DDFS and ΣΔ Approaches for Fractional Frequency Synthesis in Terahertz Instruments

This paper compares direct digital frequency synthesis (DDFS) and ΣΔ CMOS-based frequency synthesizers for terahertz instrument systems. This paper first discusses the need for fractional synthesis in terahertz systems, and provides a brief overview of reference clock frequency selection, and design choices related to implementation of fractional functionality. ΣΔ-multistage-noise-shape-based and DDFS-based fractional synthesis approaches and architectures are discussed and CMOS synthesizer modules based on both of these approaches are presented. Finally, the paper compares two different 100-GHz CMOS fractional frequency synthesizer chips, which employ the same millimeter-wave circuits but different fractional generators (third- and sixth-order ΣΔ, and DDFS-based). Performance in terms of phase-noise, spurious components, and power consumption are directly compared. Measurements suggest ΣΔ synthesis is more suitable than DDFS for THz fractional synthesizers due to the high-phase multiplication of spurs.