Post modulator filtering in Δ-Σ fractional-N frequency synthesis

This paper presents an analysis of the post modulator filtering impact on synthesizer noise performance. A simple low pass filter applied at modulator output results in narrower phase error distribution. This improves synthesizer immunity to inherently present nonlinearities. In addition, the charge pump ON time is reduced increasing resistance to noise coupling from power supply and substrate. This technique improves the performance of modulator architectures with maximum attenuation at DC, like MASH. MASH is commonly used in Δ-Σ frequency synthesis.