[ntp:questions] Home Time Server
David L. Mills
mills at udel.edu
Thu Oct 11 00:26:38 UTC 2007
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.
Terje Mathisen wrote:
> David L. Mills wrote:
>> In regards to how the clocks work, all the above WWV clocks use the
>> munutes and seconds pulses as well as he 100-Hz subcarrier modulation.
>> The PSTI/Traconex clock uses a comb filter for the seconds pulse and
>> brute force for the subcarrier demondulation. The NTP WWV driver is a
>> mathematically optimum linear receiver with a ton of DSP algorithms.
>> I have no experience with European and Japanese broacasts but assume
>> the DCF-77, MSF, HBG and JJY services are equivalent to WWVB. It looks
> DCF-77 is not equivalent at all, it is in fact much better! :-)
> With the right gear, you can sync to the gps-like pseudorandom
> modulation of the DCF-77 carrier phase, this allows you to determine
> pulse transitions at approx 15-30 us level.
> You still have to correct for transmission delay, which means that such
> a fancy receiver is only useful when you're close enough to the
> transmitter that the direct ground wave dominates the received signal.
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