[time-nuts] Re: GPSDO/GNSSDO project: STM32G4 + u-blox ZED-F9T + TDC7200

[Bob kb8tq]

Hi

> On Aug 7, 2022, at 6:08 AM, Carsten Andrich via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi Bob,
> 
> On 06.08.22 18:50, Bob kb8tq via time-nuts wrote:
>> As many have found, dealing with OCXO’s in changing environments is a bit
>> exciting. They most certainly do go in a variety of settings. Something around
>> 99% live in a fixed / bench sort of install. The spec sheet data on a catalog part
>> is inevitably talking about a very stable situation. Performance is always degraded
>> if things like shock / vibration / acceleration / temperature change are involved.
> 
> For my prototype I just picked a readily available, not too expensive OCXO. If the prototype works out, of course an OCXO with low g-sensitivity is due for the next iteration. Specialized 10/100 MHz OCXOs with <0.2 ppb/g sensitivity on the worst axis are available, but not exactly cheap.

There are devices out there with much lower G sensitivities. 

> 
>> Without a target accuracy spec on the device, there really is no way to know if
>> this or that approach is going to work. Is this a time source? Is it just a frequency
>> reference? What is the target accuracy in the real world?  What is the “vehicle”
>> and what are its characteristics
> I require time (and therefore implied frequency) synchronization with <10 ns accuracy. 1 ns would be preferred, as I already reached that for stationary applications with SRS FS740s. My use case is coherent sampling for spatially distributed software-defined radios and millimeter wave frequency converters. The vehicle is primarily a car, but may also be airborne. 

If your cars are in an urban area, even getting 10 ns ( let alone 1 ns) will be difficult,
That’s regardless of the GPS used. Vibration is likely to be a pretty big deal in an
airborne application. This is one of the reasons various outfits make vibration 
compensated oscillators. 

Static compensation is a different animal than full up vibration compensation. If you
(slowly) tip the OCXO over and it shifts < 0.x ppm then it passes. Put it on a vibe
table and shake it at 10 Hz and you likely get a *very* different result. 

> 
>> and we never even made it
>> to the problematic filter on the DAC
> Could you elaborate on that, please?

A 1 Hz filter implies a certain amount of time delay. That time delay gives you in 
loop phase shift. That phase shift will result in peaking in the noise. This is not
what you want in a GPSDO ….

Bob

> 
> Thanks and best regards,
> Carsten
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