[time-nuts] Re: Temperature accuracy / repeatability / drift of double ovens

[Bob Camp]

Hi

Since we started the thread with a question about double ovens …. I’m guessing that the
structure being considered is some sort of oven in a oven sort of thing. In that case, you 
are right into a very similar rabbit hole as the OCXO.

Bob

> On Mar 31, 2023, at 3:26 PM, Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi,
> 
> There is a few things which differs. The oven for crystals is quite different from the cooling of DFB lasers. You regulate your built in Peltier element. Also, the temperature gradient over the crystal blank isn't the same as the temperature of the DFB laser diode.
> 
> So, there is a bit difference in these rabbit holes.
> 
> Cheers,
> Magnus
> 
> On 2023-03-31 21:03, Bob Camp via time-nuts wrote:
>> Hi
>> 
>> There are a lot of ways to rate this sort of thing. One is to look at the level of control
>> at the temperature sensor. It might be a thermistor, it could easily be something else.
>> For a long time OCXO designers tossed around this as their “control level”.
>> 
>> You would look at the variation on the thermistor bridge over some period. Convert
>> the volts to resistance and then to C, F, or K. Folks would get numbers like “12 micro
>> kelvin” as a result.
>> 
>> A fairly simple step back, scratch head, and think about it sort of analysis ultimately
>> showed this to be a bit useless or any sort of real performance evaluation.
>> 
>> What matters is the level of temperature control over some volume ( the inside of
>> an OCXO or ….). Real world devices are not completely located at a single point.
>> It’s a bit amazing just how much things change over a CM or so of distance in an
>> enclosure. Toss in variable heat sources and it really gets wild.
>> 
>> One route down this rabbit hole:
>> 
>> http://www.karlquist.com/oven.pdf
>> 
>> One also might note that the author is floating around here on the list.
>> 
>> Bob
>> 
>>> On Mar 31, 2023, at 2:13 PM, Ed Marciniak <ed at nb0m.org> wrote:
>>> 
>>> Linear technology made a thermoelectric full H bridge temperature controller. At the time, I’d like to say it came as a lot of two components. The second was an op-amp that may have been selected or trimmed unless there was a good reason to include it with samples (maybe less commonly available).
>>> 
>>> It was supposed to be able to achieve 10 millikelvin stability easily, and with more care in design, 1 millikelvin stability was supposed to be not too difficult to achieve.
>>> 
>>> It was a moderately expensive part…about $20 for the two components. A handful of passives and a a pair of N-channel and a pair of P-channel MOSFETs were required.
>>> 
>>> If memory serves correctly it was also switched mode rather than linear.
>>> 
>>> Get Outlook for iOS <https://aka.ms/o0ukef>
>>> From: Bob Camp via time-nuts <time-nuts at lists.febo.com>
>>> Sent: Thursday, March 30, 2023 6:01:55 PM
>>> To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
>>> Cc: Bob Camp <kb8tq at n1k.org>
>>> Subject: [time-nuts] Re: Temperature accuracy / repeatability / drift of double ovens
>>>  Hi
>>> 
>>> Simple answers (and likely not much help):
>>> 
>>> Thermal gain on a single oven is likely in the 300 to 600 range.
>>> 
>>> Double oven boosts that by 10 to 30X
>>> 
>>> Thermal gain in this case: take the ambient change divide it by the thermal gain
>>> and you get the oven temperature change.
>>> 
>>> Long term stability:
>>> 
>>> low enough that it does not impact aging. ( = that’s how you test it). Since that gets back
>>> to the specific thermistor being used, you are into the ‘likely not much help” range.
>>> 
>>> Best guess info: The crystal + circuit is < 1x10^-9 / C in the vicinity of the turn. The operating
>>> point is offset from turn to minimize the combination of crystal + circuit. Just what the net
>>> is ….
>>> 
>>> Still, if you can “see” aging at the parts in the (low?) 10^-11 range, it sort of kind of gets
>>> you to a number. Maybe 0.01C / day.
>>> 
>>> Since you are canceling the circuit with the crystal, if things got to far off, a year later the
>>> temperature performance would not be any good. That likely gets you to something
>>> in the < 1C per year range.
>>> 
>>> Bob
>>> 
>>>> On Mar 30, 2023, at 6:16 PM, Gerhard Hoffmann via time-nuts <time-nuts at lists.febo.com> wrote:
>>>> 
>>>> Temperature accuracy / repeatability / drift of double ovens:
>>>> 
>>>> What can be reached here, with a tolerable effort?
>>>> Is there anything known about that in the public? mK-numbers?
>>>> 
>>>> The application is not a crystal oven this time, but diode lasers
>>>> that interrogate Rb atoms and must be kept in lockstep at a certain
>>>> beat frequency. The frequency tuning is mostly by temperature;
>>>> laser diode current plays a minor role, but probably via changing the
>>>> local temperature in the junction and a little bit later in the cavity.
>>>> 
>>>> Are there any GoTo thermistors / circuits for stability?
>>>> 
>>>> Cheers, Gerhard
>>>> (stumbling into new territory)
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