[time-nuts] WWVB PM Time Questions

[paul swed]

Well John perhaps there is some interest in your receiver. I see the vcTCXO
is down by 5 devices from yesterday. Make that 6 now. For anyone else usps
is cheapest at $4.99.
Regards
Paul

On Thu, Jul 30, 2020 at 10:29 AM paul swed <paulswedb at gmail.com> wrote:

> 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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>