When I measure the output frequency of a DDS with a very accurate frequency counter the frequency appears to drift even though I haven't changed the frequency tuning word. Why?
A DDS relies on an external frequency source to produce its output signal. Therefore, if the frequency of the external source drifts, so will the DDS output frequency. In other words, the DDS output frequency is only as stable as the frequency of its external source.
You can think of the DDS as a digital frequency divider. It divides the frequency of the external source such that the output frequency is a fraction of the frequency of the external source. So, if the external source drifts, then the DDS output drifts by the same relative amount. For example, if the external source drifts by 0.002%, then the DDS output frequency drifts by 0.002%.
Note that 0.002% is the same as 20 parts-per-million (ppm). If you use a typical quartz crystal oscillator as the external frequency source, then you are likely to see frequency drift in the 20ppm range.