[time-nuts] Tbolt temperature sensor
SAIDJACK at aol.com
SAIDJACK at aol.com
Thu Feb 5 19:54:10 UTC 2009
>The reason I'm asking this is because I can't figure out from
>your descriptions or from the plots a clear way to distinguish
>between unintended oven current-induced changes in applied
>EFC voltage vs. direct temperature-induced changes in OCXO
>frequency output. It seems both would have the same effect.
As long as all effects are fairly linear over a given temperature range,
then it doesn't matter where the thermal sensitivity is coming from. You may
even have canceling effects (if for example the oven has a negative tempco, and
the DAC/Vref have a positive tempco). So in other words, even the unintended
temp-induced EFC changes will be compensated, because we measure absolute
current versus absolute EFC voltage when the unit is locked, and calculate the
parameters from this measurement, which includes all unintended thermal
>I wonder if there is a clear test that would tell you one way or
You could test specifically for the unintended effects: turn off temperature
compensation and GPS locking, and put the unit into a thermal chamber and
see how it performs. On our double ovens we usually achieve better to
significantly overall performance than the OCXO thermal spec itself. This shows that
we don't "make things worse" by the DAC, DAC-reference, or grounding thermal
Since our PCB can perform better than the double ovens we use, this means
the PCB is about 50x to 100x or more better than the performance of the single
ovens, since that's the relationship of double/single oven performance.
One caveat: since the double ovens are so extremely good, it can actually
happen that the electronic compensation could under some circumstances make
things worse than if it was just turned-off. This can happen if you operate your
oven in a small thermal range (say +/-5C) where the current/temp relationship
is nice and linear, and then almost sudden go -50C lower than usual, where
the relationship may be parabolic etc.
For applications that require large thermal ranges (say fighter aircraft
that can descend 30000 feet in mere minutes or less) we actually disable the
tempco statistics, and rather measure the unit over a 100C range in a thermal
chamber, and establish the best tempco parameter that will work from -25C to
+75, and will improve the units performance over the entire temperature range
without adding additional error anywhere.
In all of our Fury and FireFly GPSDO's the user has the option to put a
hard-coded tempco value, and disable statistics gathering.
To check for ground loops: solder a wide (say 1/2 inch) copper string from
the OCXO case to the power supplies' ground pin so the oven ground current will
flow through this instead of the OCXO ground pin. We do not see any change
in performance doing this, and with a current clamp you can verify that most
of the current is actually flowing through this new ground.
>Maybe change oven voltage quite suddenly. That should cause
>a change in oven current. If the measured EFC voltage or the
>measured frequency also changes suddenly then that might
>indicate it's not really temperature that's being compensated for.
This would not work, since it would change the oven current for sure, and
this would end up causing an immediate EFC voltage change since it would be
interpreted as a thermal change (which it is not over the long term of course
after the oven settles down). That's why we have a highly accurate, low noise
10.45V regulator on our boards to generate the OCXO voltage, and don't use the
raw supply voltage.
>Can I do these tests on my double oven Fury? Or is this something
>only for the single oven version.
You can do these tests on the double oven Fury. The tempco numbers are
usually around 10 - 50 for a good double oven, and around 1500 to 4000 for a good
single oven. This means the compensation is usually 100 to ~300 times more
aggressive on a single than on a double oven unit. You may want to upgrade the
firmware to the last release (rev 1.21) for these types of tests.
On a typical double oven we thus see very little correlation between OCXO
current and EFC voltage locked or unlocked :)
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