A coupled circuit and device simulator is presented for the accurate simulation of phase noise in RF MEMS VCOs. The numerical solution of device level equations is used to accurately compute the capacitance of a MEMS capacitor. This coupled with a circuit simulator facilitates the simulation of circuits incorporating MEMS capacitors. In addition, the noise from the MEMS capacitor is combined with a nonlinear circuit-level noise analysis to determine the phase noise of the RF MEMS VCO. Simulations of an 800 MHz MEMS VCO implemented in a HP 0.8/spl mu/m CMOS technology show good agreement with experimentally observed behaviour.