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 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. 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? 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.
