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

John Gibbons jcg66 at case.edu
Mon Aug 14 14:43:48 EDT 2023


The absolute accuracy of the 1PPS synchronization lies exactly between 1-2
sync clock cycles (crossing over clock domains requires this to prevent
metastability - you must do this as well!)
so my 1PPS is always between 125-250 nSec (8 MHz clk) from the 50 nSec
window that the UBLOX gives me for absolute timing of the 1PPS signal.

The derived sample clock, however, is deadly accurate as it is also derived
from the UBLOX freq output so it will easily hold 1x10^-10 accuracy and on
an 8 KHz sample clock it guarantees me to be within 12.5 aSec (yes 10^-15
sec).  Since I presume you're using the on board clock for your A/D card
you will be at least 4 orders of magnitude worse as quartz xtals (or even
worse a resonator!) are not so good (not to mention temp drift that will
eat you alive...).

The sample clock is very important as it determines your A/D sample freq
hence directly affects any timing / frequency measurements you extract from
your data.
For us it directly affects the freq measurement of the carrier freq and
needs to be pretty darn good (12.5 aSec is pretty respectable).

What is your freq ref standard for the A/D sample clock?  Is it GPS DO'd?
Is it synced to the 1PPS?

John N8OBJ

On Mon, Aug 14, 2023 at 12:13 PM Jonathan <emuman100 at gmail.com> wrote:

> John,
> 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.
> Jonathan
> 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
>>> KC3EEY
>>> 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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