Low power mode card detection

1. A circuit comprising:

an in-phase/quadrature down converter configured to be coupled to an antenna of a first NFC (near field communication) device;

DC (direct current) cancellation circuitry coupled at outputs of the in-phase/quadrature down converter; and

detection circuitry configured to analyze DC signals output by the DC cancellation circuitry to detect the presence of a second NFC device, the detection circuitry comprising:

amplifiers coupled to receive DC signals from the DC cancellation circuitry,

an analog to digital converter coupled to receive amplified signals from the amplifiers, and

a digital signal processing unit coupled to receive digital signals from the analog to digital converter.

2. The circuit according to claim 1 , further comprising low pass filters between the converter and the DC cancellation circuitry.

3. The circuit according to claim 1 , wherein the DC cancellation circuitry is configured by the detection circuitry, based on DC levels indicative of no second NFC device being present in a field generated by the first NFC device.

4. The circuit according to claim 1 , wherein the digital signals provide information related to the phase and amplitude of a variation of an RF (radio frequencey) signal sensed by the antenna.

5. The circuit according to claim 1 , further comprising the antenna and a front end circuit coupled between the antenna and the converter.

6. A method comprising:

emitting an electromagnetic field by a first NFC device;

sensing an RF signal at an antenna located in the electromagnetic field in response to a change in the electromagnetic field;

converting the RF signal into in-phase and quadrature signals;

performing DC cancellation of the in-phase and quadrature signals; and

detecting the presence of a second NFC device by analyzing DC signals resulting from the DC cancellation, wherein the emitting, sensing, converting, DC cancellation, and detecting steps are performed periodically.

7. The method according to claim 6 , further comprising low pass filtering the in-phase and quadrature signals.

8. The method according to claim 6 , wherein the DC cancellation removes DC levels measured when no second NFC device is present in the electromagnetic field.

9. The method according to claim 6 , wherein analyzing the DC signals is performed by a receiver of the first NFC device.

10. The method according to claim 6 , wherein a time taken for the sensing, converting, DC cancellation, and detecting steps is small compared to a time between successive emitting steps.

11. The method according to claim 6 , wherein the first NFC device is in a standby mode between two successive emitting steps.

12. The method according to claim 6 , further comprising initiating, by the first NFC device, a near field communication with the second NFC device in response to a detection of the presence of the second NFC device in a range of the first NFC device.

13. A near field communication (NFC) device, comprising:

a resonant circuit configured to generate an electromagnetic field;

a field detection circuit configured to sense an RF signal in response to a change in the electromagnetic field;

a processor; and

a non-transitory computer-readable storage medium storing a program to be executed by the processor, the program including instructions for implementing a method of:

converting the RF signal into in-phase and quadrature signals;

performing DC cancellation of the in-phase and quadrature signals; and

detecting the presence of a second NFC device by analyzing DC signals resulting from the DC cancellation, wherein the generating, sensing, converting, DC cancellation, and detecting are performed periodically.

14. The NFC device according to claim 13 , wherein the analyzing comprises detecting a change in a phase of the electromagnetic field, an amplitude of the electromagnetic field, or a combination thereof.

15. The NFC device according to claim 13 , wherein the method further comprises low pass filtering the in-phase and quadrature signals.

16. The NFC device according to claim 13 , wherein the DC cancellation removes DC levels indicative of no second NFC device being present in the electromagnetic field.

17. The NFC device according to claim 13 , wherein the method further comprises initiating a near field communication with the second NFC device in response to a detection of the presence of the second NFC device.

18. The device according to claim 13 , wherein a time taken for the sensing, converting, DC cancellation, and detecting steps is small compared to a time between successive generating steps.

19. A circuit comprising:

an in-phase/quadrature (IQ) down-converter configured to be coupled to an antenna of a first NFC (near field communication) device;

DC cancellation circuitry coupled at outputs of the IQ down-converter; and

detection circuitry configured to analyze DC signals output by the DC cancellation circuitry to detect the presence of a second NFC device, the detection circuitry comprising:

a chopping circuit coupled to receive DC signals from the DC cancellation circuitry, and

a reader/writer receiver of the first NFC device.

20. The circuit of claim 19 , wherein a frequency of the chopping circuit corresponds to a frequency of filters of the reader/writer receiver.

21. The circuit of claim 19 , wherein the analyzing comprises detecting a change in a phase of the electromagnetic field, an amplitude of the electromagnetic field, or a combination thereof.