[time-nuts] Measure GPSDO stability with minimum resources?

Bob Stewart bob at evoria.net
Thu Oct 6 13:49:48 EDT 2016

Hi Tim,
Thanks for the document.  I have been doing some 12 hour holdover tests.  But as I mentioned, the HP quick projections had fooled me into thinking there was some trick to this other than actually capturing the DAC over multiple days to get a real projection.


      From: Tim Shoppa <tshoppa at gmail.com>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Thursday, October 6, 2016 12:37 PM
 Subject: Re: [time-nuts] Measure GPSDO stability with minimum resources?
The HP Smartclock app note will help you a lot:

There are lots of Z3801A EFC curves on the web for you to see what typical range of unit-to-unit variation is.
Of course to actually test holdover, you do that by opening the PLL loop (unhook GPS antenna) and letting the EFC extrapolation in your software run for a day (or whatever), watching the phase difference. Then you have to test that the system recovers back into phase lock smoothly after getting hooked back up.
Tim N3QE 
On Wed, Oct 5, 2016 at 1:37 PM, Bob Stewart <bob at evoria.net> wrote:

For my GPSDO, I need to calculate the OCXO aging for holdover projection purposes as well as get some figure of merit for the recent past of the OCXO stability.  The latter is so that I can determine that the PLL has (or soon will have) a good lock.  I'm developing on a dfPIC33FJ128MC802, and I've used about 70% of the code space,  I could probably set aside 4K bytes of data space for this calculation. 

I have a rather primitive way of doing part of this, but I was hoping someone would steer me to something a bit better.

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