The AM-FM conversion mechanism caused by the nonlinear behaviors of the varactor and switching transistor capacitances, respectively, with respect to the output voltage swing is analyzed. The varactor effect is analyzed and explained, based on the Fourier series analysis, by deriving the effective capacitance variations as a function of output voltage amplitude and normalized varactor bias. The principle of switching transistor effect is explained understanding the change of effective capacitances caused by the transistor mode transition. Circuit techniques that can resolve the varactor and switching transistor capacitance effects, respectively, are introduced and the effectiveness of each technique are tested based on simulations. An AM-FM conversion free VCO topology is proposed and fabricated in 0.18$\mum$ CMOS technology for 1.9GHz operation and verifying measurement results are reported.