Abstract: In this paper, a digitally controlled ring oscillator (DCRO) for ultra-wideband impulse radio (IR-UWB) communication system is presented.Three-stage tuning method is applied in the DCRO, including coarse voltage tuning and fine switched varactor tuning.A novel delay cell is proposed and multiple-loop architecture is utilized for ring oscillator, allowing to obtain high operation frequency and low power consumption.Under CSMC 0.18μm CMOS process, when the supply voltage is 1.8V, the DCRO has a wide frequency tuning range from 3 to 8.6GHz and a frequency resolution of 2MHz.At 8.6GHz, the circuit achieves a phase noises of-112.4dBc/Hz at 10MHz offset.The total power consumption is 20mW.