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?

Tim.

Tim,

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
would expect.

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
itself.

Dave

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?
> 
> Tim.

"David L. Mills" <mills@udel.edu> wrote in message news:<3FA95802.B5C2365C@udel.edu>...
> 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.

Thanks, I now know more about what's inside those than I did before!  It's
good to know that they are more than the WWVB-receiver-on-a-chip things
that are in the quote-unquote "atomic clocks".

(Incidentally, for a good response to all the ads for "atomic clocks"
and "atomic watches", see
  
  http://www.leapsecond.com/pages/atomic-bill/index.htm 

for the Real Deal.)

> 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
> would expect.
> 
> 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.

A disciplined HP 10811E OCXO will run for a day with less than 5 microseconds
of drift, after becoming "unlocked", as in a Z3801A.  That's 6x10^-11
by my calculations.  But the disciplining takes several continuous days
of lock to GPS, and without GPS disciplining the spec is indeed just
a little better than 10^-8.  Those of us on the East Coast are stuck
with *maybe* a few hours each night of clean WWVB reception, probably not
good enough to achieve that level of discipline.

> 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.

That's a good and fairly painless idea.  You are correct that most
modern digital radios have very little sensitivity at 60kHz.

> The crystal filter can be a drag, unless you can find suitable rock.

60kHz rocks are readily available, about a buck each, and presumably would
be part of the TRF antenna/preamp.

Tim.