IEEE Transactions on Circuits and Systems I: Regular Papers | 2019
A Programmable Sustaining Amplifier for Flexible Multimode MEMS-Referenced Oscillators
Abstract
This paper presents a programmable single-chip sustaining amplifier for MEMS-referenced oscillators. The chip integrates a low-noise amplifier, variable-gain amplifiers (VGAs), band-pass filters (BPFs), all-pass filters (APFs), automatic level control, and output buffers. It also contains a second signal path for background cancellation, i.e., eliminating the effect of electrical feedthrough capacitance of the MEMS resonator. Important parameters of each circuit block (e.g., VGA gain, BPF center frequency and quality factor (<inline-formula> <tex-math notation= LaTeX >$Q$ </tex-math></inline-formula>), APF phase shift, and so on.) can be set using bias currents, thus enabling fine-grained control over the overall transfer function. The amplifier was designed and fabricated in <inline-formula> <tex-math notation= LaTeX >$0.5~\\mu \\text{m}$ </tex-math></inline-formula> CMOS. The chip was integrated with board-level current DACs (programmable via an I<sup>2</sup>C bus) and tested inside a vacuum chamber with various polysilicon (poly-Si) and single crystal (SC-Si) silicon comb-drive MEMS devices with resonant frequencies in the 10–90 kHz range. Oscillators referenced to a high-<inline-formula> <tex-math notation= LaTeX >$Q$ </tex-math></inline-formula> (~13 000) SC-Si device had 20 dBc/Hz lower close-in phase noise (at 10 Hz) than those referenced to the low-<inline-formula> <tex-math notation= LaTeX >$Q$ </tex-math></inline-formula> (~320) poly-Si device. Experimental results highlight advantages of the design, including automatic mode selection, background cancellation, control of feedback phase to optimize phase noise, and frequency locking to an external reference.