Harmonic biasing in a double-sided comb-drive resonator, for resolving feed-through issues in low-power driving

We demonstrate a novel technique for operating a differentially-driven and differentially-sensed double-sided comb-drive resonator. The resonator is driven by application of two out-of-phase ac signals to the drive stators, while the rotor is subjected to a pure harmonic signal at the same frequency as of the ac driving signals. In this mode of operation, no dc bias is necessary. This strategy results in two frequency-mixing operations, such that a clear response can be sensed at the 3rd harmonic of the drive signal. Besides the responses in the 1st and 3rd harmonics of the drive frequency, no other higher-harmonic components are predicted from the analysis or observed in the measurements. With harmonic biasing, the resonator is insensitive to both feed-through currents that result from direct capacitive cross-coupling of sense and drive pads, and to feed-through currents that result from imbalanced sense ports due to packaging and setup. We show that a clear response can be achieved for low-power drive signals, whereas if the system were driven as a classic resonator with a dc bias on the rotor, the response would have been overshadowed by feed-through. We also demonstrate that the phase of the sensed response can be modified by varying the phase of the harmonic bias on the rotor.