Microelectromechanical system resonator devices and oscillator control circuits
Reference oscillators are ubiquitous in timing applications generally, and in modern wireless communication devices particularly. Microelectromechanical system (MEMS) resonators are of particular interest due to their small size and potential for integration with other MEMS devices and electrical circuits on the same chip. In order to support their use in high volume low cost applications it would be beneficial for MEMS designers to have MEMS resonator designs and manufacturing processes that whilst employing low cost low resolution semiconductor processing yield improved resonator performance thereby reducing the requirements of the oscillator circuitry. It would be further beneficial for the oscillator circuitry to be able to leverage the improved noise performance of differential TIAs without sacrificing power consumption.
1. A microelectromechanical systems (MEMS) device comprising:
a MEMS resonator; wherein
the MEMS resonator comprises:
a central resonator element suspended by a plurality of anchor beams disposed at predetermined points on the periphery of the central resonator element in dependence upon the resonance mode the central resonator element is designed to resonate in;
a plurality of electrostatic actuators disposed at first predetermined locations around the periphery of the central resonator element, each electrostatic actuator comprising a first movable portion disposed towards the central resonator element, a first fixed portion disposed distal to the central resonator element, and a first central portion disposed between the movable portion and the fixed portion;
a plurality of sensor structures disposed around the periphery disposed at second predetermined locations around the periphery of the central resonator element, each sensor structure comprising a second movable portion disposed towards the central resonator element, a second fixed portion disposed distal to the central resonator element, and a second central portion disposed between the movable portion and the fixed portion;
a first separation between the central resonator element and each first movable portion of each sensor structure of the plurality of sensor structures is electrically adjustable by application of an appropriate DC bias voltage between the central resonator element and each sensor structure of the plurality of sensor structures; and
a second separation between the central resonator element and each first movable portion of each electrostatic actuator of the plurality of electrostatic actuators is electrically adjustable by application of an appropriate DC bias voltage between the central resonator element and each electrostatic actuator of the plurality of electrostatic actuators.
2. The MEMS device according to claim 1 , further comprising
a plurality of first stoppers, each first stopper disposed relative to the central resonator element and an electrostatic actuator of the plurality of electrostatic actuators to limit motion of the moveable portion of the electrostatic actuator of the plurality of electrostatic actuators towards the central resonator element.
3. The MEMS device according to claim 1 , further comprising
a plurality of first stoppers, each first stopper disposed relative to the central resonator element and an sensor structure of the plurality of sensor structures to limit motion of the moveable portion of the sensor structure of the plurality of sensor structures towards the central resonator element.
4. The MEMS device according to claim 1 , further comprising
a plurality of first stoppers, each first stopper disposed relative to the central resonator element and an electrostatic actuator of the plurality of electrostatic actuators to limit motion of the moveable portion of the electrostatic actuator of the plurality of electrostatic actuators towards the central resonator element; and
a plurality of second stoppers, each second stopper disposed relative to the central resonator element and an sensor structure of the plurality of sensor structures to limit motion of the moveable portion of the sensor structure of the plurality of sensor structures towards the central resonator element.
5. The MEMS device according to claim 1 , wherein
each movable portion of each electrostatic actuator of the plurality of electrostatic actuators is movable from a first position with a first gap between the movable portion and the central resonator element to a second position with a second gap between the movable portion and the central resonator element under application of a DC bias to the central resonator element; and
the second gap is smaller than the first gap.
6. The MEMS device according to claim 1 , wherein
each movable portion of each electrostatic actuator of the plurality of electrostatic actuators is movable from a first position with a first gap between the movable portion and the central resonator element to a second position with a second gap between the movable portion and the central resonator element under application of a DC bias to the central resonator element;
the second gap is smaller than the first gap; and
the second gap is smaller than the minimum feature size of the MEMS device defined by the manufacturing process for the MEMS device.
7. The MEMS device according to claim 1 , wherein
a size of a gap between the moveable portion and the central resonator element is adjustable under application of a DC bias to the central resonator element; and
a resonant frequency of the MEMS device varies according to the size of the gap between the moveable portion and the central resonator element.
8. The MEMS device according to claim 1 , wherein at least one of the stoppers and the movable portions of the plurality of electrostatic actuators comprise dimples formed upon them such that contact between the stoppers and the movable portions of the plurality of electrostatic actuators is only through these dimples.
9. A microelectromechanical systems (MEMS) device comprising:
a MEMS resonator comprising:
a central resonator element suspended by a plurality of anchor beams disposed at predetermined points on the periphery of the central resonator element in dependence upon the resonance mode the central resonator element is designed to resonate in;
a plurality of electrostatic actuators disposed around the periphery of the central resonator element, each electrostatic actuator comprising a movable portion disposed towards the central resonator element, a fixed portion disposed distal to the central resonator element, and a spring disposed between the movable portion and the fixed portion; and
a plurality of stoppers, each stopper disposed relative to the central resonator element and an electrostatic actuator of the plurality of electrostatic actuators to limit motion of the moveable portion of the electrostatic actuator of the plurality of electrostatic actuators towards the central resonator element; wherein
the movable portions of the plurality of electrostatic actuators form gaps relative to the central resonator element defined by the minimum gap feature of manufacturing process employed to form the MEMS resonator which is larger than the gaps when the moveable portions of the plurality of electrostatic actuators are driven under electrostatic actuation into contact with the stoppers, which is limited only be manufacturing process employed.
10. The MEMS device according to claim 9 , wherein
at least one of the stoppers and the movable portions of the plurality of electrostatic actuators comprise dimples formed upon them such that contact between the stoppers and the movable portions of the plurality of electrostatic actuators is only through these dimples.
11. A microelectromechanical systems (MEMS) device comprising:
a MEMS resonator comprising a pair of sensing electrodes and a pair of driving electrodes; and
the MEMS device further comprises:
a fully differential transimpedance amplifier (TIA) providing a feedback loop between the sensing electrodes and the driving electrodes, the TIA comprising
an input stage having differential inputs and differential outputs, wherein the differential inputs are coupled to the sensing electrodes of the MEMS resonator and the input stage employs a regulated cascade topology;
a variable gain amplifier (VGA) having differential inputs and differential outputs, wherein the differential inputs of the VGA are coupled to the differential outputs of the input stage and the gain of the VGA is controlled by an automatic gain control (AGC) circuit; and
an output stage having differential inputs and differential outputs, wherein the differential inputs of the output stage are coupled to the differential outputs of the VGA, the differential outputs of the output stage are coupled to the driving electrodes of the MEMS resonator and the output stage employs a super source follower.
12. The MEMS device according to claim 11 , wherein at least one of:
at least of the MEMS resonator is a Lame mode resonator and the AGC comprises a peak detector coupled to the output stage and a comparator; and
the differential inputs of the input stage and the differential outputs of the output stage each present low impedance to compensate for the large parasitic capacitance of the MEMS resonator and push the dominant pole of the MEMS oscillator beyond the oscillation frequency of the MEMS oscillator.
13. A microelectromechanical systems (MEMS) device comprising:
a MEMS resonator comprising:
a piezoelectric MEMS resonator having a pair of sensing electrodes disposed on a first side of the piezoelectric MEMS resonator and a pair of driving electrodes disposed on a second side of the piezoelectric MEMS resonator; and
a fully differential transimpedance amplifier (TIA) providing a feedback loop between the sensing electrodes and the driving electrodes, the TIA comprising
an input stage having differential inputs and differential outputs, wherein the differential inputs are coupled to the sensing electrodes of the MEMS resonator and the input stage employs a regulated cascade topology;
a variable gain amplifier (VGA) having differential inputs and differential outputs, wherein the differential inputs of the VGA are coupled to the differential outputs of the input stage and the gain of the VGA is controlled by an automatic gain control (AGC) circuit; and
an output stage having differential inputs and differential outputs, wherein the differential inputs of the output stage are coupled to the differential outputs of the VGA, the differential outputs of the output stage are coupled to the driving electrodes of the MEMS resonator and the output stage employs a super source follower; wherein
the piezoelectric MEMS resonator is coupled to the fully differential TIA on its inputs and outputs via capacitors; and
the piezoelectric MEMS resonator is driven without any DC signal applied to it.
14. The MEMS device according to claim 13 , wherein
at least one of:
at least one of the MEMS resonator is a disk resonator and the AGC comprises a peak detector coupled to the output stage and a comparator; and
the differential inputs of the input stage and the differential outputs of the output stage each present low impedance to compensate for the large parasitic capacitance of the MEMS resonator and push the dominant pole of the MEMS oscillator beyond the oscillation frequency of the MEMS oscillator.
15. A microelectromechanical systems (MEMS) device comprising:
a MEMS resonator; and
either:
a plurality of other MEMS resonators each operating over a predetermined first frequency range of a plurality of first frequency ranges; and
a multiplexer coupled to the MEMS resonator and the plurality of other MEMS resonators; wherein
the MEMS resonator operates over a predetermined second frequency range; and
the MEMS device provides at least one of:
an oscillator providing multiple output frequencies, each output frequency being within at least one of the plurality of first frequency ranges and the second frequency range; and
a single output frequency over a bandwidth established by the plurality of first frequency ranges and the second frequency range;
or:
the MEMS resonator is one of a plurality of MEMS resonators each having a different resonant frequency;
the plurality of MEMS resonators are coupled to a plurality of inputs of an electrical multiplexer;
a control circuit provides at least one of power and control signals to the plurality of MEMS resonators;
an output of the electrical multiplexer is coupled to a transimpedance amplifier; and
the MEMS device provides a programmable output at one frequency of the plurality of different resonant frequencies of the plurality of MEMS resonators under an action of the control circuit.