Hi > On Oct 18, 2020, at 10:07 AM, Andy Talbot <[email protected]> wrote: > > Hi Folks > I've just joined this list, but have had an interest in accurate frequency > measurement for decades, ever since finding a fully operational HP5061A in > skip with what appears to be plenty of Cs life left in the tube. It only > gets turned on when needed here. Anyway : > > This morning, mainly to prove something it to myself I made a plot of the > off-air UK Droitwich LF transmitter whose carrier is supposed to be a > national standard - although I believe it uses a Rb source that is > periodically updated with a Cs one - manually. > > I have a Leo-Bodnar GPSDO had been turned on only some 20 minutes before > the plot was started, as teh reference into a custom own-design LF receiver > that gives a digitised baseband I/Q output to a PC. Its LO is tuned using > an AD9852 48 bit DDS clocked at 10MHz and set by an algorithm that allows > a micro-Hz but completely deterministic tuning error - it was this error I > was trying to show by doing a plot of Droitwich off air using the GPSDO as > a reference. > > The plot at http://www.g4jnt.com/DropF/clipboard_202010181333.png shows the > result - the phase is the red line. My calculated tuning error at this > frequency is 3uHz, but there is a slope to the line showing around 20 > degrees of phase shift in a little under 2 hours. This corresponds to > about 7.8uHz error. If my tuning error contributes 3uHz of this, that's > still 4.8uHz on 198kHz , or 0.024 PPB. It was well after sun rise, and > the transmitter is only 100km away from me, so oughtn't to expect > propagation anomalies
It is far enough to see transitions ….. > > My question, does anyone know if a GPSDO, has any inherent drift over its > first hour or so? I wouldn't have thought there was any mechanism for > that, but who knows. Typically GPSDO’s are only rated for accuracy after 24 hours on power. Many of them continue to improve ( = tweak their filtering) for weeks after turn on. They are best left on full time / connected to a good antenna / with a good sky view. > > Finally, I guess 2.4E-11 isn't really outrageous for a rubidium source, > even one that is 'supposed' to be corrected from 'time-to-'time' Or Is it? A lot depends on how well the temperature of the device is controlled … If the Rb is rated 2x10^-10 over 0 to 70 and has the typical “snake path” compensation curve, you could easily see 2 or 3 x10^-11 for a 3C delta. Bob > > > Andy > www.g4jnt.com > _______________________________________________ > 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.
