Ron is spot on there. It has an internal PLL, so you just need a reference clock.
A 555 would have issues running at 15Mhz+ (if at all?!), and even if it did it, it would hardly be accurate enough, especially for DDS. Many links on the web for DDS circuits, as far as I remember, most seem to use those canned oscillator modules. A bit more expensive than the crystal alone (we're talking, what? 2USD max?), but they a CMOS/TLL output, and are very stable and accurate (all the driving stuff is in there). 25 and 50Mhz ones are freely available and are ideal for this task.
Just my two cents,
Blueteeth