This paper presents a theoretical study of CMOS differential Colpitts VCOs. The objective is to provide a deep understanding of the different mechanisms that impact the performances of these VCOs, namely the Gm-boosting and cyclostationary noise. The developed methodology and expressions can be used to analyze, optimize and build new VCO topologies. A novel topology with an optimized gate to source (GS) feedback is proposed. It exhibits a figure of merit (FOM) better than -190 dBc/Hz/mW for all the frequency offsets.