[time-nuts] phase noise in digital divider, 2nd harmonic

Attila Kinali attila at kinali.ch
Fri Jul 29 08:29:19 EDT 2016


On Thu, 28 Jul 2016 23:39:24 +0000 (UTC)
life speed via time-nuts <time-nuts at febo.com> wrote:

> The DDS tunes over more than an octave, so obviously the single low pass 
> filter isn't going to cut it.  I am noticing up to 3 dB phase noise 
> degradation at the output of the divider as the DDS frequency decreases and 
> the 2nd harmonic is in-band to the LPF.  I suspect this is disturbing the 
> threshold crossing in the high speed digital logic divider, as described in 
> "The Effect of Harmonic Distortion on Phase errors in Frequency Distribution 
> and Synthesis" by F.L. Walls et al at NIST.
> 
> What do you think?  I should probably put in a switched filter to get my 3 dB back  ;)

Without being an expert in DDS and knowing your circuit, this sounds
like the most likely cause and the easiest solution. All other harmonics
reduction systems I am aware of are much more complicated.


			Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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