Dear Bob, The capacitors are 47n NP0/C0G types (Kemet C0805C473K3GAC7800), picked for low tempco (and low DF and other non-ideal behavior). I've not spotted any hysteresis artefacts in these in previous designs, but I haven't measured their performance in this circuit.
Forgot to mention in the previous message: the baluns are transformers (M/A-COM MABAES0060), so the only DC the ADC should see is its own input offset (plus offset current across the 25R input filter resistors). Full schematic is here ( http://www.lartmaker.nl/time-nuts/DMTD_rev0.99.pdf ; needs cleanup, but all connections are there). JDB. On Sun, Feb 23, 2020 at 10:13 PM Bob kb8tq <[email protected]> wrote: > 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 < > [email protected]> 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 < > > [email protected]> wrote: > > > >> Good evening! > >> > >> I'm going through some old stuff... > >> > >> > >> On Wed, 27 Nov 2019 00:29:19 +0100 > >> Jan-Derk Bakker <[email protected]> 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@tau=1s versus 3.25e-13@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. > >> > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] > >> To unsubscribe, go to > >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
