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

Wed Nov 16 17:28:36 UTC 2022

Magnus, Attila,
I went ahead and did a fairly simple implementation of the side channel. 
Both channels show, apart from the noise, the same phase drift in the 
order of some ps. This seems to confirm the drift originates in the 
mixers. Now I need a 3 channel DSS to test this. Maybe I can use the 
10MHz ref out as the 3rd channel. And I have to find a way to inject the 
side channel without creating additional leakage between the two main 
channels.
Erik.

On 16-11-2022 13:05, Magnus Danielson via time-nuts wrote:
> 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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