[time-nuts] Predicting clock stability fromthevariouscharacterization methods

Ulrich Bangert df6jb at ulrich-bangert.de
Fri Dec 1 12:34:17 UTC 2006


Hi folks,

> (4) With four points you get change in drift over time.
> The standard deviation of the drift prediction errors is
> called the Hadamard Deviation.

you know that i like to advertise from time to time for it: 

My PLOTTER utility does compute the normal as well as the overlapping
Hadamard deviation. It may be downloaded from

http://www.ulrich-bangert.de/plotter.zip

Cheers
Ulrich, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Van Baak
> Gesendet: Donnerstag, 30. November 2006 19:26
> An: Mike Feher; 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] Predicting clock stability 
> fromthevariouscharacterization methods
> 
> 
> > Tom -
> >
> > Excellent description of the process. Glad you took the time to 
> > explain
> this
> > so clearly. While I do understand the process, I do not believe I 
> > could
> have
> > stated it so well. Not to nit pick, but you did make a 
> small typo in 
> > that you interchanged the predicted and measured value of 
> P2 in your 
> > example.
> For
> > most of us that will be obvious, and non relevant, but, to 
> some it may 
> > be confusing. Regards - Mike
> 
> Ah, right. In the example, the prediction, P2', should
> be 32 and the actual, P2, is 35; a prediction error of
> 3 us. Thanks.
> 
> ----
> 
> By the way, here's extra credit for some of you:
> 
> (1) With one point you get phase, or time error.
> 
> (2) With two points you get change in phase over time,
> or frequency.
> 
> (3) With three points you get change in frequency over
> time, or drift. The standard deviation of the frequency 
> prediction errors is called the Allan Deviation.
> 
> This is a measure of frequency stability; the better the 
> predicted frequency matches the actual frequency the lower 
> the errors. A little bit of noise or any drift causes the 
> errors to increase; the ADEV to increase. In the summation 
> you'll see terms like P2 - 2*P1 + P0. You can see why 
> constant phase offset or frequency offset doesn't affect the sum.
> 
> (4) With four points you get change in drift over time.
> The standard deviation of the drift prediction errors is
> called the Hadamard Deviation.
> 
> This is a measure of stability where even drift, as long
> as it's constant, is not a bad thing. In the summation
> you'll see P3 - 3*P2 + 3*P1 - P0. You can see why
> constant phase, frequency, or even drift doesn't affect
> the sum.
> 
> ----
> 
> So imagine a situation where you're making a GPSDO
> and very long-term holdover performance is a key design 
> feature. What OCXO spec is important?
> 
> In this application phase error is easy to fix - you just
> reset the epoch.
> 
> Frequency error is easy to fix. After some minutes or
> perhaps hours you get a good idea of the frequency
> offset. You then just set the EFC DAC to a calculated
> value and maintain it during hold-over. In this case the
> OCXO with the lowest drift rate (best Allan Deviation)
> is the one to choose.
> 
> But with a little programming even drift is also easy to
> fix. After some days or perhaps weeks you get a pretty
> good idea of frequency drift over time and so you ramp
> the EFC DAC over time to compensate.
> 
> The only limitation to extended hold-over performance
> in such a GPDO is irregularity in drift rate.
> 
> In this example, the Hadamard Deviation would be a
> good statistic to use to qualify the OCXO you need.
> Drift, as long as it's constant (e.g., fixed, linear, even
> log, or other prediction model) is not the limitation.
> 
> /tvb
> 
> 
> 
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com 
> https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
> 





More information about the Time-nuts_lists.febo.com mailing list