measurement at the terminals of a capacitive element ( 31 ) of said oscillating circuit, an electric signature of a transponder.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to systems using electromagnetic transponders, that is, transmitters and/or receivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from a high-frequency field radiated by an antenna of the read-write terminal. 
   2. Discussion of the Related Art 
     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 inductive resistor L 1  in series with a capacitor C 1  and a resistor R 1 . This series oscillating circuit is controlled by a device  2  including, among others and non-limitingly, an amplifier or antenna coupler, and a control circuit exploiting received data, especially provided with a modulator/demodulator and a microprocessor for processing the control signals and the data. Circuit  2  generally communicates with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power necessary for their operation from a supply circuit (not shown) connected, for example, to the electric supply system. 
   A transponder  10 , intended for cooperating with a terminal  1 , essentially includes a parallel oscillating circuit formed of an inductive resistor L 2 , in parallel with a capacitor C 2  between two input terminals  11 ,  12  of a control and processing circuit  13 . Terminals  11 ,  12  are in practice connected to the input of a rectifying means (not shown), outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor, a demodulator of the signals that may be received from terminal  1 , and a modulator for transmitting information to the terminal. 
   The oscillating circuits of the terminal and of the transponder are generally tuned on the same frequency corresponding to the frequency of an excitation signal of the terminal&#39;s oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a transmission carrier but also as a remote supply carrier for the transponder(s) located in the terminal&#39;s field. When a transponder  10  is located in the field of a terminal  1 , a high-frequency voltage is generated across terminals  11  and  12  of the transponder&#39;s resonant circuit. This voltage, after being rectified and possibly clipped, is intended for providing the supply voltage of electronic circuits  13  of the transponder. 
   The high-frequency carrier transmitted by the terminal is generally modulated in amplitude by said terminal according to different coding techniques to transmit data and/or control signals to one or several transponders in the field. In return, the transmission of data from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L 2 , C 2 . This load variation is performed at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal in the form of an amplitude variation or of a phase variation by means, for example, of a measurement of the voltage across capacitor C 1  or of the current in the oscillating circuit. 
   When idle, that is, when no transponder has entered its field, a terminal  1  periodically transmits a data message modulated on the high-frequency signal. This message is a request message intended for possible transponders. This request, or general call, belongs to a process needed for the initialization of a communication between a transponder and a terminal. 
   A difficulty in establishing a communication to one or several transponders is due to the fact that several electromagnetic transponders may be simultaneously located in the terminal&#39;s field. 
   Another difficulty is that transponders of different types or families (for example, transportation card, access card, electronic purse) use the same frequencies and, at least partly, the same communication protocols, which are normalized. Accordingly, a terminal must be able to determine, not only the number of transponders present in its field, but also the type of transponders, to select that or those with which it must communicate. 
   Such constraints require a loop operation of a control program of the terminal until all the transponders present in its field have been properly identified. 
     FIG. 2  schematically shows, in a simplified manner, a flowchart of initialization of one or several communications by a read-write terminal of the type to which the present invention applies. 
   After a starting, initialization and testing phase, a read-write terminal  1  of transponders starts (block  20 , ST) a stand-by procedure during which it waits for the completion of a communication with at least one transponder. This procedure includes periodically sending (block  21 ) a request sequence (REQ) to the possible transponders present in the terminal&#39;s field. Upon each sending of an interrogation request  21 , the reader monitors (block  22 ) the reception by its demodulator of an acknowledgement message (ATQ) coming from a transponder having entered its field. In the absence of any acknowledgement, the reader loops on the sending of a request  21 . When its receives an acknowledgement ATQ, it switches to a mode of checking whether the transponder really is a transponder intended for it (block  23 , TYPE), as well as to a possible anti-collision mode (block  24 , ANTICOLLISION) to individualize several transponders that may be present in the field. Indeed, if several transponders are present in the terminal&#39;s field, they may respond at the same time or with a sufficiently short time interval to make the result of the demodulation by the reader unexploitable. Said reader must then either select a transponder with which it wishes to communicate, or assign different channels to the different transponders. 
   A communication only starts when the initialization and anti-collision process illustrated in  FIG. 2  is over (block  26 , E), that is, when the reader has detected (block  25 , ALL) that it has identified all the transponders present in its field. Determining that a transponder belongs to a given family is part of this identification. As long as all the transponders have not been identified, interrogation requests are sent. If a transponder has been properly identified, it is placed in a state where it no longer acknowledges interrogation requests to avoid polluting the detection of the other possible transponders. 
   An initialization and anti-collision process such as briefly described in relation with  FIG. 2  is well known. Illustrations of conventional methods are for example to be found in French patent applications no. 2,760,280 and 2,773,627 which are incorporated herein by reference. 
   The method illustrated in  FIG. 2  is most often implemented by setting the maximum number of cards likely to be present in the transponder&#39;s field. As described, in particular, in French patent application No. 2,760,280, this number can be modified by the reader according to the results of exploitation of the anti-collision process (block  24 ) to increase the detection probabilities and to reduce the duration of the initialization process. 
   A disadvantage of conventional systems is that the determinations of the number and/of the type of transponders use a transmission of messages by the transponders. To determine the transponder&#39;s family, a software-implemented comparison of an identification transmitted by the transponder is used. In addition to the problems of program execution time, it is thus not checked whether the software identification is transmitted by a transponder or by a pirate device placed close to the terminal. 
   SUMMARY OF THE INVENTION 
   The present invention aims at overcoming the disadvantages and at solving the problems posed by known systems in relation with the identification of one or several transponders in the field of a terminal. 
   The present invention more specifically aims at providing a novel mode of identification of an electromagnetic transponder which is not based on an analysis of a transmitted data message. 
   The present invention also aims at reducing the time required for the initialization and the completion of communications between a read-write terminal of electromagnetic transponders and one or several transponders having entered its field. In other words, the present invention aims at reducing the duration necessary for the determination and the identification, by the read-write terminal, of all the transponders present at a given time in its field. 
   The present invention further aims at providing a solution that does not require use of the detection results of the terminal&#39;s demodulator. 
   To achieve these and other objects, the present invention provides a terminal for generating a high-frequency electromagnetic field by means of an oscillating circuit, adapted to cooperating with at least one transponder when said transponder enters this field, and including: 
   means for regulating the signal phase in the oscillating circuit with respect to a reference value; and 
   means for detecting, based on a measurement of the current in the oscillating circuit and on a measurement of the voltage across a capacitive element of this oscillating circuit, an electric signature of a transponder. 
   According to an embodiment of the present invention, the electric signature enables determining the type to which the transponder belongs from among a set of types identified by characteristic manufacturing parameters. 
   According to an embodiment of the present invention, the terminal includes means for comparing the signature of a transponder having entered its field with at least one signature sample stored in a learning phase. 
   According to an embodiment of the present invention, the terminal includes means for determining and storing characteristic information relative to the voltages across the capacitive element of its oscillating circuit and to the currents in this oscillating circuit, in several determined configurations of the distance separating a reference transponder from the terminal, and for taking into account this characteristic information to determine the transponder&#39;s signature. 
   According to an embodiment of the present invention, said characteristic information includes, among other things: 
   the voltage across the capacitive element when no transponder is present in the terminal&#39;s field; 
   the voltage across the capacitive element when a transponder is in a relation of maximum closeness with the terminal; 
   the current in the oscillating circuit when no transponder is present in the terminal&#39;s field; and 
   the current in the oscillating circuit when a transponder is in a relation of maximum closeness with the terminal. 
   According to an embodiment of the present invention, the signature detection is performed without interpreting any data message carried by the high-frequency field. 
   According to an embodiment of the present invention, the terminal includes means for evaluating, based on the measurement of the current in the oscillating circuit, the minimum number of transponders present in the field. 
   According to an embodiment of the present invention, the terminal further includes means for, based on the measurement of the voltage across the capacitive element of the oscillating circuit, evaluating the maximum number of transponders present in the terminal&#39;s field. 
   The present invention also provides a method for establishing at least one communication between a terminal of generation of a high-frequency magnetic field and an electromagnetic transponder, including determining the type of a transponder from among a set of types identified by characteristic electric parameters, before exchanging any data message between the transponder and the terminal. 
   According to an embodiment of the present invention, said determination includes of comparing the current measured in the terminal&#39;s oscillating circuit and the voltage across a capacitive element of this oscillating circuit with previously calculated and stored values to establish the set of identified types. 
   According to an embodiment of the present invention, the method includes periodically sending a request sequence until at least one transponder entering the field sends an acknowledgement, and evaluating based on the current measurement a minimum number of transponders likely to be present in the field. 
   According to an embodiment of the present invention, the method further includes, based on the evaluation of the minimum number and on a measurement of the voltage, evaluating the maximum number of transponders likely to be present in the terminal&#39;s field. 
   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 and 2 , previously described, are intended to show the state of the art and the problem to solve; 
       FIG. 3  shows, in the block diagram form, an embodiment of an electromagnetic transponder read-write terminal according to the present invention; 
       FIG. 4  is a simplified flowchart illustrating an embodiment of a learning phase of the method for evaluating the number of transponders according to the present invention; 
       FIG. 5  is a flowchart illustrating, in a simplified manner, a first embodiment of the method for determining the number of transponders in the field of a terminal according to the present invention; 
       FIG. 6  shows characteristics of current variation in the oscillating circuit of a terminal of the present invention with respect to the distance at which are located one or several transponders, to illustrate a learning phase of the method of evaluating the number of transponders according to the present invention; 
       FIG. 7  is a simplified flowchart of a step of the method of the present invention illustrated by  FIG. 5  according to a second embodiment of the method of the present invention; and 
       FIG. 8  shows a characteristic of the current in the terminal&#39;s oscillating circuit according to the coupling coefficient, and a characteristic of the voltage across the capacitor of this oscillating circuit, to illustrate the second embodiment of the method of the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been referred to with the same reference characters in the different drawings. For clarity, the characteristics of  FIGS. 4 and 8  have been drawn out of scale and only those elements of a terminal or of a transponder and those steps of the method which are necessary to the understanding of the present invention have been illustrated in the drawings and will be described hereafter. In particular, the means used to implement the calculations in the method of the present invention have not been detailed, since it is conventional. It may be, for example, the microprocessor generally provided in a read-write terminal. Further, reference will often be made to “cards” to designate transponders. The present invention however applies whatever the type of transponder, be it or not provided with a microprocessor (card of credit card type, electronic label, etc.). 
   A feature of the present invention is to provide an identification of a so-called electric signature of a transponder. Each transponder type has specific electric characteristics that are determined by its manufacturing based on integrated circuit manufacturing technologies. These characteristics are linked to the sizing (shape and size) of the inductive resistor and capacitor of the transponder&#39;s resonant circuit. Once the transponder has been manufactured, these characteristics cannot be changed (neglecting the influence of the component aging). Further, the very characteristics of the resonant circuit influence the oscillating circuit of a terminal. Accordingly, it can be considered that each transponder family has its “signature” from the point of view of its oscillating circuit. Further, advantage can be taken of the technological dispersions of these components between manufacturing batches to individualize transponder families. Such dispersions are however negligible within a same batch, or at least on a same wafer. 
   Another feature of the present invention is to provide a determination of the family to which the transponder present in the field of a read-write terminal belongs, based on physical measurements performed on the terminal&#39;s oscillating circuit. More specifically, the values of the current in the terminal&#39;s oscillating circuit and the voltage across the capacitor of this oscillating circuit are compared to values measured and/or calculated in a learning phase that precedes putting the terminal into service. 
   According to the present invention, the determination of the transponder type is performed (upstream of the demodulator) without requiring exploitation of the received data signal (that is, downstream of the terminal&#39;s demodulator). The present invention provides evaluating this number based on the sole electric current and voltage determinations, and on calculations of these variables. 
   Determining the family to which a transponder belongs without it being necessary to interpret (to demodulate) a data message considerably improves the system security. Indeed, to be “accepted” by the terminal, the transponder must have the same characteristics as a reference transponder. This means that the pirate transponder has to have the same structure (to be manufactured in the same way in integrated form) as a valid transponder. Now, it is more difficult to reproduce intrinsic electric characteristics (linked to the technology used and to the integration method) than data message characteristics. Risks of fraud are thus considerably reduced. 
   Another feature of the present invention is to provide a phase regulation of the terminal&#39;s oscillating circuit with respect to a reference value. According to the present invention, this phase regulation is performed by means of a loop. The response time of this loop is chosen to be sufficiently long to avoid disturbing the possible back modulation from a transponder and to be sufficiently short as compared to the passing speed of a transponder in the terminal&#39;s field. This can be called a static regulation with respect to the modulation frequencies (for example, the 13.56-MHz remote supply carrier frequency and the 847.5-kHz back-modulation frequency used in the data transmission from the transponder to the terminal). 
   The phase regulation enables reliable exploitation of the performed measurements of physical variables. Indeed, the load formed by a transponder on the oscillating circuit depends on the distance separating this transponder from the terminal. Now, the variation range, evaluated across the capacitor of the terminal&#39;s oscillating circuit (capacitor C 1 ,  FIG. 1 ) depends on the tuning of this oscillating circuit as well as on the tuning of the transponder&#39;s oscillating circuit on the remote supply carrier frequency. Now, in conventional circuits, the tuning is not perfect. In particular, in conventional circuits, the tuning of the resonance frequency to the carrier frequency is performed manually by means of a variable capacitor, once the terminal has been manufactured. The tuning requires adjusting, especially, due to manufacturing tolerances of capacitive and inductive elements, to guarantee the chosen phase operating point between a reference signal provided by an oscillator of the terminal and the received signal, sampled for example across capacitor C 1 . A detuning of the terminal&#39;s oscillating circuit has several consequences and, in particular, that of modifying the signal amplitude in this oscillating circuit and, accordingly, modifying the amplitude of the signal available for a possible evaluation. 
     FIG. 3  shows in the form of blocks an embodiment of a terminal  30  according to the present invention, equipped with a phase regulation loop of the oscillating circuit. 
   Conventionally, terminal  30  includes an oscillating circuit formed of an inductive resistor or antenna L 1 , in series with a capacitive element  31  and a resistive element R 1 , between an output terminal  32  of an amplifier or antenna coupler  33  and a terminal  34  at a reference potential (generally, ground). An element  35  for measuring the current in the oscillating circuit is interposed, for example, between capacitive element  31  and ground  34 . Measurement element  35  belongs to the phase regulation loop, which will be described hereafter. Amplifier  33  receives a high-frequency transmission signal E, coming from a modulator  36  (MOD), which receives a reference frequency (signal OSC), for example, from a quartz oscillator (not shown). Modulator  36  receives, if necessary, a signal Tx of data to be transmitted and, in the absence of any data transmission from the terminal, provides the high-frequency carrier (for example at 13.56 MHz) adapted to remotely supplying a transponder. Capacitive element  31  is a variable-capacitance element controllable by a signal CTRL. 
   In terminal  30 , a phase regulation of the current in antenna L 1  is performed with respect to a reference signal. This regulation is a regulation of the high-frequency signal, that is, of the carrier signal corresponding to signal E in the absence of data to be transmitted. This regulation is performed by varying the capacitance of the oscillating circuit of terminal  30  to maintain the current in the antenna in a constant phase relation with the reference signal. This reference signal corresponds, for example, to signal OSC provided by the modulator&#39;s oscillator. Signal CTRL originates from a circuit  37  (COMP) having the function of detecting the phase interval with respect to the reference signal and accordingly modifying the capacitance of element  31 . The phase measurement is performed, for example, from a measurement of current I in the circuit by means of current transformer  35  connected in series with element  31 . This transformer generally is formed of a primary winding  35 ′ between element  31  and terminal  34 , and of a secondary winding  35 ″, a first terminal of which is directly connected to ground  34  and a second terminal of which supplies a signal MES providing the result of the measurement. A current-to-voltage conversion resistor R 35  is connected in parallel with secondary winding  35 ″. The result of measurement MES is sent to comparator  37 , which accordingly controls capacitive element  31  by means of signal CTRL. 
   According to the preferred embodiment illustrated in  FIG. 3 , comparator  37  uses the same phase demodulator (not shown) as that which is used to demodulate the signal coming from the transponder, possibly received by the oscillating circuit. Accordingly, comparator  37  provides a signal Rx restoring a possible back modulation of data received from a transponder to a block  38  symbolizing the rest of the terminal&#39;s electronic circuits. 
   The practical implementation of the phase regulation loop is within the abilities of those skilled in the art by using conventional means and based on the functional indications given hereabove. As an alternative to the current transformer of  FIG. 3 , other conventional measurement means may be used. An example of a phase regulation terminal is described in document EP-A-0,857,981. 
   Regulating the phase of the terminal&#39;s oscillating circuit on a reference value enables getting rid of possible problems of sizing tolerances for the oscillating circuit components and of the drift of these components in operation. This further enables performing reliable measurements relative to the magnetic coupling between the terminal&#39;s oscillating circuit and that of one or several transponders which is incorporated herein by reference. 
   Current and voltage measurements in the terminal&#39;s oscillating circuit can now be exploited to deduce therefrom, according to the present invention, information relative to the transponder type. This information takes into account, in particular, the coupling between each of the cards and the terminal, that is, the coupling coefficient between the terminal&#39;s oscillating circuit and that of one of the cards. This coupling coefficient essentially depends on the distance separating the transponder from the terminal. Coupling coefficient k between the oscillating circuits of a transponder and of the terminal is always between 0 and 1. The distance separating the antennas from the oscillating circuits is, as a first approximation, proportional to 1-k. Accordingly, in the following description, reference will be made either to distance or to the coupling coefficient. 
   In an application of the present invention in which several transponders are likely to be present at the same time in the field of a terminal, another feature is to also provide an evaluation of the number of cards present in the terminal&#39;s field. According to the present invention, this determination is here again performed based on physical measurements performed on the terminal&#39;s oscillating circuit. As for the signature determination, the number of cards in the terminal&#39;s field is evaluated by comparing the values of the current in the terminal&#39;s oscillating circuit and the voltage across the capacitor of this oscillating circuit to values measured and/or calculated in a learning phase that precedes putting the terminal into service. 
   The evaluation of this number of transponders, without requiring exploitation of the received data signal, considerably reduces the number of loops of the communication initialization method ( FIG. 2 ). Indeed, even if a method dynamically adapting the number of loops based on the measurements is used, the initial number taken into account is refined as compared to a conventional implementation. 
   The present invention originates from an interpretation of different relations linking the electric variables measurable by the terminal in different operating configurations with one or several transponders. 
   In particular, current I in the terminal&#39;s series oscillating circuit (for example, measured by transformer  35 ) is linked to the so-called generator voltage (noted Vg) exciting the oscillating circuit, and to the apparent impedance Z 1   app  of the oscillating circuit, by the following relation: 
   
     
       
         
           
             
               
                 I 
                 = 
                 
                   
                     Vg 
                     
                       Z1 
                       app 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
     
   
   Further, the fact of regulating the oscillating circuit phase on a reference value enables the distance variation of a transponder entering the terminal&#39;s field to only translate as a modification of the real part of the impedance of this oscillating circuit. Indeed, all variations which would tend to modify the imaginary part of this impedance by the load formed by the transponder (or the transponders) are compensated for by the phase regulation loop. Thus, it is ensured that, in static operation, the imaginary part of impedance Z 1   app  is null. Accordingly, impedance Z 1   app  becomes equal to apparent resistance R 1   app  and can be expressed as: 
   
     
       
         
           
             
               
                 
                   
                     Z1 
                     app 
                   
                   = 
                   
                     
                       R1 
                       app 
                     
                     = 
                     
                       R1 
                       + 
                       
                         
                           a 
                           2 
                         
                         ⁢ 
                         
                           L2 
                           
                             R2 
                             · 
                             C2 
                           
                         
                       
                     
                   
                 
                 , 
                 
                   
 
                 
                 ⁢ 
                 
                   with 
                   : 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
           
             
               
                 
                   
                     a 
                     2 
                   
                   = 
                   
                     
                       
                         k 
                         2 
                       
                       · 
                       
                         ω 
                         2 
                       
                       · 
                       L1 
                       · 
                       L2 
                     
                     
                       
                         X2 
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             L2 
                             
                               R2 
                               · 
                               C2 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
                 , 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ) 
               
             
           
         
       
     
   
   where ω represents the pulsation, X 2  represents the imaginary part of the impedance of the transponder&#39;s oscillating circuit (X 2 =ωL 2 −1/ωC 2 ), and where R 2  represents the load formed by the transponder circuits on its own oscillating circuit, modeled in  FIG. 1  by a resistor R 2  shown in dotted lines, in parallel with inductive resistor L 2  and capacitor C 2 . In other words, resistor R 2  represents the equivalent resistor of all the transponder circuits (microprocessor, back-modulation means, etc.), added in parallel on capacitor C 2  and inductive resistor L 2 . In above formula 2, the series resistance of inductive resistor L 1 , which adds to the two other terms, has been neglected. It may also be considered that the value of this series resistance is, for simplification, included in the value of resistance R 1 . 
   The above formulas 2 and 3 have been established in the case where a single transponder is present in the terminal&#39;s field. By combining formulas 1, 2, and 3, current I can be expressed as: 
   
     
       
         
           
             
               
                 I 
                 = 
                 
                   
                     Vg 
                     
                       R1 
                       + 
                       
                         
                           k 
                           2 
                         
                         · 
                         
                           
                             
                               ω 
                               2 
                             
                             · 
                             L1 
                             · 
                             
                               L2 
                               2 
                             
                           
                           
                             
                               Z2 
                               2 
                             
                             · 
                             R2 
                             · 
                             C2 
                           
                         
                       
                     
                   
                   . 
                   
                     
 
                   
                   ⁢ 
                   
                     with 
                     : 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ) 
               
             
           
           
             
               
                 
                   Z2 
                   2 
                 
                 = 
                 
                   
                     X2 
                     2 
                   
                   + 
                   
                     
                       
                         ( 
                         
                           L2 
                           
                             R2 
                             · 
                             C2 
                           
                         
                         ) 
                       
                       2 
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
         
       
     
   
   In above expressions 4 and 5, only values Vg and k are likely to vary for a given terminal and transponders of a given family. 
   Among the measurements of electric variables that can be easily performed on the read-write terminal side, the present invention provides using off-load and maximum coupling values that correspond to the following cases. 
   The off-load values represent the current and the voltage when no transponder is present in the terminal&#39;s field. In this off-load operation, the apparent impedance Z 1   off-load  of the terminal&#39;s oscillating circuit now only depends on components R 1 , L 1 , and C 1  of the terminal. Further, since, due to the phase regulation, the imaginary part of this impedance is always null, one may write: 
   
     
       
         
           
             
               
                 
                   I 
                   
                     off 
                     ⁢ 
                     
                       - 
                     
                     ⁢ 
                     load 
                   
                 
                 = 
                 
                   
                     Vg 
                     R1 
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
                 ) 
               
             
           
         
       
     
   
   Further, it is known that imaginary part X 1   app  of apparent impedance Z 1   app  can be expressed as: 
   
     
       
         
           
             
               
                 
                   
                     X1 
                     app 
                   
                   = 
                   
                     X1 
                     - 
                     
                       
                         a 
                         2 
                       
                       · 
                       X2 
                     
                   
                 
                 , 
                 
                   
 
                 
                 ⁢ 
                 
                   with 
                   : 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 ) 
               
             
           
           
             
               
                 X1 
                 = 
                 
                   
                     ω 
                     · 
                     L1 
                   
                   - 
                   
                     
                       1 
                       
                         ω 
                         · 
                         C1 
                       
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
                 ) 
               
             
           
         
       
     
   
   Since imaginary part X 1   app  is null, one can write:
 
X 1 =a 2 .X 2 .  (formula 9)
 
   Based on these relations, the difference between the current and off-load values can be expressed as follows:
 
 X   1 − X   1   off-load   =a   2   .X   2 − a   off-load   2   .X   2 .  (formula 10)
 
   Now, coefficient a off-load  is null since the off-load coupling is null. Further, voltage VC 1  across element  31  (neglecting the influence of current transformer  35 ) can be written as I/ωC 1 , I being, for example, measured by transformer  35 . As a result, formula 10 hereabove can be written as: 
   
     
       
         
           
             
               
                 
                   a2 
                   · 
                   X2 
                 
                 = 
                 
                   
                     
                       VC1 
                       
                         off 
                         ⁢ 
                         
                           - 
                         
                         ⁢ 
                         oad 
                       
                     
                     
                       I 
                       
                         off 
                         ⁢ 
                         
                           - 
                         
                         ⁢ 
                         load 
                       
                     
                   
                   - 
                   
                     
                       VC1 
                       1 
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
                 ) 
               
             
           
         
       
     
   
   Another easy-to-determine operating condition corresponds to maximum coupling kmax. In this condition, that is, in a relation of minimum distance between a transponder and the terminal (for example, the transponder being laid on the terminal as close as possible to antenna L 1 ), current I max  in the terminal&#39;s oscillating circuit can be measured as a transponder of the involved family or type is laid on the terminal. 
   By applying formula 10 to the current value and to the maximum coupling, and by transferring these applications into formula 11, one may write, for a card: 
   
     
       
         
           
             
               
                 
                   
                     
                       a 
                       2 
                     
                     · 
                     X2 
                   
                   
                     
                       a 
                       max 
                       2 
                     
                     · 
                     X2 
                   
                 
                 = 
                 
                   
                     
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                       - 
                       
                         VC1 
                         I 
                       
                     
                     
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                       - 
                       
                         
                           VC1 
                           max 
                         
                         
                           I 
                           max 
                         
                       
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   12 
                 
                 ) 
               
             
           
         
       
     
   
   Now, by applying formula 3 to the above formula, one obtains: 
   
     
       
         
           
             
               
                 
                   
                     
                       a 
                       2 
                     
                     · 
                     X2 
                   
                   
                     
                       a 
                       max 
                       2 
                     
                     · 
                     X2 
                   
                 
                 = 
                 
                   
                     
                       k 
                       2 
                     
                     
                       k 
                       max 
                       2 
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   13 
                 
                 ) 
               
             
           
         
       
     
   
   Thus, ratio k/k max  between the current and maximum coupling coefficients can be expressed, when a transponder is present in the terminal&#39;s field, as: 
   
     
       
         
           
             
               
                 
                   k 
                   
                     k 
                     max 
                   
                 
                 = 
                 
                   
                     
                       
                         
                           VC1 
                           I 
                         
                         - 
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       
                         
                           
                             VC1 
                             max 
                           
                           
                             I 
                             max 
                           
                         
                         - 
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
                 ) 
               
             
           
         
       
     
   
   By app lying formula 11 to maximum coupling k max  and by combining formulas 1, 2, and 6 to express coefficient a max   2 , one may write: 
   
     
       
         
           
             
               
                 
                   
                     X2 
                     · 
                     R2 
                     · 
                     C2 
                   
                   L2 
                 
                 = 
                 
                   
                     
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                       - 
                       
                         
                           VC1 
                           max 
                         
                         
                           I 
                           max 
                         
                       
                     
                     
                       Vg 
                       · 
                       
                         ( 
                         
                           
                             1 
                             
                               I 
                               max 
                             
                           
                           - 
                           
                             1 
                             
                               I 
                               
                                 off 
                                 ⁢ 
                                 
                                   - 
                                 
                                 ⁢ 
                                 load 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   15 
                 
                 ) 
               
             
           
         
       
     
   
   Now, for a given transponder family, ratio 
             X2   ·   R2   ·   C2     L2         
is constant. Further, excitation voltage Vg of the terminal&#39;s oscillating circuit is determined. A constant K 2  respecting the following relation can thus be defined:
 
   
     
       
         
           
             
               
                 K2 
                 = 
                 
                   
                     
                       - 
                       Vg 
                     
                     · 
                     
                       
                         X2 
                         · 
                         R2 
                         · 
                         C2 
                       
                       L2 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             VC1 
                             max 
                           
                           
                             I 
                             max 
                           
                         
                         - 
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       
                         
                           1 
                           
                             I 
                             max 
                           
                         
                         - 
                         
                           1 
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   16 
                 
                 ) 
               
             
           
         
       
     
   
   Above formula 15 can apply to any value of current I and of voltage VC 1  (replacing the values at the maximum coupling). Accordingly, having stored, in a learning phase, the off-load voltage and current as well as, for a given transponder family, the same variables at the maximum coupling, the belonging of a transponder present in the field to this family can be deduced from the constant measurements of these variables. It is sufficient to check that the following relation is respected: 
   
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           VC1 
                           I 
                         
                         - 
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       ) 
                     
                     · 
                     
                       ( 
                       
                         
                           1 
                           
                             I 
                             max 
                           
                         
                         - 
                         
                           1 
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       ) 
                     
                   
                   
                     
                       ( 
                       
                         
                           
                             VC1 
                             max 
                           
                           
                             I 
                             max 
                           
                         
                         - 
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       ) 
                     
                     · 
                     
                       ( 
                       
                         
                           1 
                           I 
                         
                         - 
                         
                           1 
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       ) 
                     
                   
                 
                 = 
                 1. 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   17 
                 
                 ) 
               
             
           
         
       
     
   
   If, conversely, the result is different from one, this means that the reader has a card of another type in its field (for example, a card from another manufacturer). 
   In the application of the present invention to the case where several cards are likely to be present at the same time in the field, the above checking is not sufficient. The respective participations of the transponders to the apparent impedance (more specifically, to the apparent resistance) on the terminal side must then be summed up. Accordingly, for n transponders present in the terminal&#39;s field, one may write, by applying formulas 2 and 3: 
   
     
       
         
           
             
               
                 
                   Z1 
                   app 
                 
                 = 
                 
                   R1 
                   + 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         a 
                         i 
                         2 
                       
                       · 
                       
                         
                           
                             L2 
                             i 
                           
                           
                             
                               R2 
                               i 
                             
                             · 
                             
                               C2 
                               i 
                             
                           
                         
                         . 
                       
                     
                   
                 
               
             
             
               
                 (formula  18) 
               
             
           
         
       
     
   
   Considering that the transponders are of the same type, that is, they have the same intrinsic electric characteristics (L 2 , R 2 , and C 2 ), above formula 18 becomes: 
   
     
       
         
           
             
               
                 
                   Z1 
                   app 
                 
                 = 
                 
                   R1 
                   + 
                   
                     
                       L2 
                       
                         R2 
                         · 
                         C2 
                       
                     
                     · 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         n 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           a 
                           i 
                           2 
                         
                         . 
                       
                     
                   
                 
               
             
             
               
                 (formula  19) 
               
             
           
         
       
     
   
   The only term which is then variable is that depending on the coupling between oscillating circuits, and thus on the distance between each transponder and the terminal. 
   For n cards having different coupling coefficients k i , one may write: 
   
     
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     n 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     a 
                     i 
                     2 
                   
                 
                 = 
                 
                   
                     
                       
                         ω 
                         2 
                       
                       · 
                       L1 
                       · 
                       L2 
                     
                     
                       Z2 
                       2 
                     
                   
                   · 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         k 
                         i 
                         2 
                       
                       . 
                     
                   
                 
               
             
             
               
                 (formula  20) 
               
             
           
         
       
     
   
   As a first approximation, it may be considered that, as seen from the terminal, everything occurs as if it saw n transponders having the same coupling coefficient corresponding to an average coupling coefficient. Accordingly, an average coupling coefficient per card can be defined as being given by the following relation: 
   
     
       
         
           
             
               
                 
                   k 
                   av 
                   2 
                 
                 = 
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     n 
                   
                   ⁢ 
                   
                     
                       
                         k 
                         i 
                         2 
                       
                       n 
                     
                     . 
                   
                 
               
             
             
               
                 (formula  21) 
               
             
           
         
       
     
   
   This amounts to defining a coefficient a av  by the following relation: 
   
     
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     n 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     a 
                     i 
                     2 
                   
                 
                 = 
                 
                   n 
                   · 
                   
                     
                       a 
                       av 
                       2 
                     
                     . 
                   
                 
               
             
             
               
                 (formula  22) 
               
             
           
         
       
     
   
   It can be considered that, for n cards or transponders present in the terminal&#39;s field, current I measured by its current transformer depends on the number of cards and on their respective coupling coefficient expressed as a function of an average coupling coefficient as follows: 
   
     
       
         
           
             
               
                 I 
                 = 
                 
                   
                     Vg 
                     
                       R1 
                       + 
                       
                         n 
                         · 
                         
                           k 
                           av 
                           2 
                         
                         · 
                         
                           
                             
                               ω 
                               2 
                             
                             · 
                             L1 
                             · 
                             
                               L2 
                               2 
                             
                           
                           
                             
                               Z2 
                               2 
                             
                             · 
                             R2 
                             · 
                             C2 
                           
                         
                       
                     
                   
                   . 
                 
               
             
             
               
                 (formula  23) 
               
             
           
         
       
     
   
   Intuitively, it can be seen that if all transponders have the same coupling coefficient with the terminal, that is, if they all are at the same distance, current I decreases with an increase of the number of transponders present in the field. Similarly, for a same current measured in the terminal&#39;s oscillating circuit, a decrease in the coupling coefficient of each transponder implies an increase in the number of transponders in the field. In other words, the product of the number of transponders by the square of the average coupling coefficient per transponder can be considered as being constant. 
   Considering that, in above formula 23, only values Vg, n, and kav are likely to vary for a given terminal and a given family of transponders, and writing this relation, at maximum coupling, for one card and for n cards, the following can be deduced: 
   
     
       
         
           
             
               
                 
                   
                     I 
                     
                       max 
                       
                         ( 
                         n 
                         ) 
                       
                     
                   
                   = 
                   
                     Vg 
                     
                       R1 
                       + 
                       
                         n 
                         · 
                         
                           
                             ( 
                             
                               
                                 k 
                                 av 
                               
                               
                                 k 
                                 max 
                               
                             
                             ) 
                           
                           2 
                         
                         · 
                         
                           ( 
                           
                             Vg 
                             
                               
                                 I 
                                 
                                   max 
                                   
                                     ( 
                                     1 
                                     ) 
                                   
                                 
                               
                               - 
                               R1 
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 , 
               
             
             
               
                 (formula  24) 
               
             
           
         
       
     
   
   with I max (I) and I max (n) representing the currents at the maximum coupling respectively for 1 and n cards. 
   By combining formulas 23 and 24, the following relation is obtained: 
   
     
       
         
           
             
               
                 
                   I 
                   
                     max 
                     
                       ( 
                       n 
                       ) 
                     
                   
                 
                 = 
                 
                   
                     1 
                     
                       
                         n 
                         
                           I 
                           
                             max 
                             
                               ( 
                               1 
                               ) 
                             
                           
                         
                       
                       + 
                       
                         
                           1 
                           - 
                           n 
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                     
                   
                   . 
                 
               
             
             
               
                 (formula  25) 
               
             
           
         
       
     
   
   Now, as previously indicated, the off-load and maximum coupling currents for a card can be measured in a learning phase of the reader (using a sample card for the current at maximum coupling I max(1) ). Accordingly, the reader is able to calculate the different values of the current at maximum coupling for 2, 3, 4, etc. cards, the maximum number of calculated values being linked to the application and to the maximum number of cards estimated, in absolute, to be likely to be found in the reader&#39;s field. 
     FIG. 4  shows a flowchart of an embodiment of a learning and preparation phase according to the present invention. 
   At the beginning (block  40 , ST), the reader is turned on and it is configured by means of its internal computer system for a learning phase. 
   In a first step (block  41 ), the off-load voltage and current VC 1   off-load  and I off-load  are measured and stored. These measurements are performed while no card is present in the terminal&#39;s field. 
   In a second step, voltage VC 1  and current I at maximum coupling are measured for all the types of cards that the terminal is desired to recognize. In  FIG. 4 , this is illustrated by the setting of an index j (block  42 ), followed by the measurements of the variables (block  43 ), by using a sample card which is placed at a minimum distance from the terminal, ideally at a null distance. Current I(j) max  is for example measured by an intensity transformer ( 35 ,  FIG. 3 ) while voltage VC 1 (j) max  is measured, for example, across capacitor  31 . Based on the variables measured at blocks  41  and  43 , the computer means of the reader then calculate (block  44 ) the constant K 2 (j) of the considered card family (or any other appropriate value, linked to constant K 2 ) and this characteristic value of the family is stored (block  45 ). As long as there remain types of cards to be recorded (test  46 ), the counter is incremented (block  47 ) and steps  43  to  45  are repeated. 
   When all the transponder types have been recorded, the learning phase is over (block  48 , E) and the reader is able to determine, each time a transponder appears in its field, whether it belongs to an identified type. 
   Of course, the order of the measurements between the off-load operation and the maximum coupling is arbitrary. The off-load measurements (which are independent from the family or type of cards meant to operate with the terminal) may even be performed independently from the maximum coupling measurements (which can be renewed, for example, to change the type of cards intended for operating with the terminal). Further, any other magnitude representative of ratio K 2  may be stored instead of it (for example, value K 2 /Vg). 
     FIG. 5  shows a simplified flowchart of an embodiment of the method of card type identification according to the present invention. 
   According to this embodiment, after the end (block  48 ,  FIG. 4 ) of the learning phase, the reader switches to a conventional stand-by operating mode where it transmits periodic requests to monitor the occurrence of a transponder in its field. 
   As soon as the reader has detected the possible presence of a transponder, it starts a determination phase such as illustrated in  FIG. 5 . After a step (block  50 , ST) of program initialization, the current values of current I and of voltage VC 1  are measured (block  51 ). Then, the characteristic constant K 2  of the current transponder is calculated (block  52 ). It is then searched (block  53 ), in the set of values stored in the learning phase, whether one of the constants corresponds to the current value. 
   If such is the case, the recognition of the current card is validated (block  54 ). This validation may come along with various actions. For example, the reader may be adapted to the card type (block  55 ), as will be seen hereafter, or the validation may be considered as a first reader access control. Afterwards, a conventional anti-collision method is applied (block  57 , illustrating the initialization INIT of a transmission). 
   If a transponder is not recognized, there again, several actions (block  56 ) may be taken according to the purpose that is desired to be given to the recognition. For example, the exchanges may be stopped by blocking the request procedure (block  57 ), then considering that the transponder is a pirate transponder. In the case where the present invention is only implemented to adapt the reader to the transponder type, no specific adaptation is provided. Afterwards, the conventional anti-collision method is applied (block  57 ). 
   An advantage of the present invention is that it enables a first access control to a reader which is not based on a demodulation of a data message but on the intrinsic electric characteristics of the transponder. Since the signature used is of “electric” type, it is much more difficult to falsify (capacitance C 2  and equivalent resistance R 2  originate from elements integrated in a chip). Further, considering that the intrinsic differences of transponders according to their type are not sufficient, it can easily be provided, upon manufacturing of the transponders, to slightly modify their components per type to give them different signatures. 
   Another advantage of the present invention is that this access control can be implemented before the beginning of a communication and anti-collision process initialization. In practice, the measurements and calculations necessary to the present invention are very fast with the computer means provided in the reader. 
   Another advantage of the present invention is that it enables, if necessary, adapting the reader to the transponder with which it must communicate before implementing the transmission initialization process. For example, a transponder type may be dedicated to a coupling type (remote or close) with a reader. If the reader can accept several operating modes according to the coupling relation with the transponder, the reader and, more specifically, its oscillating circuit, can be adapted to the dedicated operating mode of the transponder. 
   Indeed, it can be deduced from the learning phase whether the transponder type is dedicated to an operation in extreme closeness or in remote coupling. For example, how to structurally dedicate a transponder to an operation respectively in extreme closeness and in remote coupling is described in European patent applications no. 00410077.2 and 00419078.0 of the applicant which are incorporated herein by reference. This translates in the position of optimal coupling point k opt  of the transponder with respect to maximum coupling k max , where optimal coupling k opt  is defined as being the coupling position where the voltage recovered by the transponder is maximum. It is known that if k opt &lt;k max , the transponder is dedicated to an operation in extreme closeness. If k opt &lt;k max , the transponder is dedicated to an operation in remote coupling. Now, the ratio between these two coupling coefficients depends on current I off-load and at maximum coupling. Indeed, as a first approximation, one may write, by combining formulas 1, 2, and 3 in which X 2  is, at first order, close to zero: 
   
     
       
         
           
             
               
                 
                   
                     k 
                     opt 
                   
                   
                     k 
                     max 
                   
                 
                 = 
                 
                   
                     1 
                     
                       
                         
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             max 
                           
                         
                         - 
                         1 
                       
                     
                   
                   ⁢ 
                   
                       
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   26 
                 
                 ) 
               
             
           
         
       
     
   
   Accordingly, based on the learning phase, it is known to determine whether the transponder type is dedicated to an operation in extreme closeness or in remote coupling. 
   On the other hand, the position of capacitor C 2  of the transponder with respect to the value of this capacitor at the tuning C 2 tun, that is, when the transponder&#39;s oscillating circuit is tuned on the frequency of the remote supply carrier, can be deduced from ratio K 2 /Vg. 
   Indeed, for a card present in the field of a reader, one may write: 
   
     
       
         
           
             
               
                 
                   
                     X2 
                     · 
                     R2 
                     · 
                     C2 
                   
                   L2 
                 
                 = 
                 
                   R2 
                   · 
                   
                     
                       ( 
                       
                         
                           ω 
                           · 
                           C2 
                         
                         - 
                         
                           1 
                           
                             ω 
                             · 
                             L2 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                         
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   27 
                 
                 ) 
               
             
           
         
       
     
   
   Based on this expression and on formula 16, one may write: 
   
     
       
         
           
             
               
                 C2 
                 = 
                 
                   
                     1 
                     
                       
                         ω 
                         2 
                       
                       · 
                       L2 
                     
                   
                   + 
                   
                     
                       
                         
                           
                             VC1 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                         - 
                         
                           
                             VC1 
                             max 
                           
                           
                             I 
                             max 
                           
                         
                       
                       
                         
                           R2 
                           · 
                           Vg 
                           · 
                           
                             ω 
                             2 
                           
                           · 
                           
                             ( 
                             
                               
                                 1 
                                 
                                   I 
                                   max 
                                 
                               
                               - 
                               
                                 1 
                                 
                                   I 
                                   
                                     off 
                                     ⁢ 
                                     
                                       - 
                                     
                                     ⁢ 
                                     load 
                                   
                                 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         
                             
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   28 
                 
                 ) 
               
             
           
         
       
     
   
   Now, when the transponder is tuned on the remote supply carrier frequency, there is: 
   
     
       
         
           
             
               
                 
                   
                     ω 
                     2 
                   
                   · 
                   
                     C2 
                     tun 
                   
                 
                 = 
                 
                   
                     1 
                     
                       
                         ω 
                         2 
                       
                       · 
                       L2 
                     
                   
                   ⁢ 
                   
                       
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   29 
                 
                 ) 
               
             
           
         
       
     
   
   Accordingly, term 
           term   ⁢           VC1     off   ⁢     -     ⁢   load         I     off   ⁢     -     ⁢   load         -       VC1   max       I   max           R2   ·   Vg   ·   ω   ·     (       1     I   max       -     1     I     off   ⁢     -     ⁢   load           )               
translates the transponder&#39;s detuning.
 
   The following may thus be written: 
   
     
       
         
           
             
               
                 
                   C2 
                   = 
                   
                     
                       C2 
                       tun 
                     
                     - 
                     
                       
                         K2 
                         
                           R2 
                           · 
                           Vg 
                           · 
                           ω 
                         
                       
                       ⁢ 
                       
                         
                             
                         
                         , 
                       
                     
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   30 
                 
                 ) 
               
             
           
         
       
     
   
   where term 
           K2     Vg   ·   ω           
gives an image proportional to the transponder&#39;s detuning. The above may of course be translated in terms of quality factor Q 2  of the transponder. In this case, ratio K 2 /Vg gives an image proportional to the quality factor variation.
 
   Afterwards, by measuring the current ratio K 2 /Vg of a transponder in the reader&#39;s field, an information about the value of the transponder&#39;s capacitance as compared to its theoretical tuning capacitance corresponding to the ratio K 2 /Vg stored in the learning phase is obtained. 
   In the case of a transponder dedicated to an operation in extreme closeness and the quality factor of which is smaller than the quality factor at the tuning, the quality factor of the reader&#39;s oscillating circuit can be decreased (for example, by increasing the values of C 1  and/or R 1 ). The transmission rate is thus increased. Further, by increasing the value of resistance R 1 , the transmission power of the terminal, and thus its remote supply range, is reduced, which enhances an operation in extreme closeness. 
   If, still in the case of a transponder dedicated to extreme closeness, quality factor Q 2  is greater than the quality factor at the tuning, the value of R 1  may be increased to reduce the transmission power and to decrease the remote supply range. 
   In the case of a transponder dedicated to a remote coupling operation, the terminal can adapt its values of C 1  and R 1  according to whether the transponder has a relatively low or high transmission flowrate. 
   More generally, advantage can be taken from the learning phase and from the use of a reference transponder to have said transponder transmit to the terminal information relative to its operation (for example, operation in extreme closeness or in remote coupling, back modulation flowrate, minimum voltage level, memory size and content type, identification type, etc.). All these characteristics are then associated to the type of transponders stored in the terminal. For example, this avoids transmitting this information at the beginning of each communication between a transponder and the terminal. 
   Generally, all the actions taken by the terminal can be adapted according to the information previously gathered by the terminal in the learning phase with a reference transponder of the concerned family. The information extracted from the “electric” identification can in particular be combined with “software” information transmitted by the transponder to the terminal. 
   In a preferred application of the present invention, the number of transponders present in the terminal&#39;s field is also evaluated. For this purpose, additional calculations are performed in the learning phase. However, as will appear from the following description, the measurements taken into account are the same as those performed to determine the type of transponders. 
     FIG. 6  illustrates an example of a network of curves of current I in the oscillating circuit according to distance d or, more specifically, to the average coupling coefficient per card in the reader&#39;s field. The average coupling scale decreases towards the right while the distance scale increases towards the right. 
   As illustrated by the curve network of  FIG. 6 , the maximum current at maximum coupling kmax, that is, at a null distance, decreases as the number of cards in the terminal&#39;s field increases. Further, all the curves have substantially the same shape and join the horizontal shape of the off-load current as the distance increases (or the decrease of the average coupling coefficient per card). It should be noted that the curves do not cross one another. 
   Accordingly, by measuring, upon operation of the reader, the current in its oscillating circuit, the minimum number of cards in the field can be determined by comparing this measured current to the different values calculated during the learning phase. It should be noted that, as compared to the curves of  FIG. 6 , the calculations of the learning phase do not lead to really plotting the curve network, but merely to calculating the different possible maximum currents. 
   To calculate the minimum number of curves, an additional step (not shown) is provided in the learning phase ( FIG. 4 ), in which a set of current values corresponding to the maximum coupling for several cards is calculated. These values of I max (n&gt;1) correspond to those illustrated in  FIG. 6 , the maximum number of calculated values depending on the application. 
   The reader is then able to determine, each time one or several transponders of the same type appear in its field, the minimum number of transponders. This number will enable adapting the request procedures and, more specifically, adapting the number of anti-collision steps to be implemented upon initialization of a transmission. 
   Indeed, the test corresponding to above formula 17 can be generalized to n cards of the same type. In this case, the measured values VC 1  and I 1  correspond to the contribution of the n cards and the test to be performed is the following: 
   
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         VC1 
                         I 
                       
                       - 
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                     
                     ) 
                   
                   
                     ( 
                     
                       
                         1 
                         I 
                       
                       - 
                       
                         1 
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                     
                     ) 
                   
                 
                 = 
                 
                   K2 
                   ⁢ 
                   
                       
                   
                   . 
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   31 
                 
                 ) 
               
             
           
         
       
     
   
   This test is performed instead of test  53  of  FIG. 5 . If the result is incorrect, this means that several cards of different types are present. Their respective types then cannot be determined based on the mere current and voltage measurements. In this case, the conventional software request, initialization and anti-collision method must then be used ( FIG. 2 ). 
   If the result is correct, this means that one or several cards of the same type are present. A method of evaluation of the number of cards according to the preferred embodiment of the present invention can then be implemented, to find not only the type, but also the minimum and maximum numbers of cards present in the reader&#39;s field. 
   Validation step  54  ( FIG. 5 ) then corresponds to a determination, based on the values previously calculated in the learning phase, of minimum number nmin of transponders present in the terminal&#39;s field. 
   For example, if the measured current I is included between off-load current I off-load  and the maximum current for one card I(j) max(1)  (corresponding to measurement I(j) max  of the learning phase,  FIG. 4 ), two cases are possible. Either there is a single card in the reader&#39;s field and this card has a given coupling k (smaller than k max ). Or there are n cards in the reader&#39;s field, which all individually have couplings k smaller than coupling k of the first case. 
   If the measured current is between two maximum current values for n and n+1 cards, it is certain that there are at least n cards in the reader&#39;s field. There may however be more than n+1 cards if the average coupling per card is smaller than in the case where only n cards are present. 
   A number n R  of request cycles can then be chosen in the anti-collision method, which is a function of this minimum number. Based on this number, a conventional anti-collision process is then applied. 
   A first advantage that already appears with the simplified embodiment such as described hereabove is that by knowing the minimum number of cards, the number of anti-collision requests can already be adjusted and time is already saved with respect to the conventional method. 
     FIG. 7  shows a simplified flowchart of a preferred embodiment of the present invention in which the determination of the number of request phases of the transmission initialization method is refined. The flowchart of  FIG. 7  overlaps that of  FIG. 5 , for example instead of validation step  54 . 
   The minimum number n min  is first determined (block  72 ) as in the simplified method. 
   Then, a set of values VC 1 (th,n min ) corresponding to a set of theoretical values of the voltage across capacitor C 1  (element  31 ,  FIG. 3 ) are calculated (block  73 ) for each of the minimum numbers of cards likely to be present in the terminal&#39;s field. 
   The calculation of these theoretical values is performed as follows. 
   Due to the phase regulation loop provided on the reader side, for each previously-determined minimum number of cards, the theoretical voltage across the reader&#39;s capacitive element can be calculated, then the maximum number of cards present in the field can be deduced therefrom. 
   Indeed, formula 16 previously established for one card remains valid for any value of current I, of voltage VC 1 , and of the coupling. Accordingly, the current voltage VC 1  can be expressed as: 
   
     
       
         
           
             
               
                 VC1 
                 = 
                 
                   
                     ( 
                     
                       
                         K2 
                         · 
                         
                           ( 
                           
                             
                               1 
                               I 
                             
                             - 
                             
                               1 
                               
                                 I 
                                 
                                   off 
                                   ⁢ 
                                   
                                     - 
                                   
                                   ⁢ 
                                   load 
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                     
                     ) 
                   
                   · 
                   
                     I 
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   32 
                 
                 ) 
               
             
           
         
       
     
   
   Constant K 2  has been calculated and memorized in the learning phase based on the measured values (blocks  41  and  43 ,  FIG. 4 ). 
   Equation 32 hereabove remains valid for several cards present in the terminal&#39;s field. Accordingly, based on the current measurement of the current (block  51 ,  FIG. 5 ) and on the determination of the minimum number of cards n min  (block  72 ), a theoretical value VC 1 (th,n min ) of voltage VC 1  across capacitive element  31  can be deduced for nmin cards, by applying the following relation that is deduced from formula 32: 
   
     
       
         
           
             
               
                 
                   VC1 
                   
                     ( 
                     
                       th 
                       , 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         min 
                       
                     
                     ) 
                   
                 
                 = 
                 
                   
                     ( 
                     
                       
                         K2 
                         · 
                         
                           ( 
                           
                             
                               1 
                               I 
                             
                             - 
                             
                               1 
                               
                                 I 
                                 
                                   off 
                                   ⁢ 
                                   
                                     - 
                                   
                                   ⁢ 
                                   load 
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           VC1 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                         
                           I 
                           
                             off 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             load 
                           
                         
                       
                     
                     ) 
                   
                   · 
                   
                     I 
                     . 
                   
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   33 
                 
                 ) 
               
             
           
         
       
     
   
   It is then possible to compare (block  74 ) the current value of the voltage measured across element  31  to this theoretical value calculated for the previously-determined minimum area. 
   If the measured voltage is smaller than or equal to the theoretical value calculated for number n min , this means that the number of cards present in the field is equal to the minimum number. In this case, number n R  of request sequences of the anti-collision process is chosen (block  75 ) based on this number n min , which is known to now correspond to the exact number of transponders. 
   If the measured value is greater than the theoretical value, this means that there are more than n min  cards in the terminal&#39;s field. 
   It is then proceeded to another calculation phase including determining, based on the voltage measurement, the maximum number of cards present in the field. For this purpose, the calculation of the voltage VC 1  that should be obtained to correspond to the current I measured for an increasing number nmin+i is iteratively performed. Indeed, since the minimum number of cards has been determined based on the current measurement, and since it is known that the real number of cards does not correspond to this minimum number, the number of cards in the terminal&#39;s field is greater than the minimum number but the average coupling coefficient of these cards is smaller than the maximum coupling coefficient. In the opposite case, the measured current would have provided a greater minimum number of cards. 
   As illustrated by the example of embodiment of  FIG. 7 , a counter representing the additional increment i with respect to number n min  of cards is initialized (block  76 ). By having set variable i to 1, a first voltage value VC 1  is calculated for the measured current I and the minimum number of cards +1. The relation applied for the calculation of this value (block  77 ) is obtained in the following way which will be better understood in relation with  FIG. 8 . 
     FIG. 8  shows an example of shapes of current I in the oscillating circuit according to distance (or to coupling coefficient k) associated with a scale of values of voltage VC 1  across capacitive element  31 . 
   As illustrated by this drawing, the calculations (block  44 ,  FIG. 4 ) performed during the learning phase enable determining points of origin of curves  81 ,  82 ,  83 , and  84  that are all located at the maximum coupling coefficient kmax. Each curve corresponds to an increasing number of cards for a decreasing current. The measurement (block  41 ) of the off-load current defines the asymptote of the different curves. The plotting of the currents of  FIG. 8  is to be compared with the plotting of  FIG. 6 . The determination of the different points on the current axis, at the position of the maximum coupling coefficients (or null distance) enables determining areas (horizontal in the representation of the drawing) where the minimum number of cards is different from one area to the other. The measurement of voltage VC 1  enables, afterwards, determining the maximum number of cards. 
   The calculation (block  73 ) of the theoretical voltage value, for the minimum number of cards obtained at value I of the measured current, determines an intersection point which, in the example shown in  FIG. 8 , is on curve  82  representing the shape of two cards. 
   The calculation performed at block  77  of  FIG. 7  corresponds to determining the points of intersection of the line of measured current I with curves  83  and  84  corresponding to greater numbers of cards. 
   After having calculated a first value VC 1 (I, 3 ) for a unity increment i, the obtained value is compared with the measured value VC 1  (block  78 ). As long as measured value VC 1  is not greater than the calculated value, increment i is increased (block  79 ) and the calculation is resumed for a greater number of cards (value VC 1 (I, 4 )). In the example of  FIG. 8 , it is assumed that the measured value VC 1  is included between the values for three and four cards. Taking the measured current into account, this means that the number of transponders is at most four, and thus included between 2 and 4. It is then possible to set (block  80 ) the number of requests of the anti-collision process to this maximum number (or to the maximum number +1 for reasons of determination security). 
   Then, whether the number of requests has been determined by block  75  or by block  80 , a conventional request process is performed, taking this number into account (block  57 ). 
   It should be noted that, although the representation of  FIG. 8  illustrates complete theoretical curves, the calculations necessary to the implementation of the present invention only concern points of these curves that, in practice, need not be precisely determined. 
   The calculation of value VC 1 (I,n min +i) is performed based on the following formula: 
   
     
       
         
           
             
               
                 
                   
                     VC1 
                     
                       ( 
                       
                         
                           I 
                           , 
                           
                               
                           
                           ⁢ 
                           
                             n 
                             min 
                           
                         
                         + 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               
                                 n 
                                 min 
                               
                               + 
                               i 
                             
                             
                               n 
                               min 
                             
                           
                           · 
                           
                             ( 
                             
                               
                                 
                                   VC1 
                                   
                                     ( 
                                     
                                       th 
                                       , 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         n 
                                         min 
                                       
                                     
                                     ) 
                                   
                                 
                                 I 
                               
                               - 
                               
                                 
                                   VC1 
                                   
                                     off 
                                     ⁢ 
                                     
                                       - 
                                     
                                     ⁢ 
                                     load 
                                   
                                 
                                 
                                   I 
                                   
                                     off 
                                     ⁢ 
                                     
                                       - 
                                     
                                     ⁢ 
                                     load 
                                   
                                 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           
                             VC1 
                             
                               offl 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                           
                             I 
                             
                               off 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               load 
                             
                           
                         
                       
                       ) 
                     
                     · 
                     I 
                   
                 
                 , 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   34 
                 
                 ) 
               
             
           
         
       
     
   
   which is deduced from the application of formula 14 to an average coupling coefficient per card in the minimum number area, considering that, for a given measured current, the average coupling coefficient is given by the following relation: 
   
     
       
         
           
             
               
                 
                   
                     
                       k 
                       2 
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           I 
                           , 
                           
                               
                           
                           ⁢ 
                           n 
                         
                         + 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             n 
                             min 
                           
                           + 
                           i 
                         
                         
                           n 
                           min 
                         
                       
                       · 
                       
                         
                           k 
                           2 
                         
                         ⁡ 
                         
                           ( 
                           
                             I 
                             , 
                             
                                 
                             
                             ⁢ 
                             th 
                             , 
                             
                                 
                             
                             ⁢ 
                             
                               n 
                               min 
                             
                           
                           ) 
                         
                       
                     
                     , 
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 ( 
                 
                   formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   35 
                 
                 ) 
               
             
           
         
       
     
   
   where k(I,th,n min ) designates the average coupling coefficient per card corresponding to value nmin. 
   An advantage of the present invention is that it is now possible to determine at least the minimum number of transponders present in the field. 
   In the preferred embodiment illustrated hereabove, even the exact number or, at least a maximum number of transponders present in the field is determined. Knowing these numbers enables adapting the initialization algorithms of a communication when at least one transponder acknowledges an interrogation request transmitted by a terminal. 
   The fact of knowing a priori the number of cards in the reader&#39;s field enables evaluating the optimal number of request phases. The exchange time dedicated to the anti-collision protocol that must enable either selecting a card from among several cards presented at the same time in front of the reader, or identifying the cards in the reader&#39;s field, or allowing a selection sequencing of the different cards with which the reader must communicate, can thus be optimally reduced. 
   The adaptation of the expectable number of cards in the reader&#39;s field is performed, according to the present invention, as soon as a transponder acknowledges a request from the terminal. Initially, it can be provided to arbitrarily or conventionally set this number if a predetermined number is required to implement the request process. 
   As soon as the reader detects the presence of a transponder, it performs the procedure of determination of the number of transponders by using the data calculated during the learning. For the rest, a conventional request, anti-collision, and initialization process can be used. 
   Number n R  of requests does not necessarily correspond to the maximum or exact number determined by the present invention, but is a function of this number (for example, product or quotient by a predetermined coefficient, sum or subtraction of a predetermined number). 
   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, the practical implementation of the present invention based on the functional indications given hereabove is within the abilities of those skilled in the art. It should only be noted that they will generally resort to usual techniques of programming of digital processing circuits present in transponder read/write terminals. Thus, since the present invention basically implements calculation processes, it may be used with minor modifications of a conventional terminal having current and voltage information in the terminal&#39;s oscillating circuit. 
   Among the applications of the present invention are contactless chip cards (for example, identification cards for access control, electronic purse cards, cards for storing information about the card holder, consumer fidelity cards, toll television cards, etc.) and read or read/write systems for these cards (for example, access control terminals or porticoes, automatic dispensers, computer terminals, telephone terminals, televisions or satellite decoders, etc.). 
   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.