[time-nuts] Testing frequency using NTP

[Ulrich Bangert]

Steve,

> Hmmm... If I measured a 10MHz oscillator for a 1/10 second, I 
> could achieve, at best, 1ppm accuracy. Now my measuring 
> system has a non accumulating error in the ms range, say <1s, 
> so this would be totally unworkable. If I sampled for 1s, 
> best would be .1ppm accuracy...

this is a common misbelieve but simply not true. You need not buy the
most advanced stuff. Even a moderate surplus counter like the RACAL DANA
1991/1992/1996 will get you < 1ppb (b, not m!) resolution @ 1 s. Since
more than 30 years counters do no more simply "count" but work with lots
clever electronic tricks inside!

73s de Ulrich, DF6JB 


> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Steve Rooke
> Gesendet: Donnerstag, 2. Oktober 2008 07:37
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Testing frequency using NTP
> 
> 
> Hmmm... If I measured a 10MHz oscillator for a 1/10 second, I 
> could achieve, at best, 1ppm accuracy. Now my measuring 
> system has a non accumulating error in the ms range, say <1s, 
> so this would be totally unworkable. If I sampled for 1s, 
> best would be .1ppm accuracy, but my measuring errors would 
> still swamp the result. To make my measuring errors small, I 
> could make the 1s overhead very small compared to the measing 
> time period by, say, sampling for 10^6s thereby making the 1s 
> error 1ppm. Now the 1s error is probably considerably smaller 
> than that, probably by the order of at least a decade or two. 
> If it was just .1s error, I could get a result to 1ppm in 
> about a day and better if I sample of a number of days.
> 
> Cheers,
> Steve
> 
> 2008/10/2 Scott McGrath <scmcgrath at gmail.com>:
> > It depends on how accurately you want to measure the oscillator 
> > frequency with your approach short term you probably would 
> not be able 
> > to measure the oscillator offset any better than a few 
> parts in 10-5 
> > longer term probably a few parts in 10-7 might be possible as you 
> > could compute the allen deviation and fit a curve through 
> the median 
> > values.
> >
> > NTP from a stratum 3 clock is only going to be precise to a few 
> > milliseconds and for meaningful accuracy you need another order of
> > magnitude.   This is part of the function of the drift file in xntpd
> > in which the daemon attempts to compensate for the drift and offset 
> > inherent in cheap oscillators used in computer applications.
> >
> >
> > - Fellow nuts am I all wet here or have I missed a technique
> 
> -- 
> Steve Rooke - ZL3TUV & G8KVD
> Omnium finis imminet
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
> and 
> follow the instructions there.
>