Where does the AD9910 spurs come from?

[Tracy95]

When using AD9910 recently, it is found that it is under 960MHz clock. AD9910 outputs 300MHz, 290MHz, and 302MHz (all in single tone mode), 3 dot frequency signals. Among them, the 300MHz signal has better spurious in the 100MHz bandwidth, which is basically above 80dB. 290MHz and 302MHz are very poor, and there are some spurs in the 100MHz bandwidth. Among them, when 290MHz output, 310MHz spurious maximum about 60dBc appears, but the spurious energy is unstable, with fluctuations of about 10dB. Can you help me analyze where this spur is generated? Why are there such big ups and downs in energy?
Here is the article of AD9910 Parameters for reference.
https://www.utmel.com/components/ad9910 ... out?id=260