A micromechanical frequency reference with parts-per-trillion holdover stability

TL;DR

This paper presents a MEMS-based frequency reference achieving record fractional stability of ~8 parts-per-trillion over 8 hours, rivaling atomic clocks, through innovative electronic and temperature stabilization techniques.

Contribution

The work introduces a gain-insensitive dual-frequency resonance tracking method that significantly improves long-term stability of MEMS clocks.

Findings

Achieved 8 parts-per-trillion stability at 8 hours.

Implemented a gain-insensitive frequency locking architecture.

Demonstrated stability comparable to chip-scale atomic clocks.

Abstract

Microelectromechanical (MEMS) resonators are widely used in timekeeping applications, and recent advances in fabrication, materials, and encapsulation technology have advanced their potential as high stability frequency references. However, for holdover applications that require the highest levels of long-term frequency stability, compact vapor atomic clocks remain dominant. In this work, we demonstrate a 268 MHz MEMS clock that achieves record fractional frequency stability of ~8 parts-per-trillion at an averaging time of 8 hours, competitive with chip-scale atomic clocks. We achieved this using a single-crystal silicon electrostatic resonator that has no currently known intrinsic drift mechanism and is protected from the environment with a wafer-level encapsulation. We specifically identify gain variations in the sustaining electronics as the dominant limitation in conventional phase-locked oscillator architectures -- originating from temperature sensitivity and drifts in the electronic components -- and overcome this by implementing a frequency-locked loop architecture based on dual-frequency resonance tracking (DFRT). This novel approach removes the specific gain of the supporting electronics as a frequency determining variable in the oscillator. When combined with dual-mode tracking and ratiometric temperature stabilization of the resonator, this approach enables a dramatic enhancement to long-term frequency stability and establishes gain-insensitive DFRT locking as a general paradigm for high-stability MEMS clocks.