[time-nuts] Droitwich Frequency Error

[Andy Talbot]

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