A micro-electromechanical system (MEMS) comprises a substrate
incorporating an oscillatory ring, forcing electrodes for driving the
ring into resonance, and sensing electrodes providing an electrical
output signal dependent on oscillation of the ring as a result of such
forcing and any externally applied force. A positive feedback circuit is
provided for feeding back a signal dependent on the output signal of the
sensing electrodes to the forcing electrodes in order to sustain
oscillation of the ring. The use of positive feedback to drive the
forcing electrodes in order to sustain oscillation of the ring is highly
advantageous in such an application since it produces a system which
exhibits very low phase noise of a magnitude considerably less than the
phase noise experienced in use of a phase-lock loop circuit to sustain
oscillation.