On 18.10.2020 16.07, Andy Talbot wrote: > > 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. >
The stability of UK Droitwich has had its good times and its bad times. In 2008 I did a similar measurement, and got behaviour like this (from Silkeborg, Denmark): https://n1.taur.dk/timenuts/droit.png https://n1.taur.dk/timenuts/droit2a.png (apology for unlabeled axes, Y is offset in seconds, X i sampletime in seconds) Compared to those, your plot is very well behaved, and I recall talking to a retired BBC engineer who had the company they had outsourced the operation to in rather low regard. So performance has improved greatly. The phase change of about 20 degrees is around 280ns - that's really good. The 680km to DCF77 goes up and down by about a microsecond over the same period. On the other hand, unless your antenna and amplifiers are really broadband, the phase change could come from temperature drift in your antenna/filers/amplifiers/.. The way you test for that is to build a little test transmitter - maybe another DDS clocked from the same reference. Considering that you are comparing an error in microseconds to the GPSDO, unless the GPSDO is unlocked or broken, the error from it will not be significant. (this is also where you realise that you need at least two more clocks:-) Careful, building coherent VLF receivers is a gateway drug. /Kasper Pedersen OZ2KKP _______________________________________________ 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.
