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1. (WO2019048736) FREQUENCY REFERENCE OSCILLATOR DEVICE AND METHOD OF STABILIZING A FREQUENCY EFERENCE SIGNAL
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Claims

1 . A frequency reference oscillator device comprising

- a first oscillator comprising a first resonator having first long-term stability and a first frequency-vs-temperature turnover temperature, the first oscillator being capable of providing a first frequency signal,

- a second oscillator comprising a second resonator having second long-term

stability, which is inferior to the first long-term stability, and a second frequency-vs- temperature turnover temperature, the second oscillator being capable of providing a second frequency signal,

- a thermostatic controller for adjusting the temperature of the first resonator

essentially to said first turnover temperature and the temperature of the second resonator essentially to said second turnover temperature, and

- a stability control circuit configured to use the first frequency signal for adjusting the frequency of the second oscillator for providing a temperature stabilized and long-term stabilized output frequency signal.

2. The device according to claim 1 , wherein the stability control circuit is adapted to use the first frequency signal and a feedback loop utilizing the second frequency signal for adjusting the frequency of the second oscillator, whereby said stabilized output frequency signal and the second frequency signal are obtained at output of the second oscillator.

3. The device according to claim 1 or 2, wherein the stability control circuit is functionally independent of the temperature of the first resonator.

4. The device according to any of the preceding claims, wherein the thermostatic controller is functionally independent of the first and the second frequency signals.

5. The device according to any of the preceding claims, wherein the stability control circuit is configured to operate the first oscillator and to use the first frequency signal

intermittently for said adjusting of the second oscillator.

6. The device according to any of the preceding claims, wherein the first resonator is a degenerately doped electrostatically actuated single-crystal MEMS resonator.

7. The device according to any of the preceding claims, wherein the second resonator is a degenerately doped piezoelectrically actuated composite MEMS resonator, such as an aluminum nitride thin film actuated silicon resonator.

8. The device according to any of the preceding claims, wherein the first and/or second oscillator comprises

- a resonator comprising silicon doped to an average doping concentration of at least 9*1019 cm"3,

- an actuator for exciting the resonator into a resonance mode having a

characteristic frequency-vs-temperature curve having a high-temperature turnover point at a turnover temperature of 85°C or more.

9. The device according to claim 8, wherein said doping concentration of the resonator is at least 1 .1 *1020cnr3 and the frequency-vs-temperature curve has two turnover points, one of which is said high-temperature turnover point, the other optionally being a low-temperature turnover point located at a temperature of less than 85°C.

10. The device according to any of the preceding claims, wherein both of said first and second turnover temperatures are 85°C or more.

1 1 . The device according to any of the preceding claims, wherein the second turnover temperature is substantially different, in particular at least 5°C different, from the first turnover temperature.

12. The device according to any of claims 1 - 8, wherein the second turnover temperature is substantially equal to the first turnover temperature, in particular differing at most 5°C therefrom, and the thermostatic controller is adapted to adjust the temperatures of the first and second resonator essentially to the same temperature.

13. The device according to any of the preceding claims, wherein the second oscillator has lower characteristic phase noise than the first oscillator when the frequency offsets from the respective carrier frequencies are above a certain frequency, such as 100 Hz, offset from the respective carrier.

14. The device according to any of the preceding claims, wherein the first resonator is a plate resonator adapted to oscillate in Lame mode or a beam resonator adapted to oscillate in length extensional mode.

15. The device according to any of the preceding claims, wherein the first and/or second resonator is a plate element with an in-plane aspect ratio different from 1 or a beam element, and is adapted to oscillate in the square extensional/width extensional, in-plane flexural, out-of-plane flexural, or length-extensional/Lame modal branch.

16. The device according to any of the preceding claims, wherein the first and/or second resonator is a silicon-based resonator doped to an average concentration of 1 .1 *1020cnr3 or more, such as 1 .3*1020cnr3 or more.

17. The oscillator according to any of the preceding claims, wherein the second resonator comprises

- a silicon base layer having an n-type dopant concentration of 1.3*1020 cm"3 or above,

- an aluminum nitride transducer layer and a conductive electrode layer

superimposed on top of the base layer,

and wherein the resonator is shaped as a plate or beam, whose geometry yields for the element in said resonance mode an essentially zero TCF1 along with TCF2 and TCF3 characteristics that provide said at least two turnover points.

18. The device according to any of the preceding claims, wherein the stability control circuit comprises a phase locked loop (PLL) -based circuit, such as a fractional-N PLL circuit or a microcontroller -based circuit.

19. The device according to any of the preceding claims, wherein the first and second resonators are degenerately doped silicon resonators adapted to resonate in different resonance modes.

20. A method of stabilizing a frequency reference signal, the method comprising

- providing a first oscillator comprising a first resonator having first long-term stability and a first frequency-vs-temperature turnover temperature, the first oscillator being capable of providing a first frequency signal,

- providing a second oscillator comprising a second resonator having second long- term stability, which is inferior to the first long-term stability, and a second frequency-vs-temperature turnover temperature, the second oscillator being capable of providing a second frequency signal,

- using thermostatic control for heating the first resonator to said first turnover

temperature and the second resonator to said second turnover temperature,

- using the first frequency signal to adjust the second oscillator for providing a

stabilized output frequency signal.

21. The method according to claim 20, wherein a frequency reference oscillator device according to any of claims 1 - 19 is used.