An integrated CMOS micromechanical resonator high-Q oscillator

TLDR

The authors present a fully monolithic high‑Q oscillator fabricated by combining CMOS and surface micromachining, where a polysilicon micromechanical resonator sets the oscillation frequency to achieve high stability. They model the micromechanical resonator’s circuit and noise characteristics and design a series‑resonant oscillator that employs a gain‑controllable transresistance sustaining amplifier. The oscillator’s steady‑state amplitude is highly sensitive to the dc‑bias voltage, and although the resonator’s high Q improves stability, its limited power handling prevents the device from reaching the short‑term stability typical of high‑Q oscillators.

Abstract

A completely monolithic high-Q oscillator, fabricated via a combined CMOS plus surface micromachining technology, is described, for which the oscillation frequency is controlled by a polysilicon micromechanical resonator with the intent of achieving high stability. The operation and performance of micromechanical resonators are modeled, with emphasis on circuit and noise modeling of multiport resonators. A series resonant oscillator design is discussed that utilizes a unique, gain-controllable transresistance sustaining amplifier. We show that in the absence of an automatic level control loop, the closed-loop, steady-state oscillation amplitude of this oscillator depends strongly upon the dc-bias voltage applied to the capacitively driven and sensed /spl mu/resonator. Although the high-Q of the micromechanical resonator does contribute to improved oscillator stability, its limited power-handling ability outweighs the Q benefits and prevents this oscillator from achieving the high short-term stability normally expected of high-Q oscillators.