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Message-ID: <3.0.1.16.20011204101542.2bff5384@pb623250.kuleuven.be>
Date: Tue, 04 Dec 2001 10:15:42
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: LF: Improving extreme weak signals
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Hello group,

First of all my congrats to the ZL6QH team and W4DEX for setting a new
milestone. Besides the new distance record it also shows that 136kHz DX
progation isn't limited to northern (southern) latitudes.

Now another item :
In the recent 136kHz trans-pacific tests and the 73kHz trans-atlantic tests
the mode was DFCW with 2 minute dotlength.
I believe that both DFCW and these long dotlengths could make it possible
to gain a few more dB SNR :

1. Assuming we use a 2 minute dotlength where the dot starts exactly on an
even minute the receiving software could take advantage of this
'synchronisation' as it would exactly know over what period the signal has
to be integrated. This would not only allow to get rid of the 'blur' but
also strongly reduce the FFT calculation time (that could be used for other
SNR improving handlings). Further it might be possible that the knowledge
of the exact 'dot period' can allow a better discrimination between
presence/absence of a signal.

2. With the current receiving software what you see on the screen is the
strength of a signal compared to the noise. Using DFCW with a exactly known
shift would make it possible to show the 'differential' signal. This should
give a 3dB gain over QRSS.

Eg : Assume that we could agree to a standard shift of 0.1Hz. Further
assume that the receiving software shows a 4Hz segment (let's say 798 to
802Hz). So at the 800Hz line on the screen we would not see the absolute
strength of the 800Hz component but the difference between the 800.0 and
799.9Hz components. If a 'dash' is transmitted (on 800.0Hz) the
differential signal would be positive, if a 'dot' is transmitted (on
799.9Hz) the differential signal would be negative and if nothing is
transmitted the differential signal would be (close to) zero.
If we connect these differentential values to a good color pallette a DFCW
signal would be shown on a single line (the 'dash' frequency) where a 'dash
could appear as one color (eg. red) and a dot as another color (eg. blue)
while noise (no signal) would appear as the inbetween color (eg. brown).
We would have to get used to this kind of appearance of a DFCW signal, but
the profit would be a 3dB better SNR.

I wonder if the above makes some sense (I'm not a DSP specialist) ?

73, Rik  ON7YD