Hi, Been a while since I visited, I recall there are many, well, time nuts here. I am trying to track down a source of phase noise in a frequency synthesizer design. One part of the frequency reference upconverts a DDS and then divides it down again using a digital divider - standard technique for DDS angle modulation spurious reduction. 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 ;) - Lifespeed

		Attila Kinali

On Thu, 28 Jul 2016 23:39:24 +0000 (UTC) life speed via time-nuts <time-nuts@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

It is not traditional phase noise, and not a normal Nyquist filtering problem, but, yes, presence of significant second harmonic energy in your DDS output will shift/dither your squaring input. It is hard, with practical filters and filter components to get much better coverage than one-half octave per low-pass filter. It will be set by how much second harmonic content you can tolerate. That is, how far down the low-pass filter slope the second harmonic needs to be to get the performance you want. So using the one-half octave per filter "rule of thumb" then you might need three or more low pass filters, depending on the actual frequency span output you are dealing with. --- Graham On Thu, Jul 28, 2016 at 6:39 PM, life speed via time-nuts < time-nuts@febo.com> wrote: > Hi, > > Been a while since I visited, I recall there are many, well, time nuts > here. I am trying to track down a source of phase noise in a frequency > synthesizer design. One part of the frequency reference upconverts a DDS > and then divides it down again using a digital divider - standard technique > for DDS angle modulation spurious reduction. > > 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 ;) > > - Lifespeed > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >