[time-nuts] Low cost synchronization
mikec at ciholas.com
Sat Aug 20 18:25:29 EDT 2005
On Sat, 20 Aug 2005, David Forbes wrote:
> At 4:07 PM -0500 8/20/05, Mike Ciholas wrote:
> >One wonders if you can build some sort of long term reception
> >processing that would pick out the signal from the noise.
> There are two ways to proceed: One is to try to actually learn
> the time of day form WWVB, and the other is to just keep your
> local clock synchronized to the one second pulses from WWVB.
I can't stay sync'ed to WWVB. I don't have the power to run the
receiver all the time, and you loose the signal during the day in
many places. So we're into trying to get the signal during some
period of time.
> The standard time signals such as WWVB use a one-bit-per-second
> coding scheme where the signal is amplitude modulated, so you'd
> need to measure the phase of this modulation to perhaps 5% of a
> cycle, or 0.05 seconds. Your 1PPM oscillator will be off by .05
> seconds in 50,000 seconds or 14 hours. So you could integrate
> data for 14 hours. That should give you sqrt(50,000) improved
> S/N, or 200 times better S/N ratio (23dB).
A >20dB improvement is huge! You might almost be able to get
some station anywhere on the globe. Of course, we might start to
get mutual interference between WWVB, MSF, and JJY when we are
between those two stations. Maybe a single receiver on DCF 77.5
KHz is better?
> If you want to learn the time, then the next problem is that
> you're integrating data which is changing, so you have to deal
> with that by some fancy integration tricks such as using
> separate bins for 1 and 0 level bits.
Yes, this will be necessary. Not really that bad if you expect
to be within, say, 4 minutes. You don't need to bin all the
minute bits and then you can apply a moving window on the binary
pattern to find the best "match".
> I hope this makes some sense. If not, then read up on pulsar
> detection and timing techniques. --
This was helpful. Thanks.
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec at ciholas.com
Evansville, IN 47713 http://www.ciholas.com
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