Abstract:
An electromagnetic transponder comprising: an oscillating circuit; a first rectifying bridge having its A.C. input terminals connected across the oscillating circuit and having its rectified output terminals connected at least to a voltage regulator in charge of providing a supply voltage; and a second rectifying bridge, of dimension smaller than that of the first bridge, having its two A.C. input terminals connected across the oscillating circuit and having at least one output terminal connected to a demodulator of data sensed by the oscillating circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to electromagnetic transponders and, more specifically, an electromagnetic transponder with no autonomous power supply.  
         [0003]     The present invention applies for example to smart cards or electronic tags communicating without any contact with a terminal and extracting the power necessary for the supply of the circuits that they comprise from the high-frequency electromagnetic field radiated by the terminal. The present invention especially applies to electronic tags or smart cards in applications of electronic passport type.  
         [0004]     2. Description of the Related Art  
         [0005]     Systems for electromagnetic transponders are based on the cooperation between an oscillating circuit on the read-write terminal side and a resonant circuit on the electromagnetic transponder side (generally, a portable element), to exchange information by using a high-frequency field radiated by the oscillating circuit of the terminal. In the transponders to which the present invention applies, the high-frequency carrier is also used as a remote-supply carrier providing the transponder supply power.  
         [0006]     An example of application of the present invention relates to transponder systems based on standards ISO 14443 and 15693 according to which the remote-supply carrier radiated by the terminal is 13.56 MHz, while a back-modulation sub-carrier may be used by the transponders to transmit information to the terminal with a 847.5-kHz frequency. In the terminal-to-transponder transponder direction, the carrier is modulated in amplitude with a modulation factor generally on the order of 10%. The modulation factor defines as being the amplitude difference between the transmission of a state  1  and the transmission of a state  0 , divided by the sum of these amplitudes. In fact, standards set a range of acceptable modulation factors that transponders are supposed to be able to interpret and that terminals are supposed to respect. This range, in the case of the above-mentioned standards, ranges between 8 and 14%.  
         [0007]      FIG. 1  very schematically shows in the form of blocks an example of an electromagnetic transponder system to which the present invention applies. A transponder  1  (TR) is intended to be placed in the electromagnetic field of a terminal  2  (TERM) having an inductive element L 2  of an oscillating circuit emitting a high-frequency radiation sensed by an antenna L 1  of transponder  1 .  
         [0008]      FIG. 2  very schematically shows in the form of blocks an example of a conventional architecture of an electromagnetic transponder  1 . The transponder comprises an oscillating circuit  10 , formed of an inductive element L 1  forming an antenna, in parallel with a capacitor Cl at the A.C. input terminals of a rectifying bridge  11 . The rectified output terminals of bridge  11  are connected by a storage capacitor Cs. The signal sensed when transponder  1  is in the field of a terminal is used upstream and downstream of rectifying bridge  11 .  
         [0009]     Downstream, the rectified voltage recovered at the output of bridge  11  is especially used by a regulator  12  (REG) to extract a D.C. supply voltage Vdd of the transponder circuits. The output of bridge  11  is also connected to the input of a circuit  13  (DEM) for demodulating the received data, generally, an amplitude demodulator. Finally, a modulator  14  (MOD) of the information to be transmitted to the terminal by back modulation connects a microprocessor  15  (μP) or any other circuit of control and of digital interpretation of the transmissions, to a back-modulation stage connected to the output of bridge  11 . For simplification, the back-modulation stage has been assumed to be integrated to block  14 .  
         [0010]     Upstream of rectifying bridge  11 , the high-frequency signal is sampled toward, especially, a circuit  20  (CLK GEN) of generation of a clock from the high-frequency carrier and a circuit  21  (RF DET) of detection of the presence of a radio-frequency excitation across resonant circuit  10 .  
         [0011]     A disadvantage of conventional transponders is that supply voltage Vdd provided by regulator  12  is contaminated by noise due to the switchings performed downstream (especially by the microprocessor). This noise is propagated to the input of regulator  12  on the rectified signal of bridge  11 , which adversely affects the signal-to-noise ratio of the system and thus makes the data extraction by demodulator  13  more difficult.  
         [0012]     This problem is all the more present as the modulation factor is low. Now, a low modulation factor (typically, less than 20%) is further required to provide a sufficient remote-supply power. Indeed, an amplitude modulation transmission in all or nothing would divide approximately by two the remote-supply capacity.  
       SUMMARY OF THE INVENTION  
       [0013]     An embodiment of the present invention aims at overcoming all or part of the disadvantages of known transponder systems, in particular of transponders intended to receive amplitude-modulated data with a modulation factor smaller than one.  
         [0014]     An embodiment of the present invention especially aims at improving the signal-to-noise ratio at the input of an amplitude demodulator of an electromagnetic transponder.  
         [0015]     An embodiment of the present invention also aims at providing a solution which requires no modification of the terminal of the electromagnetic transponder system.  
         [0016]     To achieve all or part of these aims, as well as others, one embodiment of the present invention provides an electromagnetic transponder comprising:  
         [0017]     an oscillating circuit;  
         [0018]     a first rectifying bridge having its A.C. input terminals connected across the oscillating circuit and having its rectified output terminals connected at least to a voltage regulator in charge of providing a supply voltage; and  
         [0019]     a second rectifying bridge, of dimension smaller than that of the first bridge, having its two A.C. input terminals connected across the oscillating circuit and having at least one output terminal connected to a demodulator of data sensed by the oscillating circuit.  
         [0020]     According to an embodiment of the present invention, the surface area ratios of the first and second rectifying bridges to the total surface area taken up by the bridges formed in integrated form range between 80-20 and 98-2, such as between 93-7 and 97-3.  
         [0021]     According to an embodiment of the present invention, a bandpass filter is interposed between the second rectifying bridge and the demodulator.  
         [0022]     According to an embodiment of the present invention, a low-pass filter is interposed between the first rectifying bridge and the voltage regulator.  
         [0023]     According to an embodiment of the present invention, reference terminals of the rectified outputs of the bridges are isolated from each other or connected by a resistive element.  
         [0024]     The foregoing and other features of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1 , previously described, very schematically shows in the form of blocks an example of an electromagnetic transponder system of the type to which one embodiment of the present invention applies;  
         [0026]      FIG. 2 , previously described, very schematically shows in the form of blocks an example of a conventional architecture of an electromagnetic transponder; and  
         [0027]      FIG. 3  very schematically shows in the form of blocks an embodiment of an electromagnetic transponder according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0028]     Embodiments of an electromagnetic transponder with no autonomous power supply are described herein. In the following description, numerous specific details are given to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0029]     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
         [0030]     Same elements have been designated with same reference numerals in the different drawings. For clarity, only those elements necessary to the understanding of embodiments of the present invention have been shown and will be described hereafter. In particular, the respective structures of the different transponder elements have only been detailed when they are not conventional structures. Further, the structures of the digital circuits (microprocessors or circuits in wired logic) of interpretation of the transmitted data have not been detailed, one embodiments of the present invention being compatible with conventional circuits.  
         [0031]      FIG. 3  very schematically shows in the form of blocks an embodiment of an electromagnetic transponder according to the present invention.  
         [0032]     As previously, the transponder comprises an oscillating circuit  10  formed of an inductance L 1  forming an antenna in parallel with a capacitor C 1  to sense the electromagnetic field of a read/write terminal. As previously still, the transponder comprises a digital unit  15  (for example, a microprocessor μP) of control and interpretation of transmissions as well as, directly connected to the oscillating circuit, a clock generation circuit  20  (CLK GEN) and a radio-frequency excitation detection circuit  21  (RF DET) having its respective outputs sent onto microprocessor  15 .  
         [0033]     A feature of the present invention is to provide two rectifying bridges  11 ′ and  22  having their respective A.C. inputs connected to terminals of oscillating circuit  10 . Bridge  11 ′ is of greater capacity than bridge  22 . The respective dimensions of bridges  11 ′ and  22  are a function of the current needs of the downstream circuits which are connected thereto.  
         [0034]     Bridge  22  of reduced size is only intended for a demodulator  13  (DEM) of structure conventional per se, which is connected to the rectified output of bridge  22  and which provides the demodulated data to microprocessor  15 .  
         [0035]     Bridge  11 ′ is at least intended to provide the power necessary to a regulator  12  (REG) of structure conventional per se so that it generates supply voltage Vdd of the transponder circuits. As previously, a capacitor Cs connects the rectified outputs of bridge  11 ′ and voltage Vdd is provided across a capacitor Ca at the output of regulator  12 .  
         [0036]     The fact of dedicating a rectifying bridge  22  to demodulator  13  enable isolating the signal to be demodulated from the switching noise downstream of bridge  11 ′. Indeed, such noise does not pass into the reverse rectifying elements and is thus not present on the side of oscillating circuit  10 . The signal-to-noise ratio at the input of demodulator  13  is thus improved.  
         [0037]     An embodiment of the present invention takes advantage from the fact that the current needs of the amplitude demodulator are very low as compared to the current needs of the transponder. Typically, the demodulator (which interprets a voltage signal) needs a few tens of microamperes, while the transponder supply requires up to 100 milliamperes, or even more. The needs thus are by a ratio of 1 to 1,000, or even of 1 to 10,000. The surface area ratios of bridges  11 ′ and  22  to the total surface taken up by the bridges made in the integrated form range between 80-20 and 98-2. For example, bridge  22  amounts to between 3 and 7% of the total surface area taken up by the bridges (ratios between 97-3 and 93-7).  
         [0038]     According to the type of technology used to integrate the circuit (especially the use or not of separate wells), reference terminals M 1  and M 2  of bridges  11 ′ and  22  may be isolated from each other or be connected by a resistor, to avoid feedback of switching noise through the ground.  
         [0039]     The fact for circuits  20  and  21  to be directly connected to oscillating circuit  10  is not disturbing. Indeed, the switching noise is sent back by the supply voltages and, since these circuits are supplied by voltage Vdd, the noise rises up to regulator  12 , then are blocked by bridge  11 ′.  
         [0040]     A modulator  14  (MOD) of conventional structure performing the back modulation of the data to be transmitted to the transponder is connected on bridge  11 ′ rather than on bridge  22  to, here again, avoid polluting the demodulator.  
         [0041]     According to a variation illustrated in dotted lines, a band-pass filter  23  (BP) is interposed between the output of bridge  22  and the input of demodulator  13 , to only let through the frequency band of the data signal expected by the demodulator.  
         [0042]     According to another variation illustrated in dotted lines, a low-pass filter  24  (LP) is interposed upstream of voltage regulator  12 , to filter the high-frequency noise generated by the circuit on the supply voltage.  
         [0043]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the practical implementation of the present invention based on the functional indications given hereabove is within the abilities of those skilled in the art. Further, other circuits may be connected downstream of bridge  11 ′ according to applications, provided that bridge  22  is dedicated to demodulator  13  and that the supply of the possible circuits connected upstream of bridges  11 ′ and  22  originates from bridge  11 ′.  
         [0044]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.  
         [0045]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.