Design and characterization of a monolithic CMOS-MEMS mutually injection-locked oscillator for differential resonant sensing

•Simple design guidelines for mutually injection-locked oscillators (MILOs) are given.•Monolithically co-integrated MEMS-based MILOs are fabricated.•Their performance in terms of sensitivity enhancement and drift-rejection is experimentally assessed.•The results compare well to theory and highlight the robustness of the proposed approach. This paper presents a proof of concept of a differential sensor based on the phase-difference of two injection-locked MEMS resonators, strongly coupled through their actuation voltages by a digital mixer. For the first time the feasibility of a fully monolithically co-integrated CMOS-MEMS differential resonant sensor, exploiting the capabilities of the injection-locked synchronization is proved. The principle of the system is first presented, from which optimal design guidelines are derived. The design of the different blocks of the system is then addressed. Our experimental results demonstrate the sensitivity enhancement of the proposed solution, as predicted by theory, and partial thermal drift rejection in a 70°C range. The simulated and experimental results highlight the critical points of the system design, on which the emphasis of this article is placed.