The biggest problem with 99.9% of the diy AD98XX designs around is that nobody has read the datasheets properly as far as I can tell.
There is information in most of these datasheets about harmonic and spur suppression, the simplest of which, to considerably reduce even order harmonics, is to use the transformer style output coupling from the DDS chip (This was not in all datasheets, especially some of the earlier ones but is in the later revisions).
It also has the benefit of increasing the output level and with properly matched filters applied, reduces the variation in output level seen especially with the formerly cheap '9850 and '9851 modules.
Spur reduction (as opposed to harmonic reduction) is a whole different kettle of fish and involves, amongst other things, the use of different Xtal/clock frequencies to drive the DDS chip depending on what output frequency range is desired so as to move the spurs further away for proper filtering.
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Terry VK5tm
www.vk5tm.com ( )
