[time-nuts] Re: Thunderbolts ...

[Bob kb8tq]

Hi

The OCXO “as shipped” on units after about 2021 or 2022 is in the low parts 
in 10^-9 over -20 to +70. Is yours 1 x 10^-9 or 2 x 10^-9? there’s no easy way
to tell. 

If you assume the TC is linear ( it may not be …) that gets you into the 10 to
20 ppt / C range. With a room that swings a couple of degrees, that’s going to
be tough to measure ….. Even if the TC is a bit non-linear, it’s unlikely you are
over 40 ppt / C. 

Bob

> On Jul 24, 2021, at 2:30 PM, Stewart Cobb <stewart.cobb at gmail.com> wrote:
> 
> 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
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