[time-nuts] Cheap jitter measurements
hmurray at megapathdsl.net
Mon Apr 9 16:53:21 EDT 2018
kb8tq at n1k.org said:
> Somewhere in the NTP algorithm, there is a âzero errorâ estimate. GPS
> modules have the same thing buried in them. A GPS module (like NTP as
> described above) can only *express* a PPS modulo some clock rate. GPS
> modules get around this with a firmware hack. They simply tell you what the
> error was. It is a simple way to get out of the âI need a 10 GHz clock
> sourceâ problem. No need for FPGAâs or any other guck. You just do an
> estimate and report it. It then would work on any hardware and let you do
> the sort of measurements weâre talking about.
The GPS offset is a kludge to work around not being able to control the local
> Now - *can* that be done with NTP?? Who knowsâ¦.
The kernel clock has a knob so the same concept doesn't apply.
The API for the kernel clock can be read to a ns. I don't see ntpd having
much use for finer grain than that. I should look at the source to see what
the internal details look like.
If ntpd decides the clock needs correcting, it tells the kernel to do the
work. The kernel offsets the clock rate knob by 500 PPM, so it takes 2000
microseconds to adjust the clock by 1 microsecond. It would be possible to
read the correction-left and adjust the time by that amount.
I think it would be possible to make similar adjustments by post processing
the data. I'd have to double check to make sure I understand what is in
loopstats. If now, we could fix it.
These are my opinions. I hate spam.
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