[time-nuts] phase noise in digital divider, 2nd harmonic
Graham / KE9H
ke9h.graham at gmail.com
Fri Jul 29 09:31:57 EDT 2016
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.
On Thu, Jul 28, 2016 at 6:39 PM, life speed via time-nuts <
time-nuts at febo.com> wrote:
> 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
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