DDS Spurs

A simple example for showing how spurs move around with a DDS.
In all cases, the DDS has a 16 bit phase accumulator and a 5 bit phase truncation.
The spectrogram x-axis shows frequency scaled by clock rate (i.e. 0 to 0.5, because the simulation is real only)
The y axis is the DDS set frequency, also scaled by clock rate.  The spectra were computed with a Hamming window, and a 4096 point transform.

First, for frequencies near zero

Then, for frequencies near fclock/15


Finally, for frequencies near fclock/4

You can see the "big spurs" move in nice lines, but the small spurs are essentially random (little speckles throughout the image)