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

[Graham / KE9H]

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 at 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
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