I think you'll have to say what is acceptable to you and define what you
mean by "spectral grass". In terms of phase noise, you'll do better with a
normally designed DDS than you will with a normally defined PLL. For data on
what a DDS can do rephase noise, look at various models of synthesizers on
the PTS web site.

Now of course there are spurs to contend with in any DDS design, but
discrete spurs are not what I would call "spectral grass". If spurs are
indeed your issue, you can go to a 14 bit DDS (AD9951 or others) and get at
least spurs at least 80 db down, or add filtering to the DDS output.


Chuck