A Temperature-to-Digital Converter for a MEMS-Based Programmable Oscillator With < ±0.5-ppm Frequency Stability and < 1-ps Integrated Jitter

MEMS-based oscillators offer a silicon-based alternative to quartz-based frequency references. Here, a MEMS-based programmable oscillator is presented which achieves better than 0.5-ppm frequency stability from 40 C to 85 C and less than 1-ps (rms) integrated phase noise (12 kHz to 20 MHz). A key component of this system is a thermistor-based temperature-to-digital converter (TDC) which enables accurate and low noise compensation of temperature-induced variation of the MEMS resonant frequency. The TDC utilizes several circuit techniques including a high-resolution tunable reference resistor based on a switched-capacitor network and fractionalfrequency division, a switched resistor measurement approach which allows a pulsed bias technique for reduced noise, and a VCO-based quantizer for digitization of the temperature signal. The TDC achieves 0.1-mK (rms) resolution within a 5-Hz bandwidth while consuming only 3.97 mA for all analog and digital circuits at 3.3-V supply in 180-nmCMOS.