Varactor modeling methodology for simulation of the VCO tuning sensitivity

A modeling methodology for CMOS varactors is presented using a simple test structure and a new special device target function. The device target function is based on the definition of the differential frequency tuning sensitivity K_vco of a voltage controlled oscillator (VCO). The parameter extraction applying the new methodology is demonstrated for a BSIM4 MOS model. A good agreement between measurement and simulation of K_vco for a 4 GHz VCO built in a 0.25 μm BiCMOS technology is achieved using a varactor model extracted with the new methodology. The modeling approach directly gives Kvco for different oscillation amplitudes of the VCO. The good agreement between modeled and measured Kvco characteristics can be used to determine the oscillation amplitude of the VCO which is difficult to be measured directly at RF frequencies.