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@febo.com> wrote:
On Nov 22, 2015, at 7:47 AM, paul swed <paulsw...@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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