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

[Hal Murray]

nealix at gmail.com said:
> And, I assume that since we have no idea if the used rubidium oscillator from
> ebay is working properly anymore (aside from output seen on a counter), then
> we should take that rubidium oscillator to a calibration vendor and pay them
> to test it, correct? 

I think an amateur can do a reasonable job of calibrating a rubidium.  I 
wouldn't adjust anything, just measure the actual frequency and use that as a 
correction factor when processing other data.

The idea is to assume it has good long term stability and compare it with GPS 
(which has very very good long term stability).

Do you know about TVB's picDIV and picPET?
  http://www.leapsecond.com/pic/

If you have a scope, use a picDIV to make a PPS from the rubidium.  Compare 
the PPS from GPS with the PPS from the rubidium.  Reset the picDIV until they 
are close together.  They will drift slowly.  Hopefully very slowly.  Come 
back in an hour or day and see how far they have drifted.  Now do the math.  
Repeat.

I have a Rigol scope.  I'm pretty sure I could program it to automate the data 
collection.

If you have a counter/timer that your PC can talk to, measure the time from 
one PPS to the other.


If you have a PC, get a picPET, clock it from the rubidium and watch the PPS 
from a GPS.  The picDIV is only accurate to 400 ns.  You can get a lot better 
than that if you pick your times to match when the picPET shifts across a 
clock edge.

Mumble.  I don't know how to describe that in words that will be easy to 
understand.  It will be obvious after you see it.  Consider reading a clock 
that only ticks once per second, like the typical CMOS clock on a PC.  Assume 
your PC has a clock and you want to know how far off it is.  Spin reading the 
CMOS clock until it changes.  Then grab your PC clock.  That gets you your PC 
clock close to the beginning of a second.

For the picPET, instead of using 2 samples 24 hours apart, adjust the 
start/stop times for your 24 hour run to start right after the picPET 
fraction-within-a-second changes.


For more accuracy, get a TAPR TICC.  $229
  https://tapr.org/product/tapr-ticc/
Thanks John.  You did a wonderful job.


-- 
These are my opinions.  I hate spam.