Abstract: A wideband voltage-controlled oscillator (VCO) with low phase noise is presented.A Class-D topology with a periodically time-varying inductor is adopted in this VCO, and the common-mode resonance extension technique is applied to the resonator. Wideband harmonic shaping is thus achieved, and the phase noise performance across the full operating bandwidth is optimized. Furthermore, the conventional N-channel metal-oxide-semiconductor pair is replaced with a P-channel metal-oxide-semiconductor cross-coupled pair to reduce the superior flicker noise and lower channel thermal noise. Fabricated in SMIC 55-nm CMOS technology, the chip occupies an area of 0.47 mm², including pads. Measurement results demonstrate that the VCO achieves a continuous tuning range from 3.5 GHz to 5.1 GHz (38.4%). The output power ranges from 7.5 dBm to 7.1 dBm, and a phase noise of -125.8 dBc/Hz at a 1 MHz offset is measured at 3.5 GHz. Operating with a 1.8 V supply voltage, the VCO core consumes 21.3 mA to 23.0 mA, while the output buffer draws 14.4 mA to 15.3 mA. The corresponding figure of merit with tuning range (FoM_T) is between 192.4 dBc/Hz and 189.6 dBc/Hz.