[ntp:questions] Home Time Server

David L. Mills mills at udel.edu
Sun Oct 14 04:59:31 UTC 2007


Rob,

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.

Dave

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.
> 
> 
> Regards,
> Rob




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