Re: VLF: Experiments - anyone?

[Markus Vester <markusvester@aol.com>]

 Hi Warren, Paul,

 

I'm pasting two earlier mails to the group and to Dex, which should give an idea of the transmit setup using Paul's FEC-encoded BPSK.

 

Best 73,

Markus (DF6NM)

Von: Paul Nicholson <vlf0403@abelian.org>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mo, 1 Dez 2014 9:25 am
Betreff: Re: VLF: Experiments - anyone?

...

I think so, Markus devised a way to send an encoded message
using Spectrum Lab.  It is necessary to arrange something like a
DPDT relay to reverse the phase of the tx signal under control
of Spectrum Lab.  The PC needs reasonably good clock discipline
(0.1 seconds or so) so that the bits are synchronous with UT
and the carrier has to be GPS or Rubidium locked, or maybe
from a very good OCXO.

I'll dig out the details in a follow-up.
--
Paul Nicholson
-- 

-----Ursprüngliche Mitteilung-----
Von: Markus Vester <markusvester@aol.com>
An: dexter.mc <dexter.mc@gmail.com>
Cc: Paul Nicholson <vlf0403@abelian.org>
Verschickt: Mo, 2 Jun 2014 11:30 pm
Betreff: Re: 8971 khz - SpecLab setup for PSK drive

Hi Dex,

 

I'll try to give some hints regarding speclab setup for binary audio output.

 

First you'll need to encode your text message using Paul's web script http://abelian.org/fec/encode.cgi . Let Paul suggest an encoding (probably either r 1/8 K=25 or r 1/16 K=25), a symbol duration (eg. 30 seconds), and a UTC start time. Copy and paste the resulting binary sequence to a text file.

 

To setup SpecLab, you can start from a config file which was derived from my current VLF grabber.

http://dl.dropboxusercontent.com/u/26404526/_vlf_wide_twoloops_tx-psk-30s_140602.USR

http://dl.dropboxusercontent.com/u/26404526/SpecLab_setup_to_send_binary_sequence.png

You can ignore or disable all that stereo receive stuff. All that's needed is a digimode terminal instance, which should be configured as shown on the right of the screenshot. Important entries are Encoding: None (one-to-one) and the speed setting, eg. 0.0333 sym/sec. Then you can load the prepared text file to the TX window, but don't press the big TX (red) - RX (green) - OFf (grey) button yet.

 

The transmission should be initiated automatically from the scheduled actions tab (top left), issuing the di.tx("") command precisely at the assigned sequence start time (eg. 5:00 UT). This will then activate the dormant digimode terminal on time, based on the PC clock which should be kept to UTC by some NTP tool.

 

The audio output from the soundcard will be on-off keyed dashes at eg. 1300 Hz, which can simply be rectified to drive a small relay, commuting between 0° and 180° outputs of a small transformer: http://df6nm.bplaced.net/VLF/pictures/VLF_PSK_140510/psk_relay.jpg

 

Good luck!

Markus

 

-----Ursprüngliche Mitteilung----- 
Von: Markus Vester <markusvester@aol.com>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: So, 11 Mai 2014 12:11 pm
Betreff: Re: VLF: Coherent BPSK at 8270 from DF6NM

Thanks Paul, for devising this experiment in the first place, and encouraging me to take part in it. It looks like we are now getting real close to the ultimate Shannon limit, making best use of your superb receive capabilities.

 

The transmit hardware setup is similar to the previously used one, with a couple of minor extensions. Here are some pictures: df6nm.bplaced.net/VLF/pictures/VLF_PSK_140510/ 

 

The 8270 Hz carrier comes from a Rubidium-referenced decadic synthesizer, with frequency calibrated to Loran-C to within about 1e-10. For the test, an encoded bit sequence for the five-character message was obtained beforehand from Paul's script on his website. SpecLab's digimode terminal (configured similar as previously for Opera, and initiated by "scheduled actions") is used to generate a keyed sidetone, with bit timing controlled by NTP to within a second. The tone is being rectified at the soundcard output and drives an ancient and sensitive telephone relay, commuting the carrier between two complimentary outputs of a small pot core transformer.

 

The phase-keyed signal is then amplified by the usual "Nitro" audio PA to about 60 Watts (28 V rms). A large ferrite transformer (in the brown chocolate box) bought the voltage up to 200 V rms, feeding 0.3 A antenna current to the venerable seven-bucket coil which is sitting in the blue rubber bins outside the window.

 

To keep the antenna on resonance, the phase detector from my LF automatic tuner (basically an XOR comparing the phase of voltage and current, with sensors modified for the lower frequency) was inserted between the amp and the transformer. It drives a little DC motor, pulling on a nylon string, which then rotates a 10x10x0.5 cm^3 ferrite plate inside the coil. This effectively mitigates current reductions and phase variations caused by wind, temperature and humidity. During operation, one can hear the whistle of the coil and transformers, occasionally interrupted by a short "krk" sound when the relay is bouncing during phase changes at full or half minute.

 

All the best,

73, Markus (DF6NM)

 

From: Paul Nicholson

Sent: Sunday, May 11, 2014 8:36 AM

To: rsgb_lf_group@blacksheep.org

Subject: VLF: Coherent BPSK at 8270 from DF6NM

I am pleased to report reception of two test transmissions from
DF6NM on 8270.000 Hz which took place on Saturday 2014-05-10
morning.

Markus was sending coherent BPSK with UT synchronous symbols
using a symbol period of 30 seconds.  The FEC is a terminated rate
1/4 convolutional code with constraint length 21, cascaded
with an outer error detecting code using a 16 bit CRC.

Two transmissions of 46 bits were made, each lasting 132
minutes with a 20 minute carrier test in between.  ERP was
probably around 5 or 10 uW and the range is 1028 km.

Eb/N0 was about -0.5dB in the first test and about -1.5 dB
in the second, which is below -7dB in the symbol bandwidth
of 33.3 mHz.

Both messages were decoded with some margin to spare.

The decoder is a list Viterbi decoder using the tree trellis
algorithm with a list length of 2000 and stack size 20000.
Both messages decoded at the top of the list so the list
decoding wasn't actually necessary for this strength of signal.

The transmitter uses a rubidium source and the receiver is
GPS timed.   A reference phase at the receiver is obtained by
averaging the phase of the squared signal but at such low signal
strengths the resulting reference is unreliable and the decoder
makes a search for the correct phase and phase drift rate.

The signal is completely invisible in any spectrogram at the
receiver.   In a spectrogram running at the symbol bandwidth
the signal is too far below noise and the bandwidth of the
transmission is such that, in a resolution capable of seeing
the signal above noise, the signal is too wide.

For example, this spectrogram uses the symbol bandwidth,

  http://abelian.org/vlf/tmp/df6nm_140510a.png

Maybe someone can see some very faint line at 8270.000 ?

The first test ran 07:02 to 09:12 and the second
from 09:32 to 11:44.   The 20 minute carrier in between is
also below noise at this resolution.

Following the second test, an hour of carrier is visible in
the 278 uHz spectrogram at

  http://abelian.org/vlf/fbins.shtml#p=1399780800&b=110&s=sp

More information on the FEC codes, trials, and search for
good polynomials can be found at

  http://abelian.org/fec

Stronger codes with constraint length up to 25 are available
and hopefully further tests will be made soon.
--
Paul Nicholson
--