Re: VLF: First VLF detection DL > EA

[Paul Nicholson <vlf0403@abelian.org>]

> what is the reason to use a 3kHz filter before the noise
> blanker and not a narrower one?

For lowest noise floor out of the blanker, narrower is better
so that less out-of-band energy is going into the blanker.
The blanker is an amplitude modulator with a random modulating
signal and thus creates random sidebands around every frequency
component going in, which averages out to a raised noise floor.

Too narrow and the sferics become stretched in time and reduced
in amplitude by the filter.  They become harder to discern
against the threshold and the blanking duration becomes longer
than necessary. So wider is better in that respect.

Somewhere then, in the middle there is an optimum.  Fortunately
a broad optimum.  I standardised on 3kHz after some trial and
error at about 9kHz.   Perhaps at higher frequencies a narrower
filter might work better.   Some trials are called for and I can
imagine a shell script already being prepared at SQ5BPF.

--
Paul Nicholson
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