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

From: Albin Stigö
Subject: Re: [Discuss-gnuradio] Tuning in VLF with a sound card
Date: Wed, 8 May 2019 10:15:44 +0200

Hi Brad,

Just some random ideas... What you are trying to do is very doable. Ive seen a lot of people do it for VLF reception... Usually along with some kind of FET amplifier before the mic...

The frequency xlating FIR filter doesn't have great performance on the rbpi at the moment. 
The frequency xlating FFT filter would be better in your case.

I'm working on a patch that will make these blocks 14 times faster on raspberry pi so that will also improve things...


On Wed, May 8, 2019, 06:06 Brad Hein <address@hidden> wrote:

On Tue, May 7, 2019 at 4:19 PM Marcus D. Leech <address@hidden> 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 =)

I should perhaps have entered this discussion earlier, and pointed out one of my early applications using a sound-card for VLF work:


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.

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.

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