On 6/25/18 6:05 AM, Dana Whitlow wrote:
A useful hardware demonstration of DDS phase truncation is to do step a DDS
through a long
series of frequencies at a relatively low stepping rate, and view the
output with an FFT-based
(real-time) spectrum analyzer set for spectrogram display. Watch this for
a while and it becomes
quite easy to reach a practical understanding of what's going on and how
the spurs behave.
Note that to avoid confusion from aliasing products, it is a good idea to
constrain the range of
frequencies swept to the first Nyquist zone (zero to < 1/2 the DDS clock
frequency).
There's a fairly simple regularity in the spur behavior which is not
particularly obvious from
looking at equations and tables (at least, not at my math skill level), but
which jumps out at
you in the spectrogram display (or even without the spectrogram).
Here you go, as an example:
http://jim-lux.blogspot.com/2018/06/dds-spurs.html
16 bit DDS, 5 bit phase, 4096 samples for FFT spectra, etc.
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