[time-nuts] WWVB PM Time Questions

[paul swed]

Hello to the group.
Poul has done some very fine work and you can learn a lot from him.
But several comments that will help. Its easy to create all kinds of
solutions that look for phase shifts. I spent quite a bit of time doing
that. But the nasty reality is without accounting for the noise, signal
fades, and delay shifts they generally fail. Or work for short periods of
times.
Simplistically if you have a 1 second image of the incoming signal its easy
to see the phase shift.
With respect to zero crossings it works really poorly. Thats why on Loran C
they were very clear the slice point was as I recall 30% up the envelope.

Humor on the d-psk-r. The new unit does not have an output that contains
the phase shifts of wwvb. The units intention is to remove all phase shifts
so that all old style phase tracking receivers and clocks work. They all
do. Have 7 of them.
So to experiment with Johns fine KB2DB receiver I need the raw phase
flipping wwvb signal.
I have built his receiver and now that there is an answer to the TCXO issue
I need a raw feed. Chuckle. When I built the new unit I really debated
adding that BNC. Hindsight is always really clear.
Best regards
Paul
WB8TSL


On Thu, Jul 30, 2020 at 4:48 AM Poul-Henning Kamp <phk at phk.freebsd.dk>
wrote:

> --------
> rcbuck at atcelectronics.com writes:
> > Paul,
> > "The new de-psk-r I built has no raw wwvb outputs." What do you mean by
> > raw?
> >
> > I have been thinking about how the phase shift could be detected in
> > software instead of hardware. Could something like this maybe work:
>
> Back when I played with VLF, I did this on DCF77/Rugby etc.
>
> In my case I used a 12 bit 1MSPS ADC, and (exponentially) averaged
> the RF signal into per-station circular buffer, this is very cheap
> and fast to do in an interrupt handler.
>
> In your main code you can demodulate that buffer to DC by multiplying
> and summing with precomputed sin&cos tables.
>
> That gives you baseband I & Q from which you can trivially calculate
> phase and amplitude.
>
> You can make the buffer as short or long as you want, I did the
> trivial thing and made it a full second long:
>
>         http://phk.freebsd.dk/loran-c/CW/
>
> The trick to that is that you can recover many stations from the
> same circular buffer, by using different sin&cos tables.  All the
> above plots came out of the same single 1sec buffer snapshot.
>
> This obviously works for any buffer length which is a full number
> of carrier cycles for all the stations you are interested in, in
> principle you can recover all stations on N*kHz, N << 500 from from
> a single 1000 sample buffer at 1MSPS.
>
> The advantage of using a 1second buffer was that I could extract
> what the stations thought was top of the second from their modulation.
>
> (I actually calculated my position based on DCF77, Rugby, HBG,
> France Inter and the strange 200/3 kHz station in Moscow, the result
> I got was about half a kilometer wrong.)
>
> To recover the per-second modulation you simply need to shorten the
> buffer so it resolves the modulation, which probably means no longer
> than 1/20 second for WWVB, but 1/100, if you have the S/N for it,
> is much easier in terms of signal analysis code.
>
> An alternative strategy, which I used for DCF77 phase recovery, is
> to detect the duration of the AM pulse and pick one of two 1-second
> long buffers based on that.
>
> And you don't need much CPU power at all, I did Loran-C time/freq
> on a Analog Devices Aduc7206:
>
>         http://phk.freebsd.dk/AducLoran/
>
> And that included a graphical display, (watch the animation.gif :-)
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
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