Hi Amchardy and Quang,
Unfortunately the effect of using the image frequency to do initial ramp-down is you are dealing with lower power frequency due to the sinc response inherent to the DAC internal to DDS. Regarding spurs and phase noise performance, you can improve them by using a powerful narrow BPF.
It was stated in the datasheet, and I'll quote it here:
"During no-dwell low operation, a negative transition of the DRCTL pin initiates a negative slope ramp, which continues uninterrupted (regardless of any further activity on the DRCTL pin) until the lower limit is reached."
For visualization, I have created figure when no-dwell low bit is set:
That is right, it would produce negative sweep w/o having positive sweep first. If you want that method, that would be okay too.
-Sittie