[time-nuts] Basic question re adev measurements

[Bob Camp]

Hi

My first guess is that your limiter is (for what ever reason) doing better
at say 1 Hz offset than it is at 10 Hz. Second guess would be that the
higher offset is closer to a noise source / spur in your lab. Past that, you
get into a lot of "that depends" things. If you are doing cross correlation
then you may be better correlated at the lower offset....

To directly answer the question - no there nothing about a lower offset that
by it's self should improve ADEV, provided the effective measurement
bandwidth is not changed. 

If you change the measurement bandwidth then lower bandwidth means less
noise. Since a lot of "modern" gizmos do a post filter on the data, you do
see data plots done that way. Simple answer is that it mostly impacts the
shortest Tau for "normal" sources.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Mark Spencer
Sent: Monday, March 26, 2012 5:07 PM
To: time-nuts at febo.com
Subject: [time-nuts] Basic question re adev measurements

Greetings,  I was reviewing some older adev plots of mine and noticed that
there may be a correlation between lower adev numbers and lower frequency
off set between the reference source and the device under test.   It's my
understanding that the adev calculations remove constant frequency off sets
but I'm wondering in practice this degrades the the measurements. 

It occurs to me that if I am comparing two 10 MHz signals with a TIC that
the available dynamic range of each measurements will be 100 ns.   Would a
constant frequency off set effectively reduce the precision of the
measurements by eating up some of this dynamic range ?

To put this in perspective frequency offsets of say one or two parts per
trillion seem to result in better adev readings than off sets of say ten or
more parts per trillion.  

Sorry if I have missed something obvious here.

Thanks in advance
Mark Spencer

Sent from my iPod
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