[time-nuts] WWVB PSK demodulation; simple carrier regeneration?

Tue Nov 20 23:39:01 UTC 2012

Hi,
	I am have thinking / tinkering / planning a wwvb receiver for longer
than I am willing to admit!!  It seems to me that one could take a
ferrite loop, JFET buffer, ADC and a small microprocessor to get down to
baseband - say 20 Hz complex sampling rate - and then output on a serial
port for more processing - initially on a PC - but after the  algorithms
are developed -  on the same micro or another one.  My current tinkering
plan is an ADC operating at 80 KHz sampling rate into an AVR.
Alternately one could easily use an ARM with the built in ADC like
Poul-Henning Kamp and his last Loran receiver.  (but I have the AVR and
the experience with it) 

	I don't think the signal processing to get down to 20 Hz complex is
that hard.  It would allow the use as a amplitude based receiver -  take
the absolute value of the complex signal or one could do both the
amplitude and phase receiver.  Did anyone notice that the phase changes
are delayed 1/10 of a second from the beginning of the 'bit' so that the
phase change is always in a 17 dB down RF state?  

Dave Armstrong


On Tue, 2012-11-20 at 08:50 -0500, paul swed wrote:
> Great thread.
> Bob in the d-psk-r thats exactly what I am doing is flipping a switch on
> the incoming signal.
> That normalizes it. However in its current approach it is random as either
> 0 or 180 out always. No attempt has been made to determine 0. But it does
> run the phase tracking rcvrs fine. That said for Pete. The time tracking
> rcvrs like the spectracom 8170 does require a non PSK carrier to work. So
> thats why  I preserve the AM for those types of rcvrs.
> But on to what you have written. I tinkered with a soundcard and spectrum
> lab and it showed some promise. If you are willing to have a big power
> sucking pc/laptop running 24 X 7 thats going to be a great solution as you
> have shown. Just take the output of the 1s and 0s and when you see a 1 flip
> a inverting amplifier. Thatthen feeds the old rcvrs. Done.
> I do not have matlab so would love to see what you did to do the decoding.
> I strongly believe your approach is completely as valid as mine. Like you
> say there are some very interesting thoughts on how to do this.
> By the way big issue much of the US actually has very poor wwvb reception.
> Thats what I have had to struggle with.
> Regards
> Paul.
> 
> On Tue, Nov 20, 2012 at 7:56 AM, Bob Camp <lists at rtty.us> wrote:
> 
> > Hi
> >
> > I believe the "lowest cost" approach is to take the RF and run it through
> > an simple switch. The switch either has a 0 degree or a 180 degree phase
> > shift. Drive the control of the switch with a computer generated track of
> > the known modulation format. Let the computer get time via NTP and just
> > generate the (really slow) switch drive waveform. A diode ring and a pair
> > of transformers will do for the switch.
> >
> > Is it a bit noisy? - yes. Should it bother a receiver that already deals
> > with a lot of noise? - probably not if your NTP is within <10ms.
> >
> > Bob
> >
> > On Nov 20, 2012, at 2:52 AM, Peter Monta <pmonta at gmail.com> wrote:
> >
> > > Here are a few demodulated frames of WWVB's new BPSK bits:
> > >
> > > 0011101101000 01101 0(0)001100111(0)011011011(0)1010110 00 0 000 000000 0
> > > 0011101101000 00100 0(1)001100111(1)011011011(1)1010111 00 0 000 000000 0
> > > 0011101101000 01000 0(0)001100111(0)011011011(0)1011000 00 0 000 000000 0
> > >
> > > The fields are described in the NIST document [1]:  sync word, parity
> > word,
> > > time in binary minutes, and various metadata.  The bits in parentheses
> > are
> > > the "marker" bits which have less power; it appears they're using all
> > three
> > > as duplicates of time[0], the LSB of the time word.
> > >
> > > Strong signal here in California, even during daytime.  My receiver is
> > just
> > > a hacked-up ferrite loop, JFET buffer, sound card, and Matlab.  About 300
> > > Hz single-sided bandwidth.
> > >
> > > So it's an interesting question:  what is the simplest device that can
> > > change this signal into something the legacy WWVB receivers can track,
> > > without any modification at all to the legacy receiver?  By "simplest" I
> > > mean avoiding any intelligence like carrier acquisition, timing recovery,
> > > or bit demodulation; but it should still have good noise performance to
> > the
> > > largest extent possible.
> > >
> > > One candidate might be this:  multiply the signal by an estimate of its
> > > phase one minute ago.  The frames are very similar minute-to-minute, as
> > can
> > > be seen above.  The exceptions are the parity word, changes in the
> > > metadata, and rollovers in the time word involving large numbers of bits.
> > > (I wish they had Gray-coded the time word, or, better, scrambled it in
> > some
> > > way so that the time word can't nearly emulate the sync word for many
> > > minutes running, which looks like a risk with the current format.)  But
> > > aside from this, the legacy receiver would be seeing carrier *
> > > xor(minute_i, minute_(i-1)), which is mostly carrier, and should result
> > in
> > > good tracking.  It's like a differentially-coherent receiver in reverse.
> > >
> > > The local oscillator would have to be accurate to a fraction of an RF
> > cycle
> > > over one minute, which works out to ~30 ppb, OCXO territory.  Is there
> > some
> > > way to do this with just a TCXO?  Maybe stability is all that's needed
> > > rather than accuracy.
> > >
> > > As for timing receivers, it's not clear to me that the BPSK helps at all.
> > > The timing marker is the amplitude modulation, and, assuming no cycle
> > slips
> > > in the carrier loop, this can be averaged for as long as one likes to
> > > refine the position of the falling edge (something like the "Hatch
> > filter"
> > > for GPS, carrier-aided code tracking).  The BPSK just helps the bit
> > > demodulation; but the bits are so predictable---was help really needed
> > > here, assuming a minimally-DSP-capable receiver?
> > >
> > > Cheers,
> > > Peter
> > >
> > > [1] NIST-Enhanced-WWVB-Broadcast-Format-sept-2012-Radio-Station-staff.pdf
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