[TangerineSDR] [hamsci-grape] Re: 3-Channel VLF SDR Backend System

John Gibbons jcg66 at case.edu
Mon Aug 14 11:21:16 EDT 2023

That has already been designed and built and hardware tested (with better
timing for data sampling) - it's called the Grape 2

John N8OBJ

On Mon, Aug 14, 2023 at 9:57 AM Jonathan <emuman100 at gmail.com> wrote:

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