[time-nuts] WWVB PM Time Questions

Fri Jul 31 18:25:10 UTC 2020

Back in December 2018 there was a WWVB thread.

>From Poul-Henning's post on 2018-12-05 quoting John N8UR:

"While everyone's been talking :-) , I recorded some WWVB IQ data for
folks to play with.  You can download it from

http://febo.com/pages/wwvb/

The receiver ran at 48 ksps and was centered on 80 kHz (to allow a 20
kHz IF to move away from 0 Hz crud).  The data was taken in early
afternoon in Dayton, Ohio.  WWVB was easily visible in an FFT."

I ran the python code posted by Poul-Henning with the WWVB IQ data.
The resulting file 'out.txt' has columns for sample time, amplitude,
and phase.  It was used for the plots below.  There is about 10
minutes of data.

initial data: n8ur_rx_center=0.08MHz_rate=48ksps_start=2018.12.05.13.57.54.bin
 (236 MB)
plotted data: out.txt   (2.4 MB, 61,000 datapoints)

I hadn't looked at phase data before, and it took a while to make any
sense of it.  It's surprising how well the bit per second data came
through, compared to amplitude modulation.  Different filtering will
likely improve the AM performance, but the phase plot looked good now,
so here it is.

The first graph shows the +90 degree phase shift (top line) and -90
(bottom line).  The SDR clock appears relatively stable at first, but
drifts lower in frequency at a fairly steady rate until about 550
seconds, when it flattens out again.  Overall a bit over 180 degrees
more than expected.  A half cycle at 60 kHz, or about 8.3e-06 drift in
10 minutes.  The unpleasantness at about 470 seconds is reflected in
the 10 minute plot during minute 7 (3 lines down from the top).  I
removed the 'drift' in a crude manner, and it's a wonder it worked as
well as it did.

The middle two graphs are the same data at different time scales.  I'm
not sure if the short term phase variation has any meaning, or if it's
'just noise' at this level.  The WWVB phase change actually takes
place 0.1 seconds after the start of the second, but it fit just right
for display as is.  The real minute begins at second 6, and this is
adjustment is made in the 10 minute waterfall plot.  The first few
seconds of data are at the bottom left corner.

Green denotes binary 1, phase shift +90 degrees.
Red is binary 0, phase shift -90 degrees.

A simplified description of WWVB phase-modulated time code:

seconds  0 - 12  fixed sync  0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 0, 0, 0
seconds 13 - 18  time parity (ECC)
seconds 19 - 46  binary minute of century
seconds 47 - 58  DST and leap sec
second  59       fixed sync  0

these seconds are notable here:

43, 44, 45, 46 contain changing binary minutes 3, 2, 1, 0
19  a copy of binary minute 0
29, 39  Reserved, but also a copy of binary minute 0 in this sample.

I'm using python, numpy, matplotlib, and Pandas.  It all runs pretty
quick on my machine, only a few seconds to load, calculate, and
display the graphs.

I'm going to try looking directly at John's IQ data to see the effect
of the larger dataset on results and calculation time.  It's a big
help knowing what the processed IQ should look like.  Thanks for the
data and code,

Rob

On Fri, Jul 31, 2020 at 7:00 AM paul swed <paulswedb at gmail.com> wrote:
>
> Ray Yes you have it correct. The flipper is bidirectional. It either adds
> flips or removes them.
> It does read the nema second per minute in the 59th second. Then depends on
> the 1 pps for an accurate phase flip. The flip is 100 ms into the second
> and by propagation delay to the east 5-7ms. Though the reality is if the
> flips not exact the phase tracking receivers work just fine.
> But beyond the simple reading of the nema sentence its the magical
> conversion to the new wwvb message format thats quite difficult. Then they
> turned on teh slow code and Rodger and I worked that issue. That actually
> took some months to resolve the fast and slow code integration.
> So as a transmitter put the software on an arduino or bluepill. Since you
> are just doing work locally the nema sentence can be repeated over and over.
> Regards
> Paul
> WB8TSL
>
> On Fri, Jul 31, 2020 at 12:01 AM <rcbuck at atcelectronics.com> wrote:
>
> > Paul,
> >
> > I wasn't talking about putting the d-psk-r software on the Blue Pill
> > board. The d-psk-r software will be on the Arduino board.
> >
> > What I was thinking of doing was putting a single NEMA message on the
> > Blue Pill board. Then connect the UART transmit line to the Arduino in
> > place of the GPS UART line. That way I could send the same NEMA message
> > without having it change every minute. I would count 60 (or maybe 62)
> > pps coming from the GPS and I would know the Arduino was ready for the
> > next NEMA message.
> >
> > Initially I will use the GPS UART transmit line to confirm the
> > Arduino/GPS combo works. Then play with sending my own NEMA GPRMC
> > message from the Blue Pill board.
> >
> >
> > It appears your software simply reads the TX buffer of the GPS until it
> > is empty once per minute and then proceeds to parse the NEMA time/date
> > information out one bit per second. However, the La Crosse Ultratomic
> > clock would not work with this data stream since you are stripping the
> > phase reversals out. Is that correct?
> >
> > I want to extract the time information only. So I would feed the 60 kHz
> > sine wave into the preamp wire on the flipper board. Is that correct?
> >
> > Ray,
> > AB7HE
> >
> > -------- Original Message --------
> > Subject: Re: [time-nuts] WWVB PM Time Questions
> > From: paul swed <paulswedb at gmail.com>
> > Date: Thu, July 30, 2020 5:14 pm
> > To: Discussion of precise time and frequency measurement
> > <time-nuts at lists.febo.com>
> >
> > Ray
> > Are you speaking to the d-psk-r software? Its quite a bit more involved
> > than what you mentioned. Its in the coding sequence. But could it be put
> > on
> > a bluepill absolutely. Its worked on all of the arduinos we have tried.
> > The
> > other thing is that by feeding a 60 KHz CW signal into the d-psk-r
> > modulator/flipper it turns the signal into a wwvb bpsk signal. No AM
> > modulation. But for bench testing its a noise free signal.
> > But I am interested in the approach you are trying to develop that I
> > guess
> > might be a SDR solution. Good luck looking forward to your success.
> > Regards
> > Paul.
> >
> > _______________________________________________
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