[time-nuts] Discipline an oscillator with NTP?

[Chris Albertson]

On Fri, Jul 22, 2011 at 2:30 PM, Javier Herrero <jherrero at hvsistemas.es> wrote:
> I've found a plot of the ntp-synthesized GPS output compared with the
> UTC-aligned GPS from a Thunderbolt. The generated PPS output was 1us wide,
> and it is represented in infinite persistence to get an idea of the jitter.
> The offset was around 50us, and the jitter around 8us, so not very bad (it
> was at least one order of magnitude better than my requirements, so I did
> not bother to optimize it further).
>
> The ntp source was a M12-based ntp server (a blackfin running uClinux, not a
> Soekris :) ).
>
> Driving the PPS output to a serial port from the ntp is not as trivial as
> you think. This PPS output is from an "oscillator" disciplined to the system
> clock - really a stearable divider from the system clock (it is an embedded

The software PPS on the serial port that i suggested would be an
intermediate step.  What you measured was the output of what I called
a NTPDXO decided down.   I think we are talking about the same kind of
design.  I guessed that the jitter on the final PPS would be 1000x
worse than from a GPS and your 8uS measurement is spot on that.   The
M12 has a handful of nS error

Chris Albertson
Redondo Beach, California