[time-nuts] [?? Probable Spam] Re: frequency stabilty question

[Ulrich Bangert]

Jim,

pardon to correct you but

> It's if you measured the frequency (instantaneously) at one second 
> intervals, and calculated the standard deviation, that would 
> be the ADEV for tau=1 second.

is simply wrong in at least two aspects. 

First: The measurements need to be the AVERAGE frequency measured over a time interval of one second, which most easily is done by setting the counter's gate time to one second. 

Note that the misunderstanding about the difference between an instantaneous measurement of frequency (which is possible too, but not with counter based methods) and an averaging measurement of frequency over a given time interval was one of the sources of the upsetting discussion between Bruce and WarrenS in this group about the tight PLL method. For that reason I think we need to be very carefull in not to use terms as "instantaneous" in the wrong sense.

Second: It may be due to that you wanted to explain something as easy as possible but I hope that it is clear to you that Allan's fame relates to the fact that he found out that the standard deviation is exactly the WRONG tool to use in this case and that he needed to formulate a new species of deviation that is today called after him.

Regards
Ulrich Bangert



> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Jim Lux
> Gesendet: Montag, 15. August 2011 15:31
> An: time-nuts at febo.com
> Betreff: [?? Probable Spam] Re: [time-nuts] frequency 
> stabilty question
> 
> 
> On 8/14/11 8:10 PM, Paul Cianciolo wrote:
> > Folks,
> >
> > I amtrying to understand some of the terms used here quite often. I 
> > quoted this from Wikipedia
> >
> > An Allan deviation of 1.3×10−9 at observation time 1 s 
> (i.e. τ = 1 s) 
> > should be interpreted as there being an instability in frequency 
> > between two observations a second apart with a relative root mean 
> > square(RMS) value of 1.3×10−9.
> >
> > Does this mean the observations made were at the very 
> begining and the very end of the 1 second time.
> > If so what value about all the values in between?   What 
> happens if the oscillator deviated far worse than this during 
> the interrim.
> >
> 
> A measurement at tau=1 second doesn't say anything about what 
> happened 
> at shorter intervals.
> 
> >
> > Or does the measurement consist of making  measurements every cycle 
> > during that 1 second and then entering all those values 
> into a formula 
> > that accounts for them all??
> >
> 
> Nope..
> It's if you measured the frequency (instantaneously) at one second 
> intervals, and calculated the standard deviation, that would 
> be the ADEV 
> for tau=1 second.
> 
> measure at 10 second intervals and you get ADEV(Tau=10sec), etc.
> 
> 
> That's why you typically see an ADEV as a series of performances at 
> different taus.
> 
> You can also fill in the gaps in the curve, to a certain 
> extent, because 
> physical oscillators have constraints on what the ADEV can do (i.e. 
> you're not likely to see 1E-9 at tau=1 second, 1E-5 at tau=2 seconds, 
> 1E-10 at tau=3 seconds)
> 
> In fact, if you do the ADEV measurement and it's NOT a nice curve and 
> has spikes and weirdnesses, that starts to tell you have either a 
> measurement system problem or a problem with your frequency standard.
> 
> (sort of like spurs in a phase noise plot from 120Hz line 
> interference, 
> or reference clock leakage)
> 
> As an example of measurement system problems, it's pretty 
> common to see 
> a "hump" in ADEV around 500-1500 seconds (around 15-20 
> minutes) because 
> of temperature effects on the test equipment or unit under 
> test as the 
> airconditioning/heating cycles on and off.
> 
> 
> 
> 
> 
> > Maybe a very basic tutorial on this topic would help but I 
> cant find 
> > one
> >
> >
> > Signed very confused,
> > Thank You
> >
> > PauLC
> > W1VLF
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