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A transformer-feedback voltage-controlled oscillator (TF-VCO) is proposed to achieve low-phase-noise and low-power designs even at a supply below the threshold voltage. The advantages of the proposed TF-VCO are described together with its detailed analysis and its cyclo-stationary characteristic. Two prototypes using the proposed TF-VCO techniques are demonstrated in a standard 0.18-μm CMOS process. The first design using two single-ended transformers is operated at 1.4 GHz at a 0.35-V supply using PMOS transistors whose threshold voltage is around 0.52 V. The power consumption is 1.46 mW while the measured phase noise is -128.6 dBc/Hz at 1-MHz frequency offset. Using an optimum differential transformer to maximize quality factor and to minimize the chip area, the second design is operated at 3.8 GHz at a 0.5-V supply with power consumption of 570 μW and a measured phase noise of -119 dBc/Hz at 1-MHz frequency offset. The figures of merits are comparable or better to that of other state-of-the-art VCO designs operating at much higher supply voltage.