Methods and apparatus are provided for calibrating a voltage controlled oscillator, such as an N-stage voltage controlled ring oscillator. The voltage controlled oscillator comprises a power supply input and at least one gate delay element and has a frequency that is a function of a delay of the gate delay element and a voltage applied to the power supply input. A voltage controlled oscillator is calibrated by varying an output voltage of a programmable voltage source through a range of values; applying the output voltage to the power supply input of the voltage controlled oscillator; comparing an output clock frequency of the voltage controlled oscillator to a reference frequency clock for each of the output voltage values; and selecting a value of the output voltage that provides an approximate minimum frequency difference between the output clock frequency and the reference frequency clock.