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

[Attila Kinali]

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