Abstract: A novel way to improve the frequency resolution of digitallycontrolled oscillator(DCO) for quadband global system for mobile communication(GSM) was proposed. The characters and principle of DCO are described at first. Then the proposed novel series switched varactor model is analyzed. The fine tuning capacitor bank in DCO is then implemented using this varactor to achieve better frequency resolution. A DCO in 90 nm complementary metal oxide semiconductor(CMOS) process using this new structure achieves a frequency resolution of 1.6 kHz without dithering and a phase noise of -152 dBc/Hz at 20 MHz offset when oscillating at 3.1 GHz. The total power consumption is 8.16 mW. The simulation shows this DCO satisfies quadband GSM requirement and is capable of alldigital phaselocked loop(ADPLL) application.