Instrumentation and Measurement Technology Conference
IMTC®/2001
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.
Keywords:
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.DDS: background | simulations (Mathcad) | products
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