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

Wed Nov 16 12:05:34 UTC 2022

Erik,

The side-channel to track the side-carrier can be made much simpler. You 
do not need to FFT it, but you can do a direct PLL lock setup in 
software and with that relate the clocks to each other. It consumes a 
few cycles per sample, but really not much.

Cheers,
Magnus

On 2022-11-16 07:41, Erik Kaashoek via time-nuts wrote:
> Magnus,
> One of the articles referenced by Attila mentioned inserting a second 
> known calibration input signal into both channels with a frequency 
> offset big enough so it becomes invisible in the regular DSP phase 
> measurement channel but by adding a second DSP phase measurement 
> channel at the offset of the inserted signal they had a real-time 
> measurement of the drift and where able to compensate for it.
> Its rather compute intensive and I'm not sure what the offset has to 
> be to become invisible but could you imagine this could work in a 
> rather limited HW?
> Maybe I should test it by inserting a calibration signal and see the 
> impact, but I can not imagine the short "FFT length" I'm usingÂ  to be 
> long enough to give 100dB or more suppression of the calibration signal.
>
> Erik.
>
>
> On 15-11-2022 22:59, Magnus Danielson via time-nuts wrote:
>> 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 JÃ¶rdens' 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.
>>>
>>> Â Â Â Â Â Â Â Â Â Â Â  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 JÃ¶rdens, 2016
>>> http://dx.doi.org/10.1063/1.4950898
>>>
>>> [7] 
>>> https://semiengineering.com/knowledge_centers/low-power/architectural-power-issues/ir-drop/
>>>
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