[time-nuts] ADEV

jimlux jimlux at earthlink.net
Sat Nov 13 15:18:27 UTC 2010


Magnus Danielson wrote:
> Hi Bob,
> 
<snip>

> I think the system applications for short-term stability measurements 
> was quite clear, and was brought out specifically. 

Basically, these are all systems where there is some "storage" or 
"delay" and you are comparing a signal generated/recorded at some time t 
with that signal at some time t+delta, where delta is in the seconds to 
minutes range.  (well, milliseconds in the radar case)...

(the really slow speed comm application is slightly different.. there, 
it's more of a reciprocal mixing of close in noise and a frequency 
stability in the hours/days sense issue.. although for one-way doppler 
measurements ADEV is important)

> 
> ADEV addresses the oscillator noise issues, but isn't particularly well 
> suited for the numerous of systematic effects that comes on top of the 
> oscillator noise in the system.

yes. but some familiarity with how ADEV is measured/computed, and the 
fact that there are "test sets" out there that make a plot of ADEV means 
that you can look at a ADEV plot for the system, and if it deviates from 
that for the underlying oscillators, you can make some educated guesses 
about the system issues.  Not that it's best, by any means, but at least 
it exists.

When buying or building a complex system, one is often faced with the 
problem of "how do we write a testable specification or requirement" to 
show that the completed widget works.  And, further, you generally don't 
want to spend more developing the test than the thing you are buying. 
It does happen, though..probably part of the game for developing at the 
ragged edge of performance.  However, if you can sort of back your way 
into an analysis that shows that a performance of X on test Y (no matter 
how unsuited philosophicaly) means that you will get performance Z on 
important requirement Q, then all is good.

At work, we refer to the developing of a whole subproject to understand 
and test how the primary object is working as "doing a science 
project".. but that's because I work in an "engineering" area as opposed 
to one of the "science" areas, where science projects are their lives. 
By the time we get it, we're on a schedule and budget that doesn't allow 
for much contemplation, reflection, and thinking about how to best do 
things: planetary motion waits for no man, and the launch period is a 
few weeks long, at best.


> 




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