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A theoretical analysis of the constraints posed by a tapped inductor on a dual-band fourth-order voltage-controlled oscillator (VCO) is presented. The analysis provides guidelines for frequency band selection, tapped-inductor design, and VCO optimization. The guidelines are utilized in a VCO design example simulated on a 45-nm CMOS process. An adaptive frequency-tuning scheme exploits unique features of the fourth-order tank to optimize VCO performance.