[ntp:questions] WWVB DSP decoding (like audio refclocks)?

Tim Shoppa shoppa at trailing-edge.com
Tue Nov 4 14:15:07 UTC 2003


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.



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