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Low phase noise monolithic oscillators are in high demand in this age of wireless communications. Although LC oscillators generally have better phase noise performance, there is motivation to design ring oscillators with comparable phase noise compared to LC oscillators. The advantages of ring oscillators include significantly less die area and generally wider tuning range. Ring oscillator phase noise analysis and simulation, however, often ignore power supply noise, which is a major and possibly dominant contributor of phase noise. This paper presents a method of determining a given oscillator's sensitivity to both intrinsic and power supply noise sources and provides a means for comparing different oscillator architectures based on this information.