A Low-Phase-Noise Quad-Core Millimeter-Wave Fundamental VCO Using Circular Triple-Coupled Transformer in 65-nm CMOS

The minimal achievable phase noise (PN) of millimeter-wave (mm-wave) voltage-controlled oscillators (VCOs) is bounded by the smallest realizable inductor with a reasonable high quality factor ( ${Q}$ ). A multi-core VCO reduces the total equivalent inductance of the tank by synchronizing multiple VCO cores, therefore improving the PN performance. This article presents a 60-GHz fundamental VCO based on a mode-rejection circular triple-coupled-transformer tank, which improves the PN performance of the VCO from three aspects. 1) the circular topology improves ${Q}$ of small inductors in the mm-wave frequency range by alleviating the inner edge deconstructive coupling; 2) compared with the circular inductor topology, the triple-coupled transformer provides the flexibility to suppress the unwanted oscillation mode at the center taps of the gate coils, which enables the NMOS-only VCO configuration to avoid PN degradation due to limited PMOS performance in the mm-wave frequency range; and 3) the triple-coupled transformer boosts the gate-to-source voltage swing of ${G}_{m}$ transistors. The quad-core VCO prototype is designed and fabricated in a 65-nm CMOS process. It achieves a measured PN of –104.7 dBc/Hz at 1-MHz offset and a corresponding figure of merit (FoM) of 186.5 dBc/Hz at 59.1-GHz carrier frequency.