"Mauro" writes:

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

Assuming your source is not so good as to be below the noise floor of the
AD9951, you will get slightly better performance without multiplying by
two. The rule-of-thumb for perfect conversion is 6dB for each doubling.
But it won't be perfect, so by doubling you'll be a little worse than 6dB
and then dividing again you will not get as much back. If the doubling was
perfect and the performance of the AD9951 was the same at 200MHz and 400MHz,
you get the identical results.

Of course, this might be bogus if there are other artifacts.

Make sense?

If you can build a doubler, then you can measure phase noise. Mix your 20MHz
output with a known good 20MHz source in a double-balance mixer. Adjust the
phase to get zero DC (quadrature). If you low-pass filter the output, the
noise is an indication of phase noise. If you have a low frequency spectrum
analyser available, you can get a nice plot. Otherwise you can use an AC
voltmeter to get relative performance.


Steve.
--
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