I still don't see what you're trying to say with the above statement, about 4.x of DAC resolution? The AD9850 has a 10 bit DAC. And proper filtering should remove any alias images or harmonics, resulting in a sine wave. Again, if you look at page 9 of the datasheet (http://www.analog.com/media/en/technical-documentation/data-sheets/AD9850.pdf), you'll see that the images should be low pass filtered, after which only the fundamental (sine) remains.
Works nice...in theory...
4.x BITS of DAC resolution, call it 4 bits if the X bothers you. (Note...I actually meant 2.x)
MCLK runs at 125Mhz. You want 30Mhz output. You have to add to the accumulator a number that'll swing the DAC fast enough to give you that 30Mhz output, and it has to be a large number. As the desired output frequency gets higher, the number of 'steps' in your output drops.
Say you want a 62.5Mhz sine wave output. Well, it ain't gonna happen. 125Mhz / 62.5Mhz = 2 steps, high and low. 1 bit sine wave = square wave. Filter that...
Want a 31.25 sine wave output? Ain't gonna happen again. 125Mhz / 31.25Mhz = 4 steps, high, 1/2 high, 1/2 low, and low. 2 bit sine wave? No...
You want the full 10 bit DAC resolution, you have to divide your MCLK by 1024...125Mhz / 1024 = 122,075.3125 Hz. Double the Hz for every bit you can stand to lose in DAC resolution.
Simple math.
As far as filtering goes... Sure...design all the filters you want. Try it at 30Mhz and you'll end up with garbage. Even if you do get a decent sine wave output that's free of distortion, it'll likely only work well within a very narrow set of parameters.
Sure...go ahead and buffer the output of that filter. Sounds good. By the time you get done with all of that, may as well have gone and bought a proper 30Mhz sine wave generator.