PhD defence of Ali Gorji Zadeh - "Temperature compensation of oscillators using a phase-locked loop with a thermal feed-back"
Using an integrated micro-heater and a phase-locked loop (PLL) configuration, a technique to create a temperature-insensitive area on a die is described. The proposed PLL configuration employs thermal feedback through the micro-heater to significantly improve the temperature stability of the die area. Two oscillators are employed in the PLL to act as temperature sensors to detect the ambient temperature variations and command the thermal loop to compensate them. The temperature stability of these oscillators located in the temperature-compensated die area is improved significantly and can provide a stable timing signal needed for the system. The temperature compensation of oscillators provided by this technique is studied in more detail. Design methodologies and system-level analysis are presented. An application on temperature compensation of MEMS-based oscillators is studied with analysis and simulations. Based on these preliminary steps, an all-CMOS test-chip for proof-of-concept is designed and fabricated using a standard 130 nm CMOS technology. Analysis and experimental results show that the micro-heating system employed in this technique can provide a low pass filtering effect enough to replace the electrical low pass filtering effect provided by a charge pump and a loop filter. Therefore, the charge pump and the loop filter existing in the conventional PLL loops can be eliminated in this system. Test characterization of the realized proof-of-concept chip shows that the temperature stability of the die area can be improved by a factor as large as 50x for an ambient temperature range of 36°C to 52°C. Simulation shows that this range can be improved by many folds using more heat preserving technologies like SOI CMOS instead of bulk CMOS.