Oscillation sensor with calibration unit and measurement device

1. Oscillating sensor for a measurement device comprising:

an oscillator comprising:

a resonance circuit for providing an oscillation signal;

a gain stage configured to provide a feed-back to the resonance circuit to inject energy for excitation of the oscillator to maintain oscillation;

at least one calibration element to adjust an open loop gain of the oscillator;

a calibration unit to provide a modulated calibration control signal to selectively adjust an electrical measure of the at least one calibration element based on at least one predetermined duty cycle,

characterized in that

a frequency counter is provided to measure an oscillation frequency of the oscillation signal and to provide an indication of the oscillation frequency to the calibration unit, and that

the calibration unit is further configured to provide the modulated calibration control signal with at least one cycle frequency which depends on the oscillation frequency of the oscillation signal.

2. The oscillating sensor according to claim 1 , wherein the modulated calibration control signal corresponds to a pulse width modulation signal.

3. The oscillating sensor according to claim 1 , wherein the at least one calibration element is configured to be controlled by a modulated calibration feedback control signal wherein particularly the at least one calibration element includes a passive electrical component, particularly a resistor, and a switch controlled by the modulated calibration feedback control signal.

4. The oscillating sensor according to claim 1 , wherein the at least one calibration element includes a gain control element to be controlled by a modulated calibration gain control signal for adjusting a gain of the gain stage wherein particularly the at least one calibration element includes a resistor and a switch controlled by the modulated calibration gain control signal.

5. The oscillating sensor according to claim 1 , wherein the calibration unit is configured to maintain the at least one predetermined duty cycle while varying the at least one cycle frequency.

6. The oscillating sensor according to claim 1 , wherein the calibration unit is configured to set the at least one cycle frequency to a value where an amplitude modulation of the oscillation signal is lower than 1%, preferably lower than 0.1% of a total oscillation amplitude of the oscillation signal.

7. The oscillating sensor according to claim 5 , wherein the calibration unit is configured to set the at least one cycle frequency to a value where an amplitude modulation of the oscillation signal is lower than 1%, preferably lower than 0.1% of a total oscillation amplitude of the oscillation signal.

8. The oscillating sensor according to claim 1 , wherein the calibration unit is configured to set the at least one cycle frequency to a value which is selected from one of frequency ranges which are determined by a plurality of cycle times which are selected from a range in which cycle time is different from (n+½) multiplied by the oscillation cycle time by more than 2% and not more than 25%, preferably by more than 5% not more than 15% of the oscillation cycle time, wherein the oscillation cycle time corresponds to the oscillating frequency.

9. The oscillating sensor according to claim 6 , wherein the calibration unit is configured to set the at least one cycle frequency to a value which is selected from one of frequency ranges which are determined by a plurality of cycle times which are selected from a range in which cycle time is different from (n+½) multiplied by the oscillation cycle time by more than 2% and not more than 25%, preferably by more than 5% not more than 15% of the oscillation cycle time, wherein the oscillation cycle time corresponds to the oscillating frequency.

10. The oscillating sensor according to claim 7 , wherein the calibration unit is configured to set the cycle frequency to a value which is selected from one of frequency ranges which are determined by a plurality of cycle times which are selected from a range in which cycle time is different from (n+½) multiplied by the oscillation cycle time by more than 2% and not more than 25%, preferably by more than 5% not more than 15% of the oscillation cycle time, wherein the oscillation cycle time corresponds to the oscillating frequency.

11. The oscillating sensor according to claim 1 , wherein the resonance circuit has a sensing coil and/or a sensing capacitor.

12. The oscillating sensor of claim 1 , further comprising an analyzing circuit which is configured to receive an indication of the oscillation signal and to provide an amplitude signal as a sensor signal indicating an amplitude of the oscillation signal.

13. The oscillating sensor of claim 12 , wherein the resonance circuit has a sensing coil for detecting a conductive object being in a sensing range of the sensing coil.

14. Method for operating an oscillation sensor, the oscillation sensor comprising an oscillator, the oscillator comprising a resonance circuit for providing an oscillation signal, a gain stage configured to feed back to the resonance circuit to inject energy for excitation of the resonance circuit to maintain oscillation, the oscillation sensor further comprising at least one calibration element to adjust a feedback coupling between the gain stage and the resonance circuit, wherein an electrical measure of the at least one calibration element is adjusted based on at least one predetermined duty cycle of a modulated calibration control signal

characterized in that

a frequency counter is provided to measure an oscillation frequency of the oscillation signal and to provide an indication of the oscillation frequency to the at least one calibration unit, and that

the modulated calibration control signal has at least one cycle frequency, which depends on the oscillation frequency of the oscillation signal.