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This letter presents a low phase noise quadrature ring oscillator with new start-up circuit. The oscillator architecture is a two-stage differential ring with an additional pair of transition-assistance transistors. The circuit was implemented in 0.18 mum CMOS technology and the measured tuning range of the prototype device is from 1.7 GHz to 5.5 GHz and figure of merit (FOM) is -162 dB. The proposed area of application is the core of the local oscillator in a multi-standard wireless transceiver.