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Design of Low-Power Zero Temperature Coefficient (ZTC) CMOS Oscillators

Shahidani, Mohammad Aref | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56989 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Akbar, Fatemeh
  7. Abstract:
  8. The increasing demand for autonomous vehicles and reliable communication protocols and hardware interfaces, such as CAN bus and USB, underscores the necessity for stable clock sources that maintain a low temperature coefficient (TC) over wide temperature ranges. This demand is particularly emphasized in applications such as wearables, network sensors, downhole devices, WSNs, and IoT, where long-lasting battery life and frequency-stable clock sources over a broad temperature range (e.g. -20 °C to 100 °C) are crucial. Traditionally, variations in frequency caused by temperature have been mitigated by employing off-chip components like crystals or ceramic based oscillators, but this approach results in increased size and power consumption. An alternative solution to achieve an integrated stable clock reference is to implement phase-compensated LC tank oscillators (TNULL) which possess high Q factors and can offer superior frequency stability. However, high Q inductors are typically bulky and integrating them into the design would result in on-chip area consumption. Other potential candidates for fully-integrated compact clock references include RC oscillators and ring oscillators. Despite their simplicity, RC oscillators suffer from weak temperature stability caused by comparator errors. To address this issue, RC oscillator typically employ high power comparators or additional circuitry for thermal compensation. On the other hand, ring oscillators are generally more robust than RC oscillators; however, temperature compensation remains essential for their performance. Regarding ring oscillators, utilizing single-ended inverters are preferred over fully differential counterparts due to their superior phase noise characteristics. This thesis presents a novel, compact, energy-efficient, and frequency-stable CMOS oscillator suitable for automotive and IoT applications. To ensure consistent frequency performance despite temperature variations, the oscillator employs temperature compensation techniques. Specifically, a ring oscillator integrated within an OpAmp feedback loop incorporates a varactor at each of its inverter gate outputs. These varactors are controlled by a tunable voltage generated by a PTAT voltage source. The OpAmp is designed as a folded-cascode to provide substantial differential gain and sufficient input common-mode range (ICMR), ensuring satisfactory operation despite input common-mode variations with temperature. All transistors are biased in the subthreshold region to enable low-power operation. The temperature-compensated oscillator performs at a frequency of 2 MHz with an accuracy of 79.5 ppm/°C across a temperature span from -10 °C to 200 °C, while consuming only 2.3 µW. The phase noise at a 100 kHz offset is also equal to -91.7 dBc/Hz. Similar works have been compared and the proposed circuit maintain the best FoM among the compared similar works with FoM of 188 dB
  9. Keywords:
  10. Temperature Coefficient of Resistance ; Subthreshold ; Varactor Diode ; Ultra Low Power Receiver ; Low Voltage ; Autonomous Vehicles (AVs) ; Wireless Sensor Network ; Internet of Things

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