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

Ulrich Bangert df6jb at ulrich-bangert.de
Tue Dec 26 07:02:55 EST 2006


Poul,
 
> The field in the oncores serial data-stream called negative 
> sawtooth correction must be taken into account to get optimum 
> performance out of the receiver.  Ignoring it in toto or 
> simply averaging it will generally not do for OCXO disciplining.

thank you for your clear words! While my posting has produced a lot of
different points to discuss over, this is basically what I had expected
from a experienced person. I hope you needn't cross your fingers while
writing it down!

Best regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> Gesendet: Dienstag, 26. Dezember 2006 03:02
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] TIC resolution impact on GPSDO's performance
> 
> 
> In message <45906C9A.806 at xtra.co.nz>, Dr Bruce Griffiths writes:
> 
> >Poul-Henning
> >
> >You are using a different meaning for coherence than the 
> standard one. 
> >The standard meaning is something like: Two signals or waves 
> are said 
> >to be */coherent/ *if their behaviour at various 
> times/places is linked 
> >in a deterministic way.
> 
> The term coherence originates in optics and was briefly used 
> in radiotelegraphy and is no almost exclusively used in the 
> context of lasers and waveguides.
> 
> When two signals are coherent, you can predict one from the other.
> 
> We are dealing with three signals here:
> 
> 	GPS_PPS:	the 1 Hz signal as the Oncore wants to do it.
> 
> 	HW_PPS:		the 1 Hz signal as the Oncore can do it.
> 
> 	NEG_SAW:	(GPS_PPS - HW_PPS)
> 
> None of the above three signals are coherent, you need two 
> out of three to predict the last one.
> 
> And since the first one is a virtual signal, not available 
> except as the sum of the other two, talking about coherence 
> is especially pointless.
> 
> 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.
> 
> And to conclude this by now surely far too tiresome debate 
> before both of us gets booted from the list:
> 
> The field in the oncores serial data-stream called negative 
> sawtooth correction must be taken into account to get optimum 
> performance out of the receiver.  Ignoring it in toto or 
> simply averaging it will generally not do for OCXO disciplining.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by 
> incompetence.
> 
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