For yesterday's EbNaut transmission, I have not employed the
usual Rubidium oscillator and automatic antenna variometer
hardware, but reverted to the software modulation scheme which
had been originally used for our first tests in April 2014. This
is an extension of the "green line" anti-glitch method, with
1pps-sync in SpecLab and RF feedback of the analog output to the
ADC input. To generate PSK modulation, the phase prescription
for the software PLL is no longer fixed at 180° (green in the
colour spectrogram), but switched between 90° (red) and 270°
(sky-blue) by a SpecLab script which periodically reads 1 or 0
symbols from a text file.
The primary advantage of this method is that no external
hardware modulator is needed. With the feedback picked up from
the antenna, it also intrinsically compensates for wind-induced
resonance variations and can do without an automatic variometer.
Thirdly, the phase switching occurs smoothly by frequency
variation, reducing switching transients and far-away sidebands.
The main disadvantage is that the phase ramping takes time
(about 1 to 3 seconds) and also introduces some jitter, so this
method cannot be used effectively for fast signaling with
symbols shorter than 5 seconds.
To use it, you first need to set up GPS 1pps and audio
feedback as described in
http://df6nm.bplaced.net/VLF/VLF_Transmit_phase_correction.txt
This can be done either with a stereo input soundcard (1pps on
the right, RF feedback on the left channel), or mono with both
signals combined through resistors (RF feedback about 5% of the
1 pps level). Make sure that the 1pps samplerate detector is
locking properly (i.e. mostly green), and your PC time is close
to the nearest second.
Then you need to encode your message using Paul's ebnaut-tx
software
http://abelian.org/ebnaut/
and press "Save" to make a text file. For use with the SpecLab
script, you then need to insert spaces as separators between the
0 and 1 digits, e.g. converting "1001..." into " 1 0 0 1..." by
replacing 0s and 1s in notepad (note that "1 0 0 1 ..." works
just as well but may produce a surprising result when reopening
the file - search for "bush hid the facts" ;-) As an example, my
recent transmission is at
http://df6nm.bplaced.net/VLF/psk_8k19a_73.txt
The file should be saved as psk.txt and placed in your SpecLab
home directory.
Then you will want to select
- the TX frequency in the second oscillator of the test signal
generator,
- the symbol duration as periodic actions interval (eg. 10
seconds),
- the start time, by letting scheduled actions open psk.txt a
couple of seconds before. You should also close the file after
the transmission,
- and finally tick "send unmodulated test tone at TX frequency"
in the digimode terminal to let the signal appear on the left
output.
A couple of notes regarding frequency selection: For straight
carrier experiments, staying within very few mHz of 8270 Hz can
show the signal on various grabbers. However EbNaut PSK
reception would be susceptible to co-channel interference over a
wider bandwidth, eg. +- 0.1 Hz for 10 second symbols. As the PSK
signal will anyway be invisible on the grabbers, it might be
advisable to choose e.g. 8970.1 or 8969.9 Hz for future PSK
transmissions, thereby nulling potential carriers near 8270.000
but still staying inside the decimated IQ bandwidth for EbNaut
recordings (+-0.25 Hz here).
As SpecLab aligns the absolute phase to time-of-day, the TX
frequency for overnight transmissions should be a multiple of
1/86400 Hz to avoid an Alpha-like midnight phase step.
Hope this may help to get a few more EbNaut signals on air...
Big thanks to Paul and Wolf for making all this possible.
All the best,
Markus (DF6NM)
