[time-nuts] NAA experiments as a reference

paul swed paulswedb at gmail.com
Sat Sep 13 10:33:50 EDT 2014

Charles I literally just sat down to do some math. What you say is the same
thoughts I have. The information I have on NAA says that for 200bit msk its
a total of a 100 hz shift +/-50 Hz. That makes no sense I would think it
would be at least +/- 100 Hz. They had in the past run a 100 bit msk. I
somewhat wonder about the documentation accuracy in the tracor manual and
the net. Hey it has to be true on the internet.
Agree the whole tracor d-msk-rs goal is to recreate a 100 Hz signal for the
PLL. The signal is only there when its on one of the signals so again you
are right and that was expected.
I had very high hopes that without going to the next step I would get a
clue as to the quality of the underlying NAA reference to better understand
if the next step had value. The jitter is so high the answers not apparent.

On Sat, Sep 13, 2014 at 12:56 AM, Charles Steinmetz <csteinmetz at yandex.com>

> Paul wrote:
>  OK have been experimenting with a simple vlf receiver for 24 Khz. Using
>> an HP 3335a as the LO. The Tracor 900 d-msk-r circuit.   *  *  *   I was
>> hoping to see a 100 Hz somewhat steady signal in phase relationship to my
>> local 100 Hz reference. Thats absolutely not apparent. Sorry Paul. I think
>> this method may be a bust.
> So what you have is a 100 Hz signal that comes and goes according to the
> mark/space timing.  (Or if you didn't filter the other frequency out you'd
> have 100 Hz and 300 Hz signals representing mark and space.)
> Chapters and volumes have been written about carrier recovery with FSK
> signals.  Read up and you'll find more than enough possible solutions to
> make your head spin.
> Given what you have so far -- a 100 Hz signal that comes and goes
> according to the mark/space timing -- you could try using the 100 Hz as the
> reference to a PLL with a very long time constant (compared to the symbol
> rate), much the way that color televisions used to keep their color
> subcarriers locked to the video signal's color burst, or a synchronous AM
> detector rides out carrier fading.  I'm not saying that will be the best
> you can do with the incoming NAA signal -- for that, read up on carrier
> recovery -- but it is the next logical step on your current path.
> Also of note, since the MSK signal is FSK, not binary (180 degree) PSK,
> the frequency multiplication didn't do anything magical for you (like
> removing the modulation, which it would have done with a BPSK signal such
> as WWVB).  What it did for Tracor was produce the 100 Hz signal needed by
> the rest of the box, when the existing LO was adjustable only in 100 Hz
> increments and the mark and/or space frequencies would, therefore, have
> been 50 Hz using the existing LO.  (Doubling also widened the separation of
> the mark and space frequencies from a delta of 100 Hz to a delta of 200 Hz,
> thus making the filtering a bit easier -- but that is not the primary
> reason for it.)  Finally, once you are no longer tied to the Tracor 900 and
> its existing 100 Hz IF and detector chain, you are free to use whatever LO
> frequency you want.
> Best regards,
> Charles
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