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#### An Octave Tuning Range Quad-Core VCO Using a Compact Quad-Mode Transformer-Based Inductor

This work presents an octave tuning range quad-core voltage-controlled oscillator (VCO) using a quad-mode transformer-based inductor with compact size. The proposed inductor comprises outer and inner transformers, leveraging both even- and odd-mode excitation current from the quad VCO cores. In comparison to other works, this approach significantly simplifies design complexity and reduces chip area because the proposed quad-mode inductor involves only four design parameters. Furthermore, quad-mode operations of the proposed inductor are achieved without switch loss by controlling mode switches between adjacent and distant VCO cores. Therefore, wideband inductive frequency tuning can be realized without Q-degradation and high power consumption. The VCO, implemented in a 28-nm CMOS technology, achieves a frequency tuning range of 72.6% from 17 to 36.4 GHz. The chip consumes 9.7–13.3 mW at a 0.6-V supply voltage and core area of 0.065 mm². The proposed VCO attains a peak FoMT of 196.3 dBc/Hz and peak FoMTA of 208.1 dBc/Hz.