[time-nuts] Basic question re adev measurements

[Mark Spencer]

Thanks that's helpfull.  What happened was one of my first measurement devices and the software I was using didn't handle phase wraps properly so I took pains to limit the frequency off set so I could gather data for 40,000 seconds or more without phase wraps.   Now I have equipment and software that can handle phase wraps so I haven't paid as much attention to the frequency off set.   From a noise floor perspective the newer setup seems to be as good or better than the old one but I suspect there is another source of error that I am missing.  After a few runs at lower frequency off sets with the newer equipment the adev numbers seemed to go down but it is likely a co incidence.  I'll have to see what other error sources I can find.  Something to keep me busy in my non work time. 

Regards
Mark Spencer


------------------------------
On Mon, 26 Mar, 2012 5:26 PM EDT Bob Camp wrote:

>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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