[time-nuts] new WWVB BPSK dev board

[Attila Kinali]

On Tue, 4 Dec 2018 21:42:41 +0000
Gregory Maxwell <gmaxwell at gmail.com> wrote:

> On Tue, Dec 4, 2018 at 9:22 PM Attila Kinali <attila at kinali.ch> wrote:
> > known bits in the BPSK signal are the first 12 bits of each minute.
> 
> This seems transparently incorrect to me.  If your receiver has access
> to only a tiny chunk of signal and no idea of anything else then yes,
> but that isn't a realistic restriction.  Given that we know the signal
> is a clock all the bits are almost perfectly predictable.  Unless I'm
> confused about something about the signal.

Yes. True. Once you aquired the signal you can guess quite a few bits
of the next minute. But this gives at best 60 bits per minute of
information for the BPSK correlator. Compare this to 512 bits per _second_
of DCF77. Ie DCF77 has 500 times more information to work with and
to pull the signal out of the noise than WWVB. 

Or to put it differently: you could say the same for the AM signal.
Once you know what bits will be transmitted, you can do a matched filter
and get an increase in signal quality. Of course, AM is not as easy to
use this technique on and the gain is less, but at these low rates,
it's basically identical. 

As I explained in one of the mails I linked to, the reason why BPSK
helps with getting signal out of the noise is the pseudo-random signal
modulated ontop of it, that gives _a_lot_ of additional information
to the signal and de-correlates it from the usual jammers (single frequency
narrow band and wide-band noise-like jammers)[1]. The WWVB does not do
that. The additional spreading of the spectrum of the signal is minimal
and in the order of the spreading due to the AM modulation.

			Attila Kinali


[1] This is not tied to BPSK as modulation, but works with
any modulation. The key point here is making the spectrum wide
and predictable. See https://en.wikipedia.org/wiki/Spread_spectrum
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