Fellow time-nuts, As I now have a hydrogen maser sitting here, it triggers me to measure things. Essentially I try to measure the stability of the maser, and well, that will be very hard since one need very quiet reference sources to do that, but that then triggers the question of how quiet are my sources anyway. So, I decided to start a measuring campaign to figure this out and to measure both ADEV stability and phase-noise stability, and with that build reference for the future of what tools to use for a particular measurement. It also has the side-effect that it forces me to get more of my lab in order.
Anyway, so as I measure, I find that I have problem getting good ADEV measure. I operate my TimePod such that I can do the three-corner hat. I see some disturbance in the ADEV with a re-occurrence of about 0.5 s and that part of the ADEV never converges to something useful even when doing a 48 h test. At the upper end, the BVAs I use for reference is clearly drift limiting, so I have to restort to Hadamard to see somewhat beyond the drift limit, but that is not helpful. So, I decide to setup phase-noise measurments and do that while the BVAs settles, as drift does not obstruct the phase-noise as much as ADEV. As I measure phase-noise of my 5065, OSA8600, OSA8601 and EFOS10 it becomes very clear that the EFOS10 does not have even the same shape of the phase noise as the other three, and being far worse than the reference BVAs despite the output oscillator is a OSA8600 and for higher frequencies this should completely dominate the phase-noise behaviors. It comes to the point where I decide I need to break a measurement and investigate this deeper. So, a tell-take of the phase-noise plot is that it has some strange spikes at about 0.1 Hz and 0.3 Hz. It then has ripples that eventually goes smooth as the filter-bandwidth of the phase-noise measurement increase, so it just becomes an almost smooth noise energy which slopes down. To put this in context, it is about 30 dB higher than expected when it is as worst. Surely, this is something other than the intended noise of the maser. It now becomes time to shift view again, remember we started with long-term stability in ADEV and HDEV and then shifted to phase-noise. So, I shift over to view the phase data, and in particular the residue phase as the linear phase-slope has been removed (press 'r' in the phase view of TimeLab). I then see regular phase-spikes at 10 s appart. I measure these to be in the range of 10 ps high. For a while I have my suspicions falsly on the PFD detector in the PLL, as I have a compassionate dislike/hate for them, but Bob and John talked me out of doing that hack right now. Better first collect more evidence. Then it hits me. Could it be... what happens in 10 s interval here, the only process going on with that type of rate is the logging of data from the maser. So, I pull the cable and start new measurements... and that is it. I now have ADEV operating smoothly in the region where it was clearly fighting, and I have a nice smooth 1/tau slope down into the abyss from the maser, clipping the 1E-13 line at tau=3 s at which time it is so quiet that as it progresses into the E-14 range of measures it becomes hard to measure using BVAs. Phase-noise measurement is a spitting image of the BVA I have, and those matches up with TvBs measurements of OSA8600. Now, I have already took the precaution to sprinkle the cables with common mode suppression chokes. I have not used any fancy power-supplies, but just a standard HP/Agilent bench supply for the BVAs, and they are being fed from the same supply even. The EFOS-B masers was shipped with optical isolators for RS232, and this may help for common mode and "ground loop" but not for the transitions themselves. I will have to investigate this further to figure out how the main action that makes the bursts of data on the serial port to appear in the measured 5 MHz, and then how to isolate it. Until then I now know I need to pull that cable for precision measurements, but that is not a satisfactory solution as I want that log-data as an intact series. I send this note, to also indicate the importance of monitoring the data in different views, that phase-noise measurement may be giving a significant clue as to what goes on, and then the phase (and/or frequency) view to further locate it. Amplitude noise and pure spectrum analysis may have been additional methods to also consider, just to illustrate that one needs a wide selection of tools to locate the root cause of a disturbance and mitigate it. I've heard people say "I'm only interested in stability", well, that may be the end goal of the measurement, but to get there and to improve the setup other measurement discipilnes may be necessary. Thanks goes to Bob Camp and John Miles as discussion partners, and to Ole Petter Rønningen who helped me a lot with the EFOS10 maser. I will continue my measurement campaign, and see if there is more things to clean up. I will also investigate more on the serial link. My initial measurement campaign is to just establish roughly the noise of various sources and for various measurements, in order to be prepared for a second round allowing for more qualitative measurements with the best sources. This also raises the awareness of those sources having issues, so that I will for instance work harder to look at my 5065s, put them all into operational state and see what is the main limitations in their performance. The main point being, it's fun to be back in a more functional lab again. :) Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.