AD9957 - FAQ
Does Analog Devices offer a list of manufacturers of oscillators for DDS devices?
We do not recommend a specific manufacturer, but Ecliptek, Epson, Vari-L, Connor-Winfield, Wenzel, and Valpey-Fisher are known as sources for high quality parts.
Where can I find some good background material on direct digital synthesis?
The following DDS tutorials should be helpful:
A Technical Tutorial on Digital Signal Synthesis and
Tutorial MT-085, Fundamentals of Direct Digital Synthesis