A MEMS-Assisted Dual-Resonator Temperature-to-Digital Converter

Many applications demand precision oscillators, which are usually generated through employing resonators with high quality factor (Q). Quartz and MicroElectroMechanical systems (MEMS) are the most common precision resonators in today’s industry. Although quartz has been used in timing products for a long time, however, in the past couple of decades, MEMS resonators are gradually taking over a big portion of the market due to some advantages they have compared to quartz. With today’s resonator’s technology, precision oscillators require some types of compensation in order to achieve frequency stability of < ±1ppm over temperature. This work presents a high resolution temperature sensor employed in a MEMS-based programmable oscillator suitable for telecom applications. For the target application, the clock should have an Allan Deviation (ADEV) of < 1e−10 and frequency stability of < ±0.1ppm across the temperature range of −45˚C to 105˚C. Given the fact that in MEMS-based programmable oscillators the output clock is generated by a frequency synthesizer whose reference clock is made from the MEMS resonator, the frequency stability requirement necessitates utilization of a temperature sensor in order to apply a proper correction signal to the frequency synthesizer at each temperature. However, it must have enough resolution to ensure its noise does not violate the target output clock jitter. Also, it should have enough temperature tracking bandwidth to maintain frequency stability and ADEV in breezy conditions. This work presents a temperature-to-digital converter (TDC), designed in a 0.18µm CMOS process, based on a novel technique that achieves a resolution of 20µK over a bandwidth of 100Hz. By burning 19mW power, it gains an energy/resolution FOM of 0.04pJK2, showing around a 16x improvement on the previous state-of-the-art resolution FOM number among all the best temperature sensors reported so far. This TDC enabled us to successfully implement a MEMSbased programmable oscillator whose frequency stability and phase noise performance satisfy the telecom application requirements.