Low-power high-accuracy clock harvesting in inductive coupling systems

1. An apparatus, comprising:

a front-end circuit, which comprises an antenna and a modulation switch; and

a digital processing circuit, which is configured to transmit data to a remote unit using inductive coupling of an Alternating Current (AC) magnetic field generated by the remote unit, by modulating a load impedance of the antenna using the modulation switch,

wherein the front-end circuit is configured to supply to the digital processing circuit a voltage signal, which has a frequency of the AC magnetic field and which has a non-zero envelope both during intervals in which the modulation switch is closed and during intervals in which the modulation switch is open, and wherein the digital processing circuit is configured to derive a clock signal from the voltage signal.

2. The apparatus according to claim 1 , wherein the front-end circuit comprises a voltage clamping element connected in series with the modulation switch.

3. The apparatus according to claim 2 , wherein the front-end circuit further comprises a diode connected in series with the modulation switch.

4. The apparatus according to claim 2 , wherein the antenna is connected in parallel to the serially-connected modulation switch and voltage clamping element.

5. The apparatus according to claim 2 , wherein the voltage clamping element is configured to clamp a voltage signal induced by the AC magnetic field across the antenna, and wherein the front-end circuit is configured to provide the clamped voltage signal for powering the digital processing circuit from a connection point between the modulation switch and the voltage clamping element.

6. The apparatus according to claim 2 , wherein the voltage clamping element comprises a transistor biased by a Zener diode.

7. The apparatus according to claim 1 , wherein the digital processing circuit is configured to derive the clock signal solely from the voltage signal without an oscillator and/or a Phase-Locked Loop (PLL).

8. The apparatus according to claim 1 , and comprising a pressure sensor, wherein the digital processing circuit is configured to read a pressure reading from the pressure sensor, and to transmit the pressure reading to the remote unit using the derived clock signal.

9. The apparatus according to claim 1 , wherein the antenna comprises a coil.

10. The apparatus according to claim 9 , wherein the front-end circuit comprises a capacitor connected in parallel with the coil.

11. The apparatus according to claim 1 , wherein the digital processing circuit is configured to modulate the load impedance by alternately opening and closing the modulation switch in a pattern that depends on the data.

12. The apparatus according to claim 1 , wherein the digital processing circuit is configured to derive the clock signal from the voltage signal both during the intervals in which the modulation switch is closed and during the intervals in which the modulation switch is open.

13. A method, comprising:

using a digital processing circuit, transmitting data to a remote unit using inductive coupling of an Alternating Current (AC) magnetic field generated by the remote unit, by modulating a load impedance of an antenna using a modulation switch;

supplying to the digital processing circuit a voltage signal, which has a frequency of the AC magnetic field and which has a non-zero envelope both during intervals in which the modulation switch is closed and during intervals in which the modulation switch is open; and

in the digital processing circuit, deriving a clock signal from the voltage signal.

14. The method according to claim 13 , wherein modulating the load impedance and supplying the voltage signal are performed by a front-end circuit that comprises a voltage clamping element connected in series with the modulation switch.

15. The method according to claim 14 , wherein the front-end circuit further comprises a diode connected in series with the modulation switch.

16. The method according to claim 14 , wherein the antenna is connected in parallel to the serially-connected modulation switch and voltage clamping element.

17. The method according to claim 14 , and comprising clamping, using the voltage clamping element, a voltage signal induced by the AC magnetic field across the antenna, and providing the clamped voltage signal for powering the digital processing circuit from a connection point between the modulation switch and the voltage clamping element.

18. The method according to claim 14 , wherein the voltage clamping element comprises a transistor biased by a Zener diode.

19. The method according to claim 13 , wherein deriving the clock signal is performed solely from the voltage signal without an oscillator and/or a Phase-Locked Loop (PLL).

20. The method according to claim 13 , and comprising reading a pressure reading from a pressure sensor, and transmitting the pressure reading to the remote unit using the derived clock signal.

21. The method according to claim 13 , wherein the antenna comprises a coil.

22. The method according to claim 21 , wherein a capacitor is connected in parallel with the coil.

23. The method according to claim 13 , wherein modulating the load impedance comprises alternately opening and closing the modulation switch in a pattern that depends on the data.

24. The method according to claim 13 , wherein deriving the clock signal from the voltage signal is performed both during the intervals in which the modulation switch is closed and during the intervals in which the modulation switch is open.

25. A method, comprising:

applying an external unit to form a magnetic field, having a frequency and an accuracy, around a remote system comprising a modulation switch and a digital processing circuit;

in the remote system, deriving from the magnetic field a clock signal having the frequency and the accuracy;

applying the digital processing circuit to generate a digital signal carrying data synchronized with the clock signal;

applying the digital processing circuit to manipulate the modulation switch in a series of alternations between an open state and a closed state in a pattern derived from the digital signal, thereby encoding the data by way of load modulation; and

applying the external unit to decode the data by extracting the load modulation with the clock signal.

26. The method of claim 25 , and comprising powering the digital processing circuit solely from the magnetic field by way of inductive coupling.

27. The method according to claim 25 , wherein deriving the clock signal is performed both during intervals in which the modulation switch is in the closed state and during intervals in which the modulation switch is in the open state.

28. The method according to claim 25 , a voltage clamping element is connected in series with the modulation switch.

29. The method according to claim 28 , wherein a diode is connected in series with the modulation switch.

30. The method according to claim 28 , wherein an antenna of the remote system is connected in parallel to the serially-connected modulation switch and voltage clamping element.

31. The method according to claim 28 , and comprising clamping, using the voltage clamping element, a voltage signal induced by the magnetic field across the antenna, and providing the clamped voltage signal for powering the digital processing circuit from a connection point between the modulation switch and the voltage clamping element.