[time-nuts] First few measurements of my Arduino Due GPSDO
andrew at cleverdomain.org
Sun Sep 14 13:23:35 EDT 2014
On Sun, Sep 14, 2014 at 4:29 AM, Andrew Rodland <andrew at cleverdomain.org> wrote:
> My clock is quantized at 10MHz, so you wouldn't expect better than
> 100ns accuracy. But I added -50ns to the offset in software, making it
> zero in on the edge where the offset is 0 counts 50% of the time and
> -1 count the other 50%. (Dithering provided by noise in the system and
> the Resolution-T's own sawtooth). This seems to have worked better
> than expected.
I'm in the middle of measuring a free-run with the control loop
disabled and I realized that this explanation is pretty well mistaken.
The thing is, even though I can only *measure* time (the PPS offset,
or NTP timestamps) in multiples of the 10MHz clock, my PPS *output* is
actually quantized by the 84MHz crystal clock on the Due. +/- 1 cycle
at 84MHz = ~24ns, and that agrees much better with the noise I see at
very short times.
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