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.
[snip]
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
The point of my message was that you cannot make a SW radio out of a
laptop alone ( the question he asked) but could make a VLF receiver out
of a laptop that could be used with a downconverter to tune SW. Many
have done so. The downconverter is typically a superhet radio whose
last IF is downconverted through an add-on to the audio frequency
range.
The audio frequency laptop receiver need not have especially high
sampling frequency or sample size when used with a downconverter for
SW. A 10 kHz wide passband is adequate for most transmission modes and
is readily provided by a 48 kHz sample rate - even 24 kHz could be
enough. Because the front end tuner has AGC and can regulate levels
into the sound card, the latter's A/D converter having something
approaching 16 bit resolution will have more than adequate dynamic
range - even 8-bits could be sufficient.
Of course, the state of the art is for higher resolution A/D converters
and higher sampling rates for direct conversion for DSP. As discussed
elsewhere, great care must be taken in controlling interference from
the SDR to itself, via the antenna or other unintended coupling.
Tom