A programmable on-the-fly Fractional-N PLL is required to lock to an incoming clock source and produce an output clock. The generated clock needs to be locked to the input source but maintain a high-degree of precision, hence an on-the-fly programmable fractional feedback divider is required (“on the fly” meaning the frequency transition and re-obtaining lock process for small frequency adjustment is glitch free and contains limited frequency over/undershoot).