[ntp:questions] Home Time Server

Rob van der Putten rob at sput.nl
Sat Oct 13 18:41:37 UTC 2007


Hi there


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.


Regards,
Rob
-- 
Lifestyle sux




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