Pages: 47-55

Abstract

The design principles of a fractional-N delta-sigma frequency synthesizer and the procedure of choosing the proper digital delta-sigma modulator (DDSM) are presented in this paper. Three previously presented spur reduction techniques are addressed and their effect on four types of DDSMs is explored. Furthermore, other factors which are important in choosing a DDSM and their evaluation method are introduced here. Simulation shows that between MASH 1-1-1, single quantizer error feedback modulator (SQ_EFM) and single quantizer multiple feedback modulator (SQ_MFM), the output feedback is the most suitable approach for reducing sputs. And, between these three DDSMs, the one that requires minimum hardware is the best option. If out of band phase noise is critical, single quantizer multiple feedforward modulator (SQ_MFFM) is preferred, although it degrades in-band phase noise.