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

Wed Nov 16 15:58:02 UTC 2022

Magnus,
As often with your advice, I'm not smart enough to understand.
The digital down mixing to zero Hz is done with I/Q mixers where sin/cos 
of the internal LO is multiplied with the input signals and then average 
over some samples to get I and Q of the signal vector as the output 
frequency is zero Hz.
I would expect I would need a separate set of I/Q mixers for the side 
channel and by summing over many samples the noise would be reduced 
sufficiently to get a relevant I and Q signal even if the side channel 
has 40dB lower amplitude. I may have used the word FFT to describe the 
operation of the I/Q mixers as computationally they look a lot like a 
single bucket FFT
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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