An X-band quadrature voltage-controlled oscillator (QVCO) is fabricated using 0.18-mum CMOS technology. The high impedance of a current source allows a highly effective Q-factor of the LC tank . The proposed X-band CMOS quadrature VCO has low phase noise of - 117.5 dBlHz at 1-MHz offset frequency, despite the absence of a current source, because a transformer can separate the gate bias of series-connected transistors from cross-coupled RF feedback. The VCO can operate under 1.8 V of power supply. The figure of merit (FOM) is 187.5 dB at a 10.3-GHz oscillation frequency, which is the best value among X-band VCOs. (C) 2005 Wiley Periodicals, Inc.