[time-nuts] Re: What phase variations to expect in a DMTD due to temperature fluctuations?

[Erik Kaashoek]

Antilla
Using a narrow tube and a soure of cold air the biggest contributor has
been identified as the Gilbert cell active mixers used in the first stage.
In the current HW they are not used in a balanced way so a common mode
change, e.g. temperature change, is very visible.
I will read the the documents you referred to.
Erik

On Mon, Nov 14, 2022, 21:00 Attila Kinali via time-nuts <
time-nuts at lists.febo.com> wrote:

> Good afternoon,
>
> On Sun, 23 Oct 2022 18:05:40 +0200
> Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
>
> > During the testing of a DMTD there appears to be a "large" sensitivity
> > to temperature variations.
> > Opening a door in the room with the DMTD causes changes in the phase
> > difference in the order of 1 ps
> > Blowing cold air over the encased DMTD causes phase variations up to 10
> ps.
>
> I would like to add a few things that have not been mentioned already:
>
> Most electronics seem to have a tempco of 1-10ps/K. It is not clear
> where this tempco comes from, i.e. nobody fully explained it. It
> is remarkable, though, that the range is pretty narrow and quite
> stable over various technologies. Of course, analog filters have
> a larger variation of tempco.
>
> My guess (read: totally unscientific assumption, not backed by
> any data or experiments) is that a major source of tempco are
> mechanical stresses due to different linear expansion coefficients.
> How exactly mechanical stresses affect delay in electronics is
> not quite as simple as it would seem at a first glance. So it's
> difficult to come up with a decent model that can be tested in
> experiments.
>
> Summa summarum: The few-ps tempco you are seeing is what I would
> expect. See also [1] where they measured the tempco of a mixer
> setup (the numbers boiled down to 1-2ps/K IIRC) and proposed
> a way how to measure and compensate the drift.
>
> I also recommend having a look at [2] for a more general treatment
> of the issue of temperature coefficients in time/frequency measurement
> systems.
>
> On Mon, 24 Oct 2022 14:43:43 +0200
> Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
>
> > The PTFE cables have been replaced with semi-rigid coax cables and the
> > stability, both mechanical and temperature, have improved.
>
> Please keep in mind that the problem with PTFE is not the external
> insulation of the coax cables, but the dielectric between the core
> and the screen. A lot of semi-rigid still uses PTFE because it's
> reasonably cheap and gives good performance. See [3-5] for more
> information on this topic.
>
> On Mon, 24 Oct 2022 10:10:27 +0200
> Carsten Andrich via time-nuts <time-nuts at lists.febo.com> wrote:
>
> > only the ADC clock should matter and the used ADC should be of the
> > simultaneous sampling type. If it's not, its multiplexer may have a
> > detrimental temperature-dependent effect on the phase measurement.
>
> It's a bit more complicated than that, unfortunately.
> The mixer and their LO already add already some temperature dependence
> due to inevitable asymmetries. The ADC themselves have a tempco too.
> And it's not just direct temperature effect on the circuitry but also
> indirect effect from power supplies. Even if using a dual-channel ADC
> there are effects that affect the two channels differently. If you look
> at Sherman and Jördens' paper [6], who looked at phase stability in SDR
> systems for frequency / stability measurements, then you see that there
> is a lower limit of a few 10's of fs in ADC sample timing. My guess is
> that at least some of that is due to noise on the power grid in the
> chip that causes IR drop [7]. Which is, by its nature, not symmetric.
> It is also very likely that even small mechanical stresses due to minute
> temperature variations at short time scales already cause timing
> differences
> and phase shifts in the 10s of fs.
>
> Figuring out where all these small temperature coefficients come from
> is difficult, to say the least, and very tedious. Once you reach <10ps/K
> I would, personally, call it a day and do the rest by proper enclosure
> design and keeping everything at a stable temperature. This way it is
> easier to reduce the tempco than to hunt for it in the electronics.
>
>                         Attila Kinali
>
>
> [1] "2Ï€ Low Drift Phase Detector for High-Precision Measurements"
> by Jablonski, Czuba, Ludwik and Schlarb, 2015
> https://doi.org/10.1109/TNS.2015.2425733
>
> [2] "Environmental Effects in Mixers and Frequency Distribution Systems",
> by Nelson and Walls, 1992
>
> [3] "Current Innovations In Phase Stable Coaxial Cable Design",
> by Times Microwave Systems
> https://www.timesmicrowave.com/downloads/tech/phasearticle.pdf
>
> [4] "Understanding Phase Versus Temperature Behavior",
> by Micro-coax
>
> http://www.micro-coax.com/wp-content/themes/micro_coax/includes/pdf/applications_notes/13-MIC-0012.Phase_vs_Temp_Behavior_FINAL.pdf
>
> [5] "Temperature Stability of Coaxial Cables",
> by Czuba and Sikora, 2011
> http://przyrbwn.icm.edu.pl/APP/PDF/119/a119z4p17.pdf
>
> [6] "Oscillator metrology with software defined radio",
> by Jeff A. Sherman and Robert Jördens, 2016
> http://dx.doi.org/10.1063/1.4950898
>
> [7]
> https://semiengineering.com/knowledge_centers/low-power/architectural-power-issues/ir-drop/
>
> --
> In science if you know what you are doing you should not be doing it.
> In engineering if you do not know what you are doing you should not be
> doing it.
>         -- Richard W. Hamming, The Art of Doing Science and Engineering
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