Hi Yung-Nien Chiu,
Those unwanted tones are the image frequencies as a consequence of having a DAC output on our DDS. It is usually located at (system_CLK/2 - Fout) + (system_CLK/2). The ways to mitigate this effect is to either introduce a high order filter with a cut off freq at system_CLK/2 or input a very high system_CLK frequency. High enough that it can push the image frequencies as far as possible and be naturally attenuated due to the sinc envelope. However it is common to add a filter at the output of the DAC even with high frequency clocks. If you want to push the frequencies even farther, you can use the interpolation filters for that.
Best Regards
Louijie