[time-nuts] KD2BD WWVB receiver/decoder in QEX

[Bob Camp]

Hi

The most basic gotcha with WWVB is that propagation can (and does) shift the 
carrier more than a full cycle over the course of a day. That’s at 60 KHz, so one 
cycle is a lot (as in 16.666 ppm). Even at one second with a not so great receiver
and a poor antenna, GPS should give you ~0.01 ppm. Right up front, you have a bit of a problem. 
(Yes I’m mixing measurements in that comparison, but the point is still valid). 

What I keep wondering is - There is no big mystery about the WWVB transmitter's 
 location. You likely know your own location as well. Part of demodulating the data
gives you day of the year. From that you can figure out some of the basics of the 
propagation effects (sunrise is at X:XX sunset is at … etc). You also could grab stuff 
like weather data fairly easily (no idea if that actually helps). If you fit out the basic
propagation impacts, WWVB could get a lot better. At the very least, you would know
when to ignore the signal. 

So yes, you could do better today than they could back in the good old days in terms of 
the propagation coarse effects. 

Unfortunately, there also is data on 24 hour comparisons of WWVB carrier (same time of 
day, one day apart). If you pick your time right (noon or midnight), the variable propagation 
can be reduced quite a bit. Based on that data, you are doing well at 100 ppt over 
24 hours. Might the new modulation help that by 10X? ..maybe.  GPS over a 24 hour
period should be giving you something in the 0.1 to 0.01 ppt range (same sort of pick a likely
stable ionosphere time slot and compare).

Does that make a WWVB device un-interesting? Not by any means. If you stretch out the
time, both systems get down into the “I have nothing else that good” range. Checking one
against the other is indeed an interesting thing to do. You just need a *lot* of time to do it.

Bob    


> On Nov 22, 2015, at 12:41 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> 
>> On Nov 22, 2015, at 7:47 AM, paul swed <paulswedb at gmail.com> wrote:
>> 
>> As mentioned a nice answer to the wwvb modulation change.
>> I looked up the parts and it seems that they have gone into the NOS state.
>> Though you can get some from digikey and such especially in the SOIC
>> package. Also the VCO isn't available.
>> It appears that the Chinese sight has the lmc6484 and LM387n at reasonable
>> prices for small quantities. Most likely will order from there.
>> Have not checked out the PIC chip yet.
>> The 74HCXX are common and reasonable.
> 
> That’s kind of a shame. I’m sure a redesign with modern SMD parts could be accomplished.
> 
> The big question is how the stability of a wwvb disciplined oscillator would compare to a GPS disciplined one (all other things being equal). Well, it’s a big question for me, since I have no idea, but I imagine simply asking here will give an immediate answer. :)
> 
> I’d have to guess that the PLL would behave better given a 60 kHz reference rather than a 1 Hz one. But how stable is that 60 kHz reference after going through, what, a thousand miles of ionosphere or so?
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