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

Jonathan emuman100 at gmail.com
Mon Aug 14 12:13:29 EDT 2023


Did you take measurements of the sampling and timestamping accuracy of the
Grape 2? I don't believe you included it in the other email.


On Mon, Aug 14, 2023 at 11:21 AM John Gibbons <jcg66 at case.edu> wrote:

> 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
>>> KC3EEY
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