[TangerineSDR] 3-Channel VLF SDR Backend System

Jonathan emuman100 at gmail.com
Mon Aug 14 09:57:41 EDT 2023

My apologies, the pictures did not attach inline. The attachments are all
in order of what I describe.


On Mon, Aug 14, 2023 at 6:20 AM Jonathan <emuman100 at gmail.com> wrote:

> I have been working on a 3-channel VLF backend system similar to the
> single channel system I built in 2020. It's based on a Raspberry Pi 3,
> Audio Injector Octo Soundcard, Trimble Resolution SMTx GPS timing receiver,
> and VLF preamp interface board and power distribution. It's designed to
> capture VLF spectrum from an E-field receiver and an orthogonal loop dual
> channel H-field receiver for triple axis reception of the VLF band. With
> it, bearing can be determined and the loops can be synthesized for any
> bearing based on how the loop signals are mixed. This provides additional
> analysis of VLF signals using the powerful vlfrx-tools software. Everything
> in mounted in a Hammond dicast aluminum enclosure. In the center is the
> Raspberry Pi 3B, Audio Injector Octo Soundcard with audio breakout board,
> and TTL<>RS232 adapter for the serial console. On the left are power,
> capture, and timing status indicator LEDs as well as a safe shutdown button
> to safely unmount the data USB drive. On the right is the Trimble
> Resolution SMTx and interface board. On the bottom is the VLF receiver
> interface board.
> This is the Pi 3B with Audio Injector Octo soundcard. It has 6 audio
> inputs and can sample up to 96 kHz. The audio breakout board breaks out the
> audio inputs to RCA jacks, which I removed, for a direct solder connection.
> The PPS from the GPS gets get through a potentiometer for adjustment to 80%
> of the soundcard’s full scale. I will be feeding it through an RC network
> to shape the 125 us pulse. The PPS is also connected to a GPIO pin for use
> with the ppsgpio driver, GPS Daemon, and ntp and functions as a networked
> stratum 1 time server as well. Data is stored on a 512 MB USB drive. The
> console port is accessible via TTL<>RS232 adapter (in blue heat shrink) for
> complete headless operation, especially when the network is not available.
> Both the Ethernet and RS232 are connected to RJ45 bulkhead couplers for
> panel jack connection. The indicator LEDs, shutdown button, console port,
> and GPIO PPS all connect through a 40-pin female header.
> The GPS is a Trimble Resolution SMTx GPS timing receiver. I used it
> because it was cheap and what I had on hand, but still performs well for an
> older model of the Trimble/Protempis GNSS timing receiver line. The PPS
> time pulse width is 125 us. It’s powered using the handy PPS Piggy
> interface board for Trimble/Protempis receivers. The antenna is connected
> through an SMA to SMB pigtail with bulkhead SMA jack. The other hole in the
> enclosure is for the Raspberry Pi WiFi antenna jack which I will add later.
> The indicator LEDs are panel mounted as well as the safe shutdown button.
> These provide an indicator for power, soundcard capture, and GPS timing,
> with the later two controlled by GPIO pins and series resistors. The safe
> shutdown button will issue “shutdown -h now” when pressed for longer than 3
> seconds to safely unmount the USB drive if no network or console access is
> available. Data will constantly be written to the USB drive during normal
> operation in bursts. The USB drive is ext2 fornated. The LED indicators and
> safe shutdown button are monitored via script.
> Lastly, this is the VLF receiver interface board. It provides power to the
> Pi and GPS receiver using an adjustable 3A DC-DC converter set to 5.1V.
> Power to the E-field and H-field VLF receiver channels is through 24V
> isolated DC-DC converters. Main power comes in via 12V unregulated wallwart
> and drives both the adjustable DC-DC converter and the isolated DC-DC
> converters. The VLF receiver channels also have audio isolation
> transformers to maintain isolation between the backend system and VLF
> preamp and connect to the audio inputs on the audio breakout board. Both
> the power and signal paths have 10M bleeder resistors to bleed off any
> excess charge on the feedline as well as gas discharge arrestors for surge
> protection. The feedline is shielded cat5 or cat 6 cable pairs and connect
> to the green screw terminals. The isolated DC-DC converters are plugged
> into pin sockets and are removable in case the feedline is too long and 48V
> DC-DC converters are used to maintain the voltage at the end of the
> feedline due to the voltage drop of a long feedline. The board also
> provides a connection to power LED indicator as well.
> My next step is to fine tune the shaped PPS pulse for more accurate
> timing. Once complete, I will start work on the dual channel H-field
> receiver.
> Jonathan
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