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 <[email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
