[time-nuts] WWVB PM Time Questions

[Bob kb8tq]

Hi


> On Jul 30, 2020, at 3:01 PM, rcbuck at atcelectronics.com wrote:
> 
> So the $64 million dollar question is this. How do the La Crosse
> distributors sell the ULTRATOMIC clock for $35-$40. That means La
> Crosse's manufacturing cost is probably around $15-$20. Building a
> million clocks would get the cost down, but still..... I'm sure there
> are a lot of transistors in their IC to handle all the phase tracking
> and time decoding. It is obvious they don't have a vcTCXO in the clock
> so they must be doing everything in software.


Since they have no interest in extracting the carrier phase for timing,
there is no real need for a fancy oscillator …… Their definition of 
“precision” and the TimeNut definition are pretty far apart.

Bob


> Or maybe the IC is a
> combination micro and FPGA. Any ideas how they would approach that?
> 
> If you read the online reviews of the clock they are about 99% positive.
> A lot of reviews say their clocks based on the AM modulation method
> would not sync but the phase modulation ones always work.
> 
> Ray,
> AB7HE
> 
> -------- Original Message --------
> Subject: Re: [time-nuts] WWVB PM Time Questions
> From: paul swed <paulswedb at gmail.com>
> Date: Thu, July 30, 2020 10:39 am
> To: Discussion of precise time and frequency measurement
> <time-nuts at lists.febo.com>
> Cc: rcbuck at atcelectronics.com
> 
> 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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