Electromagnetic transponder operating by very close coupling

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a system using an electromagnetic transponder. An electromagnetic transponder is a (generally mobile) transceiver that can receive questions in a non-contact wireless manner from a (generally fixed) device called a read / write terminal. More specifically, the present invention relates to transponders that do not have an independent power supply. Such transponders extract the power required by the electronic circuits contained therein from the high frequency electromagnetic field radiated from the antenna of the read / write terminal. The present invention applies to read-only transponders adapted to work with terminals that only read transponder data, or read / write transponders that contain data that can be modified by the terminal.

【0002】

[Conventional technology]

Electromagnetic transponders are based on the use of oscillating circuits on the transponder side and on the read / write terminal side that include windings that form the antenna. These circuits are coupled by a dense magnetic field when the transponder enters the magnetic field of the read / write terminal. The range of the transponder system, that is, the distance from the terminal where the transponder is activated to the farthest point, is particularly the antenna size of the transponder, the excitation frequency of the coil of the oscillator circuit generating the magnetic field, and the intensity of excitation. , And the power consumption of the transponder.

【0003】

FIG. 1 shows an example of a conventional data exchange system that exchanges data between a read / write device 1 (STA) and a transponder 10 (CAR) in a functional and very schematic manner.

【0004】

In general, device 1 is substantially located between the output terminal 2p of the amplifier or antenna coupler 3 (DRIV) and the terminal 2m at the reference potential (generally the ground potential) and is connected in series with the capacitor C1 and resistor R1. It is composed of an oscillation circuit consisting of an inductance L1. The amplifier 3 receives the high frequency transmission signal Tx supplied by the modulator 4 (MOD). This modulator receives, for example, a reference frequency from a crystal oscillator 5, and further receives a transmit data signal, if necessary. If no data transmission from terminal 1 to transponder 10 is performed, the signal Tx is simply used as a power source to activate the transponder when it enters an electromagnetic field. The transmitted data is generally supplied by a digital electronic system, such as microprocessor 6 (μP).

【0005】

In the example shown in FIG. 1, the node between the capacitor C1 and the inductance L1 forms a terminal for sampling the data signal Rx received from the transponder 10 and sent to the demodulator 7 (DEM). The output of the demodulator transmits the data received from the transponder 10 to the microprocessor 6 of the read / write terminal 1 (via the decoder (DEC) 8 if necessary). The demodulator 7 generally receives a clock for phase demodulation, that is, a reference signal from the oscillator 5. Demodulation may be performed based on the signal sampled between the capacitor C1 and the resistor R1 instead of across the inductance L1. The microprocessor 6 communicates with various input / output circuits (via the bus EXT) (keyboard, screen, means of transmission to the provider, etc.) and / or processing circuits. The read / write terminal circuit draws the power required for operation from, for example, a power supply circuit 9 (ALIM) connected to a power supply system.

【0006】

On the transponder 10 side, the inductance L2 in parallel with the capacitor C2 forms a parallel oscillation circuit (called a reception resonance circuit) for capturing the electromagnetic field generated by the series oscillation circuits L1 and C1 of the terminal 1. The resonant circuit (L2, C2) of the transponder 10 is tuned to the frequency of the oscillator circuit (L1, C1) of the terminal 1.

【0007】

The resonance circuits L2 and C2 terminals 11 and 12 corresponding to the terminals of the capacitor C2 are connected to two AC input terminals of a rectifier bridge 13 formed of, for example, four diodes D1, D2, D3 and D4. In the display of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of the diodes D1 and D2 form a positive rectified output terminal 14. The anodes of the diodes D3 and D4 form the reference terminal 15 for the rectified voltage. The capacitor Ca is connected to the rectified output terminals 14 and 15 of the bridge 13, stores the power supplied by the bridge, and smoothes the rectified voltage. A single half-wave rectifying assembly may be used instead of this diode bridge.

【0008】

When the transponder 10 is in the electromagnetic field of the terminal 1, a high frequency voltage is generated across the resonant circuit L2C2. This voltage is rectified by the bridge 13 and smoothed by the capacitor Ca to supply the power supply voltage to the transponder's electronic circuit via the voltage regulator 16 (REG). In general, such circuits effectively transmit information to the microprocessor (μP) 17 (coupled to memory (not shown)), the demodulator 28 (DEM) of the signal received from terminal 1, and terminal 1. Modulator 19 (MOD) to be used is included. The transponder is generally synchronized by the clock (CLK) extracted by block 20 from the radio frequency signal reproduced before rectification across the capacitor C2. In many cases, all the electronic circuits of the transponder 10 are integrated on the same chip.

【0009】

In order to transmit data from the transponder 10 to the device 1, the modulator 19 modulates the resonant circuits L2 and C2 (back modulation (back). modulation)) Control the stage. This modulation stage generally consists of an electronic switch (eg, a transistor T) and a resistor R in series between terminals 14 and 15. Transistor T is controlled at a so-called secondary carrier frequency (eg 847.5 kHz) that is much lower (generally at least 1/10) than the frequency of the excitation signal of the oscillator circuit of terminal 1 (eg 13.56 MHz). When switch T is closed, the transponder's oscillator circuit is subjected to additional damping in addition to the load consisting of circuits 16, 17, 18, 19 and 20, so the transponder is from a high frequency electromagnetic field. Pull out more power. On the terminal 1 side, the amplifier 3 keeps the amplitude of the high frequency excitation signal constant. Therefore, the power fluctuation of the transponder is converted into the amplitude and phase fluctuation of the current flowing through the antenna L1. This change is detected by demodulator 7 of terminal 1, which is a phase demodulator or amplitude demodulator. For example, in the case of phase demodulation, the demodulator detects a slight phase shift (several degrees, or less than 1 degree) of the carrier of the signal Rx with respect to the reference signal during the half cycle of the subcarrier with the transponder switch T closed. To do. The output of the demodulator 7 (generally the output of a band filter whose center frequency is the secondary carrier frequency) supplies an image signal of the control signal of the switch T. This signal can be decoded (either by the decoder 8 or directly by the microprocessor 6) to restore the original binary data.

【0010】

Data transmission alternates from one to the other and then from the other to the other (half-duplex), with the terminal not transmitting data while receiving data from the transponder.

【0011】

FIG. 2 shows a conventional example of data transmission from the terminal 1 to the transponder 10. This drawing shows an example of the shape of the excitation signal of antenna L1 for the transmission of code 0101. The modulation currently in use is amplitude modulation at a speed of 106 kbits / s (1 bit is transmitted at about 9.5 μs), much lower than the frequency of the carrier coming from oscillator 5 (eg 13.56 MHz) (period about 74 ns). Is. This amplitude modulation is performed with all-or-nothing or the difference between the peak amplitudes (a, b) of the modulation ratios less than 1 (difference between the peak amplitudes (a, b) of the two states (0 and 1)) due to the need to power the transponder 10. Is defined as the sum of these amplitudes). In the example of FIG. 2, a 13.56 MHz carrier wave is amplitude modulated at a rate of 106 kbit / s, for example with a modulation ratio of tm of 10%.

【0012】

FIG. 3 shows a conventional example of data transmission from the transponder 10 to the terminal 1. In this drawing, the control signal V of the transistor T supplied from the modulator 19 is shown._T, And an example of the shape of the corresponding signal Rx received by terminal 1 are shown. The return modulation on the transponder side is generally a resistance type using a carrier wave (called a subcarrier) having a period of, for example, 847.5 kHz (period of about 1.18 μs). This back modulation is based, for example, on BPSK type (phase-shift keying) coding with a speed of about 106 kbits / s, which is much lower than the frequency of the subcarrier. The signal Rx shown in FIG. 3 is "smoothed", i.e. does not show ripples on high frequency carriers (eg 13.56MHz). In the example of FIG. 3, it is assumed that each of the three bits shown is different from the previous bit. So this is, for example, the transmission of code 010.

【0013】

No matter what kind of modulation or return modulation (eg amplitude, phase, frequency) is used, and no matter what kind of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.) is used, this modulation or Back modulation is performed digitally, jumping between two binary levels.

【0014】

Oscillator circuits of terminals and transponders are generally tuned to the carrier frequency. That is, the resonance frequency of the oscillation circuit is set to the 13.56 MHz frequency. The purpose of this synchronization is to maximize power propagation to the transponder, which is generally a credit card sized card that integrates various transponder components.

【0015】

Signal V as shown in Figure 3_TConsists of a pulse train with a secondary carrier frequency (eg 847.5 kHz), and phase shift is performed each time the state changes as one bit shifts to the next. Looking at the signal recovered on the reader side, the shape of this signal does not look like a "digital" signal and is difficult to decode. In fact, the shape of the signal Rx initially grows non-linearly during each bit transmission time (9.4 μs) (capacitor charge) and peaks when two-thirds of the one-bit duration elapses. It reaches, and then also decreases non-linearly. The enable time, that is, the time it takes for the signal Rx to reach a level that the demodulator can decode, is related to the tuned oscillator. A high Q (quality factor) is required because of the need to transmit power for the remote power supply associated with the desired system range, and therefore the oscillator circuit needs to be tuned. The higher the Q, the smaller the passage area. This limits the data flow of the system. The Q of the reader and transponder is generally about 10.

【0016】

Transponders can be made up of a variety of objects (key rings, keys, etc.), but are now mostly in the form of credit cards that integrate all circuits and antennas or inductances L2. When exchanging information with a reader, that is, a terminal, this card is brought closer to the antenna L1 of the reader. The distance between the reader and the card varies, and depending on the application, very tightly coupled transmissions are also used, where the distance between the antennas is less than about 2 centimeters. By using such tightly coupled transmission, it is possible, for example, to allow payment by a transponder, and thus to ensure that the terminal recognizes only the transponder closest to itself.

【0017】

The problem that arises when the oscillators are very close to each other is that the power sent from the terminal to the transponder heats the transponder when the oscillators are substantially tuned (antenna L2 is generally). , Consists of one or more planar spirals around the card). This thermal effect deforms the plastic card.

【0018】

[Problems to be Solved by the Invention]

An object of the present invention is to provide a novel solution that overcomes the shortcomings of the conventional solution when the transponder is in a very tightly coupled relationship with the read / write terminal.

【0019】

An object of the present invention is, in particular, to reduce or minimize the thermal effect associated with remote power supply to a transponder by a read / write terminal.

【0020】

Another object of the present invention is to allow an increase in data transmission rate when the transponder is very close to the terminal.

【0021】

Another object of the present invention is to provide a solution that does not require modification of the structure of the reader or terminal.

【0022】

[Means for solving problems]

A feature of the present invention is that the transponder's oscillator circuit is detuned when the transponder has a very tight coupling relationship with the read or read / write terminal.

【0023】

The frequency detuning of the electromagnetic transponder is described in Document WO-A-98 / 29760. The document states that the transponder's antenna should be frequency detuned, i.e., impedance mismatched so that the transponder and its electronic circuits absorb less radiomagnetic field and power. Therefore, another transponder of this mismatch, i.e. located near the detuned transponder, can receive a sufficient radiomagnetic field and operate properly. The transmission system can detect or refer to this other transponder as if it were the only transponder in the electromagnetic field of the transmitter. According to this document, this mismatching means can also be used to "limit the absorption of power and / or electromagnetic fields by this non-selected transponder when it is in the non-selected state."

【0024】

The solution proposed in this document is to detune transponders that are relatively far from the terminal to maximize the power received by the closest transponder that is supposed to communicate with the terminal. Such a solution does not solve the tight coupling problem described above. In fact, the selected transponders are still in sync.

【0025】

Contrary to this document, the present invention provides a tightly coupled detuning operation. Therefore, one feature of the present invention is to provide a detuning operation of an oscillation circuit of an electromagnetic transponder to which electric power is remotely supplied by a terminal for the purpose of tightly coupled information transmission.

【0026】

The present invention focuses on the fact that the remote power supply recovered on the transponder side is not a monotonous function of the distance between the transponder and the terminal.

【0027】

In fact, if the transponder is brought closer to the terminal while the oscillator circuit is in tune with the remote power supply carrier frequency, the remote power supply amplitude begins to increase from the boundaries of the system range (about 10 cm). This amplitude begins to decrease again when the transponders are very close (less than about 2 centimeters) after taking the maximum value (critical coupling position). For this reason, specifically, in conventional systems, the power of the terminal cannot be subordinated to the distance at which the transponder is located.

【0028】

At this critical coupling position, when both the oscillator circuit of the terminal and the oscillator circuit of the transponder are tuned to the remote power supply carrier frequency, the remote power supply amplitude received by the transponder is maximized, and the coupling between the transponder and the terminal is established. Corresponds to the optimized distance. In other words, the critical coupling frequency corresponds to the distance at which the remote power supply is maximum for the minimum coupling coefficient. The coupling coefficient is the ratio of the square root of the product of the mutual inductance and the inductance of the oscillator circuit.

【0029】

When the transponder oscillator circuit is out of tune with the remote power supply carrier frequency, the power received by the transponder increases as the distance from the terminal decreases. However, the degree is small. In this case, there is a distance at which the received power is maximized for a given detuning condition. This is the optimum coupling, and when the two oscillator circuits are tuned to the carrier frequency, the critical coupling position is the optimum coupling condition. The optimum coupling coefficient between the two oscillators is not determined solely by the inductances L1 and L2, the capacitors C1 and C2, and the frequency (here fixed frequency, which corresponds to the carrier frequency), but the series resistance of the terminal. Note that the load on the R1 and transponder oscillators is also involved. The load of the transponder oscillator circuit referred to here is the equivalent resistance of the circuit (microprocessor, etc.) and the return modulation means (for example, the resistor R in FIG. 1) added in parallel with the capacitor C2 and the inductance L2. This equivalent resistor will be referred to as R2 hereafter.

【0030】

Therefore, even if one of the oscillator circuits is detuned, this non-contact wireless transmission system will operate if the antennas are very close to each other.

【0031】

More specifically, the present invention is of a type that includes an oscillator circuit that is upstream of a transponder and is adapted to supply a DC power supply voltage to an electronic circuit that includes a means of transmitting digitally coded information. Electromagnetic transponders and electromagnetic transponders that include means of detuning the oscillator circuit to a given frequency, which is used when the transponder must transmit information while in close proximity to the read / write terminal. I will provide a.

【0032】

According to one embodiment of the present invention, the predetermined frequency corresponds to the frequency of the carrier wave from the terminal that remotely supplies power to the transponder.

【0033】

According to one embodiment of the present invention, the means for detuning the oscillating circuit is formed from an induction component of the oscillating circuit and a capacitor with a switch in parallel, and the rectifying means is formed from a unidirectional conductive component.

【0034】

According to one embodiment of the present invention, the means for detuning the oscillating circuit is formed from two capacitors coupled to each terminal of the induction component of the second oscillating circuit, and these capacitors are switched respectively. Coupled in series with the means, the reference terminal of the switching means is connected to the reference power potential of the electronic circuit downstream of the rectifying means.

【0035】

According to one embodiment of the present invention, the means for detuning the oscillator circuit is at two positions that allow the transponder to select between an operation tuned to the predetermined frequency and an operation detuned from this frequency. Can be controlled between.

【0036】

According to one embodiment of the present invention, the means for detuning the transponder is also used to detect the distance from the read / write terminal to the transponder.

【0037】

According to one embodiment of the present invention, two resistance type modulation means connected in parallel with a capacitor for smoothing the rectified voltage supplied by the rectifying means are further included, and each modulation means is a transponder tuning operation mode. It is dedicated to one of the detuning operation modes.

【0038】

The present invention also relates to a non-contact wireless data transmission system between a terminal that generates an electromagnetic field and at least one transponder that does not have an independent power source.

【0039】

According to one embodiment of the present invention, the data transmission speed from the transponder to the terminal differs depending on whether or not the oscillation circuit of the transponder is tuned to the predetermined frequency.

【0040】

According to one embodiment of the present invention, the data transmission speed from the terminal to the transponder differs depending on whether or not the oscillator circuit of the transponder is tuned to the predetermined frequency.

【0041】

The following non-limiting description of a particular embodiment in connection with the accompanying drawings details the above objectives, features and advantages of the present invention.

【0042】

BEST MODE FOR CARRYING OUT THE INVENTION

Throughout all drawings, the same parts have the same reference numerals. The scale of drawings is not uniform. For clarity, only the parts necessary for understanding the present invention are shown in the drawings and will be described below. Specifically, the details of the digital electronic circuit are not shown regardless of whether it is on the transponder side or the reader side.

【0043】

A feature of the present invention is that the transponder oscillator circuit is detuned when it is tightly coupled to the terminal, that is, when the antennas are located, for example, less than 2 centimeters from each other.

【0044】

As a result of such detuning, the operation becomes close to the operation of the transformer, that is, the Q (quality factor) becomes small. In conventional tuning operations, Q should be as high as possible in order to optimize power transfer associated with remote power supply to the transponder.

【0045】

Detuning the transponder when the bond is very tight has several advantages.

【0046】

In the detuning operation, the terminal power, i.e. the current of the antenna (L1 in FIG. 1), may drop while transmitting sufficient power for the remote power supply of the card. In reality, the transponder is so close to the terminal that the problem of remote power supply range is eliminated. The required power is substantially determined by the transformation ratio (ratio of the number of spirals) of the inductance of the oscillator circuit. By reducing the antenna current, the thermal effect on the transponder side is suppressed.

【0047】

Figure 4 shows the shape of the signal Rx on the reader side after resistance-type return modulation on the transponder side. At this time, the oscillation circuit of the terminal is detuned from the remote power supply transport frequency (for example, 13.56 MHz). Compare this drawing with FIG. As you can see, the enable time is almost gone. The shape of the signal Rx is quite similar to that of the control signal at the gate of transistor T on the transponder side. Therefore, it is not necessary to wait for each transmission bit of the signal Rx to reach the detection threshold value of the phase demodulator of the terminal, and the transmission speed can be increased.

【0048】

Note that detuning the transponder oscillator is only desirable if the coupling is very tight. As such, the present invention can easily separate the two modes of operation of the system depending on whether the transponder is very close to the reader . It should also be noted that the coupling coefficient is reduced by detuning the oscillator circuit. This is not an obstacle, as the two oscillators are very close to each other in transformer-type operation.

【0049】

The advantage of detuning the transponder oscillator is that it does not require physical modification of existing read / write terminals. This is particularly advantageous for systems that provide terminal parks, at least in part of which are already installed. Furthermore, if there is no need to change the operating mode between tightly coupled and loosely coupled, no intervention is required on the terminal, including for software. In the opposite case, it is only necessary to reprogram the terminal to accept two different modes of operation that are located in the electromagnetic field and are determined, for example, by the transponder that sends information to the corresponding terminal in this direction.

【0050】

The first solution for detuning the transponder oscillator circuit involves connecting two capacitors in parallel with the transponder antenna. At this time, one capacitor is coupled in series with a switch that makes it controllable. This solution consists of using the type of assembly described in document WO-A-98 / 29760, which provides transponder frequency detuning with a correctable capacitance in the oscillator circuit.

【0051】

According to the present invention, the capacitance C2 must be increased relative to its tuned value for optimal coupling corresponding to the shortest possible distance between the terminal and the transponder. As a result, the resonance frequency of the transponder oscillation circuit is reduced.

【0052】

FIG. 5 shows a first embodiment 30 of a transponder based on the present invention. As before, this transponder consists of a parallel oscillator with an inductance L2 and a capacitor C2'between the two terminals 11'and 12' of the circuit.

【0053】

In the embodiment shown in FIG. 5, the rectification performed to extract the DC power supply voltage Va smoothed by the capacitor Ca is connected to the anode terminal 11'and the cathode to the positive terminal 14 of the capacitor Ca. Single-half-wave rectification by diode D. The reference voltage 15 corresponds to the negative terminal of the capacitor Ca directly connected to the terminal 12'. The voltage Va means, for example, for the electronic block 31 including the circuits 16 to 20 of FIG. A capacitor C3 is connected in series with a switch (eg MOS transistor) K1 between terminals 11'and 12'. Switch K1 is controlled by circuit 31 and is closed during tuning.

【0054】

FIG. 6 shows a second embodiment 30'of a transponder based on the present invention. According to this embodiment, the terminals 11 and 12 of the oscillation circuit are connected to the AC input terminal of the bridge 13 composed of the diodes D1 to D4 as shown in FIG. 1, for example. The two output terminals 14 and 15 of the rectified bridge 13 supply the power supply voltage Va of the electronic block 31 via the smoothing capacitor Ca.

【0055】

According to this embodiment, two capacitors C3 and C4 are connected in series with switches (eg, MOS transistors) K1 and K2, respectively, between terminals 11 and 15 and between terminals 12 and 15, respectively. Therefore, the first terminal of the capacitor C3 is connected to the terminal 11, and the second terminal is connected to the terminal 15 via the transistor K1. The first terminal of the capacitor C4 is connected to terminal 12, and the other terminal is connected to terminal 15 via transistor K2. Capacitors C3 and C4 are each coupled to the respective codes of the high frequency AC voltage V2 across the antenna L2. Therefore, the values of capacitors C3 and C4 are the same. Transistors K1 and K2 are controlled by block 31, preferably with the same signal, and are closed when this circuit must be tuned to the remote power supply carrier frequency.

【0056】

Note that the doubled capacitor allows one reference node to be used to control switches K1 and K2 (line 15). Therefore, when switches K1 and K2 are formed from N-channel MOS transistors, these switches can be controlled with all-or-nothing by the logic signal coming from block 31. This is not possible with the solution proposed by document WO-A-98 / 29760.

【0057】

For example, the capacitors C2', C3 and C4 each have a capacitance corresponding to 1/2 of the capacitance required to tune the oscillator circuit to the carrier frequency of the reader (C2 in FIG. 1).

【0058】

FIG. 7 shows a third embodiment 30 "of a transponder according to the present invention. The transponder 30" includes substantially the same components as in FIG.

【0059】

A feature of this third embodiment is that it includes an inductance L2'with an intermediate point. This midpoint is used as the reference 15 for the DC power supply of the transponder electronic circuit 31. Therefore, the first terminal 11 of the winding L2'is connected to the anode of the rectifier diode D1', whose cathode forms the positive local power supply terminal 14 of the transponder. The second terminal 12 of the winding L2'is connected to the anode of the second rectifying diode D4' whose cathode is connected to the terminal 14. As before, a capacitor Ca is connected between the terminal 14 and the reference line 15 in order to smooth the power supply voltage of the electronic circuit 31. A switched capacitor composed of switches K1 and K2 and capacitors C3 and C4 in series with switches K1 and K2 is connected between the terminals 11 and 12 and the reference line 15, for example, as in FIG.

【0060】

Note that switches K1 and K2 may be controlled by the same control signal supplied by block 31 or by different control signals, as shown in FIG. 7 (FIG. 6).

【0061】

In the embodiments of FIGS. 6 and 7, it is preferable to turn on the transistors K1 and K2 when tuning the oscillator circuit. In the circuit of Figure 7, the resonant frequency pushes back approximately twice the carrier frequency when transistors K1 and K2 are off, especially if capacitors C3 and C4 each have twice the value of capacitor C2'. Will be done.

【0062】

The transponders 30 (FIG. 5), 30'(FIG. 6) or 30'(FIG. 7) of the present invention further include a resistance return modulation circuit, which is in series with switches K3 and K4, respectively, with terminal 14. It is preferably formed from two resistors R3, R4 between 15. Resistors R3 and R4 have different values, R3 is higher and R4 is lower.

【0063】

Assuming it is located between the critical coupling and the terminal, the resistor R3 with a high value is used to perform the return modulation and the transistors K1 (or transistors K1 and K2) are turned off. The operation of the system is a detuning operation similar to the operation of the transformer.

【0064】

It is assumed that it is located far from the critical coupling position and further away from the terminal, that is, loosely coupled. Transistors K1 (or transistors K1 and K2) are turned on and resistor return modulation is performed by the smaller resistor R4. This is the conventional mode of operation.

【0065】

Note that the system range is optimized in the present invention by using low value resistors when located away from the terminal. The ratio of the respective values of the resistors R3 and R4 is, for example, 4 to 10 (R3 is 0.4 to 5 ohms, R4 is 100 to 500 ohms), and is preferably about 6 (for example, about 1500 ohms and 250 ohms). ..

【0066】

As an alternative, one or more capacitors used to detune the circuit can also be used as the back modulation means. In this case, the switches with switches R3, K3 and R4, K4 are eliminated and the values of the capacitors C2', C3 (and C4 in the embodiment of FIG. 7) are detuned when the importance of detuning is capacitive modulation. Is selected to be compatible with the phase shift detected by the terminal. This capacitive modulation does not affect its amplitude, but directly affects the phase of the voltage across the terminal inductance L1. This facilitates phase detection by the terminal. It should be noted that this type of reverse modulation does not modify the coding, i.e., the control signal of the carrier frequency of the reverse modulation switch.

【0067】

When setting the size of the oscillating circuit capacitor, pay attention to the value of the rectifying means and the smoothing capacitor Ca used. In fact, the conduction period of the bridge (Fig. 7) diode compared to the period of the remote power supply carrier is generally shorter than the conduction period of the single-wave rectifier diode (Fig. 6). Therefore, the duty cycle of operation of the return modulation means depends on the type of rectification performed. Therefore, this duty cycle specifically affects the value of the equivalent resistor R2 and thus the coupling coefficient.

【0068】

Information about the distance from the transponder to the terminal is used to detun the oscillator circuit of the transponder, which is in a very tight coupling. This distance is determined by the transponder or terminal.

【0069】

On the transponder side, one of the embodiments shown in FIGS. 5 to 7 is used. According to the present invention, each of these electronic circuits comprises an input DET that receives a local supply voltage Va. The input DET is coupled to a circuit (not shown) that measures the voltage Va and at least one component that stores this measurement. In a particular embodiment, this circuit is a microprocessor (6 in Figure 1). The memory of the measured voltage value is performed in an analog manner or in a digital manner using several bits. The number of bits depends on the desired analysis accuracy.

【0070】

According to a preferred embodiment of the invention, the following measurement cycles are periodically performed when the transponder is within range of the terminal. Preferably this is done as soon as the transponder is activated (powered) by entering the electromagnetic field of the reader. First, turn on transistors K1 (Fig. 5) or transistors K1 and K2 (Fig. 6) to tune the oscillator circuit. Measure the voltage at the terminal DET. Next, the transistors K1 and K2 are turned off. The circuit is detuned and its resonant frequency is shifted higher than twice the tuning frequency if the values of capacitors C2'and C3 are the same in the case of FIG. Measure the voltage at the terminal DET again. As an alternative, the first measurement is performed on a detuned circuit. The two values obtained are compared and the result of this comparison is stored, for example, in a single bit.

【0071】

The time required to perform the two measurements "tune" and "detuning" (eg, hundreds of milliseconds) is short compared to the transponder displacement rate, which corresponds to the hand displacement rate in most application tasks. ..

【0072】

In addition, the time that the oscillator circuit is detuned to perform the measurement is chosen to be substantially different from the half-cycle of the subcarrier, so it is preferable that the terminal does not interpret this measurement as reverse modulation. .. In fact, the detuning of the transponder oscillator circuit is interpreted as the phase shift of the terminal oscillator circuits L1 and C1 (Fig. 1), which should not be mistaken for data transmission during distance determination.

【0073】

The above measurement cycle is repeated after a short time interval (for example, about 1 millisecond), which is still fast compared to the transit time of the transponder in front of the terminal (several hundred milliseconds).

【0074】

Note that in a simple embodiment, it is sufficient to determine the position of the transponder with respect to the critical coupling before the start of each data transmission from the transponder to the terminal.

【0075】

From the change in the value of the comparison bit, it is possible to know whether the transponder is closer to or farther from the terminal than the critical coupling position. If the bit indicates that the detuning position is higher than the tuning position, it means that the transponder is very close to the terminal (tightly coupled). In the opposite case, the transponder is near the critical coupling or between the critical coupling position and the boundary of the system range.

【0076】

In a simple embodiment, the use of a dedicated distance determination input (DET) can be avoided by using the existing input of the transponder's microprocessor (included in block 31). This conventional input controls the available local supply voltage across the capacitor Ca with respect to a predetermined threshold. The microprocessor stores the state of this voltage (in the form of bits) with respect to this threshold. This bit is, for example, whether the voltage recovered by the oscillator circuit is sufficient for the transponder supply, and thus is sufficient to activate the transponder when it enters the electromagnetic field of the reader. Used to detect. This feature is found in transponder microprocessors, such as STMicroelectronics circuits ST16 and ST19, and can therefore be used without significant changes to the transponder.

【0077】

Distance determination for critical coupling has the advantage that the distance determination or region detection (tightly coupled or loosely coupled) performed is similar to differential measurement. In fact, this detection is performed on critical couplings that depend on the system and its environment. When the circuit is tuned, the recovered voltage threshold is maximized only at critical coupling. Therefore, there is no need to set a specific reference or distance threshold. In other words, the threshold distance between the two modes of operation, tuning and detuning, is self-adaptive.

【0078】

Note that the present invention does not require (modulated) information transmission from the reader to the transponder or from the transponder to the reader for distance determination. Therefore, the transponder of the present invention detects its position without the intervention of a reader. Transponders can send different messages based on their position, for example in reverse modulation, the essence of which message puts the system in one or another mode of operation.

【0079】

It should also be noted that data transmission from the transponder to the terminal can always be carried out based on the phase shift keying method, regardless of whether it is a tuning operation or a detuning operation. In fact, detuning does not change the high frequency carrier frequency (eg 13.56MHz) that detects phase shift at the subcarrier speed (eg 847.5kHz).

【0080】

Of course, the present invention has various alternatives, modifications and improvements that can be easily conceived by those skilled in the art. In particular, the sizing of different resistor and capacitive components will vary depending on the application and in particular the frequency and system range of the various carriers. Similarly, those skilled in the art can implement various circuits of transponders or terminals based on the present invention based on the functional instructions described so far. In addition, distance detection between the transponder and the terminal can be performed using other means, eg, the terminal described in WO-A-97 / 34250. However, a preferred embodiment of the present invention has the advantage that the terminal is not used for distance determination or detuning.

【0081】

The present invention relates, among other things, to non-contact chip cards (eg, ID cards for access control, electronic wallet cards, cards for storing information about cardholders, consumer fidelity cards, subscriber television cards). Etc.), and are applicable to read or read / write systems for these cards (eg, portico ie access control terminals, automatic payment machines, computer terminals, telephone terminals, televisions or satellite decoders, etc.).

【0082】

Such substitutions, modifications and improvements form part of the present disclosure and are within the spirit and scope of the present invention. Therefore, the above description is for illustration purposes only and is not intended to limit the present invention. The present invention is limited only by the definition of claims and their equivalents.

[Simple explanation of drawings]

[Figure 1]

It is a figure for showing a prior art and a problem to be solved.

[Figure 2]

It is a figure for showing a prior art and a problem to be solved.

[Fig. 3]

It is a figure for showing a prior art and a problem to be solved.

[Fig. 4]

It is a timing diagram which shows one Embodiment of the transmission method of this invention in tight coupling.

[Fig. 5]

It is a figure which shows the 1st Embodiment of the electromagnetic transponder based on this invention which includes means for detuning the oscillation circuit of a transponder.

[Fig. 6]

It is a figure which shows the 2nd Embodiment of the electromagnetic transponder based on this invention which includes means for detuning the oscillation circuit of a transponder.

[Fig. 7]

It is a figure which shows the 3rd Embodiment of the electromagnetic transponder based on this invention which includes means for detuning the oscillation circuit of a transponder.

[Explanation of symbols]

30 transponder L2, C'2 parallel resonant circuit D diode Ca capacitor K1 switch