An all software based receiver shoulnds like a neat idea until you
realize you run out of dynamic range by trying to sample such a wide
bandwidth directly.

In another reply to this thread, I mentioned commutating filters. At VLF,
one of these ahead of a good ADC would probably also yield very good
performance without downconverting to audio baseband first. As to speed of
the ADC, using undersampling you theoretically only need enough speed to
sample at twice the modulation (information) bandwidth. This is just a few
kilohertz sampling rate. I've heard of this technique, but don't recall ever
seeing it implemented in a real receiver design.

Can anyone comment and shed more light on this??

Joe
W3JDR