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A novel nonlinear model of LC oscillator is presented which is based on electrical parameters of circuit. In view of the operation condition of device in steady-state oscillation, we discuss the physical process that transforms device noise in transistors or resistors into phase noise by nonlinear spectrum conversion and linear spectrum shaping. A closed-form expression is derived for the output phase noise spectrum. This model is validated by simulation of a 2.9 GHz Colpitts oscillator using ADS. Theoretical analysis and simulation shows that phase noise performance in LC oscillator can be improved by diminishing the feedback factor.