Synchronous-phase contactless demodulation method, and associated demodulator and reader

1. A method of demodulating signals from an electromagnetic field induced by a contactless transponder in a system having a reader comprising a first antenna and a second antenna, the method comprising the steps of:

detecting, by the first antenna in the reader, modulation of the electromagnetic field induced by the transponder;

sensing, by the second antenna, a phase of the electromagnetic field;

controlling the detection of the modulation in accordance with the sensed phase of the electromagnetic field.

2. A demodulation method according to claim 1 , wherein in order to control the detection of the modulation in accordance with the sensed phase of the electromagnetic field, the flux caused by the electromagnetic field is sensed by the second antenna and the phase of the flux is used as a synchronisation element.

3. A demodulation method according to claim 1 , wherein, in order to control the detection of the modulation in accordance with the sensed phase of the electromagnetic field, the phase of the current generating the electromagnetic field is sensed and is used as a synchronization element.

4. A demodulation method according to claim 2 , wherein modulation of the electromagnetic field is detected at a sampling frequency equal to and in phase with the frequency of the electromagnetic field.

5. A demodulation method according to claim 2 , wherein a synchronization clock is generated from the sensed phase of the electromagnetic field or its generating current and this clock is used to time the signal detection time.

6. A contactless transponder reader comprising:

a main antenna for detecting modulation of an electromagnetic field;

an electromagnetic signal demodulator comprising means of sampling the modulation of the electromagnetic field detected by the main antenna; and

a synchronization means comprising a secondary antenna which is detuned with respect to the main antenna, wherein the synchronization means uses the secondary antenna to sense a phase of the electromagnetic field and controls the detection of the modulation in accordance with the sensed phase of the electromagnetic field.

7. A contactless transponder reader according to claim 6 , wherein said secondary antenna is able to sense part of the magnetic field used by the transponder; and

wherein the synchronization means further comprises a sampling control circuit, wherein the sampling control circuit is able to generate a signal controlling the sampling of the electromagnetic field.

8. A contactless transponder reader according to claim 6 , wherein the synchronization means comprises an antenna sensitive to the electrical field.

9. A contactless transponder reader according to claim 6 , wherein the synchronization means comprises a current diversion means able to sense the current flowing in the conductor forming the reading antenna and to extract therefrom phase information for synchronising the sampling.

10. A transponder reader according to claim 6 , wherein the demodulator is a synchronous detection demodulator.

11. A transponder reader according to claim 6 , wherein the synchronous detection is digital.

12. A demodulator for signals induced by a contactless transponder in an electromagnetic field, said demodulator comprising:

a detector for detecting modulation of the electromagnetic field induced by the transponder that are received via a primary antenna of a reader,

means of controlling the detection of the modulation in accordance with a phase of the electromagnetic field sensed by a secondary antenna of the reader, wherein the controlling means continuously provides synchronization of the detection of the modulation with the phase of the electromagnetic field sensed by the secondary antenna.