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A low phase noise CMOS quadrature VCO is presented. A transformer resonator coupling is analyzed. It is found that the coupled resonator has high frequency selectivity. The Q factor of a coupled resonator is (1+k) times higher than a single LC resonator. This enhancement leads to low phase noise and low power consumption of a VCO. The VCO is implemented using 0.18 μm CMOS technology. Two VCOs are linked to generate I-Q signals by the direct current modulation method. The VCO shows low phase noise performances of -110 and -117 dBc/Hz at the offset frequencies of 600 kHz and 1 MHz respectively. The oscillation frequency is 8 GHz. The tuning range of 250 MHz is achieved with a control voltage from 0 to 1 V. The VCO draws 8 mA in core circuits with 3 V supply.