[time-nuts] NAA experiments as a reference
csteinmetz at yandex.com
Sat Sep 13 00:56:55 EDT 2014
>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.
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