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

[Bob kb8tq]

Hi

The drift used in the original example is just one of many many things
that can come up. There are a lot of “corner cases” in GPSDO design. 
The constellation does “this” and they all react. Ideally they would react
to suppress whatever the issue is. It does not always work out that way.

This is not in any way unique to GPSDO’s. You might have a set of OCXO’s or
Rb’s as the elements in a three corner hat that all had the same tempco slope
and magnitude. In a normal lab, that could create a problem. 

The first time I saw the topic debated it was at FCS. It was in relation to the 
Cs standards used in national time scales ….. ( and why having them all one 
model from one company wasn’t a great idea ). 

Bob

> On Mar 31, 2022, at 10:06 PM, Chris Caudle <6807.chris at pop.powweb.com> wrote:
> 
> 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
> 
> 
> 
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