Looks like reference frequency leakage of 15.8 Mhz. Have a look at the eval board for the 9363, they likely do somethings to help this. The datasheet is a bit vague on reference leakage with no specs at 1500 but it might just be what the part does. DDS are known to have various spurs and feedthroughs at odd frequencies. ADI used to have a simulator or tool for their DDS chips that let you put in your reference, accumulator width, DAC spec and it would predict where the spurs would show up. Some of the hotter DDS had a thing called "Spurkiller" that would somehow suppress spurs in the internal DDS algorithm. DDS's are amazing but they do have their quirks.