This paper presents a transformer-based harmonic-rich-shaping voltage-controlled oscillator (VCO). Its active core features noise suppression and circulation to improve the phase noise (PN) performance, and its 2:2 transformer allows a very short common-mode (CM) return path without sacrificing the tank's quality factor. The proof-of-concept prototype is a 3.52-GHz VCO in 65-nm CMOS. It scores a −151-dBc/Hz PN at 10-MHz offset, and consumes 6.77 mW of power at a 0.7-V supply. The achieved Figure-of-Merit (FOM) is 187.8/192.3/193.7 dBc/Hz at 0.1/1/10-MHz offsets, with a 1/f PN corner of 220 kHz. Over a 22.1% tuning range, the VCO upholds a consistent FOM of >193.3 dBc at 10-MHz offset, and a 1/f PN corner of <250 kHz. It has a very low supply pushing of −10 and 37 MHz/V at the lowest and highest frequencies, respectively. The core area is 0.33 mm ² .