Re: contemplating conversion of an r-pi3b based system to a rock64
On Monday 28 August 2017 00:28:31 Alan Corey wrote:
> 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.
I was afraid of that, particularly for the 5GHz wifi band.
But aren't the huge majority of the wireless keyboards and mice just BT
at the core? Max reliable range when the dongles can see the master is
about 20 feet. I put the mouse in the box the pi is in, and had BT do a
scan with bluetoothctl, while I jiggled the mouse, nothing detected.
So to get rid of potential noise sources, I recently added a couple lines
to /boot/config.txt, turning off both the BT and wifi. It may have
helped, but still has the funkity reboot until it works requirement.
That got rid of sit, leaving only lo and eth0 as detectable by ifconfig.
Cheers Alan, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
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