A Frequency Scan Scheme for PLL-Based Locking to High-Q MEMS Resonators

Making a MEMS resonator to oscillate at its natural frequency is an essential function in vibratory gyroscopes and is done by using a PLL inside the feedback loop. In particular, because of high Q (quality factor) resonators, signal levels at start up can be very low, posing challenges to achieve PLL lock or lead to unacceptable lock times. This paper proposes a novel frequency scan technique to achieve start up and, hence, lock for the loop. The paper describes a comparator circuit to convert the input sinusoid to a clock with in-built band pass filter, dc and ac hysteresis. Also a scheme is proposed to avoid wrong phase at start up which could arise in a resonator loop using charge pump based PLL. All the above techniques are implemented in TSMC 0.18 micron process for the ADXRS290 gyroscope product and results show a reliable PLL lock with startup time <;5 ms for up to a Q of 2500.