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Meghna Misra
(From: Heritage Institute of Technology; To: Onsemi, Bangalore)
Thesis
Title: Design of building blocks for high-performance Frequency Synthesizers
The current world's ever-increasing data rate, and high-speed seamless data transmission require high efficiency and low noise clock sources for the proper operation of multiple blocks in chips. For this purpose, a stable frequency source is much needed in communication and RF systems. The crystal oscillator plays an important role in supplying such a clean stable reference clock for various circuit building blocks such as phase-frequency locked loops, frequency synthesizers, and memory elements. In this project, four 114 MHz crystal oscillators have been designed using TSMC 65 nm technology. The conventional Pierce crystal oscillator and differential crystal oscillator type architectures are used to implement the same supporting both low voltage (SI1V) and high voltage (2.5 V) supplies. We achieve a frequency tuning range of the oscillator as 7 kHz at TT () when load capacitance is varied from 2 pF to 4.3 pF. The nominal case FOMs achieved with 7 pF load at TT() are 195.15 dB, 195.95 dB, 196.86 dB, and 197.39 dB for the high-voltage Pierce, low-voltage Pierce, high-voltage differential, and low-voltage differential crystal oscillators respectively. The absolute phase noises of these oscillators under the same conditions are , , , and at 1 MHz offset respectively. The nominal power consumptions of these oscillators under the same conditions are 5.5 mW, 274 , 8.5 mW and 1.321 mW respectively. The differential oscillator thus performs better than the conventional Pierce oscillator and appears to be a better choice for frequency references in phase-locked loops.
Another important building block of frequency synthesizers is the Voltage Controlled Oscillator(VCO). The output frequency of the VCO is controlled by its input voltage and ideally should be a linear function of the same. In this project a nonlinear VCO having a tunable inductor and capacitor is linearized by using the averaging varactor method leading to reduction and linearization of the VCO gain.