A nonlinear systematic design flow for LC tank VCOs based on large signal capacitance modeling

In this work we present a novel approach for a non-linear systematic design flow for integrated VCO circuits based on the Andronov Hopf bifurcation analysis and perturbation theory. The approach allows to make an estimation of the tuning range, a stability analysis and a nonlinear calculation of the amplitude before the actual design phase. With our approach the VCO can be optimized regarding the requested amplitude specification and minimizing the effect of the higher harmonics. Using a charge based approximation for the voltage dependent varactor capacitance we obtain an analytical expression to model the large signal oscillator capacitance in dependency of the design parameters and the tuning voltage valid at any time of oscillation. The validity of the method has been verified by designing a 2.4 GHz VCO using 0.25 µm RF CMOS technology.