Abstract:
A contactless chip card includes a circuit for detecting the presence of radio frequency signals. A detector comprises at least one circuit for detecting the presence of radio signals by making direct use of the corresponding signals received by an antenna winding. In addition, a phase-shift detector detects the relative phase differences between signals provided by the antenna winding.

Description:
FIELD OF THE INVENTION 
     The invention relates to contactless chip cards in which the transmission of binary data between a user device and a contactless chip card is done via radio frequencies. In particular, the invention relates to a circuit within the contactless chip card for receiving and detecting radio frequency signals. 
     BACKGROUND OF THE INVENTION 
     In contactless chip cards, there are known ways of transmitting binary data from the user device to the chip cards by means of a signal transmitted at a carrier frequency f (i.e., 13.56 MHz) that is amplitude modulated by binary digits of a code to be transmitted. Contact chip cards work accurately only if they are able, at very high speed and with certainty, to recognize that the user device is communicating by means of radio frequency signals so the chip cards can place themselves in a mode of operation suited to recognizing signals of this type. 
     Recognizing radio frequency signals in the prior art is done by means of an electronic device of the type described in FIG. 1. This device comprises a circuit 10 for the detection of radio frequency signals which includes a resonant circuit comprising an antenna loop taking the form of a winding 20 and a capacitor C 0  which are parallel-connected. Circuit 10 provides a signal V AC  whose waveform is shown in FIG. 2a. 
     The device further comprises a rectifier and filter circuit 12 which includes a four-diode bridge 22 coupled to a capacitor C 1  to provide a full-wave rectified signal V DC  whose waveform is shown in FIG. 2b. Also included in the rectifier and filter circuit 12 is a resistor 14 and a comparator circuit 16 for comparing the potentials of V DC  and V DD  at the terminals of the resistor 14. The comparator circuit 16 provides a signal V RF  at its output indicating the presence of binary amplitude-modulated radio frequency signals. This signal V RF . informs the microcontroller 50 that the signal transmitted to the card is of the radio frequency type. 
     The invention is aimed at obtaining a detector of radio frequency signals for a chip card for determining the presence of radio frequency signals at high speed and with certainty. To this end, the detector, according to the invention, uses the signals at the carrier frequency. 
     SUMMARY OF THE INVENTION 
     The invention therefore relates to a detector for determining the presence of radio frequency signals for contactless chip cards. The detector comprises a reception antenna winding for receiving signals at a carrier frequency f transmitted by a user apparatus of the chip card. The detector comprises at least one detection circuit for detecting the presence of signals at the carrier frequency f. The input terminal of this detection circuit is connected to one end of the antenna winding. 
    
    
     BRIEF DESCRIPTION 
     Other features and advantages of the present invention shall appear from the following description of a particular exemplary embodiment, the description being made with reference to the appended drawings, of which: 
     FIG. 1 is a functional diagram of a detector of radio frequency signals according to the prior art. 
     FIGS. 2a and 2b are drawings of signals at certain points of the diagram of FIG. 1. 
     FIG. 3 is a functional diagram of a detector of radio frequency signals for chip cards according to the invention. 
     FIG. 4 is a drawing of one of the detector circuits 28 or 30 of FIG. 3. 
     FIG. 5 is a drawing of the phase detector circuit 26 of FIG. 3. 
     In the different figures, the same references indicate identical elements. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 have been described in the introduction as a reminder of the prior art, and shall not be described again. A detector according to the invention is shown schematically by the elements of FIG. 3. The detector comprises a circuit 10 for the detection of signals transmitted by a user device of a chip card (not shown) which respectively provides the signals V AC1 , and V AC0  to the ends AC1 and AC0 of the winding 20, and a circuit 24 for detecting the presence of these radio frequency signals. 
     It must be noted that the signals V AC1 , and V AC0  may also be applied to other circuits of the chip card. In particular, they can be applied to a rectifier circuit 12 according to the drawing of FIG. 1 or to a circuit for the preparation of supply voltage for the chip card from the radio frequency energy received. 
     The detection circuit 24 comprises a phase-shift detector circuit 26 whose two input terminals are connected respectively to the ends AC1 and AC0 of the winding 20 and whose output terminal is connected to a first input terminal of an AND circuit 32. The detection circuit 24 also comprises a first detector 28 of radio frequency signals whose input terminal is connected to the end AC1 of the winding 20 and whose output terminal is connected to a second input terminal of the AND circuit 32. The detection circuit 24 further comprises a second radio frequency signal detector 30 whose input terminal is connected to the end AC0 of the winding 20 and whose output terminal is connected to a third input terminal of the AND circuit 32. 
     The three input terminals of the AND circuit 32 are connected as indicated above. The output terminal of the AND circuit 32 gives a signal indicating the presence or absence of a radio frequency signal. The output terminal of the AND circuit 32 may be connected directly to a microcontroller of the chip card to inform it whether or not there is a transmission of binary data by radio frequency. Alternatively, this output terminal may be connected to a first input terminal of an AND circuit 33 whose other input terminal is connected to the output terminal of the comparator 16 of FIG. 1. 
     Referring to FIG. 4, each detector circuit 28 or 30 comprises an inverter circuit 40 whose input terminal is connected to one end AC1 or AC0 of the winding 20 depending on the detector considered. Each detector circuit 28 or 30 further comprises a diode 42 whose anode is connected to the output terminal of the inverter circuit 40, a capacitor C 2  having one of its terminals connected to the cathode of the diode 42 and its other terminal connected to ground, a resistance R 2  parallel-connected to the capacitor C 1 , and a flip-flop circuit 44. The flip-flop circuit 44 is known as a Schmitt trigger, whose input terminal is connected to the common point of the resistor R 1 , the capacitor C 1 , and the cathode of the diode 42. The inverter circuit 40 and the Schmitt trigger are supplied by the supply voltage V DD . 
     In each detector circuit 28 or 30 according to the diagram of FIG. 4, the inverter circuit is for converting the signal at the carrier frequency f into a square-wave signal with an amplitude V DD  at the same frequency. The diode 42 prevents the discharging of the capacitor C 2  when the output terminal of the inverter circuit 40 is at the ground potential. The capacitor C 2  and the resistor R 2  form an integration circuit which discharges through the resistor R 2 . 
     The voltage at the input terminal of the Schmitt trigger 44 increases as the frequency f of the signal increases. Consequently, its switching threshold and the values of the components C 2  and R 2  are chosen so that the signals having a frequency greater than a predetermined value are detected and cause the switching. 
     Referring to FIG. 5, the phase-shift detector circuit 26 detects the relative phase differences between the signals V AC1  and V AC0 . The phase-shift detector circuit 26 comprises an EXCLUSIVE-OR circuit 36 whose two input terminals AC1 and AC0 receive respectively the signals V AC1  or V AC0 . The output terminal of the EXCLUSIVE-OR circuit 36 provides a signal when the two signals V AC1  or V AC0  are in phase opposition. A capacitor 38 is connected between the supply voltage V DD  and the output terminal of the EXCLUSIVE-OR circuit 36 to filter the effects of the low-amplitude phase modulations between the signals V AC1  and V AC0 . The filtered signal is applied to a reshaping circuit comprising, for example, two inverter circuits 46 and 48. 
     The invention has been described with reference to a detector comprising a phase-shift detector circuit 26 and two radio frequency detectors 28 and 30 whose output signals are combined in the logic circuit 32 and 36. However, a detector of this kind is capable of functioning when it has only certain elements: a single detector 28 or 30; or both the single detectors 28 and 30 for improving the certainty of detection as compared with only one detector; or a single detector 28 or 30 with the phase-opposition detector 26; or finally, the two detectors 28 and 30 and the phase-opposition detector 26 as described with reference to FIG. 3. Furthermore, the different assemblies may be combined with the prior art circuit of FIG. 1 as indicated above.