This brief presents an explicit Class-F23 voltage-controlled oscillator (VCO). The square-like voltage waveform is obtained via waveform shaping, and flicker noise upconversion is suppressed by a proper common-mode (CM) return path. CM resonance at the second harmonic frequency is introduced by a compact octagonal inductor. The rms value of the impulse sensitivity function (ISF) is significantly reduced through Class-F23 operation. The VCO switches between two modes of a high-order LC resonator consisting of two identical LC tanks coupled by capacitors. A prototype of the VCO is implemented in a 28-nm CMOS. Measurements show a continuous tuning range (TR) of 4.89–7.29 GHz, with a peak figure of merit (FoM) of 190.5 dB/Hz at 5.8 GHz and better than 188.5 dB across the entire TR. The flicker phase-noise corner ranges from 70 to 400 kHz. The VCO consumes 16–19 mW from a 0.5-V supply and occupies an active area of 0.21 mm2.

This brief reports on a 13-GHz quad-core voltage-controlled oscillator (VCO) using a folded S-shaped tail inductor. The contribution of this work is that the auxiliary resonator is folded into the main inductor, so that it leads to a more compact solution than a conventional scheme. Due to the S-shaped inductor’s electromagnetic (EM) characteristics, the proposed tail filter can achieve noise suppression without EM interference to the main tank. Designed and implemented in a 28-nm CMOS process, the proposed VCO operates between 12.32 and 13.84 GHz, for an 11.6% turning range. The measurements were carried out in the free-running mode, and the results show a phase noise (PN) of 118.3 dBc/Hz at a 1-MHz offset from the central frequency of 12.32 GHz. The power consumption of the VCO core is 24.5 mW, with a 0.9-V supply voltage, and this leads to a figure of merit (FoM) of 186.6 dBc/Hz.