A −195 dBc/Hz FoMT 20.8-to-28-GHz LC VCO with Transformer-Enhanced 30% Tuning Range in 65-nm CMOS

Low quality factor (Q) of varactors and increased ratio of parasitic capacitance to total tank capacitance impede the design of high-frequency voltage-controlled oscillators (VCOs) that must attain wide frequency tuning range (FTR) and low phase noise (PN). We propose a VCO topology which instead of directly connecting a varactor to the oscillator core, leverages a transformer to magnetically couple the varactor to the core. This approach increases the tuning range of the varactor by doubling the bias range, further reduces the parasitic capacitance seen by the varactor, and boosts the resonator tank Q due to impedance transformation. Thus, both PN and FTR are improved simultaneously. Measurement results for the prototype VCO implemented in 65-nm CMOS show an FTR of 29.8% from 20.77 to 28.02 GHz while consuming 12.65 to 15.12 mW. A PN of -106.6 dBc/Hz at a 1 MHz offset and an FoM T of -195 dBc/Hz are attained.