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

[Bob Camp]

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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