Influence of automatic level control on micromechanical resonator oscillator phase noise

Clear differences in the phase noise performance of a 10 MHz MEMS-based micromechanical resonator oscillator have been measured using sustaining circuits with and without automatic-level control (ALC), and with differing mechanisms for ALC. In particular, low output power oscillators referenced to high-Q clamped-clamped beam μmechanical resonators exhibit an unexpected 1/f³ phase noise component without ALC, a 1/f⁵ phase noise component when an ALC circuit based on resonator dc-bias adjustment is used, and finally, removal of these components when an ALC circuit based on sustaining amplifier gain control is used, in which case the expected 1/f² phase noise component is all that remains. That ALC is able to remove the 1/f³ phase noise seen in non-ALC'ed oscillators suggests that this noise component emanates primarily from nonlinearity in the voltage-to-force capacitive transducer, either through direct aliasing of amplifier 1/f noise, or through instabilities introduced by spring softening (i.e., Duffing) phenomena.