This tutorial presents a detailed analysis of oscillator phase noise, focusing on the limitations of traditional linear time-invariant (LTI) models and introducing a linear time-varying (LTV) approach. LTI models, while useful for qualitative insights, lack quantitative predictive power due to the time-varying nature of oscillators. The LTV model accounts for this by recognizing that oscillators are linear time-varying systems, where the phase-noise spectra result from frequency translation of device noise. Key insights include the importance of symmetry in suppressing 1/f noise upconversion, the role of cyclostationary effects, and the AM–PM conversion. The LTV model also explains why certain oscillator topologies, like the Colpitts oscillator, perform well and suggests new designs. The model is validated through simulations and examples, including LC and ring oscillators. It also addresses the impact of amplitude noise and the need to minimize the dc value of the impulse sensitivity function (ISF) to reduce 1/f noise upconversion. The LTV model provides a more accurate description of phase noise, including the 1/f² and 1/f³ regions, and highlights the importance of symmetry and circuit design in minimizing phase noise. The tutorial concludes with practical considerations for simulation and circuit design, emphasizing the need for careful layout and design to achieve low phase noise in oscillators.This tutorial presents a detailed analysis of oscillator phase noise, focusing on the limitations of traditional linear time-invariant (LTI) models and introducing a linear time-varying (LTV) approach. LTI models, while useful for qualitative insights, lack quantitative predictive power due to the time-varying nature of oscillators. The LTV model accounts for this by recognizing that oscillators are linear time-varying systems, where the phase-noise spectra result from frequency translation of device noise. Key insights include the importance of symmetry in suppressing 1/f noise upconversion, the role of cyclostationary effects, and the AM–PM conversion. The LTV model also explains why certain oscillator topologies, like the Colpitts oscillator, perform well and suggests new designs. The model is validated through simulations and examples, including LC and ring oscillators. It also addresses the impact of amplitude noise and the need to minimize the dc value of the impulse sensitivity function (ISF) to reduce 1/f noise upconversion. The LTV model provides a more accurate description of phase noise, including the 1/f² and 1/f³ regions, and highlights the importance of symmetry and circuit design in minimizing phase noise. The tutorial concludes with practical considerations for simulation and circuit design, emphasizing the need for careful layout and design to achieve low phase noise in oscillators.