Jake Brodsky wrote:
Tom wrote:
wrote:
So I could make a short wave radio out of a laptop
using this software?

You can make an Extremely Low Frequency radio out of a computer,
tunable across the bandwidth of its sound system. You could tune up to
about 1/2 the sampling frequency of the sound systems analog-to-digital
converter. Typical sample rates are 48kHz but high end systems go up to
192 kHz so the tuning range would be up to 24 kHz and 96 kHz
respectively, provided the computer can execute the SDR DSP software
fast enough. If you connected an antenna to the microphone input, you
might hear something. To tune higher frequencies, you can use a
conventional superhet receiver as a tunable downconverter, connecting
its last IF (if higher than the computer's audio input range) to a
fixed downconverter, e.g., from 455 kHz to 12 kHz.


Err, Ummm, well, yeah, in THEORY you could. In practice you'll need to
decouple your computer from the antenna pretty well. This means you'll
need a very high Q antenna with a very low noise amplifier to isolate
it, and a very well isolated DC supply to power the amplifier. If those
things exist, you can build your very own VLF receiver from a sound
card. In fact, if your sound card can manage a sample rate of at least
120 kSamples/second then you could use it to tune in WWVB at 60 kHz or
perhaps the German equivalent at 77 kHz if you can manage to sample at
twice that rate.

Other than WWVB, I don't think much is still down there. The earth's
ionosphere resonates at about 7 Hz if memory serves, so that might be a
lower limit to what you might want to try monitoring. The Omega system
was decommissioned years ago, though I've heard rumors that some parts
of it might still be in service in some corners of the world. Finally,
if there are any old FDM coaxial systems nearby, you might detect some
leakage from their traffic (I doubt there are any who still use this
method to trunk VF traffic together, but if there are a few, you might
still hear it)

And yes, a very few radios have a third IF at 50 kHz which you could use
such a sound card with. That experiment has potential.

73,

Jake Brodsky
Amateur Radio Station AB3A

Yeah, everything looks easy in theory. Computers are serious noise
machines.

You've probably seen this website:
http://www.vlf.it/

Think of painting a room. Isn't 90% of the work the preparation? Well,
in signal analysis, conditioning the signal is a serious chunk of the
work. Once you have something clean, then digital analysis can be done.


If you do build any of those designs on vlf.it, you may want to
investigate better (lower noise) op amps.