This paper describes a mode-switching quad-core-coupled millimeter-wave (mm-wave) voltage-controlled oscillator (VCO), using a single-center-tapped (SCT) switched inductor for extension of the frequency tuning range (FTR) and improvement of the phase noise (PN). The switches not only serve for in-phase coupling among the VCO cores but also can modify the equivalent tank inductance suitable for coarse frequency tuning. The frequency gaps between the multi-resonant frequencies are controlled by the common-mode (CM) inductance that is precisely set by the lithography fabrication. Together with the tiny varactors for fine frequency tuning, a wide and continuous FTR can be achieved. It is analytically shown that tiny switches (i.e., small parasitic capacitance) for mode selection are adequate to avoid bimodal oscillation, synchronize the VCO cores against resonance frequency mismatches, and prevent PN degradation. A symmetrical layout of SCT switched inductor also aids the VCO to be immune to magnetic pulling. Prototyped in 65-nm CMOS, the VCO exhibits a 16.5% FTR from 42.9 to 50.6 GHz. The PN at 46.03 GHz is -113.1 dBc/Hz at 3-MHz offset, corresponding to a figure-of-merit (FoM) of 183.6 dBc/Hz. The die size is 0.039 mm(2).