Hi Wulf, On 05/18/2016 08:57 AM, dg...@gmx.de wrote:
This is my first contribution to the Time-Nut list, but I am a follower for several years. I have made 30 days phase difference measurements of an HP5065a versus a modified Thunderbolt GPSDO (got a MV180 oscillator) with a Fluke PM6681, and recorded the results with Timelap. Starting signal of the PM6681 was the 5MHz of the HP5065A, stop signal the 10MHz from the Thunderbolt. A single measurement was triggered once a second from the PPS of the TB. I also have a weather station at 6 km distance away, which records barometric pressure at time intervals of 10 minutes. Out from these measurements, I divided my phase measurements into 10 minute intervals, calculated the 10 min average frequencies and made a linear regression with the 10 minute pressure measurements. I got a slope of 3.86E-14/hPa frequency difference versus pressure.
Interesting!
Then I corrected the original 10 min average frequency measurements with the result from the linear regression and imported the original and the corrected datasets into Timelap. The influence of the barometric pressure to the ADEV for long time intervals is shown in the picture. Just for fun, to render the pressure sensitivity visible for the human eye, I removed the noise of the GPSDO by generating a plot of overlapping 1 day frequency averages in 10 minute time intervals. On the second picture you can compare it with the 10minute pressure data from the weather station. I think, it is quite impressive. The calculated slope out of these overlapping averages was with 3.74e-14/hPa a little bit different compared with the non overlapping dataset.
Now, after the fact fitting of curve has a drawback in that you can make the fit as really good but there you measure the reference, hence you have a closed loop. The challenge now is to use a feed-forward scheme to take the measures from the pressure and use to adjust the C-field of the 5065 and then measure against the Thunderbolt. I would guess it won't be exactly the same, not as good, but it will be then the actual performance.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
