A few days ago, Charles Steinmetz mentioned that even the "good" Thunderbolts with the 37265 OCXOs fall into two groups, one 100x worse than the other. Perhaps that difference is not due to the OCXOs themselves but to other parts on the board.
We know that, during the production run, the high-resolution board temperature sensor was replaced by a software incompatible chip which resulted in much coarser temperature resolution. We don't know how that measurement was used in the control loop. I gave a writeup of the OCXO control DAC here: <https://www.febo.com/pipermail/time-nuts/2013-November/081058.html> I forgot to mention in that writeup that the 74AC174 flip flop that implements the PWM output gets its power from a precision voltage reference chip. This stabilizes the DAC voltage over temperature. That reference, or the precision opamp that serves as a level shifter for the DAC, might have been substituted for parts with worse tempco at some point in the production run. Either of those could cause an apparent increase in observed tempco for the system as a whole. It's even possible that the "good" units had offsetting temperature coefficients between the OCXO and the DAC, and that insignificant parts changes destroyed that favorable offset, even though all the parts remained within spec. The only good way to find out would be to examine all the date codes and part numbers on the relevant parts, and to compare a group of "good" units with a group of "bad" units and look for correlations. Still if someone wants a project ... Cheers! --Stu _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
