[time-nuts] WWVB PM Time Questions

[Mark Haun]

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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