[time-nuts] Re: The STM32 GPSDO, a short presentation

[Chris Caudle]

On Thu, March 31, 2022 6:28 pm, Bob kb8tq wrote:
> Consider the case:
>
> GPS shifts ( possibly due to any of a number of issues).
> All three modules "move" forward in time by 15 or 20 ns
> over some time period.
> The GPSDO's all do their thing and shift frequency by
> the appropriate amount.
>
> Since all three devices in your three corner hat did the
> same thing at the same time, all of the delta this / delta
> that numbers just sit there.
> They do not show the frequency shift at all.

That would matter if you wanted to know how good your clock or oscillator
was (in some sort of absolute sense).
If all you wanted to know was how good your GPSDO was, wouldn't that
common mode behavior just fall into an intrinsic aspect of using GPS?
I think in the case that you want to know how good your particular GPSDO
is at being a GPS disciplined thing, compare to other models (e.g.
Thunderbolt, Jackson Labs, HP, etc.) and you can get a good idea of how
well it performs given the limitations that it has to use GPS to function.

The bigger problem I see with the original proposal were that there were
too many of the new design involved, I think three if the model under
design, and one other homebrew model, so there could be common mode
problems caused by e.g. a firmware error (frequency offset in a FLL comes
to mind) that would be hidden by having all the 3 models under test being
the same.
By testing e.g. one new model, one Thunderbolt, and one Jackson Labs it
should at least be possible to tease out what behavior is unique to the
new STM32 device.

-- 
Chris Caudle