[time-nuts] A simple sampling DMTD

[Bob kb8tq]

Hi

What does the temperature coefficient of your “hardware HPF” filter caps look like?
Are they a type that has significant hysteresis?

Bob

> On Feb 23, 2020, at 3:05 PM, Jan-Derk Bakker via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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