I have used AD9957 at single tone mode to output 250M signal(DDS), with 25M XTAL input and 40 frequency multiplication by the PLL in AD9957.
Near the fundamental frequency, spurs distribute at integral multiple of 350K frequency away from 250M.The spur nearest 250M is more than -8db. After changing the mulitplition of referance frequency and FTW, the spurs still exist and change little(350K). Actually the same spectrum and spurs is found on the SYNC_CLK which provide 1/4 sysclk. if disenable the PLL multiplier of AD9957, the spurs disappear.
How do the spurs emerge? How can I avoid this issue?
The image is spectrum nearby 250M output and the setting of PLL loop filtor.