[Lf] Improving extreame weak signals.]

[Andre Kesteloot]

James Moritz wrote:

> Dear LF Group,
>
> At 09:41 05/12/2001 +0000, Laurie wrote:
> >...but it would seem sensible to have defined time and
> >frequency "slots" into which the signal could drop,so that all the available
> >energy could be used rather than it be spread out in timeand frequency.
>
> I think we have been here before ... If you chose the optimum resolution
> for the speed of QRSS you are using, you will get the best signal to noise
> ratio between "dot" and "no dot" when all the samples containing a signal
> are used for the FFT algorithm that calculates the spectrum of the signal.
> This suggests you would get the brightest dot on the screen if the start of
> the FFT is made to coincide with the start of the dot. But in fact this
> more or less happens already; suppose you have 30 s dots and have set up
> the FFT to use 30s worth of samples. Normally, the spectrogram software
> will perform an FFT at least once every few seconds, let's say 3 seconds,
> using the received signal from the previous 30s. Even if the relative
> timing of the dots and FFT is totally un-synchronised, at least one FFT
> will start within 3 seconds of the start of the dot being transmitted, and
> so be very close to optimum. The FFTs performed on data from before and
> after the optimum time will contain less signal, so the dot displayed on
> the screen will fade in and out, reaching a peak of intensity in the
> middle. The only effect of synchronising the start of the FFT with the dots
> will be to eliminate the sub-optimum FFTs, and retain the optimum one where
> the timings coincided - the effect would be the same as superimposing an
> opaque mask on the screen with slots coinciding with the timing of the
> transmitted dots. I suppose this would not be hard to do, but would it be
> an advantage? You would get rid of some "clutter" on the display, but it
> would be harder to tell when bursts of noise and so on had occured. The
> display would not actually contain any more signal information, though.
>
> 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.
>
> BTW: I hope to get my DFCW modulator finished as soon as I get a bit of
> time to do it - should be a real advantage for someone whose callsign is
> mainly dashes!
>
> Cheers, Jim Moritz
> 73 de M0BMU