A K-band differential Colpitts voltage-controlled oscillator (VCO) is designed and implemented in a 0.13 mu m CMOS 1P8M process. The designed circuit topology consists of two single-ended Colpitts LC-tank VCOs, and the differential VCO is obtained by using a cross-coupled nMOS pair and a transformer. At the supply voltage of 1.2 V, the output phase noise of the VCO is - 113.08 dBc/Hz at 1 MHz offset frequency from the carrier frequency of 20.78 GHz, and the figure of merit is -191.49 dBc/Hz. Total power consumption is 6.24 mW. Tuning range is about 980 MHz, from 19.84 to 20.82 GHz, while the control voltage was tuned from 0 V to 1.2 V. (C) 2009 Elsevier Ltd. All rights reserved.