A 20-GHz Class-C VCO with 80-GHz Fourth-Harmonic Output in 28-nm CMOS

This letter presents a circuit solution to extract the fourth harmonic from a class-C voltage-controlled oscillator (VCO). The tail capacitor in the class-C oscillator is replaced by a dual-resonance network that allows the extraction of the fourth harmonic, while yielding low impedance at even harmonics, which is key to maintaining transistor operation in class-C. The validity of the proposed approach is demonstrated on a 20-GHz 28-nm CMOS oscillator. The circuit achieves a phase noise as low as -111 dBc/Hz at 3-MHz offset from a 78-GHz carrier while consuming 57 mW, with a figure of merit (FoM) of -182 dBc/Hz, 12% tuning range, and a dc to RF conversion of 0.7%. The undesired harmonics are suppressed by more than 70 dB.