[time-nuts] frequency stabilty question

shalimr9 at gmail.com shalimr9 at gmail.com
Mon Aug 15 14:44:04 UTC 2011

Is it correct to assume that if you collect data every 10 ms for 10 seconds (1000 data points in total), you could actually split that data set into 100 sets of data at 1 seconds for 10 seconds (10 data points in each set), which would have the same statistical quality (I am sure caveats would apply) as a single data set of 1000 data points collected at one second interval over a total period of 1,000 seconds?

Didier KO4BB

Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: Jim Lux <jimlux at earthlink.net>
Sender: time-nuts-bounces at febo.com
Date: Mon, 15 Aug 2011 06:31:21
To: <time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts at febo.com>
Subject: 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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