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.
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