On 09/26/2012 07:13 PM, Tom Van Baak wrote:
For those of you who don't dare click on encrypted Yahoo URL's, the original
NIST link is:
http://www.nist.gov/pml/div688/grp40/upload/NIST-Enhanced-WWVB-Broadcast-Format-sept-2012-Radio-Station-staff.pdf
Burt,
My reading of the document(s) is that the new format will in fact allow WWVB to be
used as a frequency standard with even greater precision then before, though not
with unmodified legacy WWVB carrier receivers. My hope is that one of you will
produce a clever reference design for such a T&F receiver make it available to
the group. It sounds like a very fun DSP project; one that we can all learn from.
Bonus points for making it an open-source Arduino shield. Making it work with both
DCF77 and WWVB would also be a plus.
If nothing else, a well-documented hack for existing Spectracom and HP WWVB
receivers would be welcome. A third idea is a translator that receives the new
carrier format and re-transmits the old carrier format; that way no mods need
to be made to legacy WWVB receivers at all, regardless of age. It would be
similar to the way the G2G (GPS to GOES) translator worked. Extra credit for
adding back the 45 degree hourly phase shift.
Looking at it, the BPSK receiver strategy often becomes the Costas loop
should maybe not be that much of a modification to some of these. The AM
and PM modulation should be fairly trivial to crank out of them.
In a Costas loop, you mix-down your input signal in both I and Q signal,
by using both the cos and sin variant of an oscillator, which can be
realized by several means. The I and Q mixer outputs is then low-pass
filtered. The I and Q is mixed to provide the phase detector signal,
which is then used for the normal PLL loop.
Modifing a standard loop into a Costas loop involves adding a 0-90 phase
splitter (see polyphase filters), a pair of mixers and a pair of
low-pass filters. If the oscillator is available at 4 times the target
rate, a cheap digital trick can be used to create an oscillator
phase-split by xoring the bits or using other alternating methods. The
Tayloe detector might be a thing to look at in that case.
The Costas loop is a good vehicle towards a MAP receiver if you as so
inclined.
There was a paper on the new signal, and some receiver strategies.
Another trick to remember is that both the AM and PM signal has a very
high degree of predictability as many bits re-occur precisely or can be
predicted out of earlier state. The redundancy bits provide redundancy
within each frame rather than between frames. Thus, the actual
information flow or "news" in the channel is essentially zero once you
have locked into the signal. What changes is your oscillators phase, and
the amplitude and phase of the transmission-path.
I'm not near the WWVB transmitter to care enough to rebuild a receiver,
but I would guess that the handy time-nuts would not have too hard to
hack their existing receivers into a Costas setup.
Cheers,
Magnus
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