On 05/08/2019 08:33 AM, Brad Hein wrote:
As an avid fan of Raspberry Pi, often putting them to use for DSP applications, I just want to say thank you for your hard work keeping gnuradio working and optimized on the platform!

I've seen some great FET preamp circuits available on the internet, a few of which I've tried out. I'll dust one off and see if I can make it work in this application.

I actually used one of these for my VLF work a few years back:

https://www.amazon.com/Behringer-MIC800-Microphone-Modeling-Preamp/dp/B000KUENNU




On Wed, May 8, 2019 at 4:15 AM Albin Stigö <albin.st...@gmail.com <mailto:albin.st...@gmail.com>> wrote:

    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...



    --Albin



    On Wed, May 8, 2019, 06:06 Brad Hein <linuxb...@gmail.com
    <mailto:linuxb...@gmail.com>> wrote:




        On Tue, May 7, 2019 at 4:19 PM Marcus D. Leech
        <patchvonbr...@gmail.com <mailto:patchvonbr...@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.





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