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In this paper, a second-order stochastic differential equation is used as a tool for the analysis of phase noise in a submicron CMOS LC oscillator. A cross-coupled topology typical of integrated CMOS designs is considered. Nonlinear limiting and mobility degradation effects in the circuit are modeled and used to predict the statistics of the random amplitude and phase deviations in terms of design variables. Assuming Gaussian noise disturbances and describing the phase noise as a random diffusion process, the average phase-noise power spectrum is derived and its accuracy verified with measurement and simulation results. Calculations for phase noise arising from stationary tank noise, nonstationary channel thermal noise, and flicker noise are discussed. The analysis is used to emphasize the fundamental power/performance tradeoff associated with compensation of tank losses via adjustments in the power supply and device size.