[TangerineSDR] Demo of Live VLF System

[Jonathan]

Hi All,

Here are some pictures of the Raspberry Pi VLF SDR box, antenna, and VLF preamp I used with this setup.


This is the Raspberry Pi box. Everything is installed in a Hammond aluminum enclosure. I added a pigtail on the Pi for an external WiFi antenna. On the left is the Trimble RES SMT360 and an interface board. In the center is the Raspberry Pi with Audio Injector Stereo soundcard. On the right is the power/audio board. It contains a DC-DC converter to power both the Pi and the Trimble interface board. It also contains another DC-DC converter that is isolated and audio isolation transformer for the VLF preamp, and connections that interface a safe “shutdown -h now” button, power, VLF stream, and timestamp-corrected VLF stream LED indicators on the enclosure cover and Pi header. The GPS PPS is fed into both one channel of the soundcard and a GPIO pin. There are connections for both GPS and WiFi antennas, as well as Ethernet and a RS232 serial console for the Pi’s console port. The VLF receiver feedline comes into a cable gland using cat5 cable with one pair for power and the other pair for audio. Everything is powered with a 12V 1.5A unregulated wall adapter. 


This is the VLF preamp board mounted on a foam insert that is inside a PVC pipe. It’s made up of a front end that connects to an E-Field antenna probe. It contains a filter to notch 60Hz a bit and also has some gain. The output is fed into an analog buffer driver and to an audio isolation transformer. The whole preamp is powered with an isolated DC-DC converter. Both power and audio isolation are required or else 60Hz mains can make its way to the front end and overload it. 


This is the antenna/preamp assembly. The antenna element is a foam insert with copper tape along the length. The copper tape is the actual element. The preamp insert goes in after the antenna insert, and a wire is soldered in between the copper tape and VLF preamp to make the antenna connection. An earth ground connection is made between the preamp and a ground strap. 

Jonathan
KC3EEY

> On Aug 30, 2021, at 3:16 PM, Jonathan <emuman100 at gmail.com> wrote:
> 
> Hi All,
> 
> I forgot to include this listserv as well. I’d like to demonstrate this on the Monday night (8/30/21) Tangerine SDR meeting. See message below. 
> 
> Thanks.
> 
> Jonathan
> KC3EEY
> 
> 
> Begin forwarded message:
> 
>> From: Jonathan <emuman100 at gmail.com>
>> Date: August 29, 2021 at 11:07:32 PM EDT
>> To: tangerinesdr at lists.tapr.org
>> Subject: Demo of Live VLF System
>> 
>> Hi All, 
>> 
>> This week I was able to test my Raspberry Pi VLF SDR box with my VLF receiver! I’m very happy with the results and I’d like to do a demo of the the preliminary setup and signal verification at the Monday night (8/30/21) meeting. This will also include a live stream of the filtered VLF audio and plots of NAA amplitude and phase. I have attached some preview screenshots. 
>> 
>> <image1.png>
>> 
>> This is the oscope program showing the GPS PPS pulse and VLF audio.
>> 
>> <image2.png>
>> 
>> The adjusted time scale shows 60Hz power and harmonics in the VLF audio.
>> 
>> <image3.png>
>> 
>> This shows the density of mains harmonics.
>> 
>> <image4.png>
>> 
>> This is the spectrogram program showing the VLF receiver audio in the frequency domain. Notice most of the spectral power is contained in the portion of the band with the mains and it’s harmonics, as well as some peaks of interesting signals. In a mains heavy environment, these could be switch-mode power supply emissions. 
>> 
>> <image5.png>
>> 
>> This is a short plot of NAA’s amplitude and phase as extracted from the timestamped VLF stream. You’ll notice the variations of both phase and amplitude that are probably local interference, due to being in a very mains heavy suburban area.
>> 
>> Jonathan
>> KC3EEY
>> 
>> 
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