Multitone-FM analysis of MEMS varactor phase noise contribution in VCOs

Commonly, varactor contribution to VCO phase noise is assessed through Leeson´s equations. Though widely applied, these equations do not show the flattening out of the SSB VCO phase noise close to the carrier. For this reason the Leeson approach only predicts the general shape of the phase noise far from the carrier, but cannot be used for precise simulations of the varactor´s absolute phase noise contribution. This paper describes a multitone-FM approach which can be used to calculate the phase noise contribution of any varactor type, but is especially suitable for the assessment of close-to-carrier noise introduced by MEMS varactors exposed to thermal noise and vibrations.