Delta-Sigma Modulation for Direct Digital Frequency Synthesis

This paper compares different DeltaSigma modulation techniques for direct digital frequency synthesis (DDS). DeltaSigma modulators such as MASH, feedforward, feedback, and error feedback have been implemented in both the phase and frequency domains in a CMOS DDS prototype IC fabricated in a 0.35-mum CMOS technology with core area of 1.7times2.1 mm² and total current consumption of 75 mA. Measured DDS performance demonstrates that the frequency domain DeltaSigma modulation technique achieves better output spectrum purity than the phase domain method. Moreover, a programmable feedforward DeltaSigma modulator is proposed to achieve different in-band and out-band noise shaping effects for DDS applications.