A 14-b direct digital frequency synthesizer with sigma-delta noise shaping

This paper describes a 14-b direct digital frequency synthesizer (DDFS) utilizing a sigma-delta noise shaping technique to reduce spurs arising from phase truncation. A new phase accumulator architecture adopting a second-order sigma-delta modulator is proposed. The sigma-delta noise shaping eliminates periodicity inherent in the phase truncation error. With the proposed phase accumulator, the significant spurs are reduced, and the spectral characteristics of the DDFS are then determined by finite precision of sine-amplitude output. A prototype DDFS IC in 0.25-μm CMOS was fabricated on 0.12-mm² die area. The measured spurious-free dynamic range (SFDR) is greater than 110 dB for 16-b phase value and 14-b sine-amplitude output. The fabricated IC consumes 100 mW with a 2.5-V supply, and correctly operates up to 250 MHz.