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A 0.18 mum CMOS 5 GHz quadrature voltage-controlled oscillator (QVCO) is demonstrated by using trifilar transformer coupling. The trifilar transformers composed of one primary coil and two secondary coils are used to separate the gate and drain bias for output voltage swing optimisation and also replace a conventional transistor-coupling method for quadrature output generation simultaneously. As a result, the trifilar-coupling QVCO achieves 180.1 dBc/Hz figure of merit (FOM) at the supply voltage of 1.2 V. The on-chip passive single sideband upconversion mixer is also demonstrated to fairly measure the quadrature accuracy of the QVCO. Thus, a 33.7 dB sideband rejection ratio is achieved.