A Transformer-Based Inverse-Class-F-Like VCO With a Digitally Controlled Common-Mode Impedance Tuner

This paper presents a transformer-based voltage-controlled oscillator (VCO) with a digitally controlled common-mode impedance tuner that adjusts the second-harmonic voltage phase. A transformer-coupled LC tank exhibits dual resonances at the fundamental frequency and the second harmonic, producing an inverse-Class-F-like voltage waveform. The proposed tuner operates on a common-mode path that is largely decoupled from the differential oscillation mode, enabling second-harmonic phase tuning with limited impact on the oscillation frequency. By tuning the second-harmonic phase, the time-domain noise-to-phase conversion is reshaped, providing an additional degree of freedom for phase-noise optimization. Fabricated in a 28-nm CMOS process, the VCO achieves a phase noise of −113.9 to −112.5 dBc/Hz at 1-MHz offset across an 11.7–13.9-GHz tuning range while consuming 13.3–14 mW of dc power. The measured peak FoM and FoMT at 1-MHz offset are 184.2 and 188.1 dBc/Hz, respectively.