Hi Chris, On 10/15/18 4:07 AM, Chris Burford wrote: > Hello All, > I've collected several data sets over the last few days from my RFS > and I'm puzzled by what I'm seeing. A link to the plot is here: > https://drive.google.com/open?id=10Kk8Cqx7es0SXf2H5AldiQkBo8nIfNMT > I'm looking at the averaging periods from about 100 thru 400 tau and > noticed a "plateau" or a brief level off period and then it proceeds > down and right again. Is this flat area a function of the averaging > algorithms for the MDEV or is it something else?
Actually, that is where you start to see real stability data. Look to the left of the plot, all 4 traces have very close traces from tau 1 s to about tau of 100 s. This is becuase of your measurement instrument noise, which falls with 1/tau^1.5 with MDEV and 1/tau with ADEV. This is just the measurement resolution obscuring your real device performance. It is only as this becomes lower than the actual DUT noise that you see the actual DUT noise. The cause of the plataue then becomes somewhat obscured as you don't see the full picture. Another aspect is that the traces is on the short side for confidence intervals of the measurements have gone down, as the truncated series in itself can cause variations of the estimated value around the actual value. One effect that can create similar behavior is a systematic signal either as a side-band or as a modulation such as a sine that cause a additive effect on top of the noise performance. This type of systematic disturbance may be an effect in the measurement setup or in the device, so care must be take to locate and eliminate it as it does not represent the systematic noise of the source. There is a few other effects that can cause similar behavior. It is recommended to monitor the phase deviation data plot. Whenever I see deviations like that I ask for the phase plot, which typically also get the frequency compenated out and only look at the residues of that prediction. It may also be convenient to look at the FFT of the data. ADEV and MDEV is a poor way to estimate stability that has systematic or semi-systematic behaviours, it's only intended for truely random noise of the noise-slope types. Other noise-types should be identified by other means and compensated out prior to ADEV/MDEV processing. > I have spent considerable time in reading "Techniques for Frequency > Stability Analysis" by W.J. Riley and a host of other publications > (NIST, etc) in preparation for this. I would enjoy hearing from any > readers that may have some insight as to what I'm seeing and or > possible causes. > Thanks for reading. While that is a good reading, it doesn't go in depth onto how to interpret the plots properly. Some have been written by David Howe as found in NIST T&F publication list. Cheers, Magnus > Chris > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
