On the design of ultra low voltage CMOS oscillators.

Wireless sensor nodes require very tight power budgets to operate from either a small battery, some energy harvesting mechanism or both. In many cases, thermal or electrochemical harvesting devices provide very low voltages of the order of 100 mV or even lower. Time-keeping functionality is required in IoT systems and the time-keeping module must be on at all times. Crystal oscillators have proven to be useful for low power time-keeping applications, and in this context supply voltage lowering is a convenient strategy. Therefore, 32 kHz crystal oscillators operating with only 60 mV supply are presented. Two implementations based on a Schmitt trigger circuit for two different crystals were designed and experimentally characterized. These crystal oscillators are based on the application of a Schmitt trigger as an amplifier. Guidelines for designing this block to be the amplifier of a crystal oscillator are provided. Furthermore, a dynamic model of the Schmitt trigger is proposed and the model results are compared against simulations. The amplifiers were experimentally characterized, providing a gain of 2.48 V/V with a 60 mV power supply. As it was intended in the design stage, for voltages above 100 mV hysteresis appears and the Schmitt trigger starts operating as a comparator. The Schmitt triggers to operate as amplifiers of the crystal oscillators are designed in a 130 nm CMOS process, requiring an area of 45 μm x 74 μm and 78 μm x 83 μm, respectively. The power consumptions of the crystal oscillators are 2.26 nW and 15 nW and the temperature stabilities attained are 62 ppm (25-62°C) and 50 ppm (5-62°C), respectively. The dependence on the supply voltage of the current consumption, fractional frequency, start-up time and oscillation amplitude were measured. The Allan deviation is 30 ppb for both oscillators. On the other hand, an LC voltage controlled oscillator (VCO) is designed in 28 nm FD-SOI for RF applications. The possibility of modeling the transistors in the 28 nm FD-SOI technology by means of the all inversion region long channel bulk transistor model used for the Schmitt trigger circuits, is studied. A crosscoupled nMOS architecture is used to build the VCO. The theoretical limit for the minimum supply voltage that enables oscillation is studied. The transistors were optimally sized to aim the minimum power consumption through a low-voltage approach and the performance of the VCO was obtained through simulations. This page is intentionally left blank. List of Abbreviations AC alternate current BLE Bluetooth low-energy CMOS complementary metal oxide semiconductor DC direct current FD-SOI fully depleted silicon on insulator IoT internet of things LC-VCO LC-tank voltage controlled oscillator LUT look up table MI moderate inversion MOSFET metal oxide semiconductor field effect transistor nMOS n-type metal oxide semiconductor PCB printed circuit board PLL phase locked loop pMOS p-type metal oxide semiconductor RF radio frequency SI strong inversion SoC system on chip UTBB ultra thin body and buried oxide WI weak inversion WSN wireless sensor network This page is intentionally left blank.