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Instrumentation and Measurement Technology Conference


Numerical distortion in single-tone DDS

by Zs. Pápay

Abstract :
Spurious performance of direct digital synthesis (DDS) is partly caused by two quantization operations in its numerical (i.e. digital) part. These errors are deterministic and periodic in the time domain, therefore they appear as line spectra (undesired components: spurs) in the frequency domain. Hence it is quite natural to analyze the effects by DFT (discrete Fourier transform). The amplitude quantization (AQ), being present permanently, causes harmonically related spurs, while phase truncation (PT) produces spurs around the output frequency by phase modulation. However, as a consequence of DDS sampling process, spurs would be folding back into the DDS bandwidth (first Nyquist zone) and possibly overlapping.
A simple procedure is presented for evaluating location and level of spurs, which are due to numerical distortion in a standard DDS system. Examples using an interactive math tool are available online.

DDS, NCO, amplitude quantization, phase truncation, spectral purity, simulation

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Copyright 2001 IEEE. Published in the Proceedings of the IEEE Instrumentation and Measurement Technology Conference, IMTC/2001, pp. 720-724, May 21-23, 2001, in Budapest, Hungary.
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DDS:  background   |  simulations (Mathcad)   |  products
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