Abstract
MEMS (Microelectromechanical Systems) oscillators commonly require gain and phase compensations to initiate oscillation. The same compensation scheme may not work due to variations from manufacturing processes and/or environmental conditions that result in a shifted resonant frequency. The use of a phase-locked loop (PLL) can effectively track the resonant frequency and ensure proper phase compensation. However, conventional PLLs are often operated in a narrow frequency range, not a wide range suitable for most MEMS applications. This study proposes a wide-range PLL for driving MEMS oscillators by using a self-biasing technique. The technique makes the damping factor and the ratio of bandwidth to operating frequency constant, independent of the manufacturing process and operating temperature. This feature can in turn provide broader operating frequency range and lower jitter performance. Measurement results show that this wide-range PLL can operate from 100 Hz to 4.5 MHz, which is a range suitable for most MEMS oscillators and resonating sensors. The circuit area is 0.115 and the power consumption is 398.2 . The proposed MEMS oscillator with a wide-range PLL is implemented in the TSMC 0.35 2P4M (two-polysilicon-four-metal) CMOS (Complementary metal oxide semiconductors) process. Both the MEMS resonator and PLL are integrated in one chip. A capacitive MEMS resonator array with different resonant frequencies was designed. In the oscillator loop, the MEMS resonator is driven to resonance and the change of capacitive induces a current which is converted to a sensed voltage. The PLL provides a driving signal to the resonator with a 90-degree phase compensation at the resonant frequency. We successfully demonstrate the operation of a 34.36-kHz and a 77.94-kHz MEMS oscillator by using the wide-range PLL. The measurement results show that the phase noises at a 1-kHz frequency offset are -88.39 dBc/Hz and -88.73 dBc/Hz, respectively.