[TangerineSDR] VLF Audio Interface for Tangerine SDR

[Bill Liles]

Jonathan, thanks for the info.

You can read about the AWESOME receive equipment at
M. B. Cohen, U. S. Inan and E. W. Paschal, "Sensitive Broadband ELF/VLF
Radio Reception With the AWESOME Instrument," in IEEE Transactions on
Geoscience and Remote Sensing, vol. 48, no. 1, pp. 3-17, Jan. 2010, doi:
10.1109/TGRS.2009.2028334.

A site you might find interesting is the Low Frequency Research Group at
Georgia Tech at http://lf.gatech.edu.

There is also a database of VLF and lower data know was the Worldwide
Archive of Low-Frequency Data and Observations (http://lf.gatech.edu). This
database has data from the mid 1970s till now.

Bill, NQ6Z

On Wed, Jul 15, 2020 at 12:47 PM Jonathan <emuman100 at gmail.com> wrote:

> Hi Bill,
>
> I appreciate all of the support from everyone!
>
> Yes, the plan is to have at least three to four input channels, depending
> on the limitation of board space. Each channel requires an analog input
> stage that is outlined in the data sheet, so I’ll try to utilize quad op
> amps. One channel will be for the E-Field probe receiver and the other two
> for the North-South and East-West B-Field loop receivers. With triple axis
> reception, bearings can be calculated for signals of interest. I invite you
> to take a look at the amazing visualization of the utility program called
> vtpolar in vlfrx-tools on the documentation page at
> http://www.abelian.org/vlfrx-tools/notes.html and click on “Polar
> Displays”. Click on the example video to show you a live polar plot of a
> three channel stream (E-Field, N-S B-Field, E-W B-Field) containing a bunch
> of sferics and whistlers. It plots the bearing and amplitude in real time
> with audio! It’s so cool! A network of these receivers can be used for not
> only lightning location, but other VLF event location, like locating the
> exit point of a whistler.
>
> The user will receive a stream of direct samples with an added timestamp
> on each sample. I’m not sure how it would be implemented, but I do also
> believe I/Q data can be generated in the FPGA. This can also open more
> doors in signal analysis. This would be something worthwhile to investigate
> after the milestones have been reached.
>
> I have not heart of the Awesome project. Do you have some information on
> it?
>
> Jonathan
> KC3EEY
>
> On Jul 14, 2020, at 5:35 AM, Bill Liles <lilesw at gmail.com> wrote:
>
> Jonathan, sounds really good. I would just like to clarify some things.
>
> 1) The board will have multiple inputs so that one can do things such as
> connect two loop antennas to get lines of bearings to the signals of
> interest. Is this true?
>
> 2) At the user level, will the use receive just a real stream of sample
> data or with the user receive I/Q data? I realize that the CS5364 (great
> selection) only outputs real data but additional FPGA processing could
> produce I/Q data.
>
> Thanks for doing this.
>
> Are you familiar with the Awesome project out of Georgia Tech?
>
> Bill, NQ6Z
>
>
> On Tue, Jul 14, 2020 at 1:06 AM Jonathan via TangerineSDR <
> tangerinesdr at lists.tapr.org> wrote:
>
>> 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
>> >>
>> >
>>
>> --
>> TangerineSDR mailing list
>> TangerineSDR at lists.tapr.org
>> http://lists.tapr.org/mailman/listinfo/tangerinesdr_lists.tapr.org
>>
>
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