From what I've seen in the data sheets, the closer you operate the RF output
to the Nyquist limit (Fref/2), the cleaner the spurs get. I've seen numbers
of -90 dB or better on even some of the cheapo Analog Devices parts.

Joe
W3JDR


"maxfoo" wrote in message
...
On Wed, 14 Sep 2005 20:29:09 GMT, "Mauro" wrote:

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


I won't be to concerned with phase noise, with that dds I'd be more
worried
about spurs. Those AD dds chips are notorious for them.