A Transformer-Based Technique to Improve Tuning Range and Phase Noise of a 20-28GHz LCVCO and a 51-62GHz Self-Mixing LCVCO

Varactors in RF CMOS processes often have significantly lower Q- factor ( Q_{VAR} ) than inductors and transformers at mm-wave frequencies. Direct connection of varactors to the outputs of an LC oscillator lowers overall tank Q, Q_{T} < Q_{VAR} , and at the same time, increases the ratio of parasitic capacitance to total tank capacitance which limits frequency tuning range (FTR). Instead, magnetically coupling a varactor to the oscillator core using an asymmetric transformer, where the core is connected to the primary and varactor to the secondary, limits the drop in Q_{T} . The ratio of parasitic capacitance to total tank capacitance is also reduced. The varactor can also be operated in accumulation-mode, with a larger Q_{VAR} . Thus, both FTR and phase noise (PN), in comparison to traditional tanks in LC VCOs, are improved simultaneously. In this paper, two VCO prototypes are implemented in 65-nm CMOS. A 60 GHz self-mixing VCO with a VCO core operating at 20 GHz shows an FTR of 18.5%, a PN of −92.5 dBc/Hz at 1 MHz offset, and an FoM _{T} of −187.1 dBc/Hz. A 25 GHz VCO shows an FTR of 29.9%, a PN of −107.9 dBc/Hz at 1 MHz offset, and an FoM _{T} of −194.2 dBc/Hz.