Abstract:
A method and a circuit for detecting a binary state supported by an analog symbol, comprising sampling the symbol with a sampling signal based on a frequency having a period shorter than the duration of a symbol, selecting a number of significant samples smaller than the number of samples which would be obtained with a sampling of the symbol at said frequency, and deciding of the symbol state based on the selected samples.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of demodulators and more specifically to radio frequency signal demodulators, the results of which must be exploited by a digital processing unit. 
     2. Discussion of the Related Art 
     An example of application of the present invention relates to electromagnetic transponder systems in which a transponder communicates with a contactless read-write terminal. 
       FIG. 1  very schematically shows, in the form of blocks, the receive demodulation portion of radio frequency signals by an electromagnetic transponder. 
     After having been received by an antenna and setup by a coupler (not shown), radio frequency signals RF are processed by an analog demodulator  1  (ANALOG DEMOD) having the function of providing a demodulated analog signal AS as well as a sampling clock CK. The sampling clock is generally taken from the transmit carrier. Signals AS and CK are sent onto an interface circuit  2  (INTERF) having the function of providing an output signal O exploitable by a microcontroller  3  (CPU) and other digital circuits. In practice, signal O is provided to a data bus with which CPU  3  communicates. In an application to electromagnetic transponders, a transmission from a terminal to a transponder is performed by using a carrier at 13.56 Megahertz on which are transmitted coded information, generally in amplitude modulation with a non-zero modulation ratio, with a 106-kilobit-per-second flow rate. On the transponder side, the carrier is used to remotely supply circuits of the transponder in the case where the latter has no battery. The same demodulation principle is used in a terminal, except that the clock signal is generally present therein without it being necessary to extract it from the received signal. 
       FIG. 2  illustrates, in timing diagrams, the principle of a demodulation performed in a circuit such as illustrated in  FIG. 1 . 
     The first timing diagram illustrates an example of data D transmitted by signal RF and to be recovered at output O of circuit  2 . In this example, it is assumed that the transmission of a bit at state 0 corresponds, within a bit time T, to a low level followed by a high level, while the transmission of a 1 corresponds to the inverse (high level followed by a low level). This is an example only and different types of codings and transmissions could be used. In the example of electromagnetic transponders using a 13.56-Megahertz carrier frequency, time T corresponds, for example, to 106 kilohertz. 
     At the output of analog demodulator  1 , a signal AS (second timing diagram) which roughly follows the shape of signal D is obtained. 
     The recovered clock signal CK (third timing diagram) corresponds to the carrier signal, that is, to a 13.56-Megahertz frequency. For clarity, the timing diagrams of  FIG. 2  are not to scale, especially in the time scale of timing diagrams AS and CK. 
     The last timing diagram of  FIG. 2  shows signal O. 
     In a conventional circuit  2 , signal AS is sampled only once in the middle of a cycle (times t 1  and t 2 ). In fact, signal AS is sent to the input of an inverter having its switching threshold TH conditioning the state provided at output O. 
     A first disadvantage of a conventional demodulation system such as illustrated in  FIGS. 1 and 2  is that, if signal AS is strongly disturbed, time t 1  or t 2  when the signal level is taken into account risks providing an erroneous result. 
     Another disadvantage is that it is not possible to bring the different times of analysis of signal AS closer to one another, since this would lead to speeds incompatible with the working speed of the CPU having its clock frequency corresponding at most to the frequency of clock CK. Accordingly, the flow rate is limited. 
     Taking the example of electromagnetic transponder systems based on a 13.56-Megahertz frequency, the limit is in practice 106 kilobits per second to respect the time necessary for a software analysis of the received data. 
     It would be desirable to be able to increase the transmission speed of such systems. For example, in an application where images are desired to be transmitted (photographs, biometric prints), a flow rate of 106 kilobits per second results in transmission times of several seconds, incompatible with the desired analysis speeds. 
     SUMMARY OF THE INVENTION 
     The present invention aims at optimizing the demodulation of RF signals and in particular at increasing the possible transmission flow rates for a given clock frequency. 
     The present invention also aims at providing a solution which overcomes possible disturbance problems in the signal originating from an analog demodulator. 
     The present invention also aims at providing a solution which is compatible with conventional electromagnetic transponder architectures and especially with systems in which the clock is transmitted at the same time as the radio frequency signal. 
     To achieve these and other objects, the present invention provides a circuit for detecting a binary state supported by an analog symbol, comprising: an analog-to-digital conversion element using a sampling signal based on a frequency having a period shorter than the duration of a symbol; means for selecting a number of significant samples smaller than the number of samples which would be obtained with a sampling of the symbol at said frequency; and means for deciding of the symbol state based on the selected samples. 
     According to an embodiment of the present invention, said decision means receive an odd number of samples and provide the binary state of the symbol by applying a majority decision criterion. 
     According to an embodiment of the present invention, the respective positions of the significant samples are determined in a training or characterization phase. 
     According to an embodiment of the present invention, the sampling signal is derived from the positions selected for the significant samples. 
     According to an embodiment of the present invention, the sampling signal corresponds to said frequency. 
     According to an embodiment of the present invention, the output of the analog-to-digital conversion element is sent to the input of a shift register having its parallel outputs provided to a multiplexer for selecting said significant samples. 
     According to an embodiment of the present invention, registers store at least the position in a symbol of a sample considered as being significant. 
     According to an embodiment of the present invention, it is applied to a system of transmission between an electromagnetic transponder and a read/write terminal. 
     The present invention also provides a method for determining a binary state supported by an analog symbol, comprising: sampling the symbol with a sampling signal based on a frequency having a period shorter than the duration of a symbol; selecting a number of significant samples smaller than the number of samples which would be obtained with a sampling of the symbol at said frequency; and deciding of the symbol state based on the selected samples. 
     According to an embodiment of the present invention, an odd number of samples is selected, the symbol state being determined by majority decision based on the respective states of the different samples. 
     According to an embodiment of the present invention, the respective positions of the significant samples are determined by a training phase. 
     According to an embodiment of the present invention, the sampling signal is derived from the positions selected for the significant samples. 
     According to an embodiment of the present invention, the sampling signal corresponds to said frequency. 
     The foregoing objects, features, and advantages of the present invention, as well as others, 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 
         FIG. 1 , previously described, shows a conventional demodulation architecture; 
         FIG. 2 , previously described, illustrates the operation of the demodulator of  FIG. 1 ; 
         FIG. 3  very schematically shows in the form of blocks an example of the architecture of a demodulator according to the present invention; 
         FIG. 4  shows an embodiment of a demodulation optimizer according to the present invention; and 
         FIG. 5  illustrates, in the form of a timing diagram, the operation of a demodulation optimizer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same references in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the circuits downstream of the demodulator have not been described in detail, the present invention being compatible with any conventional software exploitation of the signals. Similarly, the internal structure of an analog demodulator used for the present invention corresponds to a conventional structure and has not been described in detail. 
     A feature of the present invention is to select, in a symbol, several samples and to deduce from these samples state 0 or 1 of the symbol without using the CPU. According to the present invention, the number of samples taken into account is smaller than the number of samples which would be obtained with a sampling at the frequency on which the sampling signal is based. 
     Another feature of the present invention is to select a reduced number of samples per symbol as corresponding to reliable samples at positions (in time) decided in a training or characterization phase, and deducing the symbol state from these selected samples. 
     According to a first embodiment not shown, the analog signal originating from an analog demodulator is directly sampled at times in the symbol which correspond to the times providing the selected samples. This amounts to generating a sampling signal based on a frequency of period smaller than the duration of a symbol, but with a number of edges smaller than that of this frequency. 
     According to a second embodiment the sampling signal based on the sampling frequency directly is this frequency. The analog signal coming from the analog demodulator is then sampled at a frequency corresponding, preferably, to the maximum available frequency. The state 0 or 1 of the symbol is then deduced from several selected samples thereof. 
       FIG. 3  very schematically shows, in the form of blocks, this second embodiment of a demodulator according to the present invention. 
     As previously, a radio frequency signal RF is received by an analog demodulator  2  (ANALOG DEMOD) in charge of extracting an analog signal AS as well as a clock signal CK at the frequency of the carrier which here forms the sampling signal. Signal CK is transmitted to an interface circuit  2  which provides digital signals O to a CPU  3  as in the conventional circuit of  FIG. 1 . 
     According to this embodiment of the present invention, a demodulation optimization circuit  4  (DEMOPT) is interposed between the output of analog demodulator  1  providing signal AS and the corresponding input of interface circuit  2 . Circuit  4  further receives clock signal CK as well as control signals CT from CPU  3 . 
     Circuit  4  has the function of providing a signal DS corresponding to state 0 or 1 of the received symbol. Thus, interface circuit  2  directly receives the states exploitable by CPU  3 . According to an alternative embodiment and provided that signals O are electrically compatible with the bus of connection with CPU  3 , circuit  2  may be omitted. 
       FIG. 4  shows an embodiment of a circuit  4  according to the present invention. 
     Signal AS crosses an analog-to-digital converter  10  (A/D) having its sampling frequency corresponding to the frequency of clock signal CK. The output of converter  10  is sent to a series input of a shift register  11  (SREG) sized so that the length of a word that it contains corresponds to the number of samples contained in a transmitted data symbol. 
     All the bits of register  11  are read in parallel and sent onto inputs of a multiplexer  12  having the function of selecting several samples as being significant of the symbol state. 
     Preferably and as illustrated in  FIG. 4 , multiplexer  12  selects an odd number of samples (for example, three) that it provides to a decision circuit  13  (DECID) having the function of determining the symbol state by application of a majority decision method. The output of circuit  13  provides bit DS. 
     According to a preferred embodiment, the selection performed by multiplexer  12  is parameterizable. For example, two registers  13  (NREG) and  14  (dREG) contain words respectively indicating position N of a central sample in a range of samples considered as reliable and distance d between two samples in the significant range. The respective values of registers  14  and  15  are then exploited by a calculation circuit  16  providing multiplexer  12  with the respective positions N of the central sample in the range and respective positions N+d and N−d of the end samples. This is an example of generation of selection signals SEL, but any other means may be used. For example, symmetrical distance d may be replaced with intervals different from one another between the sample positions. 
     The respective contents of registers  14  and  15  are loaded into circuit  4  by CPU  3  according to predetermined parameterizing data. In a specific example where the size of register  11  is 16 bits, registers  14  and  15  respectively contain quartets indicating the positions of the significant samples. 
       FIG. 5  illustrates, in the form of timing diagrams, the operation of an optimization circuit according to the present invention. These timing diagrams show, for transmitted states 1 and 0, respectively clock signal CK, signal AS, selection signals SEL of multiplexer  12 , and output DS of circuit  13 . 
     A signal AS having a conventionally disturbed shape is assumed. This signal is sampled at the frequency of clock CK and the multiplexer selects three samples designated by their respective position N−d and N+d. In the example of  FIG. 5 , in its left-hand portion, the sampling provides a state 1 on signal DS while the right-hand portion (second symbol) provides a state 0. 
     An advantage of the present invention is that it avoids possible detection errors due to transient disturbances on the demodulated signals. Such a disturbance is illustrated in the right-hand portion of  FIG. 5  in the form of a peak p. It can be seen that, in this case, despite the fact that sample N is considered as a high state, the majority decision performed by circuit  13  enables confirming state 0 of the transmitted bit. 
     According to a first embodiment of the present invention, the most significant samples are determined in a phase of characterization or testing of the product or more specifically of a product batch. The present invention then takes advantage from the fact that the general shape of signal AS is most often repetitive for a same integrated circuit chip batch. 
     According to a second embodiment, a training phase is carried out periodically during the product lifetime to adapt the demodulator to possible drifts. These two embodiments may be combined. 
     The implementation of a training or characterization phase entails no specific difficulty. It is enough to know the nature of the transmitted message, to perform several different parameterizings of registers  14  and  15 , and to select the first parameterizing which provides a correct result over a number (considered as significant) of symbols. 
     An advantage of the present invention is that it enables accelerating the transmission rate for a given clock frequency, while remaining compatible with a software interpretation of the transmitted data. Indeed, the CPU does not intervene (outside of the control phase of registers  14  and  15 ) in the actual sample selection. It is thus possible to increase the number of transmitted bits while respecting the possibility of interpretation by the CPU. 
     Thus, taking the example of electromagnetic transponders, on a 13.56-Megahertz carrier, duration T′ of a symbol according to the present invention may correspond to a 847.5-kilobit-per-second flow rate. Such a factor 8 (with respect to the conventional 106-kbits/s flow rate) seems to be small but is in fact very advantageous. For example, an image which conventionally takes 4 seconds to be transmitted now only takes, due to the present invention, half a second. This speed becomes compatible with fast analysis needs, especially in authentication or access control applications. 
     Another advantage of the present invention is that the demodulator thus formed is parameterizable. A same hardware circuit may thus be customized to be adapted to different integrated circuits. 
     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, other frequencies and flow rates than those indicated hereabove may be exploited. 
     Further, although the present invention has been more specifically described in relation with an acceleration of the flow rate, it enables, even at constant flow rate, improving the reliability of an analog demodulator. 
     Moreover, the practical implementation of the present invention is within the abilities of those skilled in the art, be it by hardware and/or software means, based on the functional description given hereabove. 
     Finally, the adaptation of the example shown in the first embodiment in which the selection is directly performed on the sampling times rather than on the actual samples, is within the abilities of those skilled in the art by using components known per se. 
     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.