Analysis and Design of the Tank Feedline in Millimeter-Wave VCOs

This article presents a detailed analysis of the impact of the tank feedline on tuning range (TR) and phase noise (PN) in millimeter-wave LC voltage-controlled oscillators (VCOs). A robust extended TR design, with low PN and small die area, is later proposed, analyzed, and experimentally validated. A new interconnect approach for the coarse tuning capacitor bank is used to significantly reduce the LC tank routing capacitance and resistance, thus improving the TR and PN of the VCO. As a proof of concept, a 26.8-GHz VCO with a 5-bit digitally switched-capacitor bank [capacitor digital-to-analog converter (C-DAC)] is implemented using a folded feedline routing structure in a digital 45-nm CMOS silicon-on-insulator (SOI) technology. Compared to a conventional layout structure, the proposed interconnect technique yields a wider TR and lower losses while significantly improving the linearity of the C-DAC. The fabricated VCO yields a TR of 33%, covering the extended 5G frequency band with a sufficient margin from 22.4 to 31.2 GHz, with a minimum overlap of 40%. Moreover, it achieves a PN of -105.5 and -97.2 dBc/Hz at a 1-MHz offset at the low and high bands, respectively, while dissipating 6 mW from a 1.0-V supply, corresponding to an average FOM $_{{T}}$ of -192.6 dBc/Hz.