[time-nuts] TIC resolution impact on GPSDO's performance

Magnus Danielson cfmd at bredband.net
Tue Dec 26 05:13:30 EST 2006


From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] TIC resolution impact on GPSDO's performance
Date: Tue, 26 Dec 2006 08:14:12 +0000
Message-ID: <36958.1167120852 at critter.freebsd.dk>

> In message <4590BB56.5070809 at xtra.co.nz>, Dr Bruce Griffiths writes:
> >Poul-Henning Kamp wrote:
> >
> >> I think what you are trying to express is that the frequency from
> >> the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
> >> gives rise to hanging bridges.
> >>
> >>   
> >Poul-Henning
> >
> >Surely you mean harmonic? An overtone is not necessarily a harmonic.
> 
> I meant overtone, becuase they are not in any harmonic relationship,
> the XO is wandering around whereas the (ideal) PPS is, supposedly,
> rock stable.

The only real way to describe it is to say that the XO and the PPS is
asynchronous to each other. Those who has freshen up on their greek for
technicians will know that this means "not the same clock" which is quite
accuratly what we have. ITU-T Rec. G.700 has a few interesting sections in it.

The use of words such as harmonic or overtone should not be used since they
is normally used to describe the various frequencies within one signal where
as words as synchronous and asynchronous is to be used mainly for pair of
signals and describes their relative timing. Naturally, for a pair of signals
to be synchronous does not require them to have the same frequency, just a
fixed ratio between those frequencies.

It does happends that a single signal is said to be synchronous, but the
wording which should have been used is isochronous (same clock). Then we have
the lovely words of plesiochronous and mesochronous. :)

Cheers,
Magnus




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