[time-nuts] A simple sampling DMTD

[Attila Kinali]

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

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