Noise Shaping with Sigma Delta Modulators in Fractional-N Synthesizers

In this paper we present the fundamental concepts of 3^rd order single loop and Multi-Stage Shaping (MASH) sigma-delta architectures its merits and demerits and its application in fractional-N synthesizers. The emphasis in this paper is on the fractional spurs generation and how dithering can be used to mitigate the fractional spurs and the performance of each of these architectures with regards to fractional spur generation and mitigation with the dithering enabled. The paper presents both the simulated results for both the architectures with regards to the fractional spurs and measured results for the single loop topology with regards the spectral content and phase noise with and without dithering.