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Summary In this paper, a comprehensive analysis of oscillation frequency, oscillation amplitude, and phase noise is conducted for an LC cross‐coupled oscillator with unequal inductors, active and passive transconductance amplification structure, and very low phase noise. In this study, for the first time, a complete analysis of the phase noise of this oscillator is presented using a novel method that does not require complex simulations to calculate the Impact Sensitivity Function (ISF) and phase noise, and as a result, a closed‐form equation is extracted for the phase noise. The calculated results are compared with the corresponding simulations to evaluate the validity of the extracted equations. Using the closed‐form phase noise equation as well as the oscillation frequency, designers can comprehend the design trade‐offs and design an oscillator for specific phase noise and oscillation frequency. The proposed method of calculating the ISF is also applicable and valid for other second‐order oscillators. In the following, the structure of the proposed oscillator is designed and simulated using the standard 0.18‐μm CMOS technology with a 1.8‐ normalV power supply and 575.2‐MHz oscillation frequency to have the optimal coupling coefficient in terms of the phase noise and oscillation amplitude. The simulation results as well as the analyses performed based on the phase noise and oscillation amplitude parameters show that the structure proposed in this study provides a better performance with a much lower phase noise comparing with the structure of a conventional cross‐coupled oscillator as well as the proposed cross‐coupled oscillator with equal inductors.