A novel DDS array structure with low phase noise and spurs

Abstract

Spurious frequencies (spurs) resulting from phase truncation are one of the main spectrum purity issues for direct digital synthesizers (DDS). The standard approach in DDS design consists of truncating the phase word output from the phase accumulator in order to minimize the size of the lookup table. This process generates spurs and degrades the quality of signals at the output of a DDS. The truncation spur can be converted into noise throughout the available bandwidth by adding a pseudo-random sequence to the output of phase accumulator. In principle, since the noise generated by different pseudo-random sequence is quasi-uncorrelated, the combined output noise of multiple channel of dither DDSs will decrease due to their uncorrelated property. In this paper, a novel DDSs array structure on method of combine the output of multiple dithering DDS channels is proposed. First, the paper discusses the methods to suppress the truncation spurs and uncorrelated noise by using phase dithering and parallel DDSs; next, methods for eliminating the truncation spur by combining the output of multiple dithering DDS channels are analyzed to help generate spur-free outputs with low phase noise. Simulation results using MATLAB show that the novel structure can eliminate the truncation spurs without increasing the phase noise.