Amplitude demodulator for an electromagnetic transponder
An amplitude demodulator comprising a capacitive element for filtering a first D.C. component of a received signal, an element for detecting edges in the signal provided by the capacitive element, and at least one switching element for forcing, at least after detection of an edge, the returning of the signal provided by the capacitive element to a second D.C. component.
1. An amplitude demodulator, comprising:
a capacitive element to filter a first D.C. component of an amplitude-modulated signal with a modulation index smaller than one and to provide an output signal from the filtered signal, said modulation index being defined as a difference between an amplitude of a state 1 and an amplitude of a state 0 of transmitted bits divided by a sum of said amplitudes;
an element to detect edges in the output signal provided by the capacitive element; and
at least one switching element to force, at least after detection of an edge, a return of the output signal provided by the capacitive element to a second D.C. component.
2. The demodulator of claim 1 , wherein a filtering element is present upstream of said capacitive element to provide an envelope of the amplitude-modulated signal.
3. The demodulator of claim 1 , wherein a resistive element provides said second D.C. component to the output signal provided by the capacitive element.
4. The demodulator of claim 3 , wherein said switching element is formed of a switch adapted to short circuit said resistive element.
5. The demodulator of claim 1 , wherein said switching element is formed of a network of switched capacitances.
6. The demodulator of claim 1 , wherein said element to detect edges includes at least two comparators to compare an amplitude of the output signal with respect to two values surrounding said second D.C. component.
7. The demodulator of claim 6 , wherein the switching element is controlled by a circuit adapted to detect a state switching of an output of one of the at least two comparators.
8. The demodulator of claim 6 , wherein the switching element is formed of at least one switch adapted to open a branch of connection of the capacitive element to comparison inputs of said comparators.
9. The demodulator of claim 1 , wherein said second D.C. component corresponds to half a supply voltage of the demodulator.
10. An amplitude demodulator apparatus, comprising:
a capacitive element to filter a first DC component of an amplitude-modulated signal having a modulation index smaller than one, said modulation index being defined as a difference between an amplitude of a state 1 and an amplitude of a state 0 of transmitted bits, divided by a sum of said amplitudes;
a circuit coupled to the capacitive element to detect edges in an output signal provided by the capacitive element; and
at least one switching element coupled to the capacitive element to force, at least after detection of an edge, a return of the output signal provided by the capacitive element to a second DC component.
11. The apparatus of claim 10 , farther comprising a filtering element coupled upstream of the capacitive element to provide an envelope of the amplitude-modulated signal.
12. The apparatus of claim 10 , further comprising a resistive element coupled to the capacitive element to provide the second DC component to the output signal provided by the capacitive element.
13. The apparatus of claim 10 wherein the circuit comprises at least two elements to compare an amplitude of the output signal to values surrounding the second DC component.
14. The apparatus of claim 10 , further comprising a control circuit coupled to the switching element to control operation thereof based on an output of one of the compare elements.
15. A transponder system, comprising:
an oscillating circuit to provide an amplitude-modulated signal having a modulation index smaller than one, said modulation index being defined as a difference between an amplitude of a state 1 and an amplitude of a state 0 of transmitted bits, divided by a sum of said amplitudes; and
an amplitude demodulator coupled to the oscillating circuit, the demodulator including:
a capacitive element to filter a first DC component of the amplitude-modulated signal;
a detection circuit coupled to the capacitive element to detect edges in an output signal provided by the capacitive element; and
at least one switching element coupled to the capacitive element to force, at least after detection of an edge, a return of the output signal provided by the capacitive element to a second DC component.
16. The system of claim 15 , further comprising:
a filtering element coupled upstream of the capacitive element to provide an envelope of the amplitude-modulated signal;
a resistive element coupled to the capacitive element to provide the second DC component to the output signal provided by the capacitive element; and
a control circuit coupled to the switching element to control operation thereof based on an output of one of the detection circuit.
17. A method for a transponder, the method comprising:
filtering a first DC component of an amplitude-modulated signal with a capacitive element, said amplitude-modulated signal having a modulation index smaller than one, said modulation index being defined as a difference between an amplitude of a state 1 and an amplitude of a state 0 of transmitted bits, divided by a sum of said amplitudes;
detecting edges in an output signal provided by the capacitive element; and
forcing, at least after detection of an edge, a return of the output signal provided by the capacitive element to a second DC component.
18. The method of claim 17 wherein detecting the edges includes comparing an amplitude of the output signal to values surrounding the second DC component.
19. The method of claim 18 , further comprising controlling forcing of the return of the second DC component based on a result of the comparing.
20. The method of claim 17 , further comprising filtering the output signal provided by the capacitive element to provide an envelope of the amplitude-modulated signal.