I. Introduction
With the proliferation of new 5G bands for both sub-6 GHz and mm-wave frequencies, multi-standard wide frequency-tuning-range (FTR) oscillators are increasingly required. The design of wide tuning VCOs is challenging in the mm-wave regime due to the limited quality factor (Q) of the tuning capacitors and switched inductors, and routing parasitics. A VCO with transformer-enhanced tuning was recently demonstrated with 30% FTR at 20-28 GHz by placing the tuning capacitors on a secondary winding away from the core [1]. Mode-switching transformer-based VCOs [2] were also developed to improve the tuning range without sacrificing the phase noise (PN). This concept was extended in [3] to the mm-wave region for a dual-core 42% FTR VCO with a peak figure-of-merit-tuning (FoMT ) of 197.2 dBc/Hz at 25 GHz, and was limited by the parasitics of the mode-tuning capacitors.