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In this letter, we present the measured performance of a differential Vackar voltage-controlled oscillator (VCO) implemented for the first time in CMOS technology. The Vackar VCO provided good isolation between the LC tank and the loss-compensating active circuit; thus, excellent frequency stability was achieved over the frequency tuning range. The Vackar VCO was implemented using nMOS transistors and an LC tank in a 0.18 μm RF CMOS process. The oscillation frequency ranged from 4.85 to 4.93 GHz. The measured phase noise of the Vackar VCO at 1 MHz offset was -124.9 dB/Hz at 4.9 GHz with a figure-of-merit (FOM) of -188 dBc/Hz.