Rapid growth of communication systems in two recent decades has involved manufacturing transceiver with higher capabilities, smaller size, reduced power consumption and lower prices. Analog, digital and mixed-mode blocks have been usually employed in the structure of the transceiver system. CMOS technology with high capabilities has provided possibilities to develop all parts of a communication system on a single chip with the lowest cost and minimum power consumption. Most signal processing has shifted from analog to digital domain. However, due to availability of high performance digital circuits, the role of sensitive analog blocks has been more crucial. One of the most important blocks in the high frequency analog part of a transceiver is the voltage controlled oscillator with minimum phase noise in CMOS technology. For instance, in an OFDM system the local oscillator phase noise is a key parameter in the whole performance of the system. The importance of phase noise is not restricted to analog systems and even in digital systems, phase noise interpreted as jitter has a considerable significance. There are different architectures used as VCOs, among which the cross-coupled LC VCO is the most important. Comparison to other architectures, it has better specifications in terms of lower phase noise. Due to nonlinearity behavior of oscillators, the mechanism of converting noise sources to phase noise is very complex. Attempts in phase noise analysis have resulted in developing different models. One famous model is Leeson model and a recent model proposed by Hajimiri. Based on different models, various methods have been used to decrease the phase noise of oscillator. This thesis introduces operational principles of different types of high frequency oscillators. Jitter and phase noise concepts and their effects on the performance of a communication system are studied as well. It then, discuses different proposed models for phase noise of LC oscillators. The effect of each parameter on phase noise is studied, and the accuracy of the analysis is verified by simulation. More precise equations for oscillation amplitude in terms of differential pair sizing and consequently an optimum dimension for the good specifications are provided and also the effect of mutual induction between two inductors on oscillator is examined. Methods to decrease phase noise in LC oscillators are investigated and some new methods are proposed and verified via simulations. Key Words: Communication Systems, CMOS Technology, LC Oscillator, Voltage Controlled Oscillator, Phase Noise, Minimum Noise
