how to use DDS

If you don't use a sufficiently good low-pass filter after your DDS DAC, then the sinewave will have unwanted frequency spurs that, on an oscilloscope, look like little wiggles or stairsteps or jitter. You don't want that junk going into your PLL, or it will wiggle and jitter too.

Because DDS's have a huge number of spurs, and a PLL magnifies the spurs by 20 LOG N.