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We present a 5 GHz, voltage controlled quadrature oscillator, based on transformer coupling between the quadrature components. The oscillator is fabricated in a 0.18 /spl mu/m, low voltage digital CMOS process with a lossy substrate (/spl rho//spl sim/10mohm-cm) and thin, high resistivity metallization. Fully integrated low Q (/spl sim/4) spirals form the transformer windings in the resonators. The coupling has been optimized to obtain quadrature accuracy with minimum phase noise degradation. The VCO achieves a tuning range of /spl sim/1 GHz, and a phase noise of up to -123 dBc/Hz at a 1 MHz offset, while drawing 7.5 mA at 1.6 V. An image reject receiver built using the onwafer quadrature signals, provides 43 dB of image rejection, confirming better than 1/sup 0/ of quadrature matching.