Thanks for these useful comments! It seems there is no way to estimate frequency from looking at the phase data then? Does TimeLab have automated collection for the frequency-counter data on the control port?
Anders On Fri, Dec 11, 2015 at 2:51 AM, John Miles <[email protected]> wrote: > > Hi all, > > > > A 5115A phase noise test set landed in our lab and I am wondering about > the > > data collection. > > It has two telnet ports one for commands and one for phase data. > > When issuing "start" on the command-port it starts spitting out > > phase-difference values on the data port. > > > > However it seems to me that the phase data alone (REF-DUT phase, in units > > of the REF period) is almost useless without knowing the internal > workings > > of the device. > > The phase values are clearly not the true phase difference, which would > > quickly accumulate to a huge number with e.g. REF=10MHz and DUT=11MHz. > > My > > understanding is that digital downconversion (DDC) is performed on both > REF > > and DUT signals, and without knowing the LO frequencies for the DDC the > raw > > phase data alone is almost useless. > > > > The command prompt does have commands for displaying the internally > > calculated ADEV, L(f), frequency-counter readings, etc. and these seem to > > show correct and OK values, but there seems to be no way to reproduce > e.g. > > the ADEV or frequency-counter values/statistics from the raw phase values > > on the data port?? > > Is this correct? Any other experiences with the 5115A or higher end > 5120A? > > The shorter-term ADEV values on the 5120A/5125A test sets are > mathematically backed out of the phase noise data. They will never > perfectly match the ADEV values you get from plotting the phase data stream > in my experience. > > I don't actually know what they do on the 5115A, though -- since it > doesn't do cross-correlated PN, there's presumably no reason to derive the > short-term ADEV plot from the PN data pipeline. I'd expect the plots to > match in that case, apart from any errors due to different measurement > bandwidths and ADEV bin distributions. > > It's also true that there will always be an arbitrary phase slope error > due to the mismatch between the frequency estimate used to tune the > internal DDCs and the "real" frequency of the incoming data. Knowledge of > the actual DDC core frequencies would not be enough to correct for this > behavior, because you would also need to know how far off they are. The > true frequency offset can't be measured ahead of time with perfect > certainty, so it has to be estimated, and the resulting error will often > dominate the slope of the phase-difference graph. > > The TimePod and 3120A test sets allow you to specify the input and > reference frequencies explicitly to obtain a valid phase slope in residual > measurements and other cases where the exact frequencies are known by the > user. But this is something you have to remember to do prior to the > measurement, and you still don't get a calibrated absolute offset. > > (Also, note that TimeLab can read both phase and PN data from 51xx test > sets without the need to use a Telnet client. Not that it helps with this > particular issue, of course.) > > -- john, KE5FX > Miles Design LLC > > > _______________________________________________ > 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.
