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

John Gibbons jcg66 at case.edu
Wed Aug 16 12:58:01 EDT 2023


Yes - the 1PPS is spot on and the 8KHz A/D sample timing is synthesized
directly from the UBLOX 8MHz GPSDO'd output that runs the entire data
collection engine.

John N8OBJ

On Tue, Aug 15, 2023 at 12:50 PM Jonathan <emuman100 at gmail.com> wrote:

> Hi John,
> Did you take measurements of the PPS  and clock accuracy? How is the 8 kHz
> sample clock synthesized?
> The Audio Injector Octo uses a crystal oscillator, but like in the
> previous email thread about timing, vlfrx-tools is constantly making
> measurements and calibrating out the delay, then realigning each sample
> according to that series of calibrations and alignment of the centroid of
> an RC network-shaped PPS. I manually insert a calibration value consisting
> of the time between the rising edge of the rectangular pulse and the
> centroid of the RC network-shaped pulse, giving me similar PPS accuracy as
> you have achieved.
> Jonathan
> On Mon, Aug 14, 2023 at 2:44 PM John Gibbons <jcg66 at case.edu> wrote:
>> Jonathan,
>> 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
>>> KC3EEY
>>> 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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>>>>> <https://groups.google.com/d/msgid/hamsci-grape/CAOY0kB1oFMEHL2-ph5Re%3DOBnBBz%2Bv7aKhZiYnvH5gFTS0L4K-g%40mail.gmail.com?utm_medium=email&utm_source=footer>
>>>>> .
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