[time-nuts] Cheap jitter measurements

Hal Murray 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 “zero error” 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 “I need a 10 GHz clock
> source” problem. No need for FPGA’s 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 we’re talking about.

The GPS offset is a kludge to work around not being able to control the local 
oscillator.

> Now - *can* that be done with NTP??  Who knows…. 

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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