[time-nuts] Thunderbolt performance vs temperature sensor

Tom Van Baak tvb at LeapSecond.com
Sat Mar 7 23:14:58 EST 2009


See: http://www.leapsecond.com/pages/tbolt-temp/

Hi Mark,

This is very interesting work that you're doing with Thunderbolt
DS1620 temperature sensors. I hope you stick with it. I agree
with Said about the double bind idea.

I worry too that your TBolts are remembering something of the
past in spite of the hardware changes you make for each new
run. Do you do a full factory reset each time? And then let the
unit "re-learn" for several hours, or maybe several days? Do
we really know how or what it is learning and how that affects
the response to temperature?


Having tried tempco measurements in the past I have several
concerns about mythology. It's really hard to get this right and
even harder when you don't know what "smart" algorithms are
inside the box you're trying to test.

In this situation, it seems to me the main thing about temperature
is not temperature at all, but the *rate* at which the temperature
changes.

In that case, even careful cycling of room temperature every day
or cycling temperature inside a special chamber every hour will
not give you the real story -- because in both cases the focus is
on varying the temperature; not the rate at which the temperature
is changing.

Compounding the problem is that different components in the
system will react to temperature at different rates. The DS1620
is plastic and may react quickly. The OCXO is metal and will
react slowly. Who knows what additional component's tempco
are relevant to the final 10 MHz output. Some may overreact
at first and then settle down.


I guess in the ideal world you'd want to do a "sweep" where you
go through several cycles of temperature extreme at rates varying
from, say, one cycle per minute all the way up one cycle per day.

It seems to me you'd end up with some kind of spectrum, in which
tempco is a function of temp-cycle-rate. Has anyone seen analysis
like this?

For example, I'd guess that most GPSDO have low sensitivity to wild
temperature cycles every second -- because of its own thermal mass.
And I bet most GPSDO have low sensitivity to wild temperature swings
every few hours -- because the OCXO easily handles slow changes
like this well. It's for time scales in between those two that you either
hit sweet spots or get very confused and react just opposite of what
you should.


I'm thinking another testing approach is to varying the temperature
somewhat randomly; with random temperature *amplitude* along
with random temperature *rate*. Using this temperature input, and
measured GPSDO phase or frequency output, you might be able
to do some fancy math and come up with a transfer function that
tells the whole story; correlation; gain and lag as a function of rate,
or something like that. I'll do some reading on this, or perhaps
someone on the list can fill in the details?


I say all of this -- because of an accident in my lab today. Have a
careful look at these preliminary plots and tell me what you think.
It shows anything but a nice one-to-one positive linear relationship
between ambient temperature and GPSDO output.

http://www.leapsecond.com/pages/tbolt-temp/

/tvb





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