[ntp:questions] Re: WWVB DSP decoding (like audio refclocks)?
David L. Mills
mills at udel.edu
Wed Nov 5 20:05:22 UTC 2003
High-end WWVB receivers like the Spectracom Netclock/2 use a narrowband
crystal filter with bandwidth a few hundred Hz ahead of a gain block and
synchronous demodulator. The I demodulator I channel goes to a slicer
and computer; the Q channel goes to a 60-Hz VCO. All very expensive and
close to theoretical optimum.
As some folks on this list might remember, the issue has come up about
WWVB receptino here on the right coast. Conditions seem to vary somewhat
in various cities, but here in radio-free Newark, DE, conditions are
awful. My two Spectracom 8170 WWVB receivers on rackety.udel.edu (take a
look) are 22 years old and have the circuitry used today, but they are
essentially worthless most of the time. My Netclock/2 at home in a
noise-quiet location does somewhat better, but still not the rock one
You hit the nail with your comment on oscillator stability, which is the
once and for all gotcha. Even with an ovenized oscillator with 10^-8
stability, synchronous operation could not be assured beyond about 1000
seconds. You would need an oscillator some 80 times better than that for
holdover up to a day. This would be in the atomic class.
But, during periods where the signal is above the noise with a good
antenna, crystal filter and preamp, one of the modern floating-point DSP
chips like the TI 320-class would work great. Alternatively consider
using the same general lineup as the Spectracom, but don't use a slicer.
Offset the VCO by 100 Hz, so now you have moved the carrier to a
subcarrier within the soundcard passband. The signal is now very similar
to the WWV/H demodulator/decoder and the same algorithms can be used.
All you would have to do is change the seconds state machine table.
Here's a way to test the idea without building anything. Several
shortwave receivers today can tune 60 kHz with 1-Hz resolution. Wind an
antenna on a ferrite rode maybe with a preamp. Radios I know about have
atrocious low gain at and below the broadcast band. Set the BFO to
produce 100-Hz note and connect your soundcard. Rip off the WWV/H
driver, toss out the 1000-Hz stuff and synchronize directly to the
subcarrier. Weekend project.
The crystal filter can be a drag, unless you can find suitable rock. The
main hazard is the 63-kHz harmonic from TV sets and computer monitors.
Just about all the loudenboomers that used to hang out below 100 kHz are
gone on this side of the Atlantic, so WWVB has the spectrum mainly to
Tim Shoppa wrote:
> Has anyone documented an attempt at doing a DSP-like decoder for WWVB?
> Using "conventional" WWVB receiver chips here on the East Coast I only
> get reliable WWVB reception for a few hours in the middle of the night.
> During the day the signal is way below the noise.
> These receiver chips just measure the 60kHz amplitude to receive the
> timecodes. But maybe a true synchronous detector could extract the
> signal from under the noise - after all, if you get a good lock to the
> timecode once a night, maybe you can keep lock throughout the next
> day through accumulating the amplitudes with a software based
> lock-in amplifier?
> I'm thinking of something like a crystal-filtered (60kHz crystals)
> TRF front end followed by a mixer to take the 60kHz down to a few
> kHz. Then software in a PC-clone does synchronous detection, using
> technology similar to the highly succesful wwv audio decoder.
> One gotcha is that the local oscillator stability becomes a critical
> factor. Maybe this could be derived from the PC soundcard's audio
> output? PLLing a 62kHz LO to a few-kHz sine wave coming out the soundcard
> ought to be straightforward (although phase noise will probably become
> critical). This may just make the PC soundcard's clock
> stability twice as important. Maybe a careful choice could cancel out
> lowest-order variations in sound card clock rates.
> Eliminating the mixer and just putting 60kHz straight into an audio
> card probably isn't too far out of current technical capabilities.
> Any thoughts?
> Is the WWVB amplitude on the east coast during the day just too low to
> make any of this practical? Do, for example, the Spectracom or HP WWVB
> receivers make any attempt at lock-in detection?
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