"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

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


Using the built in frequency multiplier of the 9951 will increase your
phase noise, but so will driving the chip at a lower frequency. I've
been told that the built in multiplier also RAPIDLY increases the power
the chip pulls and the heat it generates, so given a choice between
doubling the frequency with the multiplier and living with 1/2 the
frequency drive, (especially if you already have a 200mhz source)
I'd not use the multiplier.