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On 05/07/2019 04:05 PM, Ben Hilburn
wrote:
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
I should perhaps have entered this discussion earlier, and pointed
out one of my early applications using a sound-card for VLF work:
https://github.com/patchvonbraun/SIDSuite
It's OLD now--I don't think it was ever converted to GR 3.7
One of the problems with mag-loop antenna is that they're very high
Q, and thus have very small fractional bandwidths, which means that
they're wildly inefficient at all but the resonant frequency. I
made up for that using a Behringer microphone pre-amp using the
balanced input.
That meant I could use a fairly "random" multi-turn mag-loop and
not worry about efficiency very much.
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.
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