[GPS_Standard] Self Test experiment with strange result

[Bert, VE2ZAZ]

Hi Dimitri.

My first thoughts are that this might be caused be the way the built-in 16-bit counter works. It is plausible that the counter could mis-behave when everything is absolutely synchronous. This is a condition that should not occur in real life, and averaging will do the work to clean this up...

You should try something else. Turn off the FLL. Do manual computing of the average offset and see whether it correlates with the described behavior. This will isolate it to the counter behavior.

Cheers,

Bert, VE2ZAZ


> Date: Wed, 4 Nov 2015 00:22:17 -0800
> To: gps_standard at mailman.qth.net
> From: dimitri at dotp.com
> Subject: Re: [GPS_Standard] Self Test  experiment with strange result
> 
> I took the 10 Mhz output from the board, run it through a divider 
> chain (74HC390s) to end up with 1Hz and used that  to feed the 1PPS input .
> 
> Montrol reports
> Avg frequency offset -3.29e-10
> Std Dev       1.46e-08
> Min -3.12e-08
> Max 2.50e-08
> 
> These values are in the same neighborhood of what I get using the 
> 1PPS from the GPS.
> 
> On frequency counters when you measure the counter's own timebase you 
> end up with 10 and as many zeroes as can fit on the display.
> 
> I was expecting values in the e-12  (or better) neighborhood.
> 
> The idea that there is a frequency "offset" when the 1PPS is derived 
> from the same 10MHz being measured  doesn't make much sense at the moment.
> 
> Can anyone shed any light on this ?
> 
> 
> Dimitri
> 
> 
> Dimitri
> 
> 
> 
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