[time-nuts] GPS orthodontics: sawteeth & hangingbridges-theeffect of time averaging

Ulrich Bangert df6jb at ulrich-bangert.de
Sat Dec 23 09:20:01 EST 2006


Tom,

as I did say in my first sentence I would have been glad to make
measurements against a H2 Maser or a Cesium. Unfortunately I don't own
one of them. 

But then let us consider what is second best: A xtal oscillator alone or
a Rb alone won't do the job because up from a certain Tau we would
measure merely the Allan Deviation of the oscillator and not the
receiver's one.

If we take a 1pps derived from a GPSDO we can be sure that the Allan
Deviation below the loop's time constant is defined by the OCXO alone
(being smaller than that of the receiver). Above the loop's time
constant the Allan Deviation will be defined by the receiver.

Conclusion: If we use a 1pps derived from a GPSDO the Allan Plot will be
completely correct for taus < the loop time constant and will display
errors up to [(squareroot(2)-1)*computed Allan] for taus > the loop time
constant which is not so bad when you consider that you can make the
experiment on your kitchen table. I guess that explains the small
difference against the USNO measurements. If the USNO made the same
measurement and came to an result close to mine I feel you are paying me
a compliment with that information. 

Now, if you think about it a few seconds: Comparing a 1pps derived from
a GPSDO to a 1pps from a receiver IS EXACTLY what is done inside the
closed loop. Taking the above mentioned restrictions into account taking
data from the closed loop is not THAT wrong!

Best regards and a Merry Christmas
Ulrich Bangert 

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Van Baak
> Gesendet: Samstag, 23. Dezember 2006 04:57
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] GPS orthodontics: sawteeth & 
> hangingbridges-theeffect of time averaging
> 
> 
> > Brooke
> > 
> > I'm not convinced that one can actually directly derive the 
> GPS timing 
> > receiver stability from measurements taken within a feedback loop 
> > where the OCXO is locked to the GPS timing receiver output. 
> Surely one 
> > has to
> 
> Yes, I'm nervous about this approach too.
> 
> > correct for the loop transfer function. As tau approaches the loop 
> > time constant the accuracy of the stability measure as 
> calculated from 
> > the uncorrected phase errors is degraded. Correcting for 
> the effect of 
> > the loop transfer function will improve the accuracy a bit. 
> The only 
> > way to measure the receiver timing stability is surely to 
> measure it 
> > against a very accurate and stable standard was done when 
> testing the 
> > M12+ receiver at USNO. When the outliers are rejected these tests 
> > indicate that the sawtooth one day stability is around 
> 5E-14 or so, ie 
> > somewhat better than Ulrichs plots show.
> > 
> > Bruce
> 
> 5E-14 is exactly the right number. Have a look:
> 
> A Comparison Between a 58503B and a CNS-II 
> http://www.leapsecond.com/pages/58503-cns2/
> 
> /tvb
> 
> 
> 
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