[TangerineSDR] VLF Audio Interface for Tangerine SDR

[Jonathan]

Hi Tom, All,

I agree, I think the CS5364 is a great choice. I also agree that the
sampling rate ranges don't seem conventional, but I believe one reason
for this is to make a hardware or standalone mode functional. This
ADC, like many other Cirrus audio ICs have both hardware and software
control modes which can make it very versatile.

I will be working on the design of this board. I know everyone is busy
with various tasks, so I'd like to contribute. No other SDR has this
sort of option, so I think this will prove valuable in the long run to
allow for VLF natural radio study and dreamers band (8.720 kHz) QSOs
on the TangerineSDR. My plan is to do a layout that will include the
ADC, analog input stages, clock routing, and digital audio interface.
I'll need help with the FPGA hardware description and low noise layout
practice.

I believe for this application, software control is the best approach,
and can be either
SPI or I2C. For configuration of the board from the data engine, I'd
like to have:
16 or 24-bit sample lengths
48/96/192 kHz sampling rates
1/2/3/4 channels for audio capturing, so I believe TDM mode is best
whether two or more than two channels are selected for capture. I2S
only allows for transmission of up to two channels at a time.

The reason why I chose the CS5364 is because a high end audio
interface used for music and audio recording utilizes a similar IC
from Cirrus, and it happens to be utilized by many in the VLF
community as a VLF SDR. It's the Behringer UMC404HD. (it's product
page is https://behringer.com/product.html?modelCode=P0BK1)
It utilizes the CS4272 stereo audio codec (datasheet at
https://statics.cirrus.com/pubs/proDatasheet/CS4272_F1.pdf). Codecs
contains audio inputs and outputs which is unnecessary for this
application and also has the limitation of using only its internal
clock and no MCLK input. Some people in the VLF community also use the
UMC202HD which has two inputs instead of four.

The purpose for my board will be to replace the soundcard,
computer/Raspberry Pi, and GNSS receiver completely and integrate
everything into my board and the Tangerine SDR. This way, the stream
can be sent over the network to any host for later processing using
either Spectrum lab (Windows) or vlftx-tools (Linux/BSD/Solaris).

An example of a permanent VLF receiver installation is Mike Smith's
setup. You can see from his site
(https://www.n4vlf.net/efield-current.html) a schematic/block diagram
of his receiving system that includes a VLF E-Field receiver,
amplitude modulated optical audio feedline (eliminates the need for
isolation transformers), and PC/soundcard/GPS setup.

On his PCs, one of them uses a 192 kHz soundcard to capture the VLF
signal and GPS PPS. He uses Linux with vlfrx-tools. The utility
program that captures audio from the soundcard, vtcard, captures audio
from the soundcard at a sample rate and sample length of your choosing
specified by runtime options. Timestamping is done by vtcard, but
using the soundcard's clock. The utility program vttime samples the
audio stream for the GPS PPS pulse and produces an offset value to
adjust the timestamps to align with GPS time. With the Tangerine SDR,
the ADC will be clocked with the GPSDO, so because of its stability,
the timestamping should not need to be adjusted.

Lastly, Mike also has a block diagram
(https://www.n4vlf.net/vlfrx.html) of all the utility programs in
vlfrx-tools used on his PCs. The audio is hum filtered, resampled, and
sent to event detectors for detecting whistlers, chorus, and sudden
ionospheric disturbances, as well as streamed for live listening,
storage, and retrieval.

vlfrx-tools is written by Paul Nicholson, one of my personal idols.
His work in VLF is absolutely amazing and myself and the rest of the
VLF community are absolutely indebted to him. I invite all of you to
look at his page (www.abelian.org) and be sure to look at his
vlfrx-tools software.

David McGaw,

The reason for choosing the extended audio range is to focus on
natural radio events in the VLF band, primarily occurring in the lower
VLF band and upper ULF band. Also, can you give some more detail on
understanding the dynamic range using 16-bit samples?

Jonathan
KC3EEY



On 7/13/20, Tom McDermott <tom.n5eg at gmail.com> wrote:
> Hi Jonathan,  this looks like a good addition to the receivers available
> for TangerineSDR.
> The CS5364 IC has some nice specifications, and the ability to receive an
> external clock,
> so it looks like a good potential choice.
>
> I notice that the frequency selectable input range is interesting (I've not
> seen a similar mode
> selection of operation on other components):
> * 2 khz - 54 khz  or
> * 54 kHz - 108 kHz  or
> * 108 kHz - 216 kHz
>
> -- Tom, N5EG
>
>
> On Sun, Jul 12, 2020 at 10:14 PM Jonathan via TangerineSDR <
> tangerinesdr at lists.tapr.org> wrote:
>
>> 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
>>
>> --
>> TangerineSDR mailing list
>> TangerineSDR at lists.tapr.org
>> http://lists.tapr.org/mailman/listinfo/tangerinesdr_lists.tapr.org
>>
>