Hey Brad - just checking in! This is an interesting experiment, and I would love to hear how it went!
Big thanks to Kevin and JMF for providing very helpful guidance, here, too =) Cheers, Ben On Thu, May 2, 2019 at 7:12 PM Kevin Reid <kpr...@switchb.org> wrote: > On Thu, May 2, 2019 at 1:22 PM Brad Hein <linuxb...@gmail.com> wrote: > >> I took a Raspberry Pi and attached a 48KHz USB sound card, with a big >> magnetic loop antenna fed into the mic. A little cheesy? yes! But I'd like >> to try and see if I can receive VLF. It's in a remote location with little >> to no interference so I'm thinking my chances should be good. The challenge >> I'm facing is that I need to write the SDR logic to "tune" throughout the >> 0-24KHz tuning range. >> >> My question is, being that a sound card source presents samples in float >> and not the usual complex data type, can I still apply the same SDR logic >> that we use for SSB/FM/AM demodulation such as those presented in the >> Gnuradio tutorials (eg. >> http://www.csun.edu/~skatz/katzpage/sdr_project/sdr/grc_tutorial3.pdf) >> and if not, how do I go about translating the float input into something I >> can use to feed existing AM/FM/SSB demodulator flowgraphs? >> > > The first thing you need to do is a "float to complex" operation (which > will leave the imaginary/Q part zero). If you were to plot the spectrum of > the resulting you would see that it is symmetric around 0 Hz, containing an > extra copy of all the signals you're receiving, but that is no worse than a > more typical received spectrum where the other half contains unrelated > signals. > > After that, the approach is exactly the same as any other receiver > flowgraph that supports receiving at an offset from the hardware > center/zero frequency. You can use either the "Frequency Xlating FIR > Filter" block (which combines a frequency shift and a low pass filter) or > the "Rotator" block (which performs a frequency shift and would usually be > followed by a separate filter), and the frequency shift of that block > should be under user control for "tuning". Then you have a baseband signal > that you can demodulate. > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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