I. Introduction
Nanomechanical resonant sensors offer unprecedented sensitivities and are used in a variety of applications, such as mass spectrometry [1]–[5] and atomic force microscopy (AFM) [6]–[10]. In mass spectrometry, the main sensing mechanism is based on tracking the resonance frequency deviations arising from the interactions of the nanomechanical resonator with the sample under study. Currently, three schemes are used for this purpose, namely the open-loop, feedback free (FF) approach, and closed-loop schemes based on frequency-locked loops (FLL) and self-sustaining oscillators (SSO). These resonator tracking architectures were analyzed and the accuracy versus speed trade-off characteristics were discussed in detail in [11], [12].