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Aggressive scaling down reduces the supply voltage of digital integrated circuits continuously. In order to realize systems-on-a-chip (SOC), analog and RF circuits should also work under the reduced voltage. For wireless sensor networks (WSN), low supply is important for communication nodes as the supply voltage must be low enough under a single solar cell as power source. Besides, the future transceiver system for WSN must be able to adapt itself to various applications and environments, so multi-mode is the trend. In multi-mode systems, it would be ideal to use a signal oscillator which can generate multiple frequencies. Therefore, a ultra-low-supply dual-band voltage-controlled oscillator (UDVCO) is proposed here. The presented UDVCO can be dynamically switched between two bands near 2.4-GHz and the 5-GHz. Its minimal operating voltage is 0.5 V by connecting the substrate of the cross-coupled NMOSFETs to the supply. A mathematical model is built for the low-supply VCOs, whose accuracy is validated by SpectreRF simulation. A series of LC tank is used as a noise filter for both the two bands. The chip is fabricated in the 0.18-mum CMOS HJTC technology. Our measurement results show that the low-frequency band can be tuned from 2.01 GHz to 2.08 GHz and the phase noise is about -82.17 dBc/Hz@1 MHz; while the high-frequency band ranging from 3.24 GHz to 3.44 GHz and the phase noise is about -74.33 dBc/Hz@1 MHz.