FR2853479A1 - Remote control device for e.g. object identification application, has portable object equipped with antenna coil that is coupled inductively to another antenna coil of fixed station and including load impedance modulation circuit - Google Patents

Abstract

The device has a portable object (1) equipped with an antenna coil (3) that is coupled inductively to an antenna coil (4) of a fixed station (2) which emits an alternative magnetic field. The portable object includes a load impedance modulation circuit (9) of the antenna coil (3). The modulation unit includes an imposing unit to impose k different levels, where k is greater than 2.

Description

Modulated inductive coupling teletransmission device

multilevel

  TECHNICAL FIELD OF THE INVENTION The invention relates to a teletransmission device comprising a portable object provided with an antenna coil inductively coupled to an antenna winding of a fixed station emitting an alternating magnetic field, the portable object comprising means for modulating a load impedance of the antenna coil of the portable object by binary data signals.

  State of the art Inductive coupling is conventionally used for the teletransmission of data between a portable object, in particular of the card, ticket, tag, etc. type, and a fixed station, for example constituted by a card reader, an interrogator of labels, etc., in the field of object identification, access control, electronic toll collection, etc. In most teletransmission devices of this type, the portable object is passive. It is therefore remotely powered by the fixed station, which has its own power supply circuit, by means of the inductive coupling which is produced by antennas constituted by coils. These devices generally use the principle of charge modulation.

  As shown in FIG. 1, when the portable object, for example constituted by a tag 1, is arranged sufficiently close to the fixed station, constituted for example by a reader 2, for inductive coupling, the windings Antenna 3 and 4 of the label 1 and the reader 2 behave like a transformer, of poor quality. Since the antenna coil 4 of the reader 2 emits an alternating magnetic field, the tag 1 interacts on this magnetic field via its antenna coil 3 by emitting a secondary magnetic field, depending on the impedance of the circuits. connected to the antenna coil 3. This induces a detectable voltage in the antenna coil 4 of the reader. A modification of the secondary impedance Z of this transformer can thus be detected by the reader 2, 10 when it is sufficient in view of the sensitivity of the reader. A binary data signal D controls the value of the impedance present at the terminals of the antenna winding 3 of the tag 1, the impedance Z can take two distinct values, respectively Z0 and Z1, depending on whether the data signal D takes the binary value O or the binary value 1.

  A capacitance Ce can be connected in parallel on the antenna winding 3 and the inductance Z, thus constituting a resonator, whose impedance can be modified either by resistive modulation, that is to say by modification of the component resistive, either by capacitive modulation, i.e. by changing the capacitive component of the resonator. The modification of the impedance Z causes a change in the amplitude of the voltage V at the terminals of the antenna winding 3 and, consequently, a change in the current flowing through the antenna winding 3 and, therefore, the magnetic field that 'he emits. The binary data signal D thus modulates the amplitude of the voltage V at the terminals of the antenna coil 3, as shown in FIG. 2, this amplitude taking, for the same magnetic field from the reader 2, a first value when the data signal D takes the binary value O and a second value, less than the first value in FIG. 2, when the data signal D takes the binary value 1.

  To enable the remote-feed of the tag 1, it preferably comprises, as represented in FIG. 3, a rectifying circuit 5, which can be constituted by a diode, a diode bridge or any suitable circuit, Connected to the output of the antenna winding 3. A DC voltage VDD is therefore produced at the output of the rectifying circuit 5, from the voltage at the terminals of the antenna winding 3, induced by the magnetic field emitted by the reader 2.

  The magnetic field seen by the label 1 varies rapidly with the distance separating the label from the reader 2. The electromotive force induced in the antenna winding 3 of the tag 1 can thus vary in large proportions. Since the tag consists essentially of an integrated circuit connected to the antenna winding 3, this circuit is generally protected by means of limiting the voltage across the antenna winding, which makes it possible to use low voltage technologies. standard, less expensive than high voltage technologies. It is known to use for this purpose a shunt type control circuit, the principle of which is illustrated in FIG. 3. The control circuit of FIG. 3 comprises a regulator 6 connected to the output of the rectification circuit 5 and controlling the value of a controlled variable impedance Ze, connected to the terminals of the antenna winding 3. In the particular embodiment illustrated in FIG. 4, the regulator comprises a divider bridge consisting of two resistors, R1 and R2, connected together. series between the output of the rectifier circuit 5 and the ground, and an amplifier 7, having an input 25 connected to the midpoint of the divider bridge and another input connected to a reference voltage Vref. The variable impedance is constituted by a transistor T whose control electrode is connected to the output of the amplifier 6.

  The voltage VDD is thus regulated to a value close to Vref (R1 + R2) / R1, which regulates and limits the voltage at the terminals of the antenna winding 3.

  Such a regulation can, however, in the absence of particular precautions, come into conflict with a modulation of the type illustrated in FIG.

  Several solutions have been proposed to solve this problem: - Limit the bandwidth of the regulation loop constituted by the rectifying circuit 5, the regulator 6 and the controlled variable impedance Ze, so that the variations of the impedance controlled by the data signal D are too fast for regulation. This is possible since field variations are due to motions, so they are low frequency variations.

  - Perform a capacitive modulation by moving the resonant frequency of the antenna winding of the label on either side of the field frequency according to the data signal D, so as to maintain constant electromotive force.

  - Use an input of the regulator 6 to perform the modulation by the data signal D. This can, for example be achieved by modulating the reference voltage Vref, which can take two values VrefO and Vrefl depending on the value of the binary signal of D data, as shown in Figure 5. In another embodiment (FR-A-2776865), illustrated in Figure 6, a voltage, variable between two values EO and El, depending on the value of the binary signal D data is interposed between the VDD output of the rectifier circuit 5 and the input of the regulator 6.

  All known teletransmission devices described above allow the transfer of binary data only at a reduced rate. However, many potential applications, for example biometric identification, require the transfer of a large amount of information, in a time that must remain low enough for a user to consider that the transmission is almost instantaneous. Increasing the frequency of the D data bit signal could increase the bit rate. However, such an increase would result in an increase in the bandwidth required. However, an increase in the bandwidth would require a reduction in the quality factor of the tag, to the detriment of the performance of the remote power supply, due to a reduction in the overvoltage coefficient of the RLC circuit.

  OBJECT OF THE INVENTION The object of the invention is an inductive coupling teletransmission device which does not have these disadvantages and, more particularly, a device having a high bit rate without any significant change in the bandwidth.

  According to the invention, this object is achieved by the fact that the modulation means comprise means for imposing k values different to the load impedance, k being strictly greater than 2.

  According to a development of the invention, the modulation means comprise a plurality of dipoles and means for selectively connecting the dipoles to the antenna coil of the portable object, so as to constitute different load resistances, associated with different words of n bits.

  According to another development of the invention, the portable object comprises a voltage regulation circuit 25 and the modulation of the load impedance can be achieved by acting on the level of one of the voltages used by the control circuit.

Brief description of the drawings

  Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of non-limiting example and shown in the accompanying drawings, in which: FIG. 1 illustrates a remote transmission device inductively coupled according to the prior art.

  FIG. 2 shows the variations of the voltage V across the antenna coil of a reader of a device according to FIG. 1 as a function of the value of the binary data signal D. FIGS. 3 to 6 respectively represent the principle and alternative embodiments of a device according to the prior art, including voltage regulation.

  Figure 7 illustrates a device according to the invention.

  FIG. 8 shows the variations of the voltage V across the antenna coil of a reader of a device according to FIG. 7 as a function of the binary data signals.

  FIG. 9 illustrates a first particular embodiment of a portable object of a device according to FIG. 7.

  FIG. 10 represents an alternative embodiment of the modulation of a device according to FIG. 7.

  Figures 11 and 12 illustrate two particular embodiments of a portable object, with voltage regulation, of a device according to the invention.

  Figures 13 to 18 illustrate various embodiments of the modulation in a portable object according to Figure 12.

  Description of particular embodiments.

  The device shown in FIG. 7 makes it possible to increase the bit rate, without significantly increasing the bandwidth. This device is distinguished from the device of FIG. 1 by the fact that the load impedance of the antenna winding 3 can take k different values, Z1 ... Zk, k being strictly greater than 2. The binary data signals are encoded, by any appropriate means, in the form of n-bit words. The data signals, in the form of n Bi ... Bn input signals, of one bit, are then applied, in parallel, to n control inputs of a load impedance circuit 9. The k different values of the load impedance are thus associated with k different words of n bits, with k = 2.

  The amplitudes of the voltage V at the terminals of the antenna winding 3 and the current in the antenna winding are thus modulated on k levels, for example on 4 different levels for data coded in the form of 2-bit words ( n = 2 and k = 4) as shown in FIG. 8 The bit rate can be multiplied by n relative to that of the device according to FIG. 1, while keeping the same transition rate from one amplitude to the other of the voltage 20 V and, consequently, a substantially identical bandwidth.

  In the particular embodiments of FIGS. 9 and 10, the n input bits, Bi ... Bn, directly control the value of an impedance connected in parallel with the antenna coil 3. The modulation circuit 9 comprises 25 then dipoles can be connected selectively to the antenna coil 3, so as to constitute different load resistances, associated with k different words of n bits.

  In the label of FIG. 9, a first resistive dipole, comprising a resistor R in series with an electronic switch constituted by a transistor T1, of the MOS type, whose gate receives a first bit Bi of data, is connected in parallel on the antenna coil 3 and an initial load resistor RO. Second and third resistive dipoles, respectively comprising resistors 2R and 4R and transistors T2 and T3, respectively controlled by Bits B2 and B3, are also connected in parallel to the antenna coil 3. Each transistor is respectively conductive or blocked according to the binary value of bit B applied to its gate. In the example shown in FIG. 9, the load impedance can thus take 8 different values (RO, RO in parallel on R, on 2R, on 4R, on R and 2R, on R and 4R, on 2R and 4R, on R, 2R and 4R), each value of the load impedance corresponding to a particular amplitude of the voltage V. The voltage V is thus modulated on 8 levels (k = 8) on the basis of 3-bit words ( n = 3). More generally, the load impedance can be modulated on k levels with n input bits (k = 2n) simultaneously controlling n transistors of n resistive dipoles.

  In the modulation circuit of FIG. 10, a load transistor TO is connected in parallel to the antenna winding 3. Its gate is connected to 4 different bias voltages, V1 to V4, via 4 circuits. of polarization. Each bias circuit comprises two transistors connected in series and controlled respectively by the bit Bi or its complement Bi and by the bit B2 or its complement B2. The complements of the signals B1 and B2 are obtained, in known manner, by inverting the signals B1 and B2. Thus, in FIG. 10, the bits Bi and B2 control the transistors of the bias circuit connected to Vi, while the bits Bi and B2 control the transistors connected to V2, the bits B1 and B2 controlling the transistors connected to V3 and bits Bi and B2 the transistors connected to V4. For each 2-bit word, only one of the bias circuits is conductive, and defines the bias voltage applied to the gate of the charge transistor TO and, consequently, the resistance thereof, which defines the load impedance of the antenna winding 3. More generally, the control electrode of the charge transistor TO can be connected to k different bias voltages by k bias circuits each having n transistors connected in series and controlled by the n bits of FIG. entry and their supplements.

  The embodiments described above, however, have a disadvantage. Indeed, load impedances Z1-Zk depend on the field level if it is not constant.

  In a preferred embodiment, which makes it possible to remedy this drawback, the load impedance modulation circuit controls one of the voltages used by a shunt type control circuit comprising, as in FIG. 5, connected to the output of the antenna coil 3, a regulator 6 connected to the output of the rectifying circuit 5 and controlling the value of a controlled variable impedance Ze connected to the terminals of the antenna winding 3.

  In a first variant embodiment, illustrated in FIG. 11, a modulation circuit 10, comprising n inputs receiving respectively bits Bi to Bn, controls the reference voltage Vref, imposing on it different values, Vrefl ... Vrefk, associated with each other. at k different words of n bits.

  In a second variant embodiment, illustrated in FIG. 12, a modulation circuit 11, comprising n inputs receiving the bits Bi to Bn respectively, is connected between the output of the rectifying circuit 5 and the first input of the regulator 6. The circuit modulation 11 causes a voltage drop that can take k different values, El ... Ek, associated with k different words of n bits.

  In an embodiment illustrated in FIG. 13, the modulation circuit 11 comprises two transistors T4 and T5 connected in series between the output voltage VDD of the rectifying circuit 5 and the first input of the regulator 6 and respectively controlled by two bits. Bi and B2. Different resistors, respectively R4 and R5, are connected in parallel on the transistors T4 and T5. Depending on the value of bit B1, transistor T4 is conductive or blocked and the voltage drop across its terminals is zero or proportional to R4. Similarly, the voltage drop across the transistor T5 may be zero or proportional to R5. The voltage drop across the modulation circuit 1 can thus take 4 different values, associated with the 4 different values of a 2-bit word.

  The resistors R4 and R5 may be replaced by diodes each having a threshold voltage Vd As shown in FIG. 14, a first diode D1 is connected in parallel with the transistor T4, while two diodes D2 and D3 connected in series. are connected in parallel to the transistor T5. The voltage drop across the modulation circuit 11 can thus take 4 values, 0, Vd, 2Vd or 3Vd, respectively associated with 4 different values of a 2-bit word. The voltage at the terminals of the antenna winding 3 is then regulated over 4 levels, respectively corresponding to Vref, Vref + Vd, Vref + 2Vd and Vref + 3Vd, associated with the 4 different values of a 2-bit word.

  More generally, the modulation circuit 11 may comprise n transistors, respectively connected in parallel with n associated dipoles, causing different voltage drops across the associated transistors when they are blocked, the n transistors being connected in series and comprising control electrodes constituting the control inputs of the modulation circuit. When the dipoles are constituted by diodes, i diodes in series are associated with a transistor associated with a control input of order i, i being between 1 and n.

  In another alternative embodiment of the modulation circuit 11, illustrated in FIG. 15, three MOS-type transistors T6, T7 and T8, whose gate is connected to the drain, are connected in series between the output voltage VDD of the 10 rectifying circuit 5 and the first input of the regulator 6. The bit Bi is connected to the substrate of a transistor T6, while the B2 is applied to the substrates of the two transistors T7 and T8. As the threshold voltage of a MOS transistor depends on its substrate potential, it is thus possible to modulate the threshold voltage of transistors T6 to T8 and, consequently, the voltage drop across their terminals. As previously, the voltage drop across the modulation circuit 11 can thus be modulated over 4 levels, associated with the 4 different values of a 2-bit word. More generally, the modulation circuit 11 may comprise a plurality of transistors connected in series, each transistor, of the MOS type, comprising a substrate, a gate, a drain connected to the gate and a source, an order control input. modulating means, i being between 1 and n, being connected to the substrates of i transistors associated with this control input.

  It is also possible to replace the modulation circuit 11 by a modulation circuit 12, inserting, between the output of the rectifying circuit 5 and the first input of the regulating circuit 6, a voltage having a predetermined value among k different values, associated respectively with k different words of n bits.

  In an alternative embodiment, the regulation circuit 12 has n dipoles connected in series. Each dipole, only one of which is shown in FIGS. 16 and 17, comprises a voltage source consisting of a battery 13 (FIG. 16) or a preloaded capacitor 14 (FIG. 17), connected in series with a transistor T9 whose grid receives the bit Bi. An additional transistor T10 is connected in parallel with the series circuit formed by the voltage source (13 or 14) and the transistor T9. The gate of the additional transistor T10 is connected to the gate of the transistor T9 by an inverter circuit 15. The voltage inserted by the modulation circuit 12 thus takes k levels associated with the different words of n 10 bits.

  In the particular embodiment illustrated in FIG. 18, a power source 16 preloaded at a predetermined voltage among k different values, associated respectively with k words of n bits, can be introduced between the output of the rectifying circuit 5 and the first input of the control circuit 6. The modulation circuit 17 then comprises the energy source 16, a charging circuit of the energy source, a switch 18 connecting the power source 16 to the charging circuit for a period of time. precharging phase and a switch 19 connecting the preloaded power source between the output of the rectification circuit 5 and the first input of the regulating circuit 6 during a modulation phase. The charging circuit comprises an analog-digital converter 20 whose inputs constitute the control inputs on which the n bits Bi to Bn are simultaneously applied.

  The converter 20 thus charges the energy source 16 at a voltage dependent on the n-bit word applied to its inputs and this voltage is then used during the modulation phase.

  The invention is also applicable when the rectifying circuit is not ideal and / or if only a fraction of the output voltage of the rectifying circuit 5 is applied to the input of the regulator 6.

Claims (16)

claims   1. A teletransmission device comprising a portable object (1) provided with an antenna coil (3) inductively coupled to an antenna coil (4) of a fixed station (2) emitting an alternating magnetic field, l portable object (1) comprising means for modulating a load impedance of the antenna coil of the portable object by binary data signals, characterized in that the modulation means comprise means for imposing different values (Zl ... Zk) at load impedance, where k is strictly greater than 2.   2. Device according to claim 1, characterized in that the binary data signals are coded as n-bit data words to form n bit input signals (B1 ... Bn), applied in parallel with n control inputs of the modulation means, k different values (Z1 ... Zk) of the load impedance being associated with k different words of n bits, the number k of distinct values being equal to 2n.   3. Device according to claim 2, characterized in that the modulation means comprise a plurality of dipoles and means for selectively connecting the dipoles to the antenna coil (3) of the portable object (1), so as to constitute k different load resistances associated with k different words of n bits.   4. Device according to claim 3, characterized in that the modulation means comprise n resistive dipoles having n distinct values, each having a resistor (R, 2R, 4R) in series with an electronic switch (T1, T2, T3), connected in parallel with the antenna coil (3) of the portable object (1), each electronic switch (T1, T2, T3) comprising a control terminal constituted by a control input (B1, B2, B3) of the modulation means.   5. Device according to claim 3, characterized in that the modulation means comprise a charge transistor (TO), connected in parallel with the antenna coil (3) of the portable object (1) and comprising a control connected to k different bias voltages (V1, V2, V3, V4) 10 through polarization circuits, each bias circuit having n series-connected transistors controlled by the n input signals and their complements binaries.   6. Device according to claim 2, characterized in that the portable object 15 comprises a rectifying circuit (5) connected to the antenna coil (3) of the portable object (1), a control circuit (6) having a first input connected to the output of the rectifying circuit (5), a second input connected to a reference voltage (Vref) and an output connected to a control input of a variable impedance (Ze), connected across the terminals of the 20 antenna coil (3) of the portable object (1).   7. Device according to claim 6, characterized in that it comprises means (10) for imposing k different values (Vref1, ..., Vrefk) to the reference voltage (Vref), associated with k different words of n bits.   Device according to claim 6, characterized in that the first input of the control circuit (6) is connected to the output of the rectification circuit (5) via a modulation circuit (11) controlled by the n input signals (B1 ... Bn) causing, between the output of the rectification circuit (5) and the first input of the control circuit (6), a voltage drop having a predetermined value among k distinct values ( El ... Ek), associated with k different words of n bits.   9. Device according to claim 8, characterized in that the modulation circuit (11) comprises n transistors (T4, T5) respectively connected in parallel with n associated dipoles causing different voltage drops across the transistors (T4, T5). ) associated when these are blocked, the n transistors (T4, T5) being connected in series and having control electrodes constituting the control inputs (Bl.Bn) modulation means.   10. Device according to claim 9, characterized in that the dipoles are constituted by resistors (R4, R5).   11. Device according to claim 9, characterized in that the dipoles are constituted by diodes (D1, D2, D3), i diodes in series being associated with a transistor associated with a control input of order i, i being understood between 1 and n.   12. Device according to claim 8, characterized in that the modulation circuit (11) comprises a plurality of transistors (T6, T7, T8) connected in series, each transistor, of the MOS type, comprising a substrate, a gate, a gate, drain connected to the gate and a source, a control input (B1, B2) of order i modulation means, i being between 1 and n, being connected to the substrates of i transistors associated with this control input.   Device according to Claim 6, characterized in that the first input of the control circuit is connected to the output of the rectifying circuit via a modulation circuit (12) controlled by the n input signals ( Bi ... Bn) and inserting, between the output of the rectification circuit (5) and the first input of the regulating circuit (6), a voltage having a predetermined value among k different values, respectively associated with k different words of n bits.   14. Device according to claim 13, characterized in that the modulation circuit (12) has n dipoles connected in series and each having a voltage source (13, 14), connected in series with a first transistor (T9) having a control electrode constituting one of the control inputs (B1) of the modulation means, and an additional transistor (T10) connected in parallel to the series circuit formed by the voltage source (13, 14) and the first transistor (T9 ) and having a control input connected to the control electrode of the first transistor (T9) by an inverter circuit (15). 15. Device according to claim 6, characterized in that the modulation circuit (17) comprises a power source (16), means for charging the energy source to a predetermined voltage among k different values, associated respectively at k different words of n bits, means (19) for insertion of the energy source (16) charged between the output of the rectifying circuit (5) and the first input of the regulation circuit (6).   16. Device according to claim 15, characterized in that the charging means comprise an analog-digital converter (20) having n inputs 25 constituted by the control inputs (Bl..Bn) of the modulation means and an output connected to the terminals of the energy source (16) during a charging phase.