[ntp:questions] Frequency adjustments in a local network

David Woolley david at ex.djwhome.demon.invalid
Mon Apr 7 13:06:27 UTC 2014

On 07/04/14 10:43, Maximilian Brehm wrote:
> David Woolley schrieb:
>> On 04/04/14 12:44, Maximilian Brehm wrote:
>>> Are there ways to achieve this using NTP that are implemented
>>>> or may be implemented? If yes, can you recommend me starting points for
>>>> either way?
>> I guess you could treat the reference system as a PPS source, but
>> there is still the problem that PPS assumes that the pulses are on the
>> absolute time seconds not at a variable and drifting offset from them.
> Is a PPS signal not supplied via a hardware link? How would I simulate
> the signal using the received timestamps?

No.  It is supplied by a software driver that gets it from a hardware link.

> I believe I read somewhere that there are PPS systems that do not
> supply a timestamp and an external NTP server has to be used to
> synchronize time. Couldn't the implemention just ignore the timestamps?

As far as I know, the timestamps are processed independent of the PPS feed.

Where you will have difficulty is that there is an assumption that there 
is some contant relationship between the timing of the timestamps and 
then PPS.  This often has quite large jitter, but the two don't drift 
relative to each other.  If you have a free running PPS source, that 
will not be true and ntpd will get confused once the drift exceeds a 
certain fraction of a second.  Also, the PPS signals will define exact 
seconds, so your driver will have to calibrate itself each time it is 
started, and you will have problems once the drift makes that 
calibration unreliable.

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