MEMS oscillators
View Patent ↗The invention provides MEMS oscillator designs in which the thermal actuation and piezoresistive detection signals are separated. A first approach splits the frequency of the loop into two distinct components, an actuation frequency and a detection frequency. A second approach modifies the design of the MEMS resonator such that the actuation signal follows a different path through the MEMS resonator than the detection signal.
1. An oscillator comprising:
a MEMS resonator having a first terminal and a second terminal;
an AC current source for providing an AC current to a resonator input coupled to the first terminal, thereby to induce an alternating strain in the resonator by means of the thermal expansion effect to drive the resonator into resonance; and
a piezoresistive read out sensor, coupled to the first terminal, for monitoring a resistance of at least a portion of the resonator to derive an oscillating output signal at an output of an oscillator circuit; and
wherein the oscillator circuit provides the ac current with a first frequency ω 1 to actuate the resonator and wherein a second different frequency ω 2 is generated in the resonator by the piezoresistive effect to derive the output signal, wherein the oscillator circuit comprises a converter in a feedback control loop between the oscillator output and the resonator input for converting from the second frequency to the first frequency; and
wherein the first terminal of the resonator is configured to both receive the AC current at the first frequency ω 1 and output the piezoresistive read out at the second frequency ω 2 .
2. The oscillator as claimed in claim 1 , wherein the oscillator circuit further comprises a phase shift element.
3. The oscillator as claimed in claim 1 , wherein the oscillator circuit further comprises a DC current source for providing a DC current to the resonator input.
4. The oscillator as claimed in claim 1 , wherein the resonator comprises two masses connected by a pair of parallel beams, with the AC or AC+DC current provided to one beam and being drained from the other beam.
5. The oscillator as claimed in claim 1 :
wherein the second terminal is coupled to a ground.
6. The oscillator as claimed in claim 1 :
wherein the first and second terminals are anchor points of the resonator.
7. The oscillator as claimed in claim 1 :
wherein the resonator is a piezoresistive resonator.
8. An oscillator comprising:
a MEMS resonator having a first terminal and a second terminal;
wherein the first terminal is configured to both input an input signal having a first frequency ω 1 and output an output signal a having a second frequency ω 2 ;
wherein the resonator is configured to resonate and generate the output signal at the second frequency ω 2 in response to the input signal at the first frequency ω 1 ; and
an oscillator circuit having a feedback control loop coupled to the first terminal and configured to convert the second frequency ω 2 to the first frequency ω 1 .
9. The oscillator as claimed in claim 8 :
wherein the input signal is an actuation current and the output signal is a piezoresistive voltage.
10. The oscillator as claimed in claim 9 , wherein the feedback control loop includes:
a frequency divider configured to convert the second frequency ω 2 to the first frequency ω 1 ; and
a transconductance amplifier to convert the piezoresistive voltage to the actuation current.