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This brief investigates the lock range of injection-locked LC oscillators using a linear control system approach. It explores the intrinsic relation between the lock range of injection-locked LC oscillators and the injection-induced variation of the impedance of the LC tank. We show that injection-locked active-inductor LC oscillators exhibit a larger lock range, as compared with their passive counterparts. We further show that the large lock range of injection-locked active-inductor oscillators is due to both the low quality factor and the large injection-induced variation of the inductance of active inductors. These findings are validated using an injection-locked passive LC oscillator and an injection-locked active-inductor LC oscillator designed in IBM 0.13- μm 1.2-V CMOS technology.