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: [email protected] [mailto:[email protected]] On >Behalf Of Mark Spencer >Sent: Monday, March 26, 2012 5:07 PM >To: [email protected] >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 >_______________________________________________ >time-nuts mailing list -- [email protected] >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. > > >_______________________________________________ >time-nuts mailing list -- [email protected] >To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
