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To lower the supply voltage for high-frequency operation, a fully integrated frequency synthesizer, together with regenerative frequency-doubling and fractional phase-rotating techniques, is presented. The frequency-doubling circuit regenerates the tail signals at twice the frequency of the quadrature voltage-controlled oscillator (QVCO) to achieve larger output swing and higher operating frequency for the synthesizer. Additionally, a hybrid circuit utilizing a new folded regime for the first-stage divider and the phase-rotating circuit is developed in the prescaler. Under full-speed operation, the QVCO with the frequency doubler and the divider can work from a 0.5-V supply, whereas the synthesizer dissipates 12 mW. At 9.1-GHz carrier frequency, the measured phase noise is -104.5 dBc/Hz from 1-MHz offset.