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

[Albin Stigö]

Thanks for the support!

Regarding your VLF experiments there's an historically interesting VLF
transmitter Grimeton SAQ sometimes active on 17.2kHz.
https://alexander.n.se/the-radio-station-saq-grimeton/saq-transmissions/?lang=en


--Albin

On Wed, May 8, 2019 at 2:34 PM Brad Hein <address@hidden> 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.
>
>
>
> On Wed, May 8, 2019 at 4:15 AM Albin Stigö <address@hidden> 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 <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 
>>>> =)
>>>>
>>>> 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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