reading your email more closely:
* the sound card generates real data with an even spectrum with components at w 
and -w
* you transfer from VLF band to baseband by multiplying by a complex NCO 
exp(-jwt) with t=[0:length(samples)]/fs
when sampling at fs
* the -w component is shifted to -2w that you filter our before decimation
* after transposition and decimation, you end up with the complex I/Q 
components you are familiar with.

You can also multiply with cos(wt) instead of exp(jwt) but then you create all 
the spectral components
at +/-2wt which might be more annoying to filter out, so you might as well work 
with a complex NCO from the
beginning.

JM

--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France

May 2, 2019 10:24 PM, "Brad Hein" <linuxb...@gmail.com 
(mailto:linuxb...@gmail.com?to=%22Brad%20Hein%22%20<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 
(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? 
I run many other flowgraphs on Raspberry Pi's remotely so the networking and 
remote aspect of the project will be easy for me. I really just need some 
initial guidance knowing which direction to go in terms of tuning. 
Thanks! 
Brad
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