A comparative study of multi-GHz LCVCOs designed in 28nm CMOS technology

Four multi-GHz LCVCOs were designed in the GlobalFoundries 28 nm HPP CMOS technology: 15 GHz varactor-tuned NMOS-only, 9 GHz varactor-tuned self-biased CMOS, 14.2 GHz digitally-tuned NMOS-only, and 8.2 GHz digitally-tuned self-biased CMOS. As a design method, analytical expressions describing tuning range, tank amplitude constraint, and startup condition were used in MATLAB to output a graphical view of the design space for both NMOS-only and CMOS LCVCOs, with maximum varactor capacitance on the y-axis and NMOS transistor width on the x-axis. Phase noise was predicted as well. In addition to the standard varactor control voltage tuning method, digitally-tuned implementations of both NMOS and CMOS LCVCOs are presented. The performance aspects of all designed LCVCOs are compared. Both varactor-tuned and digitally-tuned NMOS LCVCOs have lower phase noise, lower power consumption, and higher tuning range than both CMOS topologies. The varactor-tuned NMOS LCVCO has the lowest phase noise of -97 dBc/Hz at 1 MHz offset from 15 GHz center frequency, FOM of -172.20 dBc/Hz, and FOMT of -167.76 dBc/Hz. The digitally-tuned CMOS LCVCO has the greatest tuning range at 10%. Phase noise is improved by 3 dBc/Hz with the digitally-tuned CMOS topology over varactor-tuned CMOS.