[time-nuts] A simple sampling DMTD
Bob kb8tq
kb8tq at n1k.org
Mon Feb 24 18:42:52 UTC 2020
Hi
Yup, welcome to the online auction lottery ….. As you may have noted, I’m
struggling a bit with the Phase Station and random (likely my fault) issues.
At the same time, I’m going around in circles about the R&S FPC-COM2. It’s
a pile of money, but it would replace a lot of questionable gear I have sitting
here. Most of what I have was quite literally pulled from / saved from the
dumpster at various points over the last few decades …..
Bob
> On Feb 24, 2020, at 12:33 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
>
> I woke up this morning, check my HP105B, it's absolutely going nuts. Heater temp down, frequency ALL OVER the place. No output in any of the ports. Sigh..... eBay strikes again!
>
> ---------------------------------------
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
>
>
> On Sunday, February 23, 2020, 7:52:02 PM EST, Bob kb8tq via time-nuts <time-nuts at lists.febo.com> wrote:
>
> Hi
>
> Ok, but thats “high pass in the RF section”. You really do not have an audio high pass
> filter the way you would in a more typical DMTD.
>
> If it’s any comfort, I’m sitting here looking at a very different box. It also has “wobbles”
> as you get into parts in 10^-16. That might change a bit if the draft coming through the
> window was a bit less.
>
> Bob
>
>> On Feb 23, 2020, at 6:12 PM, Jan-Derk Bakker <jdbakker at gmail.com> wrote:
>>
>> 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 <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 <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> 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 <time-nuts at lists.febo.com <mailto: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 <mailto: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 <mailto: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 <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 <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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