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#10
June 20th 06, 04:04 AM
posted to rec.radio.shortwave
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software defined radios
Tom wrote: 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 All you are really doing is using the PC for the demod, which is technically not a software defined radio. Mind you it can be done, but it is really just an analog radio with some DSP. |
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