DDS architecture for digital frequency generation

Direct digital synthesizers (DDS) are important components in many digital communication systems(4).DDSs are now available as integrated circuits and their output waveforms up to hundreds of megahertz. Direct digital synthesis (DDS) is a technique for using digital data processing blocks as a means to generate a frequency- and phase-tunable output signal referenced to a fixed-frequency precision clock source. While DDS is slowly gaining acceptance in new system designs, methods used to improve the quality of the generated waveform are seldom used, even nowadays. From Colpitts oscillators (7) to phase locked-loops (8), methods have been proposed to improve stability, frequency resolution, and spectral purity. Among the all-digital approaches such as the one presented in (9), direct digital frequency synthesis (referred to here as DDS) appeared in 1971 (10). Three years later, this technique was embedded in a commercial unit measuring group delay of telephone lines (11). This paper provides an overview of the basics of DDS, along with simple formulas to compute bounds of the signal characteristics. Moreover, several methods are presented to overcome some of the limits of the basic DDS with a focus on improving output signal quality. This paper also describes the architecture of DDS & the various blocks involved.