A comparison of MOS varactors in fully-integrated CMOS LC VCO´s at 5 and 7 GHz

This paper examines the effect of the choice of MOS varactor on the performance of a CMOS negative resistance oscillator. The three most common MOS varactor structures (inversion, accumulation, and gated varactor) are combined with a spiral inductor over either deep trench oxide or a polysilicon patterned ground shield, to implement a matrix of six LC VCO´s in a 0.24-µm (0.18- µm Leff) SiGe BiCMOS technology[1]. Typical measured VCO phase noise is -119.7 dBc/Hz at a 1-MHz offset from a 5.67-GHz carrier, while drawing 1.6 mA from a 1.5-V supply, for a VCO figure of merit of-191 dBc/Hz.