"Mauro" wrote in message
news
So the question: is it possible to estimate if the phase noise of the
20MHz
output is getting better, worst or is it going to remain the same?

Decreasing the frequency at which you clock the DDS will increase the phase
noise. It is pretty hard to estimate as it will change quite a bit with
small changes in divisor (i.e. output frequency). But as a general rule,
you want to clock the DDS at some high multiple of the operating frequency.

Obviously, if your clock has phase noise too, that is a bad thing. I would
doubt that your frequency doubler would add phase noise, but I can't say for
sure since I haven't got your circuit. But typically doublers introduce
other forms of distortion which aren't as problematic for a DDS clock.

So just shooting from the hip, I would expect your phase noise to be a lot
worse from a 200 MHz clock than from the 400. It is still 10x the operating
frequency, though, so I wouldn't expect it to be horrible.

...