[time-nuts] Droitwich Frequency Error

[Bob kb8tq]

Hi

> On Oct 18, 2020, at 10:07 AM, Andy Talbot <andy.g4jnt at gmail.com> 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
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