[ntp:questions] Home Time Server
Rob van der Putten
rob at sput.nl
Sat Oct 13 18:41:37 UTC 2007
David L. Mills wrote:
> Not to get into a pissing contest, but both WWVB and the European LW
> services operate at similar frequencies and ERPs European services
> operate at higher latitudes where noise levels are lower, while WWVB
> operates at lower latitudes and generally longer ranges. I've lived in
> Europe and can attest the differences.
> WWVB has claimed carrier accuracy of 1e-11 and has carrier phase
> ambiguity of 16 microseconds. Electronics-wise, a narrowband crystal
> filter ahead of a phase-lock loop with a time constant of a couple
> minutes pins the zero crossings well below one microsecond. While the
> modulation bandwidth is quite narrow, finding the right on-time cycle
> crossing with 5 ms risetime (30 cycles) and a comb filter isn't hard.
> Thats how the WWV audio demodulator works.
> The Spectracom WWVB radios we have used to do an excellent job; however,
> the claimed accuracy was only 0.1 ms, mostly because the computer used
> for demodulation was so chintzy. Sadly, WWVB is dead here because of EMI
> generated by hundres of UPS systems all over campus. I'd be curious to
> know if this has also affected DCF-77 users.
AFAIK DCF-77 runs at 77.5 kHz and a lot of receivers mix this down to
625 Hz; This means a mirror at 76.875 kHz. Interference sources will
therefore be at 38.75 and 38.44 kHz (2nd harmonic).
Switching stuff often runs at 25, 30 or 33 kHz. So a frequency of 60 kHz
(WWVB) will have more interference.
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