[time-nuts] WWVB PM Time Questions

[d.schuecker at avm.de]

Hi,

> principles.  The STM32L4 series which I often use has a pretty decent
> ADC: fast (5 MSPS), with about 11 good bits in differential mode, and
> "proper" hardware downsampling (called the DFSDM in the manual).  If the

Yes, the STM32 series also have built-in OPAmps, so one could hook up a 
loop or a ferrite directly to the uC without too much external components. 
Sampling at 160ks/s should suffice to get the phase and DCF77 is in reach 
as well. At this rate you have ~500-1000 processor ticks per sample which 
should be enough to do real time demodulation.

Very nice project.

Cheers
Detlef Schücker


"time-nuts" <time-nuts-bounces at lists.febo.com> schrieb am 22.07.2020 
18:44:08:

> Von: "Mark Haun" <mark at hau.nz>
> An: time-nuts at lists.febo.com
> Datum: 22.07.2020 19:29
> Betreff: Re: [time-nuts] WWVB PM Time Questions
> Gesendet von: "time-nuts" <time-nuts-bounces at lists.febo.com>
> 
> Hi Ray,
> 
> A similar project has been on my to-do list for a couple of years now
> (so don't hold your breath!).  It should be possible to do this using
> the built-in ADC for a really nice, minimalist demonstration of SDR
> principles.  The STM32L4 series which I often use has a pretty decent
> ADC: fast (5 MSPS), with about 11 good bits in differential mode, and
> "proper" hardware downsampling (called the DFSDM in the manual).  If the
> noise is white it should be easy to get > 16 good bits into the
> demodulator.  My plan was to clock the MCU from a cheap OCXO and make it
> part of the carrier tracking loop, for a simple WWVB disciplined 
oscillator.
> 
> I also have some questions about the NIST document, but in general, I
> think you need to assume that your receiver is already locked, in which
> case you know when to expect the transitions.  Getting to this point
> (acquisition) is another story, but there are all sorts of
> correlation-based tricks that you can use, similar to GPS code
> acquisition.  As others have noted, carrier phase lock can be aided by
> squaring.
> 
> My problem is that the SDR stuff is all pretty straightforward for me,
> but the analog electronics leading up to the ADC are black magic.  I
> have a preamp circuit modeled on John Magliacane's design
> (http://www.arrl.org/files/file/QEX_Next_Issue/2015/Nov-Dec_2015/
> Magliacane.pdf),
> fed from a ~ 60-turn tuned loop made from old ribbon cable, but I am
> having problems getting anything "reasonable" looking on the scope.  I
> don't know if the problem is my suburban location or if you really can't
> see the signal until you get the bandwidth << 1 kHz. (I am using
> RC-tuned op-amp stages so the -3 dB response is necessarily a few kHz.) 
> Eventually I would like a small loopstick design with a crystal filter
> like the off-the-shelf clocks use, but I haven't a clue how to get
> there.  Maybe this project would benefit from a collaboration :)
> 
> Regards,
> Mark
> 
> On 21-Jul-20 4:37 PM, rcbuck at atcelectronics.com wrote:
> > I want to decode the WWVB time information using the BPSK information
> > that is broadcast. I will use a STM32 to do the actual decoding of the
> > bit stream. This is just an exercise in "can I do it?" as I know I can
> > buy clocks for $30 that use the BPSK method. At one time you could buy
> > an IC that output the data stream but I believe there is nothing
> > currently available to do that.
> >
> > I read the document "Enhanced WWVB Broadcast Format" by John Lowe from
> > NIST. One thing that is confusing to me is this paragraph: "Although 
the
> > phase representing the information in each symbol is shown to be
> > available before the amplitude in it transitions from VH to VL, it is
> > recommended that receivers extract it only from the high amplitude
> > portion of the symbol. This is not only because of the higher power
> > there, allowing for more robust phase demodulation, but also because 
the
> > low amplitude portion may be used in the future for additional (higher
> > rate) phase modulation."
> >
> > How would you detect the phase had changed if you don't detect it when
> > it changes at 100 msec after the carrier level drops? After the signal
> > reverses phase wouldn't any reference you are using then be in lock 
with
> > the current phase of the signal after it has changed?
> >
> > Or would you use a locally generated 60 kHz ultra stable signal as the
> > phase reference? If so, how would you keep your local source locked to
> > the 0 degree phase signal of WWVB and have it ignore the 180 degree
> > phase shift?
> >
> > I have done a lot of searching and reading from various sources. But I
> > haven't really found a good explanation of the hardware that would be
> > used to detect the phase changes. Is there a block diagram somewhere
> > that would illustrate the steps needed to detect the change during the
> > high power portion of the WWVB signal?
> >
> > I am going to put together a 60 kHz amplifier using a couple of
> > FET/transistors and a couple of high speed opamps. Then I can observe
> > the signal on my scope. Using the output of the amp I can then try to
> > figure out the best way to detect the phase changes. I can use my AWG 
to
> > supply the local 60 kHz signal source.
> >
> > I have found two articles that give me some ideas. One is the Nov/Dec
> > QEX article by John, KD2BD and the other is an article titled WWV
> > de-PSK-r by Paul, WB8TSL. Paul is a member of this list.
> >
> > Can someone make other suggestions that will get me pointed in the
> > correct direction?
> >
> > Thanks,
> > Ray, AB7HE
> >
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