Hi group an interesting experience with AD9951 and his "spures free" output come froma WoodBoxRadio member Giuliano I0CG, on this page: http://www.radioamatore.it/i0cg/add9951.html
The great improvement using a external 500Mhz reference oscillator, bypassing the internal 400Mhz PLL.
73 BeppeIK3VIG