Re: contemplating conversion of an r-pi3b based system to a rock64
Yes, software defined radio, it's the cat's meow. I have 3 of the $20
RTL2832 dongles, which aren't wonderful but they mostly work, and tune
about 25-1300 MHz. They were originally designed as DVB devices for
TV. They can also get many ham bands, police, fire, ambulance, air,
broadcast FM, CB, you name it.
There's RF gain and IF gain, with automatic options on both which are
usually a disaster. I couldn't remember how I usually set my Windows
SDR so I had to start it and look.
http://ab1jx.1apps.com/pix/misc2/gains.jpg If you overload these
dongles they splatter and give artifacts all over the place.
All or most SDRs have 2 A/D converters like sound cards do, but these
sample at points 90 degrees apart phasewise (quadrature). Do that at
2 million samples per second and you can digitize an IF. Take the
geometric mean: sqrt(I^2 + Q^2) and you demodulate AM signals. Divide
the digitized streams and you demodulate FM. Good set of articles at
https://sites.google.com/site/thesdrinstitute/A-Software-Defined-Radio-for-the-Masses
but he uses fancy equipment on ham bands under 30 MHz. You put your I
and Q stream through an FFT (Fourier transform) like FFTW and it
changes time domain data into frequency domain, so yes, it is like a
spectrum analyzer. You can get incredible selectivity that way.
There are about a dozen SDR programs for WIindows, almost nothing for
Unix. I got tired of rebooting into Windows to play with things like
HDSDR http://ab1jx.1apps.com/pix/misc2/wx_channels.jpg There's GQRX
which is written in Python or something and can barely run on a Pi. A
recent discovery is qtcsdr
http://ab1jx.1apps.com/pix/misc2/qtcsdrwb.jpg which works fairly well.
Another is sdrtrunk but it's basically only for trunked transmissions.
A real standard is rtlsdr which is non-gui. I just wanted to learn
more about how they worked by trying to write one. SDR ties up a
computer but if you can do it on something like a Pi it becomes
practical. Especially at under 2 watts total power used.
Bluetooth and wifi are about 1000 MHz too high for these dongles to
hear. Those (in HDSDR and my program) are area NOAA weather radio
transmissions around 162 MHz, always on so they make good test
signals.
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