LC-VCO Design With Dual-Gm, Boosted for RF Oscillation and Attenuated for LF Noise

A VCO topology with a high gm at RF and low gm at low frequencies (LF) is presented. A high gm(RF) improves start-up conditions, and a low gm(LF) enables a significant improvement of the phase noise and the power supply rejection. The VCO with a modified prescaler was implemented in 0.25 mu m CMOS technology. The measured phase is -128.5 dBc/Hz at 1 MH offset, when the VCO works at 3 GHz and consumes 7.5 mW. The corresponding figure of merit is 189.2 dBc/Hz/mW.