A method comprises: using a plurality of gain stages cascaded in a ring topology to form a ring oscillator configured to output an oscillation signal; controlling a supply voltage of said ring oscillator using a low-speed DAC (digital-to-analog converter) in accordance with a coarse control word; providing a capacitive load at an inter-stage node of said ring oscillator using a varactor array controlled by a control voltage array; establishing said control voltage array using a high-speed DAC array in accordance with a fine control word; adjusting the coarse control word upon a start-up to make an oscillation frequency of said oscillation signal approximately equal to target value; and adjusting the fine control word in a closed loop manner in accordance with a detection of a timing error of said oscillation signal.