It's a bit worse than that. The DUT and reference phase difference can't be allowed to exceed 50 ns per trigger interval, in the case where two ~10 MHz signals are being measured. If the frequencies disagree enough to create a phase slope greater than that -- meaning if they are more than 5E-8 * 10E6 = 0.5 Hz apart if you are triggering once per second -- your TI counts will be aliased.
You won't be able to unwrap the phase properly if your software isn't aware that the data is coming from a different Nyquist zone, so to speak. I'd expect some odd looking ADEV plots in that case. -- john > -----Original Message----- > From: [email protected] [mailto:time-nuts- > [email protected]] On Behalf Of Mark Spencer > Sent: Monday, March 26, 2012 2:08 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.
