A Temperature-to-Digital Converter for a MEMS-Based Programmable Oscillator With src='/images/tex/20587.gif' alt='< pm hbox {0.5-ppm}'> Frequency Stability and src='/images/tex/20588.gif' alt='< hbox {1-ps}'> Integrated Jitter

摘要

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 $pmhbox {0.5-ppm}$ frequency stability from $-hbox{40 }^{circ }hbox{C}$ to 85 $^{circ }hbox{C}$ 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 fractional-${rm N}$ frequency 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-nm CMOS.