[ntp:questions] Home Time Server
David L. Mills
mills at udel.edu
Sun Oct 14 04:59:31 UTC 2007
I don't think so. The WWVB receivers I have mix to baseband, not a lower
IF. The image issue you note is not a problem with these receivers. If
the DCF77 receivers mix to 625 Hz, they had beetter have very good image
rejection. That would require a very narrow passband consistent with a
crystal filter. Having said that, this has nothing to do with EMI inband
susceptability at 60 kHz.
Rob van der Putten wrote:
> 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.
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