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.

"W3JDR" writes:

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

It has to do with where the spurs end up. If you are close to Fref/2,
lots of the spurs alias down to base band. Worst case is Fref/3. With
the proper choice of filters, and band choice you can find areas where
spurs are almost non-existent. If you have a narrow band application,
your frequency planning can find those holes. Make sure you have your
peak detector turned on.

There are lots of sources of spurs-- phase truncation, amplitude
truncation, noise, etc.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA