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In this article, we propose a voltage-controlled oscillator (VCO) with a single three-coil transformer, featuring a common-mode (CM) resonance expansion and intrinsic differential 2nd harmonic output. In addition to a conventional transformer-based class-F resonator, an internal third coil is added to manipulate the CM response. On one hand, it expands the CM resonance bandwidth, eliminating the need for manual harmonic tuning to align the 2nd harmonic resonance. On the other hand, it creates a simultaneous wideband differential response at the 2nd harmonic frequency, therefore, the power consumption of the extra frequency doubler can be saved. Using the proposed stacked 8-shape inductor topology, the third coil can be placed inside the class-F transformer without consuming an extra chip area. Fabricated in a 65-nm CMOS process, the measured tuning range of the proposed VCO is 21.1%, from 12.3 to 15.2 GHz. The proposed VCO achieves a peak phase noise (PN) performance of –118.0 and –111.9 dBc/Hz at 1 MHz offset for fundamental frequency and 2nd harmonic, respectively, the corresponding peak figure-of-merit (FoM) of which is 190.4 and 190.1 dBc/Hz, respectively. The VCO consumes a power of 8.4–9.6 mW, while occupying a small area of 0.06 mm2 thanks to the stacked 8-shape inductor topology.