Abstract: Based on the 0. 35 μm 3. 3V TSMC CMOS process, a design for a 1 GHz multi-- band mixed--signal VCO is proposed. The VCO is use Analog to Digital Convertor to adjust the current of the VCO delay cells, so as to achieved multi frequency band. The simulation results presented that the VCO voltage adjustment range is 1V to 2V, and output frequency range is 823.3 MHz to 1. 061 GHz with gain of only 36.6 MHz/V. The novel VCO has a wide frequency range and also keep a very low gain, which improved the VCO noise performance.