Design Of Decentralized Digital Controlled Crystal Oscillator

Technological advancements in wireless communication made massive deployment and user adoption of cellular phones possible in the last decade. However the increase of subscribers asks for more efficient use of the limited spectrum resource, reference clock with high accuracy and stability is demanded. With the high quality factor of quartz crystal as the frequency selecting component, the crystal oscillators are widely used as clock references in communication systems such as cellular phones、global positioning systems(GPS)、and wireless local area networks(WLAN).The frequency of crystal oscillators needs to be tunable as it can be influenced by the aging of crystal and environment temperature. Digitally controlled crystal oscillator (DCXO) is able to adjust its frequency by using the automatic frequency control (AFC) signal. The capacitor bank of oscillators switched to keep the oscillator work in desired frequency.In this thesis, the theory and practice of DCXOs are studied, difficulties in design is analyzed and circuit implement is carried out.Firstly, this thesis gives a brief review on the crystal oscillator research; proposed advantages of differentially-ended DCXO; analyzed the equivalent circuit of crystal oscillator and briefly introduces the fundamental theory of oscillators.Secondly, Three analysis methods of phase noise are studied:linear time invariant (LTI), nonlinear time invariant (NTI) and linear phase time varying (LPTV). A differentially-end DCXO is presented. Small and large signal analyses are given. With linear time invariant (LTI) modeling of phase noise, the phase noise of DCXO is calculated and optimized.Finally, the realization and simulation results of DCXO in TSMC0.18μm RF CMOS process are given.