A CMOS capacitive micromechanical oscillator driven by a phase-locked loop

This work describes the design and characterization of integrated CMOS (complementary metal oxide semiconductor) oscillators comprising of a capacitively transduced micromechanical resonator and a phase-locked loop (PLL) driving circuit. Inclusion of a PLL is beneficial for sustaining oscillations at resonant frequencies within the PLL capture range. The micromechanical resonator has a measured resonant frequency of 117.3 kHz. The CMOS PLL circuit has a closed-loop bandwidth of 1.8 kHz with a capture range between 111 kHz to 118.4 kHz. The start-up times for oscillation are shortened in the scheme utilizing a PLL as it provides an initial driving signal at its free-running frequency. The measured noises for the presented two oscillator schemes are -75 dB below the resonant peak at a 10-Hz offset.