[time-nuts] Thunderbolt E failing

Mon Sep 14 14:14:20 UTC 2020

Hi

> On Sep 14, 2020, at 3:59 AM, Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> 
> On Sonntag, 13. September 2020 15:05:36 CEST Bob kb8tq wrote:
>> Hi
>> 
>> The point is that there is no need to compensate the part while it is
>> locked.
> 
> It depends. For short time constants, yes, likely the control loop is able to 
> follow the temperature-induced drift of the OCXO

At that point the part meets all relevant specifications. The customer is happy
( = it meets spec). Any further work on the part simply increases the cost of the
device. Why would you do this if it already is fully spec compliant ?

> . But you might want the TC to 
> be as long as possible. Naturally, the control loop becomes very slow to 
> react. By following the temperature, you have an additional input that allows 
> the controller to act more quickly to a changing environment. Effectively this 
> will lead to higher stability of the output.

If you spend a few years digging into this and trying various approaches, be sure
to apply the same firmware to multiple devices. If it all is a “learning” process. How
do you suggest training? The “worst case” will always be an excursion outside the
normal variation the part sees…..

If the data is collected in a formal fashion (via temperature test) and then programmed
into the device, how stable is it long term? How is it impacted by air flow and unit 
orientation? ( hint: a lot ….). 

Not cheap or easy …..

Bob

> 
>> Indeed, trying to compensate *and* learn at the same time generates even
>> more issues to deal with. The temperature effect is not a simple linear /
>> first order sort of thing ….
> 
> Maybe, but a linear approximation is probably better than nothing as it still 
> reduces the amount of change that the control loop has to counteract.
> 
> If you look at the attached screenshot - there's roughly 5500 seconds of data 
> from my GPSDO. At about 2500 seconds the temperature compensation was engaged. 
> It's just a linear correction factor for the DAC output. The factor was 
> computed using a linear regression fit to find the correlation between the 
> temperature and the OCXO drift. In the DAC chart the blue trace is the actual, 
> corrected DAC output, the orange trace is the uncorrected DAC computed from 
> the TIC input. The green trace is the OCXO drift. Without the temperature 
> compensation, the TIC takes a dive down to almost -15ns. While the temperature 
> continues its decline at an approximately constant rate, after the 
> compensation was engaged the TIC excursions are smaller and therefore the 
> stability of the output higher.
> 
> Now, of course the picture gets somewhat muddy if you keep computing the 
> compensation factor while the compensation is active, but what you still get 
> is a _residual_ factor, which you can maybe use to refine the compensation 
> subsequently. The correlation factor (r_xy) will tell if there's still 
> something to be done.
> 
>> 
>> Bob
>> 
>>> On Sep 13, 2020, at 1:41 AM, Matthias Welwarsky <time-nuts at welwarsky.de>
>>> wrote:> 
>>> On Samstag, 12. September 2020 23:53:39 CEST Bob kb8tq wrote:
>>>> Hi
>>>> 
>>>> As far as anybody knows (which *is* a big qualifier indeed ….), the TBolt
>>>> family only does it’s “temperature compensation” stuff when it’s in
>>>> holdover. While it’s locked, there *appears* to be no compensation being
>>>> done. If it’s doing anything while locked, it’s learning what the TC is,
>>>> so
>>>> it can use that information while in holdover.
>>> 
>>> That is interesting, why wouldn't they use the information if it's
>>> available? Watching the temperature gives you a prediction of what is
>>> going to happen, while watching the phase only shows what has already
>>> happened. And knowing what's about to happen is always very favorable in
>>> control theory.> 
>>>> Bob
>>>> 
>>>>> On Sep 12, 2020, at 3:52 PM, Matthias Welwarsky <time-nuts at welwarsky.de>
>>>>> wrote:
>>>>> 
>>>>> Kevin,
>>>>> 
>>>>> how long has it been in storage? I expect the GPSDO doing quite clever
>>>>> things to compensate for temperature effects. There may be some
>>>>> parameters stored on the device and the characteristics of the OCXO may
>>>>> have changed after being turned off for an extended period.
>>>>> 
>>>>> Regards,
>>>>> Matthias
>>>>> 
>>>>> On Samstag, 12. September 2020 18:13:35 CEST Kevin Schuchmann wrote:
>>>>>> Matthias,
>>>>>> 
>>>>>>  You are correct, having spent hours looking at all the stable areas
>>>>>> 
>>>>>> and cooling and heating the gpsdo I find that when it reports that it
>>>>>> is
>>>>>> 116.25 F then it is stable.
>>>>>> So now I guess I need to figure out why it is so picky. I will measure
>>>>>> the current draw from a cold start and see if I see the oven warming up
>>>>>> and then stabilizing and then heat and cool it and see how it reacts,
>>>>>> and also look at the electronics and see if an area is overly sensitive
>>>>>> to temperature.
>>>>>> 
>>>>>> Thanks
>>>>>> Kevin
>>>>>> 
>>>>>> On 9/11/2020 2:26 AM, Matthias Welwarsky wrote:
>>>>>>> On Freitag, 11. September 2020 01:08:09 CEST Kevin Schuchmann wrote:
>>>>>>>> Guess my image didn't make it, I will add it as an attachment this
>>>>>>>> time.
>>>>>>> 
>>>>>>> The temperature curve seems to show some correlation to what is
>>>>>>> happening
>>>>>>> with the DAC. Seems that the DAC and OSC jumps are mostly during
>>>>>>> periods
>>>>>>> of some thermal perturbation.
>>>>>>> 
>>>>>>> Regards,
>>>>>>> Matthias
>>>>>>> 
>>>>>>> 
>>>>>>> 
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