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Message-ID: <3.0.1.16.20011206103713.2c778a9e@pb623250.kuleuven.be>
Date: Thu, 06 Dec 2001 10:37:13
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Re. Improving extreame weak signals.
In-reply-to: <5.1.0.14.0.20011205110343.00a863e0@gemini.herts.ac.uk>
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Hello Jim,

>I think Rik's idea of displaying 2 tones differentially ought to work, but 
>it would place quite stringent demands on frequency stability. The current 
>spectrograms are not too fussy about exact frequency, so long as the drift 
>is smaller than the FFT resolution during one dot period, and the signal 
>stays on the screen. But if we were to compare two tones, it would require 
>accuracy in the frequency shift that was smaller than the resolution of the 
>FFT - a few millihertz with the longer dot lengths. Judging from last 
>year's experience with Wolf, it is quite hard for this kind of accuracy to 
>be set up and maintained throughout the transmit/receive system, when the 
>equipment being used includes amateur-type rigs, sound cards etc.

I agree that requirements will be more stringent than for normal QRSS or
DFCW. But since one can monitor a quite broad frequency range (10Hz or so)
it will be only the the accuracy of the frequency shift that is critical -
not the absolute frequency (in contradiction to WOLF).
Assuming 120 seconds dotlength the accuracy and stability of the shift has
to be better than 0.008Hz. This may sound awfull, but 0.008Hz on 136kHz is
no more difficult than a 8Hz accuracy on 2 meter. Using a properly designed
PLL or DSS system it shouldn't be a 'mission impossible'.

I believe that we have to distinguish between 2 kinds of users in the weak
signal business. On the one hand you have the normal users that just want
to use QRSS or DFCW to improve the range of their station and just want to
make QSO's all over Europe. For these users the existing DSP software is
sufficient, no need to make things more complicated.
But on the other hand there is also a smaller group that is interested in
exploring the limits (and breaking them). Since we are limited in 'brute
force' (1 Watt ERP) we will have to look for more efficient transmitting
modes and more sensitive receiving systems. It is for this second group
that I made my suggestions, assumng that they want to do more effort in
improving their station than the average user.

73, Rik  ON7YD