Abstract:
The invention concerns a reader ( 10 ) for identifying passive transponders ( 12 ) comprising an antenna for receiving a first signal (Tx) from the reader and for transmitting a second signal (Rx). The reader comprises: a time base ( 54 ) generating a sinusoidal signal, a control circuit ( 64 ), a modulator ( 56 ) for transmitting to the control circuit ( 64 ) data to modulate the sinusoidal signal so as to generate a signal carrying commands which, when it is received by the antenna of said transponder, forms said first signal (Tx), a demodulator ( 58 ), a decoder ( 60 ), a communication interface ( 62 ) and an antenna ( 14 ). The demodulator ( 60 ), which is connected to the antenna ( 14 ) to receive the second signal (Rx), comprises two multipliers ( 76, 78 ), one ( 78 ) being of the inverter type, and an adder ( 70 ) designed to combine the data derived from the multipliers ( 76, 78 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Priority is claimed to PCT/CH01/00445 filed Jul. 17, 2001, which claims priority to France 00/09625 filed Jul. 21, 2000. 
   FIELD OF INVENTION 
   The present invention relates to a reader for identifying passive transponders of the type comprising an antenna arranged for receiving a first signal Tx from the reader and for transmitting a second signal Rx to the reader. It concerns more particularly readers allowing reception of low intensity signals. 
   SUMMARY OF THE INVENTION 
   Units comprising a reader and at least one transponder, and which allow contactless identification, are well known to those skilled in the art. They are based on the principle of electromagnetic coupling. One of them is, for example, disclosed by EM Microelectronic—Marin SA (Switzerland) under the title “CID Demokit Application note on transceiver unit”. 
   The reader of this unit comprises an antenna and a control circuit which transmit a signal Lx formed of an amplitude modulated low frequency carrier, typically comprised between 9 and 150 kHz. They receive, in return, the signal Rx. 
   More precisely, the reader is also provided with a time base for generating the carrier signal, the carrier being modulated by the control signal, as well as with a demodulator for processing the picked up signals and with a microcontroller for decoding them. In the reader, a tuning capacitor is connected to the antenna to form a series resonating circuit, tuned to the frequency source. 
   The transponder receives from the reader the signal Tx, which corresponds to the damped signal Lx and which provides it, at the same time, with energy, commands and the clock. When the reader has finished addressing its commands, it continues to transmit the carrier signal, but without modulation. Consequently, the transponder is constantly supplied with energy and driven by the clock. 
   The transponder addresses signal Rx to the reader, by periodically short-circuiting its antenna in accordance with coded modulation that corresponds to the response to be given. 
   Rx is received superposed onto the carrier by the reader antenna. This combined signal is processed and decoded by the demodulator. 
   When the reader is separated from the transponder by a metal screen, the electromagnetic coupling no longer occurs directly between the two antennae of the reader and the transponder, but indirectly, through the metal screen in which induced currents (eddy currents) flow. 
   The coupling between the reader antenna and the transponder antenna then becomes very weak. Experience has shown that, in order to ensure a connection in such circumstances, the energy and modulation must be transmitted in a particularly efficient manner, and the demodulator must be very sensitive, capable of differentiating modulations in signals Rx of the order of 70 dB with respect to the carrier. 
   An object of the present invention is to propose a reader allowing such small modulations to be differentiated. Thus, the demodulator, connected to the antenna to receive the second signal that it picks up from the transponder arranged in the vicinity includes two signal multipliers, one being of the inverter type, and an adder for combining the data from these multipliers. 
   In an advantageous manner, the demodulator includes:
         a first channel formed by the first multiplier,   a second channel arranged in parallel with the first channel and including the second multiplier, as well as a low pass filter and a sampling and hold circuit,   an adder, which receives the signals from the two channels and processes them,   an amplifier-filter, and   a comparator.       

   The demodulator is arranged such that the signals derived from the adder are processed by the amplifier-filter then by the comparator. They are then addressed to the interface to deliver, at the output thereof, a signal carrying information from the transponder. 
   Particularly when the reader is separated from the transponder by a metal screen, the inductance of the reader antenna has to be as high as possible (of the order of several mH) with a high magnetic field concentration. This can be obtained by coiling the reader antenna on a core of soft magnetic material, for example ferrite. 
   Reading must also not be dependent upon problems that could be generated by variations in the features of the reader antenna, for example its time constant. Therefore, the reader is arranged such that the beginning of the pause occurs by interrupting the current, and thus the magnetic field, during its zero crossing. The voltage across the terminals of the antenna or the tuning capacitor is then at a maximum and kept for the beginning of the next transmission. 
   The reader commands require all or part of the transponders present in the electromagnetic field to send an identification return signal Rx. They are in binary form, with a series of bits that differ from each other in length. A pause, as short as possible, is inserted between the bits. 
   More precisely, the reader transmits signal Lx, of constant amplitude, with a pause time T 0  or T 1 , depending on whether it wishes to send a 0 or a 1, the ratio between T 0  and T 1  being substantially equal to ½. The pause times are reduced to a minimum, so that the energy provided is maximum and the measuring time as short as possible. The transponder antenna receives signal Tx, which is a damped image of signal Lx, the sinusoid decreasing, and then increasing progressively respectively when transmission of Lx stops and begins. This pause time must, therefore, be sufficient for signal Tx across the transponder antenna terminals to be damped to a voltage lower than the threshold voltage, in order to allow the demodulation circuit to detect it. 
   When a metal screen is capable of being located between the reader and transponder antennae, it is preferable to work at the lowest possible carrier frequency. This inevitably results in an increase in the pause duration. 
   It is also an object of the present invention to ensure a reliable connection between a reader and a transponder, in the shortest possible transmission time and to be able to work even with a metal screen arranged between the reader and the transponder. It allows, more particularly, the pause duration to be reduced to a minimum with respect to that of a command bit and to make the features of the transponder and reader antennae independent. The reader is thus arranged such that the first signal Tx, generated by the control circuit, is formed of a succession of bits each of them ending in a pause whose duration is less than or equal to six periods of the sinusoidal signal. 
   Other advantages and features of the invention will appear from the following description, made with reference to the annexed drawing, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows schematically a transponder and a reader for identifying the transponder, 
       FIG. 2  shows a part of the transponder of  FIG. 1 , and more particularly its analogue circuit, 
       FIG. 3  illustrates schematically at a and b, diagrams of the low and high level extractors, 
       FIG. 4  shows respectively, on lines a to d, the voltage curves measured at the terminals of the reader antenna, the transponder antenna and the high and low clock signals, 
       FIG. 5  shows schematically a reader according to the invention, a part of which is illustrated in more detail in  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   The unit, shown in  FIG. 1 , includes a reader  10  and a transponder  12 . Reader  10  includes, more precisely, an antenna  14  transmitting an electromagnetic signal Lx, an electronic control circuit  16 , advantageously a microcontroller, as well as a computer  18  connected to a keyboard  20  and to a screen  22 . 
   Transponder  12  is formed of an antenna  24 , an analogue circuit  25 , a logic control circuit  26  and a memory  27 . Analogue circuit  25 , which will be described in more detail hereinafter, is connected to the antenna, to receive the signal picked up by the latter. This signal carries out three functions, namely it:
         supplies the transponder with electric energy,   provides a clock signal, and   transmits commands.       

   After processing, the signal is transmitted from analogue circuit  25  to logic circuit  26 , by three connections  28   a ,  28   b  and  28   c , respectively ensuring the transmission of energy, the clock and the serial input function. Logic circuit  26  addresses data to analogue circuit  25 , via connection  29  of the “serial output” type. It introduces and will search for data in memory  27 , respectively via connections  30   a  and  30   b.    
   The commands are processed by logic circuit  26  from the data received and that contained in memory  27 . A response is addressed, by logic circuit  26 , to analogue circuit  25  so that it sends a return signal to reader  10 . 
   As  FIG. 2  shows, analogue circuit  25  includes a capacitor  32 , an AC-DC converter  34 , a low level clock extractor  35  and a high level clock extractor  36 , a demodulator  38  and a modulator  40 . 
   Capacitor  32  forms, with antenna  24 , a resonating circuit whose natural frequency is adjusted to the frequency of signal Lx transmitted by reader  10 , and to whose terminals the inputs of converter  34 , extractors  35  and  36 , and the outputs of modulator  40  are connected, via connections  33   a  and  33   b , such that the signal received by the transponder is applied to each of them and the signal transmitted by the transponder is applied to the antenna. Demodulator  38  receives, from antenna  24 , its energy through converter  34 , and the clock and commands through extractors  35  and  36 . 
   This Figure again shows supply connection  28   a , clock connection  28   b  and serial input connection  28   c , forming the outputs of analogue circuit  25 . Serial input connection  29 , which allows data to be addressed from logic circuit  26  to analogue circuit  25 , and more particularly to its modulator  40 , will also be noted. 
   Converter  34  is, in a manner well known to those skilled in the art, formed of a full-wave rectifier with a large energy capacitor, powering a voltage stabilizer. It powers all of the parts of transponder  12 . 
   The low level type extractor is made by means of two simple inverters  42  and  43  in series, as shown in  FIG. 3   a . Inverter  42  is formed of a PMOS transistor  42   a  and a NMOS transistor  42   b . It is powered by a stabilized voltage VDD provided by converter  34 . Antenna  24  applies signal Tx to its input  42   c . The dimensions of transistors  42   a  and  42   b  are calculated such that the threshold voltage Ub at which the circuit switches, is close to 1V. 
   An extractor of this type generates a clock signal each time that Tx is greater than 1V, this signal being interrupted when Tx descends below this level. Consequently, the clock signal is interrupted. 
   In normal operating conditions, the peak voltage Tx max  is generally of the order of ten volts. With an antenna quality factor comprised between 15 and 30, approximately 3 to 6 cycles are required after the pause for voltage Tx to be permanently below 1V. 
   It would of course be possible to dimension the transistors such that threshold voltage Ub is higher. In this case, however, the transponder can no longer react normally when the received signal is close to Ub. This consequently reduces its sensitivity. 
     FIG. 3   b  shows in more detail the high level type extractor  36 . It is formed of an input transistor  44 , of the PMOS type and a peak value rectifier  46 , both connected to the antenna by line  33   a , of two current sources  48  and  50  and two inverters  52  and  53 , one  52  being polarized, and the other  53  being simple. 
   More precisely, peak value rectifier  46  is formed of a diode  46   a  and a capacitor  46   b . Its input is connected to antenna  24  by connection  33   a  and its output  46   c  to inverter  52  to apply a voltage VData thereto, equal to peak voltage Tx max  of the signal received by the antenna through connection  33   a.    
   Inverter  52  includes an input  52   a  and an output  52   b , and two PMOS  52   c  and NMOS  52   d  transistors. Input  52   a  is connected to antenna  24  through transistor  44  which offsets the voltage of the antenna downwards by a value equal to its threshold value. Detection threshold Uh of the high level extractor is offset downwards with respect to VData by a value equal to the difference in the threshold voltages of PMOS transistors  44  and  52   c . The latter are dimensioned such that the threshold voltage of transistor  52   c  is several hundred mV higher than the threshold voltage of transistor  44 . Consequently, the clock signal is interrupted as soon as the voltage of signal Tx received by the antenna drops by a value equal to the difference between the two threshold voltages, whatever the value of peak voltage Tx max . 
   In order to be able to understand properly the operation of the clock extractors,  FIG. 4  shows schematically respectively on lines a, b, c and d, signal Lx transmitted by reader antenna  14 , signal Tx received by transponder  12  and the high and low level clock signals CLKh and CLKb. 
   On line a, it will be noted that the reader antenna transmits a sinusoidal signal, which is periodically interrupted, when the voltage is maximum. 
   When the voltage of signal Lx becomes constant, signal Tx at the terminals of antenna  24  of transponder  12  decreases, as can be seen on line b, more or less quickly, the speed being lower the higher the quality factor. Peak voltage Tx max  is higher, the higher the received signal. However, when the voltage exceeds a limit value, the signal is saturated. 
   At the start of a signal Lx transmitted by the reader, the high and low level clock signal extractors  36  and  35  both respond very quickly, as can be seen on lines c and d. However, in poor reception conditions, high clock signal CLKh may only appear after several periods of signal Lx transmitted by the reader. When reader  10  interrupts transmission of the sinusoidal signal, it will be noted that signal Tx received by antenna  24  is damped slowly. This is due to the fact that the quality factor of the oscillating circuit, that if forms with capacitor  32 , is high. 
   Because of this slow damping, several periods are needed before low level extractor  35  reacts, whereas signal CLKh derived from high level extractor  36  is interrupted in synchronism. 
   It thus appears clearly that if the signal received is regular and intense, one need only have a high level extractor in the transponder in order to be able to considerably reduce the pause time. However, when the received signal is not saturated, it is then desirable to have high and low level extractors, which then allows a pause of short duration to be guaranteed. It is thus possible to transmit the maximum amount of energy and a significant number of data even when the carrier frequency is low. 
     FIGS. 5 and 6  show, in more detail, the structure of electronic control circuit  16  fitted to reader  10 . 
   Electronic control circuit  16  is formed of a time base  54 , a modulator  56 , a demodulator  58 , a decoder  60 , a communication interface  62  and a control circuit  64 . 
   Time base  54  is connected to control circuit  64 , by a connection  54   a , through which it supplies a sinusoidal signal of constant frequency, advantageously comprised between 9 and 150 kHz, which acts as the carrier. Control circuit  64  receives from modulator  56 , via a connection  56   a , data which allow it to modulate the carrier signal to address data to a transponder arranged in proximity to the reader, via antenna  14  which is connected to control circuit  64  by means of a connection  64   a.    
   Antenna  14  is connected to demodulator  58  by a connection  14   a . Thus, when the transponder replies to the reader data, the signal that it addresses, picked up by antenna  14 , is received by demodulator  58 , through connection  14   a . Demodulator  58  processes this signal and the data that it contains is addressed to decoder  60  through a connection  58   a . Decoder  60  interprets this data on the basis of stored data and transmits it to interface  62  through a connection  60   a . Interface  62  is connected to the exterior, by a connection  62   a , formed for example of an RS line  232 , to ensure the transmission of commands and data to man-machine interfaces. It is also connected to modulator  56 , by a connection  62   b.    
   Thus, when an operator wishes to identify an object provided with a transponder and arranged in the field of reader  10 , he gives an order by means of keyboard  20 . This order is managed by computer  18  and sent to electronic control circuit  16  via connection  62   a . Interface  62  addresses this order to modulator  56 . The latter co-operates with control circuit  64  to modulate the carrier signal derived from time base  54 . 
   As has already been stressed hereinbefore, it is difficult to read the signals received by antenna  14 , since they are of a very low level. Demodulator  58 , shown in detail in  FIG. 6 , allows efficient reading to be ensured. It includes first and second channels  66  and  68 , arranged in parallel, an adder  70  connected to the outputs of channels  66  and  68 , an amplifier-filter  72  and a comparator  74  arranged in series at the output of adder  70 . 
   Channel  66  is formed of a multiplier  76 . Channel  68  includes an inverter type multiplier  78 , a low-pass filter  80  and a sampling circuit  82 . 
   The two channels  66  and  68  are together connected to the antenna by connection  14   a . They therefore both receive the signal UR(t) originating from antenna  14 . This modulated signal includes two components, one corresponding to the transmitted signal and the other to the picked up signal, originating from the transponder. Decoder  58  has the task of extracting the signal X(t) that corresponds to the component originating from the transponder. 
   In a first operation, the signal is multiplied by itself by multipliers  76  and  78 , the latter further inverting the resulting signal. In other words, signal US(t) derived from multiplier  76  is equal to the square of UR(t), whereas the signal derived from multiplier  78  is equal, but with the reverse sign. 
   Signal −US(t), derived from multiplier  78 , is then processed, in a conventional manner, by means of low-pass filter  80 , then by sampling circuit  82 . 
   In the device described, reader  16  is the master as regards the transponder. In other words, the reader can find out at any moment when a transponder is likely to respond to an interrogation. Just before the response signal begins, sampling circuit  82  stores the mean value of signal US−(t−Δt) provided by filter  80 . It is this stored signal that is added to signal US(t). After amplification and filtering by amplifier-filter  72 , then comparison by comparator  74 , the result of this addition allows X(t), which includes all data derived from the transponder, to be extracted, whereas the signal originating from the carrier has been removed. 
   It is quite clear that the transponder and the reader as they have been described can be subject to numerous variants, without thereby departing from the scope of the invention. 
   Thus, although the carrier frequency is relatively low, and owing to the features of the reader according to the invention, used with transponders like that described, it is possible to read passive transponders in particularly unfavourable conditions, even through a metal screen, safely and quickly.