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LF: Re: Narrow B/W Techniques

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Subject: LF: Re: Narrow B/W Techniques
From: Alberto di Bene <[email protected]>
Date: Tue, 05 Jul 2005 23:20:22 +0200
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Lawrence Mayhead G3AQC wrote:

As I understand it about 3db improvement is possible by making the FFT sample time equal to the dot length.


  that's not strictly true. Of course, the optimum is to collect data for the FFT analysis for a period of time coinciding with the actual transmission, which, for non-overlapping FFTs, implies two things : 1) the dot length must be equal to the inverse of the frequency resolution of the FFT,  2) the transmission must start synchronized with the data gathering period.  But, as shown graphically by ON7YD on his LF pages, what is usually done is to compute several FFTs spaced in time just a fraction of the dot length, so that to have the guarantee that at least one of them coincides with the actual transmission. This has a drawback, what our US friends call the "dogboning", caused by a smearing in frequency when the analysis interval and the actual data interval are only partially overlapping in time.

IMHO, a real gain could be achieved.if we would forget non-coherent detection, as that based of the computation of the magnitude of the FFT, neglecting the phase, to switch instead to a sort of matched filter, where a continuous correlation is computed between the incoming signal and a locally generated digital sinusoid of the same frequency of the expected signal. In case of random noise as input, the correlation will not show any peak. If instead a valid signal is present, a marked peak will manifest when there is phase coincidence between the local and the received signals. This technique implies that both signals have the same frequency, as the peak sharpness depends on this. This requirement in turn implies that both the Rx and the Tx are locked to GPS, with an accuracy in time and frequency much better than that 1 sec you mention. A GPSDO is mandatory, like the HP Z3801A or a self-built unit (not so difficult to do).

These are only qualitative considerations, without any computation of the gain that could be achieved by using that technique, and are meant only to stimulate further discussions on the subject.

73  Alberto  I2PHD

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