Phase Lock Loop based Temperature Compensation for MEMS Oscillators

We present a new temperature compensation system for microresonator based frequency references. It consists of a phase lock loop whose inputs are derived from two microresonators with different temperature coefficients of frequency. The resonators are suspended within an encapsulated cavity and are heated to constant temperature by the phase lock loop controller, thereby achieving active temperature compensation. We show repeated real-time measurements of two prototypes which achieve frequency stability of better than plusmn 1 ppm from -20 degC to + 80degC without calibration look-up tables and plusmn 0.05 ppm with calibration.