Communication between electromagnetic transponders

The present invention relates to systems using electromagnetic transponders, that is to say transmitters mobile receivers generally likely to be questioned, contactless and wireless, by a generally fixed unit, called terminal reading and / or writing. The invention relates more particularly, transponders no independent power supply, for example of the contactless card or electronic tag. These transponders extract the power supply required the electronic circuits comprise the high frequency field radiated by an antenna of the read terminal and writing. The invention applies to such transponders, it either of read-only transponders, that is to say own to operate with a terminal merely read data the transponder or transponders to read-write that contain data that can be modified by the terminal.

1 shows, very schematically and functional, a classic example of data exchange between a terminal 1 read-write (STA) and a transponder 10 (BECAUSE).

Terminal 1 is essentially constituted of a circuit oscillating formed of an inductance L1 in series with a capacitor C1 and a resistor R1 between a 2p output terminal an amplifier or antenna coupler 3 and a terminal 2m at reference potential (usually ground). The amplifier 3 receives a signal Tx high frequency transmission, from a modulator 4 (MOD). The modulator receives a frequency reference, for example, a quartz oscillator 5 and, if need, a DATA signal data to be transmitted. In the absence of Terminal 1 of data to the transponder 10, the Tx signal is used only as an energy source to activate the transponder 10 if it enters the field. The data transmitted generally originate from a digital system, example, a microprocessor 6 (.mu.P).

The connection point of capacitor C1 and the inductance L1 is, in the example shown in Figure 1, a terminal for sampling a signal Rx received data a transponder 10 to a demodulator (DEM). A output of the demodulator communicates (if necessary via a decoder 8 (DEC)) data received from the transponder 10 to microprocessor 6 of terminal 1. Demodulator 7 receives, generally from oscillator 5, a clock signal or reference for a phase demodulation. Where applicable, the demodulation is performed from a signal taken between the Capacitor C1 and resistor R1, and not across inductance L1. The microprocessor 6 communicates (BUS) with various circuits of input / output (keyboard, screen, means transmitting to a server, etc.) and / or treatment. The circuits of the terminal read / write draw the power necessary for their operation from a power supply circuit 9 (ALIM) connected, for example, to the electrical distribution network.

side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (Called a reception resonant circuit) for sensing the magnetic field generated by the oscillating circuit L1 series C1 of the terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned to the resonant frequency of the resonant circuit from terminal 1.

Terminals 11 and 12 of the resonant circuit L2, C2 which correspond to the terminals of the capacitor C2 are connected to two AC input terminals of a bridge rectifier 13 whose rectified output terminals 14 and 15 are connected to the terminals an energy storage capacitor Ca and smoothing rectified voltage provided by the bridge 13. The bridge 13 is mono or full wave.

When the transponder 10 is in the field of terminal 1, a high frequency voltage is generated across resonant circuit L2, C2. This voltage rectified by bridge 13 is smoothed by the capacitor Ca, which provides a voltage power to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially a microprocessor 17 (.mu.P) associated with a memory not shown, a demodulator 18 (DEM) optionally signals received from the terminal 1, and a modulator 19 (MOD) to transmit information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) derived by a block 20 of high-frequency signal recovered across capacitor C2 before recovery. Most often, all the electronic circuits the transponder 10 are integrated into a single chip.

To transmit the transponder data to 10 terminal 1, modulator 19 controls a stage of modulation (Back modulation) of resonant circuit L2, C2. This stage modulation is generally comprised of an electronic switch (E.g., a transistor T) and a resistor R, in series between terminals 14 and 15.

The transistor T is controlled at a frequency (for example, 847.5 kHz) called subcarrier, significantly lower (Usually, with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (by eg 13.56 MHz). When the switch T is closed, the oscillating circuit of the transponder is subjected to a damping Additional compared to the load formed by the circuits 16 to 20, so that the transponder samples a greater amount of power from the high magnetic field frequency. side terminal 1, amplifier 3 maintains constant the amplitude of the high frequency excitation signal. By Therefore, the transponder power variation results by a change of amplitude and current phase in antenna L1. This variation is detected by demodulator 7 the terminal which is either a phase demodulator or a amplitude demodulator.

In some cases, the floor retromodulation (transistor T, resistor R) is situated upstream of the bridge 13, that is to say Besides its AC input.

The terminal typically transmits no data for it receives from the transponder, transmission taking place alternately in one direction and then the other.

Figure 2 illustrates a classic example of transmission from terminal 1 data to a transponder 10. This Figure shows an example of shape of the excitation signal L1 antenna for transmission of a code 0101. Modulation commonly used is an amplitude modulation with a rate 106 kilobits per second (one bit is transmitted in approximately 9.5 microseconds) much lower than the frequency (eg, 13.56 MHz) of the carrier from the oscillator 5 (period about 74 nanoseconds). The amplitude modulation is effected, either all or nothing or with a modulation rate (defined as the difference of the peak amplitudes (a, b) between the two states (0 and 1) divided by the sum of these amplitudes) less than unity because of the need for supply of transponder 10. In the example of Figure 2, the carrier at 13.56 MHz is modulated at a rate of 106 kilobits per second, amplitude with a modulation rate tm, for example, 10%.

Figure 3 illustrates a classic example of transmission data from transponder 10 to terminal 1. This figure shows an example of the shape of V signal_T transistor control T, provided by modulator 19, and the corresponding signal Rx received by terminal 1. transponder side, the back modulation is generally of resistive type with a carrier said subcarrier, for example, 847.5 kHz (period of about 1.18 ms). The modulation is, for example, based on a coding of BPSK (binary coding phase jump) with a rate of the order of 106 kilobits per second significantly lower at the frequency of the subcarrier. In Figure 3, the signal Rx has was represented "smoothed", that is to say without showing the corrugations of the high frequency carrier (13.56 MHz). In the example of Figure 3, it was considered that each of the three bits represented was different from the previous bit. So, he is a transmission of a 010 code.

Whatever type of modulation or back modulation used (e.g., amplitude, phase, frequency) and regardless of the data type of coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), modulation is performed digital way, by jumping between two binary levels.

As illustrated in Figure 3, the signal V_T consists a pulse train to the frequency of the subcarrier, a phase jump occurring at each change of state of one bit next bit.

If several transponders are located in the field the same terminal, different communications can be initiated between each transponder and the terminal read-write. Most often, the transponders transmit identifiers which allow the terminal to individualize messages to their respective destinations.

In the sense terminal to the transponder, the transponders determine whether their messages are respectively for from their identifier contained in the message, they detect after demodulation.

However, it can be a problem when several transponders transmit simultaneously to of the same terminal, while these are within the scope of this thick headed. Such conflicts can be detected by the wrong terminal read-write, which is detrimental to the reliability of system.

Moreover, in some applications, it may Hopefully transponders exchange information. In such a case, the terminal mediates the communication receiving information from a transponder for retransmit to another by having the previously then demodulated remodulated.

The present invention aims to propose a new exchange solution of information between two transponders in the field of a terminal read-write.

The invention also aims at providing a solution that requires no modification of the terminal read-write existing.

The invention also aims at providing a solution that may solve conflict problems when two transponders are present in the field of a terminal of read-write with which they communicate.

To achieve these and other objects, the present invention provides an electromagnetic transponder intended for take the energy needed to operate a field radiated by a transmitting terminal to a carrier at a first frequency remote supply and rétromoduler the received signal the rhythm of a subcarrier at a second lower frequency to the first, and comprising means capable of demodulating and decoding signals modulated by said subcarrier.

According to one embodiment of the present invention, the transponder comprises an oscillating circuit upstream of a own recovery means to provide continuous voltage an electronic power supply circuit, the electronic circuit comprising means for transmitting encoded information numerically, and the transponder comprising a demodulator own differentiating information received at the rate of subcarrier retromodulation another transponder respect to information received, at the rate of a third frequency even lower, since the terminal read-write.

According to one embodiment of the present invention, said demodulator comprises two parallel branches with each a filter centered on the second and respectively third frequencies, each filter being associated with a decoder digital.

According to one embodiment of the present invention, a first decoder associated with the filter centered on the frequency of modulation is a type of decoder phase jump, a second to the third frequency being a decoder associated type decoder amplitude jump.

The invention also provides a communication system contactless and wireless between two transponders Electromagnetic no independent power supply, each transponder comprising means adapted to collect energy needed to supply of its circuits, electronic field at a first frequency remote supply radiated by at least one read-write terminal, and means for demodulate and decode the signals transmitted by another transponder modulation of a subcarrier at a second frequency.

According to one embodiment of the present invention, each transponder includes demodulators and decoders separate dedicated respectively to the reception of signals transmitted by another transponder and to the reception of signals transmitted by terminal read-write.

According to one embodiment of the present invention, the first frequency is at 13.56 MHz, the second frequency being 847.5 kHz and the third frequency being 106.5 kHz.

These objects, features and advantages, as well as others of the present invention will be described in detail in the following description of specific embodiments made in non-limiting in connection with the accompanying drawings among : <sl><li>Figures 1 to 3 described above are intended to show the state of the art and the problem encountered;</li><li>4 shows, very schematically, a communication system between transponders in a mode of embodiment of the present invention;</li><li>5 shows a first embodiment of a electromagnetic transponder according to the present invention; and</li><li>6 shows a second embodiment a transponder according to the invention.</li></sl>

The same elements have been designated by the same references in the different figures. For reasons of clarity, only those elements that are necessary for the understanding of the invention have been shown in the drawings and will be described thereafter. In particular, the creation and coding messages according to different protocols have not been detailed, are not the object of the present invention.

A feature of the present invention is to provide direct communication between two transponders electromagnetic present in the field of a terminal read-write from which they derive their power.

Figure 4 illustrates, very schematically and in block form, an embodiment of a system of communication according to the present invention.

As before, a drive 1 (STA) generates by via an antenna inductance L1 an electromagnetic field at a frequency corresponding to a carrier remote supply of transponders 10 (T1, T2). each transponder itself comprises an antenna (inductances L21, L22) for sensing the electromagnetic radiation from the terminal to draw a supply of its circuit.

The transponders 10 are, according to the invention, capable not only to communicate with the terminal 1 but directly with each other as illustrated by two-way arrows of Figure 4.

When both transponders are within the scope of the same terminal, and a transponder transmits information retromodulation, this affects the electromagnetic field available for the other transponder. It may therefore be able to detect the information transmitted subject to be capable of demodulating or decoding them.

According to a first embodiment of the invention, the same receiver is used for receiving information from the terminal (for example, at a frequency of 106 kHz) and the information from another transponder in back modulation (e.g., at a frequency of 847.5 kHz). In this case, is provided downstream of the transponder demodulator, more decoders depending on the type of information to detect. This these are then decoders that differentiate transmissions 106 kilobits transmissions at the frequency of 847.5 kHz.

5 shows, in block form and very schematically, a transponder according to the first mode of embodiment of the invention.

Include the resonant circuit composed of inductance L2 in parallel with a capacitor C2 between terminals 11 and 12 input of a bridge rectifier 13 (full wave here). The output terminals 14 and 15 straightened the bridge 13 provide the supply voltage across a capacitor storage and smoothing is also found Ca. Power regulator 16 different transponder circuits and in particular the demodulator 28, modulator 19 and logic circuits 17 to control and which are constituted either a microprocessor, or a wired logic circuit. The Figure 5 illustrates a variant of the back-modulation circuit wherein the transistor T of modulation is only placed in parallel capacitor Ca, its resistance to the state series passing up the resistance retromodulation.

For simplicity, all components were not shown in Figure 5. In particular, the clock is heard, if necessary, present. Similarly, the various connections supply circuits have not all been indicated.

According to the invention, the output of demodulator 28 is sent two decoders 25 and 26 (DEC1 and DEC2) for decoding each one of types of signals capable of being received, namely those received at the rate of 106 kilobits per second a terminal 1 and those received at the rate of 847.5 kHz another transponder.

The demodulator 28 own to the invention comprises a analog head 29 (ADH) providing the envelope of the received signal and carried by the frequency of 13.56 MHz. This envelope is, is modulated in time with the carrier retromodulation 847.5 kHz, or at a rate of transmission of the terminal (106 kHz). As a result, each decoder DEC1 and DEC2 is capable of detecting changes in levels according to whether they are at the rate of 106 kHz or at the rate of 847.5 kHz. The respective outputs of decoders provide the demodulated signals from the respectively terminal or other transponder in the field and coupling close with the transponder concerned.

6 shows a second embodiment a decoder 30 according to the invention.

According to this embodiment, the output 14 of the bridge rectifier is connected to the respective inputs of two filters 31 and 32 respectively centered on the frequency of 847.5 kHz and 106 kHz, for example, corresponding to the frequencies retromodulation and modulation of the terminal. The respective outputs filters 31 and 32 therefore restore digital signals demodulated only if the received signal contains information at the corresponding frequency. The filter 31 is followed by a 33 BPSK decoder which output supplies a bit stream to the circuit 17. The filter 32 is associated with a decoder 34 of ASK type whose output provides a bitstream decoded circuit 17.

The embodiment of Figure 6 takes advantage of in that, in transmission systems with transponders electromagnetic, transmission from the reader to a transponder is generally performed by an ASK modulation (jump amplitude) at a frequency of 106 kHz while the transmission from a transponder to a reader (or the invention to another transponder) is effected by modulation by phase shift (BPSK) with a modulation frequency of 847 kHz. We can distinguish types of modulation on the side of transponder for decoding both types of signals.

Note of course that, in both cases, it is the 13.56 MHz carrier which is modulated by the modulation transponder and reader to the transponder to the reader.

An advantage of the present invention is that it requires no modification of the transponders for which The modulation part. The invention requires only modification of the demodulation part of the transponders for be able to interpret information from a another transponder in the field of a terminal, enough close to a mutual coupling between two transponders.

Another advantage of the present invention is that allowing a direct communication between two transponders, we can resolve any conflict problems when several transponders are present in the field of the same thick headed. Indeed, from the moment a transponder picks information directly from another transponder thanks to the invention, one can provide for a one prohibition to transmit at the same time so that the management time communications transponders to the same terminal is managed directly by the transponders.

Of course, the present invention is capable of variants and modifications which will appear to man art. In particular, the practical realization of a demodulator for the implementation of the invention is within the scope of the art based on the functional indications given above and depending on the coupling scope desired between two neighboring transponders.

Note that in a simplified embodiment, it is possible by means of a single demodulator, differentiate messages transmitted by analyzing the code. However, such embodiment is reserved in case of transponders with microprocessor significant computing and storage transmitted bits. Indeed, to be able to differentiate by analysis of the received code, we need to store a length sufficient code and have a powerful calculation tool. The advantage of the preferred embodiment of the invention consisting filtering by means of two distinct analog heads demodulator avoids these elements calculation and storage Additional and makes the invention applicable in transponders using wired logic circuits to interpret transmissions.

Among the applications of the present invention, will signal particularly in contactless smart cards (For example, identification cards for access control, electronic purse cards, store cards information about the owner of the card, card consumer loyalty, pay-TV cards etc.), and reading systems or reading and / or writing these cards (for example, terminals or control gantries access, vending product, terminals computers, telephone terminals, televisions or satellite decoders, etc.). In this kind of application, present invention may allow, for example, recharging a transport card from a wallet card electronic coupling the last two in the same field (Then serving the reader does is generate a field magnetic power of these cards). Devices corresponding security can then remain in the cards, which improves the reliability of systems against hackers. The direct communication between two cards may in addition a communication priority detection when these are within the scope of the same terminal.