[time-nuts] WWVB PM Time Questions

Thu Jul 30 19:45:03 UTC 2020

Ray,

 > How do the La Crosse distributors sell the ULTRATOMIC clock for $35-$40.

That's a bit lower than a few years ago when it first came out.

 > Building a million clocks would get the cost down,

In this case, likely thousands not millions. The market for WWVB clocks 
took a hit in the 21st century when GPS, the internet, WiFi, and smart 
phones took over.

 > I'm sure there are a lot of transistors in their IC to handle all the 
phase tracking and time decoding.

Read about the ES100 chip inside the UltrAtomic clock. Also note it's 
not a "phase tracking" receiver per se. It's purpose is to get UTC via 
the subcode, not to track NIST / WWVB carrier phase. Technical documents 
can be found here:

http://everset-tech.com/

 > It is obvious they don't have a vcTCXO in the clock

Correct. For more details of the crystal(s) and typical performance see:

http://leapsecond.com/pages/ultratomic/

 > A lot of reviews say their clocks based on the AM modulation method
 > would not sync but the phase modulation ones always work.

Agreed. The amount of AM noise near 60 kHz is getting worse by the year 
so, at least in my experience, the eWWVB phase modulation decoders work 
much better than the legacy WWVB amplitude decoders.

/tvb

On 7/30/2020 12:01 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. 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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