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This letter demonstrates hollow plate microelectromechanical-system oscillators implemented into a digital phase-locked loop (PLL). The resonators feature innovative geometries: hollow square and ring plates, operating in Lamé or Kirkhope modes, respectively, with resonance frequencies comprised between 24 and 78 MHz. Advantageously, the ring-plate resonator presents better frequency stability when compared with its counterpart square plate. The PLL provides a real-time monitoring of the oscillators' frequency shift, which was tested by changing the electrostatic actuation amplitude. Ring-plate resonators show sensitivity between 38 and 232 Hz/V. Finally, the feedback loop was operated with a square-plate device filled with an aqueous solution demonstrating a sensitivity of up to 284 Hz/V.