[time-nuts] A simple sampling DMTD

[Jan-Derk Bakker]

Dear Attila,

Thanks for the heads up.

I am currently using a HPF both in hardware (capacitive coupling into the
balun driving the ADC inputs) and in software before the ZCD. This should
counteract the first-order effects of this offset, although second-order
effects (converter nonlinearity et al) will of course still be an issue.
The plots you've quoted include (different kinds of) DC offset correction
for all but the "unfiltered" data; getting an efficient DC offset
correction working in real time on this 8-bit platform was indeed one of
the main challenges of the software-only approach.

The FPGA daughterboard is currently in production at Eurocircuits; I hope
to have time to work on those the coming month. I'll also try to book some
time in our climate chamber. (I've had one of our GPSDO-designs running in
our general labs since before Christmas; surrounding it with bottles of
water works well enough to low pass filter temperature swings, but I still
see 6 degrees C swings overnight as out HVAC only runs during business
hours.)

To be continued,

JDB.

On Sun, Feb 23, 2020 at 8:11 PM Attila Kinali via time-nuts <
time-nuts at lists.febo.com> wrote:

> Good evening!
>
> I'm going through some old stuff...
>
>
> On Wed, 27 Nov 2019 00:29:19 +0100
> Jan-Derk Bakker <jdbakker at gmail.com> wrote:
>
> > This has yielded a combined "simple" signal
> > processing path of a differentiator, a double comb filter and the offset
> > estimator, which is getting very close in performance to the "ideal" band
> > pass filter (OADEV of 3.77e-13 at tau=1s versus 3.25e-13 at tau=1s for the
> BPF;
> > full plot:
> >
> http://www.lartmaker.nl/time-nuts/DMTD%20self-noise%20OADEV%20with%20PLL%20and%20various%20filters.pdf
> > for this 600000-second recording:
> >
> http://www.lartmaker.nl/time-nuts/600ksec%20run%20with%20PLL,%2010811%20through%20splitter.png
> > . OADEV past ~1000sec is severely compromised by the fact that the
> > measurement setup is in my home lab which sees temperature swings of up
> to
> > 20 degrees C and which does get bumped from time to time. Longer runs in
> a
> > more controlled setting forthcoming).
>
>
> I can offer an explanation for the large effect of the zero correction seen
> here. The LTC2140 is specified to have a +/-10µV/°C drift (at 1Vpp
> setting).
> Converted into phase error due to zero crossing shift, this turns into
> a phase shift of +/-1ps/°C @ 10MHz. Note, the shift is given as +/- and
> per channel, which means, it could very well be that the channels are
> not matched in their temperature characteristics and thus the total phase
> shift could be +/-2ps/°C ... though total shift being closer to 0.5ps/°C is
> more likely.
>
> Summa sumarum: DC offset correction is important if a zero crossing
> detector is used.
>
>                                 Attila Kinali
>
> --
> <JaberWorky>    The bad part of Zurich is where the degenerates
>                 throw DARK chocolate at you.
>
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