[time-nuts] WWVB PM Receiver

[johncroos at aol.com]

Hello All -

I am new to this forum but have read it for a couple of years. The
present fulminations on the WWVB format change should be reconsidered
in the light of prior art. As an old RfFengineer I do not see any issue
with the format and the business about patents is not really applicable
as these modulations have been in use since the 1920's.

For one thing the tube analog color TVs managed to have a PLL recover
the color sub-carrier using less than 10  cycles of the 3.5 mHz color
burst stuck on top of the horizontal sync pulse. Here we have a
continuous signal with occasional phase reversals and some amplitude
steps. This is a much simpler problem that is not begging for
sophisticated solutions, however much tun they may be to implement.

The following or variations should work.

1. Receive with a decent antenna but keep the bandwidth such that it
does not store the phase information.
2. Use preselection  with care because the filter is going to have to
flywheel down and then back up as the phase reverses. This will reduce
the carrier level during that interval. More bandwidth will shorten the
time of the ring down-ring up period. The filter must be phase stable
with temperature.
3. AGC with gain may be desired for weak signals. If so implement with
one of the Analog Devices handy
dandy VGAs. They provide 50 or more dB of voltage controlled gain with
absolutely NO phase variation as the gain is changed.
4. Now get rid of the short term amplitude variation - for this you
need a limiter with no AM to PM
conversion. Again Analog Devices has suitable parts with phase
variation of less than 5 degrees for more than 70 db dynamic range. The
AM is now gone.

These parts also output a log of the analog input - so the AM keying
could be recovered using that if
desired for some complex sampling scheme.

5. OK - now you have an amplitude stable BPSK modulated carrier.
6. Get rid of the BPSK.

Double the frequency - then divide by 2 and the carrier is recovered
less the PSK.

For the doubler you may wish to consider a Quadrature Driven Mixer
Frequency Doubler - has no Hi Q circuits and the output is easily
filtered. See my article in QEX last year March April.

Or the classic Class C or diode doubler can be used as long as they
preserve the input phase.

6a. A Costas Lopp is another classic method that should work fine.

7. Narrow the bandwidth (say 10 Hz) with a PLL  to clean up the output
of the doubler. This will get rid of wide band noise and improve the
SNR. Again the PLL must preserve phase - so some temperature test may
be a good idea. The PLL also provides a constant level clock to the
Divide by 2 to avoid phase jitter.

8. Divide by 2 - and you have a clean 60 kHz signal with no modulation.
This can the be used for frequency comparison by the phase method or to
lock up a good local standard.

So it should be possible to implement a receiver without infringing any
patents and without reams of signal processing code.

Hopefully this is helpful.    -73 John C.Roos  K6IQL  Spring Hill Kansas