A quadrature balanced voltage controlled oscillator (B-VCO) with current source switching is proposed and analyzed. This letter shows analytically that the switching improves the phase noise. A switched transistor is also used as a coupling transistor to generate quadrature signals without degrading the phase noise. To investigate the effect of quadrature coupling on the phase noise, a single B-VCO and a quadrature B-VCO are implemented with identical components in an 0.18-mu m CMOS process. Both VCO cores draw about 8.8 mA under a low bias voltage of 1.8 V. The oscillation frequencies are 10.21 GHz and 10.81 GHz. The measured phase noises of the single at an offset frequency of 1 MHz VCO is -114.83 dBc/Hz while that of the quadrature VCO is -116.67 dBc/Hz. The quadrature B-VCO is superior to the single B-VCO with respect to phase noise and oscillation frequency in the X-band.