High phase noise is a common problem in ring oscillators. Continuous conduction of the transistor in an analog tuning method degrades the phase noise of ring oscillators. In this paper, a digital control tuning which completely switches the transistors on and off, and a 1/f noise reduction technique are employed to reduce the phase noise. A 14-bit control signal is employed to obtain a small frequency step and a wide tuning range. Furthermore, multiphase ring oscillator with a sub-feedback loop topology is used to obtain a stable quadrature outputs with even number of stages and to increase the output frequency. The measured DCO has a frequency tuning range from 554MHz to 2.405GHz. The power dissipation is 112mW from 1.8V power supply. The phase noise at 4MHz offset and 2.4GHz center frequency is -134.82dBc/Hz. The FoM is -169.9dBc/Hz which is a 6.3dB improvement over the previous oscillator design.