Hello Tom, >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 effects. >I wonder if there is a clear test that would tell you one way or >the other. 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 effects. 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 :) bye, Said _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.