Re: [Discuss-gnuradio] Tuning in VLF with a sound card

On Tue, May 7, 2019 at 4:19 PM Marcus D. Leech <patchvonbraun@gmail.com> wrote:

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.

Thanks Marcus - I'll see if I can get it to compile again. In the meantime I have put together an AM receiver flowgraph using recommendations from this thread, along with what I remembered from the gnuradio tutorials and Mike Osman's video tutorials.

https://github.com/regulatre/vlfCoilEperiment

Given a 5-minute recording, which I included in the repo, I quickly found that QRM interference will be a hurdle and as you pointed out Marcus, my coil (an old VGA degaussing coil) seems to be resonant at undesirable frequencies. In its current installation it's getting overwhelmed by a steady interference source that sounds like ripples coming from a 60Hz half-wave rectifier. There are some gaps in the noise, and as I tuned around within the baseband using my flowgraph (in the repo above), I was able to tune to various parts of the baseband, but in all cases I had too much interference noise.

I have a Focusrite Si2 I could use instead, which would have more gain potential and a very low noise floor, but first I think I'll need to find a way to get away from the noise sources.