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A novel accumulation-mode MOS varactor model used for characterizing the tuning curve of LC-tank voltage-controlled oscillators (VCOs) is presented. The VCO tuning characteristic is seen to depend not only on the C-V characteristics of the varactor but also on losses in the tank. Effects of variations in the tank quality factor (Q) and oscillator's swing on the VCO tuning curve are discussed. Also a practical SPICE model using a pMOS transistor to describe an accumulation-mode MOS varactor is proposed. The model has been verified in a number of structures manufactured in a standard 0.13 μm CMOS process and is suitable for simulation within many circuit design environments.