Abstract:
A method and a system of data transmission between a terminal for generating an electromagnetic field and a transponder, the terminal and the transponder each including an oscillating circuit forming an antenna, and the transponder including an electronic circuit adapted to absorbing and giving back power provided by the terminal field, the oscillating circuits of the transponder and of the terminal being capable of transmitting radio-electric signals of determined frequency, this method including causing a detuning of at least one of the oscillating circuits with respect to the determined frequency when the transponder and the terminal are very close to each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read only transponders, that is, adapted to operating with a terminal only reading the transponder data, or read/write transponders, which contain data that can be modified by the terminal. 
     2. Discussion of the Related Art 
     Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption. 
     FIG. 1 very schematically shows, in a functional way, a conventional example of a data exchange system between a read/write unit  1  (STA) and a transponder  10  (CAR). 
     Generally, unit  1  is essentially formed of an oscillating circuit formed of an inductance L 1  in series with a capacitor C 1  and a resistor R 1 , between an output terminal  2 p of an amplifier or antenna coupler  3  (DRIV) and a terminal  2 m at a reference potential (generally, the ground). Amplifier  3  receives a high-frequency transmission signal Tx, provided by a modulator  4  (MOD). The modulator receives a reference frequency, for example, from a quartz oscillator  5  and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal  1  to transponder  10 , signal Tx is used only as a power source to activate the transponder if said transponder enters the field. The data to be transmitted come from an electronic system, generally digital, for example, a microprocessor  6  (μP). 
     The connection node of capacitor C 1  and inductance L 1  forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx, received from a transponder  10  and intended for a demodulator  7  (DEM). An output of the demodulator communicates (if necessary via a decoder (DEC)  8 ) the data received from transponder  10  to microprocessor  6  of read/write terminal  1 . Demodulator  7  receives, generally from oscillator  5 , a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C 1  and resistor R 1  and not across inductance L 1 . Microprocessor  6  communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the read/write terminal draw the power necessary for their operation from a supply circuit  9  (ALIM), connected, for example, to the electric supply system. 
     On the side of transponder  10 , an inductance L 2 , in parallel with a capacitor C 2 , forms a parallel oscillating circuit (called a reception resonant circuit) intended for capturing the field generated by series oscillating circuit L 1 C 1  of terminal  1 . The resonant circuit (L 2 , C 2 ) of transponder  10  is tuned on the frequency of the oscillating circuit (L 1 , C 1 ) of terminal  1 . 
     Terminals  11 ,  12  of resonant circuit L 2 C 2 , which correspond to the terminals of capacitor C 2 , are connected to two A.C. input terminals of a rectifying bridge  13  formed, for example, of four diodes D 1 , D 2 , D 3 , D 4 . In the representation of FIG. 1, the anode of diode D 1  and the cathode of diode D 3  are connected to terminal I 1 . The anode of diode D 2  and the cathode of diode D 4  are connected to terminal  12 . The cathodes of diodes D 1  and D 2  form a positive rectified output terminal  14 . The anodes of diodes D 3  and D 4  form a reference terminal  15  of the rectified voltage. A capacitor Ca is connected to rectified output terminals  14 ,  15  of bridge  13  to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly. 
     When transponder  10  is in the field of terminal  1 , a high frequency voltage is generated across resonant circuit L 2 C 2 . This voltage, rectified by bridge  13  and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator  16  (REG). These circuits generally include, essentially, a microprocessor (μP)  17  (associated with a memory not shown), a demodulator  18  (DEM) of the signals that may be received from terminal  1 , and a modulator  19  (MOD) to transmit information to terminal  1 . The transponder is generally synchronized by means of a clock (CLK) extracted, by a block  20 , from the high-frequency signal recovered across capacitor C 2  before rectification. Most often, all the electronic circuits of transponder  10  are integrated in a same chip. 
     To transmit the data from transponder  10  to unit  1 , modulator  19  controls a stage of modulation (back modulation) of resonant circuit L 2 C 2 . This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals  14  and  15 . Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal  1  (for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits  16 ,  17 ,  18 ,  19  and  20 , so that the transponder draws a greater amount of power from the high frequency field. On the side of terminal  1 , amplifier  3  maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current in antenna L 1 . This variation is detected by demodulator  7  of terminal  1 , which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the half-periods of the sub-carrier where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator  7  (generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder  8  or directly by microprocessor  6 ) to restore the binary data. 
     It should be noted that the terminal does not transmit data while it receives some from a transponder, the data transmission occurring alternately in one direction, then in the other (half-duplex). 
     FIG. 2 illustrates a conventional example of data transmission from terminal  1  to a transponder  10 . This drawing shows an example of shape of the excitation signal of antenna L 1  for a transmission of a code 0101. The modulation currently used is an amplitude modulation with a rate of 106 kbits/s (a bit is transmitted in approximately 9.5 μs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator  5  (period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes (a, b) between the two states (0 and 1), divided by the sum of these amplitudes) smaller than one due to the need for supply of transponder  10 . In the example of FIG. 2, the carrier at 13.56 MHz is modulated in amplitude, with a 106-kbit/s rate, with a modulation rate tm of, for example, 10%. 
     FIG. 3 illustrates a conventional example of a data transmission from transponder  10  to terminal  1 . This drawing illustrates an example of the shape of control signal V T  of transistor T, provided by modulator  19 , and of the corresponding signal Rx received by terminal  1 . On the transponder side, the back modulation is generally of resistive type with a carrier (called a sub-carrier) of, for example, 847.5 kHz (period of approximately 1.18 μs). The back modulation is, for example, based on a BPSK-type (binary phase-shift keying) coding at a rate on the order of 106 kbits/s, much smaller than the sub-carrier frequency. In FIG. 3, signal Rx has been shown as “smoothed”, that is, without showing the high frequency carrier (for example, at 13.56 MHz) ripple. In the example of FIG. 3, it has been assumed that each of the three shown bits is different from the preceding bit. Thus, it is, for example, a transmission of a code 010. 
     It should be noted that, whatever the type of modulation or back modulation used (for example, in amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), the modulation or back modulation is performed digitally, by jumping between two binary levels. 
     The oscillating circuits of the terminal and of the transponder are generally tuned on the carrier frequency, that is, their resonance frequency is set on the 13.56-MHz frequency. This tuning aims at maximizing the power diffusion to the transponder, which generally is a card of credit card size integrating the different transponder components. 
     As illustrated in FIG. 3, signal V T  is formed of a pulse train at the sub-carrier frequency (for example, 847.5 kHz), a phase shift occurring upon each state change from a bit to the next bit. As concerns the signal recovered on the reader side, it appears not to have a “digital” form, which can make its decoding difficult. Indeed, the shape of signal Rx has, for each bit transmission time (9.4 μs), a non-linear increase beginning (capacitor charge), to reach a maximum approximately at two thirds of the duration of a bit, then an also non-linear decrease. The enable time, that is, the time taken by signal Rx to reach a level decodable by the demodulator, is linked to the oscillating circuits being tuned. The need for power transfer for the remote supply, associated with the desired system range, requires a high quality factor, and thus that the oscillating circuits be tuned. Now, a high quality factor results in a small passband. This results in a limited data flow for the system. Generally, the quality factors are on the order of 10 for the reader and for the transponder. 
     The transponder may be formed by various objects (key ring, keys, etc.), but now most often is in the form of a credit card integrating all the circuits and the antenna or inductance L 2 . For an information exchange with a reader or a terminal, the card is brought closer to antenna L 1  of the reader. The distance between the reader and the card varies and, in some applications, a very close or tight coupling transmission is used, the antennas being distant from each other by less than two centimeters. Such a tight coupling transmission may be used, for example, to enable a payment by means of a transponder, and thus guarantee that only the transponder that is closest to the terminal is recognized by said terminal. 
     A problem that is raised when the oscillating circuits are very close to each other is that, if they are substantially tuned, the power transmitted from the terminal to the transponder is such that the transponder heats up (antenna L 2  is generally formed of one or several planar spirals at the card periphery). This thermal effect results in deforming the plastic card. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing a novel solution that overcomes the disadvantages of conventional solutions when a transponder is in very tight coupling relation with a read/write terminal. 
     The present invention aims, in particular, at reducing or minimizing the thermal effect linked to the remote supply of the transponder by the read/write terminal. 
     The present invention also aims at enabling an increase of the data transmission rate when the transponder is very close to the terminal. 
     The present invention also aims at providing a solution that may be implemented on the terminal side and/or on the transponder side. 
     A feature of the present invention is to detune the oscillating circuits of the terminal and/of the transponder when they are in very close or tight coupling relation. 
     A frequency detuning of an electromagnetic transponder is known from document WO-A-98/29760. This document provides for the antenna of a transponder to be “detuned in frequency or mismatched in impedance, so that the transponder and its electronic circuit absorb less radio field and power. Thus, another transponder located in the vicinity of the mismatched or detuned transponder can receive enough of the radio field to operate properly. The transmission system can then detect or consult this other transponder as if it were alone in the field” of the transmitter. Still according to this document, the mismatch means are used “when the transponder is in an unselected state to limit the power and/or field absorption by the transponder in the unselected state”. 
     The solution advocated by this document amounts to detuning the transponders that are relatively remote from the terminal to maximize the power received by the closest transponder meant to communicate with the terminal. Such a solution does not solve the above-mentioned tight coupling problems. Indeed, the transponder that remains tuned is the selected transponder. 
     Conversely to this document, the present invention provides a detuned operation in tight coupling. Thus, a feature of the present invention is to provide, for a tight coupling information transmission, a detuned operation of the oscillating circuits of a read (or read/write) terminal and of an electromagnetic transponder remotely supplied by this terminal. 
     The present invention takes account of the fact that the remote supply power recovered on the transponder side is not a monotonic function of the distance that separates the transponder from the terminal. 
     Indeed, when the oscillating circuits are tuned on the remote supply carrier frequency, if the transponder comes close to a terminal, the remote supply amplitude starts increasing from the system range limit (on the order of some ten centimeters). This amplitude transits through a maximum (critical coupling position) then starts decreasing again when the transponder becomes very close (approximately less than 2 centimeters). For this reason, in particular, it is not provided in conventional systems to make the power of the terminal dependent from the distance at which the transponder is. 
     The critical coupling position corresponds to the distance at which the coupling between the transponder and the terminal is optimized by a maximum remote supply amplitude received by the transponder when the oscillating circuits of the terminal and of the transponder are both tuned on the remote supply carrier frequency. In other words, the critical coupling frequency corresponds to the distance at which the remote supply power is maximum for a minimum coupling factor, the coupling factor being the ratio of the mutual inductance on the square root of the product of the inductances of the oscillating circuits. 
     When at least one of the oscillating circuits is detuned from the remote supply carrier frequency, the power received by the transponder increases as the distance from the terminal decreases, but with a reduced range. In this case, there also is a distance at which the received power is maximum for a given detuning condition. This is an optimal coupling, the critical coupling position being the optimal coupling condition when the two oscillating circuits are tuned on the carrier frequency. It should be noted that the optimal coupling coefficient between the two oscillating circuits depends not only on inductances L 1  and L 2 , on capacitors C 1  and C 2 , and on the frequency (which here is a fixed frequency and corresponds to the carrier frequency), but also on series resistance R 1  of the terminal, and on the load of the oscillating circuit of the transponder, that is, on the equivalent resistance of the circuits (microprocessor, etc.) and on the back modulation means (for example, resistor R, FIG.  1 ), added in parallel on capacitor C 2  and on inductance L 2 . This equivalent resistor will be designated hereafter as R 2 . 
     Thus, the contactless and wireless transmission system operates even if one of the oscillating circuits is detuned, provided that the antennas are very close to each other. 
     More specifically, the present invention provides a method of data transmission between a terminal for generating an electromagnetic field and a transponder, the terminal and the transponder each including an oscillating circuit forming an antenna, and the transponder including an electronic circuit adapted to absorbing and giving back power provided by the terminal field, the oscillating circuits of the transponder and of the terminal being capable of transmitting radio-electric signals of determined frequency, this method consisting of causing a detuning of at least one of the oscillating circuits with respect to the determined frequency when the transponder and the terminal are very close to each other. 
     According to an embodiment of the present invention, the oscillating circuits are tuned when the transponder is further away from the terminal than the critical coupling position between the two circuits, at least one of the circuits being detuned when the transponder is between this critical coupling position and the terminal. 
     According to an embodiment of the present invention, the detuning is caused when the respective antennas of the transponder and of the terminal are at less than two centimeters away from each other. 
     According to an embodiment of the present invention, both oscillating circuits are detuned. 
     According to an embodiment of the present invention, the data transmission rate from the transponder to the terminal and/or from the terminal to the transponder is modified according to whether the oscillating circuits are or are not tuned to the determined frequency. 
     According to an embodiment of the present invention, the determined frequency corresponds to the frequency of a remote supply carrier of the transponder. 
     The present invention also relates to a system of wireless and contactless data transmission between a terminal of generation of an electromagnetic field and at least one transponder having no independent supply means, the terminal and/or the transponder including means for implementing the method of the present invention. 
     The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  3 , previously described, are meant to show the state of the art and the problem to solve; 
     FIG. 4 illustrates, in the form of a timing diagram, an embodiment of the transmission method of the present invention in tight coupling; 
     FIG. 5 partially shows an embodiment of a read/write terminal according to the present invention provided with means for detuning the oscillating circuit of the terminal; 
     FIG. 6 shows a first embodiment of an electromagnetic transponder according to the present invention provided with means for detuning the oscillating circuit of the transponder; 
     FIG. 7 shows a second embodiment of an electromagnetic transponder according to the present invention provided with means for detuning the oscillating circuit of the transponder; and 
     FIG. 8 partially shows a terminal according to the present invention provided with means for determining the distance of a transponder in its field. 
    
    
     DETAILED DESCRIPTION 
     The same elements have been referred to with the same references in the different drawings, and the drawings have been drawn out of scale. For clarity, only those elements necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the digital electronic circuits have not been detailed, be it on the transponder or reader side. 
     A feature of the present invention is to provide detuning the oscillating circuit of a transponder and/or of a read or read/write terminal when in a situation of tight coupling with a terminal, that is, when their respective antennas are located, for example, at less than 2 centimeters from each other. 
     The consequence of such a detuning is that the operation becomes close to that of a transformer, that is, the quality factor is less. Now, in the conventional tuned operation, a quality factor as high as possible is desired, to optimize the power transfer associated with the transponder remote supply. 
     The fact of detuning the transponder and the terminal when the coupling is very close has several advantages. 
     In a detuned operation, the terminal power, that is, the current in the antenna (L 1 , FIG.  1 ), may be decreased while transmitting a sufficient power to the card remote supply. Indeed, since the transponder is very close to the terminal, the problem of remote supply range has disappeared. The required power then essentially depends on the transformation ratio (ratio between the number of spirals) of the oscillating circuit inductances. The current decrease required in the antenna suppresses the thermal effect on the transponder side. 
     FIG. 4 illustrates the shape of signal Rx, on the reader side, after a resistive back modulation on the transponder side, while the oscillating circuit of the terminal is detuned from the remote supply carrier frequency (for example, 13.56 MHz). This drawing is to be compared with FIG.  3 . As can be acknowledged, the enable times have almost disappeared. Signal Rx has a shape substantially similar to that of the control signal of the gate of transistor T, on the transponder side. Accordingly, it is now possible to increase the transmission rates since it is no longer necessary to wait, for each transmitted bit, that signal Rx reaches the detection threshold of the phase demodulator of the terminal. 
     It should be noted that the detuning of the transponder&#39;s oscillating circuit is only desirable in very close or tight coupling. Accordingly, the present invention enables easily dissociating two operating modes of the system according to whether the transponder is or not very close to the reader. It should also be noted that the coupling coefficient is decreased by the detuning of the oscillating circuit. This is not disturbing since the two oscillating circuits are then very close to each other in a transformer type operation. 
     Several solutions may be used to detune the oscillating circuits. Indeed, as indicated hereabove, the optimal coupling coefficient depends on the respective values of elements R 1 , L 1 , C 1  on the terminal side and R 2 , L 2 , C 2  on the transponder side. It is thus possible to detune either the transponder&#39;s oscillating circuit, or the terminal&#39;s oscillating circuit, or both. 
     According to a preferred embodiment of the present invention, it is provided, for efficiency reasons, to detune both circuits. Indeed, according to the present invention, a significant detuning has to be caused with respect to the remote supply carrier frequency. Thus, controllable means for detuning the oscillating circuits are provided, preferably, on the terminal side and on the transponder side. 
     FIG. 5 partially shows an embodiment of a read/write terminal  1 ′ of an electromagnetic transponder, provided with means for detuning the oscillating circuit with respect to the carrier frequency (for example, 13.56 MHz). This embodiment includes varying the value of the capacitance (C 1 , FIG. 1) of the oscillating circuit. 
     As previously, terminal  1 ′ includes an oscillating circuit formed of an inductance or antenna L 1 , in series with a capacitive element  24  and a resistor R 1 , between an output terminal  2 p of an amplifier or antenna coupler  3  and a terminal  2 m at a reference potential (generally the ground). Amplifier  3  receives a high frequency transmission signal Tx coming from a modulator  4  (MOD) that receives a reference frequency (signal OSC), for example, from a quartz oscillator (not shown). Modulator  4  receives, if necessary, a data signal to be transmitted. The other components of terminal  1 ′ are, unless otherwise indicated, similar to those described in relation with FIG.  1 . 
     According to the embodiment of FIG. 5, a phase regulation of the current in antenna L 1  with respect to a reference signal REF is performed. This regulation is a regulation of the high-frequency signal, that is, of the carrier signal (for example, at 13.56 MHz) corresponding to signal Tx in the absence of data to be transmitted. This regulation is performed by varying capacitance C 1  of the oscillating circuit of terminal  1 ′ to maintain the current in the antenna in constant phase relation with the reference signal. Signal REF is at the carrier frequency and corresponds, for example, to signal OSC provided by the oscillator ( 5 , FIG. 1) of the modulator. 
     As illustrated in FIG. 5, capacitive element  24  provided in series with resistor R 1  and inductance L 1  is controllable by means of signal CTRL, which is provided by a circuit  21  (COMP), the function of which is to detect the phase shift with respect to reference signal REF and to accordingly modify capacitance C 1  of element  24 . 
     Variable capacitance  24  may be formed in several ways. Generally, this capacitance must reach a few hundreds of picofarads and withstand, across its terminals, a voltage of more than 100 volts. A diode in which the capacitance of the reverse-biased junction is used as a variable capacitance that depends on this biasing may for example be used. The diode is then connected, by its anode, on the side of reference terminal  2 m and by its cathode, on the side of inductance L 1 . As an alternative, a diode-mounted MOSFET transistor may be used. Be it a diode or a MOSFET transistor, the control performed by means of signal CTRL includes modifying the voltage across the diode or the transistor to modify its capacitance. 
     The phase measurement in the oscillating circuit is performed, for example, from a measurement of the current  1  through this oscillating circuit, symbolized by an element  23 . This current measurement element is connected in series with element  24  and inductance L 1 . The result of measurement MES is sent to phase comparator  21  that then compares the phase of the current measured by block  23  with reference signal REF, and accordingly controls capacitive element  24  by means of signal CTRL. 
     It should be noted that the phase regulation loop must be sufficiently slow so as not to disturb the phase modulation at 847.5 kHz, but sufficiently fast as compared to the displacement speed of a transponder in the terminal field, which is generally the displacement speed of a hand. For example, a response time on the order of one millisecond is adequate, the displacement time of a transponder being of several hundreds of milliseconds. 
     For a detuned operation, the phase reference of comparator  21  may for example be modified to modify the control order of the loop that then regulates on another value, the signals to be compared being then phase-shifted by a value other than 90° in base value. It should be noted that, for the optimal coupling to correspond to the shortest possible distance between the terminal and the transponder, capacitance C 1  has to be increased with respect to its tuned value. This amounts to decreasing the resonance frequency of the terminal&#39;s oscillating circuit. Instead of modifying the reference phase of comparator  21 , the bias voltage of element  24  may be shifted, for example, by means of a switched resistor network, controlled by the terminal microprocessor, to make the biasing resistance of element  24  variable. 
     In the embodiment of FIG. 5, phase comparator  21  is also preferentially used to demodulate signal Rx coming from a possible back modulation of the transponder. 
     To detune the oscillating circuit of the transponder, a first solution includes connecting, in parallel with the transponder antenna, two capacitors, one of which is associated in series with a switch to make it controllable. This solution consists of using, for other purposes, an assembly of the type described in document WO-A-98/29760 that provides a frequency detuning of a transponder by means of a modifiable capacitance in the oscillating circuit. 
     According to the present invention, for the optimal coupling to correspond to the smallest possible distance between the terminal and the transponder, capacitance C 2  has to be increased with respect to its tuned value. This amounts to decreasing the resonance frequency of the oscillating circuit of the transponder. 
     FIG. 6 shows a first embodiment of a transponder  30  according to the present invention. As previously, this transponder is formed from a parallel oscillating circuit including an inductance L 2  and a capacitor C 2 ′ between two terminals  11 ′,  12 ′ of the circuit. 
     In the embodiment illustrated in FIG. 6, the rectification performed to extract a D.C. supply voltage Va smoothed by a capacitor Ca is a single-halfwave rectification by means of a diode D, the anode of which is connected to terminal  11  ′ and the cathode of which is connected to positive terminal  14  of capacitor Ca. Voltage reference  15  corresponds to the negative terminal of capacitor Ca directly connected to terminal  12 ′. Voltage Va is meant for an electronic block  31  including, for example, circuits  16  to  20  of FIG. 1. A capacitor C 3  is connected in series with a switch (for example, a MOS transistor) K 1  between terminals  11 ′ and  12 ′. Switch K 1  is controlled by circuit  31  by being closed for a tuned operation. 
     FIG. 7 shows a second embodiment of a transponder  30 ′ according to the present invention. According to this embodiment, terminals  11 ,  12  of the oscillating circuit are connected to the A.C. input terminals of a bridge  13  formed, for example, of diodes D 1  to D 4  as in FIG.  1 . Two rectified output terminals  14 ,  15  of bridge  13  provide, via smoothing capacitor Ca, voltage Va of supply of electronic block  31 . 
     According to this embodiment, two capacitors C 3  and C 4  are respectively connected, each, in series with a switch (for example, a MOS transistor) K 1 , K 2 , respectively between terminals  11  and  12  and terminals  15 . Thus, a first terminal of capacitor C 3  is connected to terminal  11 , its second terminal being connected, via transistor K 1 , to terminal  15 . A first terminal of capacitor C 4  is connected to terminal  12  while its other terminal is connected, via a transistor K 2 , to terminal  15 . Capacitors C 3  and C 4  are respectively associated with each sign of high frequency A.C. voltage V 2  across antenna L 2 . Capacitors C 3  and C 4  are thus of same values. Transistors K 1  and K 2  are controlled by block  31 , preferable from a same signal, to be closed when the circuit has to be tuned on the remote supply carrier frequency. 
     It should be noted that, due to the doubling of the capacitors, a reference node is available (line 15) for the control of switches K 1  and K 2 . Thus, if switches K 1  and K 2  are formed of N-channel MOS transistors, it is now possible, by a logic signal coming from block  31 , to control these switches in all or nothing, which is not possible with the solution advocated by document WO-A-98/29760. 
     For example, capacitors C 2 ′, C 3  and C 4  have, each, a capacitance corresponding to half the capacitance (C 2 , FIG. 1) necessary to tune the oscillating circuit on the reader carrier frequency. 
     Transponder  30  (FIG. 6) or  30 ′ (FIG. 7) also includes a resistive back modulation circuit formed, preferably, of two resistors R 3 , R 4  respectively in series with a switch K 3 , K 4  between terminals  14  and  15 . Resistors R 3  and R 4  have different values, respectively high and low. 
     Consider being between the critical coupling and the terminal, resistor R 3 , which is of high value, is used to perform the back modulation and transistor K 1  (or transistors K 1  and K 2 ) is turned off. The system then has a detuned operation close to a transformer operation. 
     Consider being far from the critical coupling position while being further away from the terminal than this position, that is, consider a loose coupling. Transistor K 1  (or transistors K 1  and K 2 ) is then turned on and the resistive back modulation is performed by means of resistor R 4  that is of smaller value. This then is a conventional operating mode. 
     It should be noted that the present invention, by using a resistance of small value when away from the terminal, optimizes the system range. The ratio between the respective values of resistors R 3  and R 4  is, for example, included between  4  and  10  (R 3  included between 0.4 and 5 kiloohms and R 4  included between 100 and 500 kiloohms) and, preferably, on the order of 6 (for example, approximately 1500 and 250 ohms). 
     As an alternative, the capacitor(s) used to detune the circuit is (are) also used as back modulation means. In this case, switched resistors R 3 , K 3 , and R 4 , K 4  are eliminated and the values of capacitors C 2 ′, C 3  (and C 4  for the embodiment of FIG. 7) are chosen so that the importance of the detuning is compatible with the phase shift to be detected by the terminal in case of a capacitive modulation. The capacitive modulation directly influences the phase of the voltage across inductance L 1  of the terminal without acting upon its amplitude. This eases the phase detection by the terminal. It should be noted that the type of back modulation does not modify the coding, that is, the control signal of the back modulation switch(es) at the carrier frequency. 
     In the sizing of the oscillating circuit capacitors, account will be taken of the rectifying means used and of the value of smoothing capacitor Ca. Indeed, the conduction periods of the diodes of a bridge (FIG. 7) are generally shorter as compared to the remote supply carrier period than the conduction periods of a single-halfwave rectifying diode (FIG.  6 ). Accordingly, the duty cycle of action of the back modulation means is different according to the type of rectification performed. Now, this duty cycle has an influence, in particular, on the value of equivalent resistance R 2 , and thus on the coupling coefficient. 
     To detune the oscillating circuits when in very close coupling, an information relative to the distance separating the terminal from the transponder is used. This distance may be determined by the transponder and/or by the terminal. 
     On the transponder side, one of the embodiments of FIGS. 6 and 7 may be used. According to the present invention, their respective electronic circuit is provided with an input DET receiving the local supply voltage Va. Input DET is associated with a circuit (not shown) for measuring voltage Va and with at least one element for storing this measurement. In a specific example of embodiment, this may be a microprocessor ( 6 , FIG.  1 ). The storage of the values of the measured voltages is performed either analogically or, preferentially, digitally over several bits, the number of which depends on the desired analysis precision. 
     According to a preferred embodiment of the present invention, the following measurement cycle is periodically performed when the transponder is in the terminal range and, preferably, as soon as the transponder is activated (supplied) by its entering the field of a reader. Transistor K 1  (FIG. 6) or transistors K 1  and K 2  (FIG. 7) are initially on, the oscillating circuit being tuned. The voltage present on terminal DET is measured. Then, transistor(s) K 1 , K 2  is (are) turned off. The circuit is then detuned, its resonance frequency being shifted to, in the case of FIG. 6, more than twice the tuning frequency if capacitors C 2 ′ and C 3  have the same value. The voltage on terminal DET is measured again. As an alternative, the first measurement is performed with a detuned circuit. The two obtained values are compared and the result of this comparison is stored, for example on a single bit. 
     It should be noted that the time (for example, on the order of a few hundreds of milliseconds) required to perform the two “tuned” and “detuned” measurements is small as compared to the transponder displacement speed that corresponds, in most applications, to the displacement speed of a hand. 
     It should also be noted that the duration for which the oscillating circuit is detuned to perform a measurement is, preferably, chosen to be substantially different from the sub-carrier half-period, so that this measurement is not interpreted by the terminal as a back modulation. Indeed, the detuning of the transponder oscillating circuit translates as a phase shift in oscillating circuit L 1 C 1  (FIG. 1) of the terminal that, during the distance determination, must not be mistaken for a data transmission. 
     The above measurement cycle is repeated after a short time interval (for example, on the order of one millisecond) that remains fast as compared to the passing time of a transponder before a terminal (several hundreds of milliseconds). 
     It should be noted that, in a simplified embodiment, it may be enough to determine, before each beginning of a data transmission from the transponder to the terminal, the position of the transponder with respect to the critical coupling. 
     The variation of the value of the comparison bit enables knowing whether the transponder is closer or further away from the terminal as compared to the critical coupling position. If the bit indicates a higher level in detuned position than in tuned position, this means that the transponder is very close to the terminal (in tight coupling). In the opposite case, the transponder is either close to the critical coupling, or between the critical coupling position and the system range limit. 
     As a simplified embodiment, the use of a dedicated distance determination input (DET) may be avoided by using an existing input of the microprocessor (contained in block  31 ) of the transponder). This conventional input controls the available local supply voltage across capacitor Ca with respect to a predetermined threshold. The microprocessor stores (in the form of a bit) the state of this voltage with respect to the threshold. The bit is conventionally used, for example, for detecting whether the voltage recovered by the oscillating circuit is sufficient for the transponder supply, and thus to activate said transponder when it enters the field of a reader. This function exists, for example, in transponder microprocessors, for example, circuits ST 16  and ST 19  of STMicroelectronics, and may thus be used with no significant modification of the transponder. 
     The distance determination with respect to the critical coupling has the advantage that the performed distance determination or area detection (tight coupling or loose coupling) resembles a differential measurement. Indeed, the detection is performed with respect to the critical coupling that depends on the system and on its environment. Only at the critical coupling is the recovered voltage threshold maximum when the circuits are tuned. It is thus not necessary to provide a specific reference or distance threshold. In other words, the threshold distance between the two tuned and detuned operating modes is then self-adaptive. 
     For a distance determination on the terminal side, a system such as described in WO-A97/34250 may for example be used. However, according to a preferred embodiment of the present invention, the determination performed does not require involving the terminal, that is, it needs no reception of data coming from the transponder. 
     FIG. 8 partially shows a terminal according to the present invention provided with means for determining the distance of a transponder entering its field. FIG. 8 is based on FIG. 5, of which it only shows capacitive element  24  and current measurement element  2 . 
     According to the embodiment of FIG. 8, the voltage across element  24  is measured by a resistive bridge (resistors R 5 , R 6 ), the midpoint of which is connected to the anode of a diode D 5 , the cathode of which is connected to a first input of a comparator (COMP)  40 . A capacitor C 5  connects the cathode of diode D 5  to ground  2 m. Capacitor C 5  thus has across its terminals a D.C. voltage, as the peak amplitude of the voltage at the midpoint of bridge R 5 -R 6 . A second input of comparator  40  receives a reference voltage Vref. As previously indicated, when the transponder enters the terminal field, the load that it forms varies the current in oscillating circuit L 1 C 1 . Since the oscillating circuits are, by default, tuned, the closer the transponder comes, the more the voltage across the capacitive element decreases. The voltage across capacitor C 1  (element  24 ) is indeed equal to the product of the voltage amplitude (constant) provided by the A.C. generator (amplifier  3 ) by the quality factor. Now, when the distance decreases, the quality factor also decreases. The output of comparator  40  thus indicates the transponder position with respect to a distance threshold (transformed in a voltage threshold Vref). The output of comparator  40  is, for example, sent to microprocessor  6 , to switch the system operation to the mode corresponding to the tight coupling and control the detuning or not of the oscillating circuits. 
     It should be noted that the fact of maintaining the tuned phase constant by means of the regulation loop enables making the distance-impedance characteristic monotonic, that is, without any point of inflexion, and thus to obtain a reliable distance determination. 
     It should also be noted that, once the distance determination has been performed by the transponder or the terminal, that of the elements which has not participated in the determination can receive the information by a data transmission from the other element. Thus, the choice of the detuning mode (transponder, terminal, transponder and terminal) is independent from the distance determination mode. 
     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 sizing of the different resistive and capacitive elements depend on the application and, especially, on the application and, especially, on the frequency of the various carriers and on the system range. Similarly, the practical implementation of the different circuits of a transponder or a terminal according to the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove. 
     Further, the choice of the distance determination mode depends on the application and on the element of the system that is preferentially desired to be modified. Preferably, the two oscillating circuits will be detuned in a very close coupling to optimize the effects of the detuning. It should be noted that, since the elements communicate with each other, the decision and the detuning may be implemented by a single element that then communicates its state to the other. 
     Moreover, it should be noted that the implementation of the present invention is perfectly compatible with conventional modulations. In particular, the data transmission from the transponder to the terminal can always be based on a phase shift keying, be it a tuned or detuned operation. Indeed, the detuning does not change the high frequency carrier frequency (for example, 13.56 MHz) on which the phase shifts are detected at the sub-carrier rate (for example, 847.5 kHz). 
     One optional modification is to increase the transmission rate when the transponder and the terminal are detuned and in tight coupling. 
     The present invention is applicable to, among other areas, 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.