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A 5-GHz CMOS voltage-controlled oscillator (VCO) integrated with a micromachined switchable differential inductor is reported in a 0.18 mum radio frequency-CMOS-based microelectromechanical system technology. The power consumption of the core is about 8 mW at the supply voltage of 1.8 V. A total tuning range of 470 MHz (from 5.13 GHz to 5.60 GHz) is achieved as the tuning voltage ranging from 0 V to 1.8 V. In the practical tuning range, the measured phase noise performances at 1 MHz offset are less than -125 dBc/Hz and -126 dBc/Hz when the inductor switch is turned on and off, respectively. The figure-of-merit is better than -190 dB. When compared with a contrast VCO circuit that utilizes a standard switchable differential inductor, this oscillator reaches a phase noise improvement of around 3 dB as the switch is turned on. Around 1-dB on-off phase noise difference can be achievable.