Abstract: A fractional-N frequency synthesizer is described that includes a voltage controlled oscillator (VCO), a programmable integer divider, and a glitch-free phase rotator. The phase select inputs of the phase rotator are controlled by a delta-sigma modulator to provide fine frequency resolution in addition to randomization and noise shaping of fractional quantization noise. The delta-sigma modulator is clocked at rates higher than the synthesizer reference clock resulting in an improvement in clock jitter at the output of the frequency synthesizer. A glitch-free phase multiplexer design is used to implement the phase rotator fractional divider to enables operation at rates higher than the reference clock. The over-sampling ratio of the delta-sigma modulator over the reference clock frequency of the PLL translates directly into an improvement in the quality of the output clock with respect to fractional quantization noise, phase mismatch, and digital noise injection.