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This letter proposes a phase-noise analysis of injection-locked frequency dividers that are based on LC oscillators. The proposed analysis method relies on the concept of the perturbation projection vector and allows one to investigate how the output phase noise is affected by the amplitude and the frequency of the input signal. Closed-form expressions for the output phase-noise spectrum under different injection conditions are provided and validated against the periodic noise analysis of a commercial circuit simulator. Indeed, the proposed semianalytical method can provide insights and guidelines to improve the circuit design.