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

[Magnus Danielson]

Hi,

Somewhere in the NIST T&F archive, there is reference to how mixers 
cause a reflection of energy and temperature coefficients then change 
phase and working-point. They use 3 dB damper on the mixer to stabilize 
that and reduce the tempco situation. The signal degradation is 
compensated for but improvement in stability significant. As I recall 
it, they refer to the cable phase stability with regard to temperature 
to be part of the culprit.

Now, DBM isn't perfect in terms of balance and nor is the Gilbert cell 
mixers that Erik is using, so milage may vary, but one should look at 
multiple aspects. Alteration of operating points, alteration of 
dielectric with temperature etc. is things to be aware of and then try 
to figure out which is the major driver for your setup and measurement 
needs and aims.

I am sure someone have attempted to temperature stabilize a mixer at 
some time.

When building synths for music, we end up temperature compensating the 
expo-converters or even ovenize them to achieve needed stability. That 
is not far from what a mixer does. Also, it is what got me into this 
time and frequency thing in the first place.

Cheers,
Magnus

On 2022-11-14 17:37, Attila Kinali via time-nuts 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/
>