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#5
June 17th 06, 03:33 AM
posted to rec.radio.shortwave
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software defined radios
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 |
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