[time-nuts] u-Blox ZED-F9T block diagram or timing knowledge available here?

[Forrest Christian (List Account)]

For a few grand you can get an off the shelf fs740 or something similar,
probably with a rubidium timebase.  But I get the impression this is more
of a learning experience.

One thing which it took me a while to wrap my head around is that for
frequency, sawtooth error generally dissappears for a lot of definitions of
reasonable accuracy.  You're averaging for such a long period that what
seems like horrible accuracy is very usable.

Hopefully some of the more experienced time nuts will correct the following
if my math is faulty.

For instance say your gps 1PPS source is accurate to +-1uS.  Over 1 second
this is 1e-6.  But over 1000 seconds this is 1e-9.  Over 100000 seconds
you're at 1e-11.   For a 10MHz clock adjusted using this long of averaging,
this error is only +-.0001Hz

Of course this is somewhat simplified and the devil is in the
implementation details.

 As I'm sure others will point out, if you're using certain receivers, then
sawtooth correction data is available to you which will enable further
correction, which makes the internal clock frequency which the 1PPS edges
are aligned to less of interest as the receiver outputs both the 1PPS and
data indicating how far the receiver believes the 1PPS edge is out of
alignment with ideal.

On Mon, Jan 6, 2020, 6:07 PM Tim S <tim.strommen at gmail.com> wrote:

> Hi, first time poster, just getting into the time rabbit-hole.
>
>     I'm looking at building my own 10MHz double-oven + Rubidium standard
> for home-lab use, and I wanted to investigate GPS disciplining.  I read
> some remarkable work using Jupiter engines and a simple N/1000 and PLL, but
> with those receivers becoming much more outdated and higher precision GPS
> receivers now fairly cheap - I thought I'd try something new from uBlox to
> not repeat prior efforts.  I've gone all through every resource I can find
> online and on the uBlox forums, but there is a general lack of public
> information about the time and clock features of the u-Blox F9 engine.
>
>     Suffice it to say, there is a section in the integration manual where
> they elude the 1PPS signal is set to the closest one of 1023 edges - which
> seems to suggest a possible sawtooth phase noise creation.  Talking with a
> person "clive1" on the uBlox forum it sounds like the internal GPS engine
> >>MAY<< now running at 384MHz there sounds like a possibility to align a
> 1PPS edge to a much finer resolution clock which is closer tied to the GPS
> solution and thus less digital divider phase noise might be possible.
>
>     Does it sound like I'm way out in left field here?  Anyone have the
> luxury of more insight into the construction of the uBlox F9 GPS engine?  I
> don't mind spending a few grand on some factory new components to get a
> decent 10MHz standard, but I'm less interested in doing so if I'm not going
> to understand what's going on inside.
>
> Thanks in advance for responses,
>
> -Tim Strommen
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