A Cryo-CMOS Wideband Mode-Switching Class-F VCO With Harmonic-Resonance Self-Alignment

This article presents a cryogenic mode-switching class-F voltage-controlled oscillator (VCO) aimed at addressing the demand for wideband, low phase noise (PN) local oscillator signals in cryo-CMOS quantum controllers. First, to resolve the issues of manual alignment of harmonics and resonances, and excessive drain-to-gate gain leading to reliability concerns in transformer (XFMR)-based class-F VCOs, a single-core class-F VCO architecture with harmonic-resonance self-alignment property was constructed by utilizing a symmetrical and XFMR-less dual-mode resonator (DMR), which features a pair of concurrently tunable and fixed-ratio (3:1) resonant frequencies. Since the pseudo-square oscillating voltage waveforms are directly fed back to the gates, a sharper class-F oscillation is achieved to reduce the impulse sensitivity function (ISF) even without the drain-to-gate gain. Then, the lossless mode-switching technique is introduced into two coupled single-core class-F VCOs to extend the frequency tuning range (FTR) and gain about 3-dB PN reduction. The coupling realized by inductive and small magnetic elements does not disrupt the self-alignment characteristics for under mode-switching operation. Large-dimension transistors were employed to design the negative transconductance cells to suppress flicker noise and channel shot noise at cryogenic temperatures (CTs). Furthermore, the worsened nonlinearity in MOSFETs at CTs is exploited to enhance the waveform shape of class-F oscillations. The designed and fabricated VCO prototype achieves 5.3- to 10.6-GHz FTR with a best PN of −153.1 dBc/Hz at 10-MHz offset at 3.7 K, corresponding to a best figure of merit (FoM) and FoM for FTR (FoMT) of 203.8 and 220.3 dBc/Hz, respectively.