Thanks to all who replied.
I think that the net is that the phase noise should remain about the same.
The source is a low noise quartz oscillator based on J.Stephensen design,
followed by a Schottkey doubler and a MAR5 amplifier.
I was in doubt to use or not the Schottkey doubler. I think i'll keep the
simplest solution: no doubler.
DDS max output will be 39MHz. A reference clock of 200MHz is still about 5
times the max output freq, high enought above the Nyquist limit.
Thanks again.
73
Mauro
i2SUH

"Mauro" wrote in message
news
I'm not an expert and need some help.
I've build a DDS around an AD9951.
I drive it at 400MHz from a source at 200MHz multiplied by 2. I get an
output freq of 20MHz.
I now drive it directly from the same source at 200MHz. I changed the
register inside DDS to still get 20MHz on output.
I do not have any possibility to measure the phase noise.
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?
Thanks for the support.
73
Mauro