This paper describes a new analytical method of tuning curves of a LC-tank voltage-controlled oscillator (VCO) with period calculation technique.In traditional analysis method

the harmonic approximation is employed

and an effective capacitance of varactors during one period is calculated by the large-signal nonlinear analysis.However

the oscillator tuning curve derived from the effective capacitance is inaccurate

and complex if higher order harmonics could not be neglected.In this paper a novel convenient method of calculating oscillating period is proposed.With this period calculation technique

the prediction of oscillator tuning curves is more accurate compared with the traditional harmonic approximation.The theoretical analyses are experimentally validated with a CMOS complementary LC-tank VCO implemented in 0.35μm 2P4M CMOS process.