[TangerineSDR] VLF Audio Interface for Tangerine SDR

[Jonathan]

Hello,

For those who don't know me, I'm Jonathan, KC3EEY, a natural radio VLF
enthusiast. I'm also a senior EE student at the University of Scranton
with Nathaniel, W2NAF. When Nathaniel and I first met, we both
discussed our projects, interests, and work. He talked about the
possibility of adding VLF capability to the PSWS and since then I was
always thinking about how to implement it.

VLF receivers are often homebrew and consist of an input stage
interfacing an H-field loop or E-field probe to a gain or driver stage
with the end result an audio output. It's convenient that  most of the
VLF band is within the audio range which means no downconversion is
needed and can be recorded directly as audio. Some examples of E-Field
receivers are Steve McGreevy's WR-3 and BBB-4, as well as the NASA
Project INSPIRE VLF3 receiver. Most of these receivers are intended
for portable use but can be used in permanent installations. For the
PSWS, VLF receivers will be used in a permanent installation, like the
HF antenna. The intended setup for this purpose will consist of an
E-field receiver and two H-field receivers with orthogonal loops for
triple axial reception. This allows for distance and bearing
calculations of VLF signals.

The VLF receivers will be setup in a location with minimal power line
interference so as not to overload the input or gain stage of the
receiver. Rural areas are ideal locations, but sometimes suburban
locations can be suitable. The receivers can be powered by batteries
or through isolating DC-DC converters operating at a high switching
frequency above VLF. The audio output from the receivers must also
have an audio isolation transformer on  the audio output on the
receiver. The audio (and sometimes power) is fed through a feedline of
cat5/6 cable or coax, and on the other end it connects to the
soundcard of a computer for audio capture. An audio isolution
transformer is also needed on the soundcard end as well (and an
isolating DC-DC converter if powered by the feedline). The reason
isolation is required is because power line interference can be
coupled into the receiver, often through earth and chassis grounds
like the chassis of a computer.

For the Tangerine SDR, I wanted to offer to design a plug in module
for one of the interfaces on the data engine. A soundcard is often an
ideal SDR for VLF, so I had a "soundcard interface" module in mind. I
wanted 192 kHz sampling with 16-bit samples and a high dynamic range,
at least 100 dB. After some research, I decoded to use the CS5364
4-channel audio analog to digital converter. Digital audio output is
accomplished with TDM or I2S which is clocked into the FPGA of the
data engine. From there, the samples are GNSS time-stamped and made
available via a TCP socket over the ethernet port.

As discussed with Tom and Scotty in the Tangerine SDR Zoom, the GPSDO
will provide the A/D master clock and time-stamping will be done in
the FPGA. The CS5364 has both hardware and software configuration mode
and looks fairly easy to use. I plan on looking into the evaluation
board and starting the design of the input stages. This is a little
new to me, but I have dabbled in digital audio in the past,
specifically with Cirrus Logic audio ICs. I plan on laying out the
board in Kicad. Details on the timestamping, possibility of hardware
or software control, and network availability will come soon.

Thanks again.

Jonathan
KC3EEY