VLF: SpecLab GPS-phaselock with faster PSK

[Markus Vester <markusvester@aol.com>]

The SpecLab GPS-PLL script can overcome soundcard glitches and provide absolute phase of the antenna voltage, but PSK modulation has been limited to slow symbol rates due to the ~ 3 second time constant of the phase swing. It appears that the PLL latency is mainly due to the digimode terminal generating the output signal, which responds to frequency commands from the interpreter script only with some delay. I have now tried to circumvent the lag by using the L3 frequency mixer which can be controlled faster, allowing phase-switching in less than 200 ms.

A couple of tricks were necessary: A constant 2 kHz signal is generated in the L1-L2 blackbox, which is then converted to the desired VLF output frequency. In addition, instantaneous phase switching is implemented by switching the bypass factor in the L4-L5 delay line. Alternatively, the phase can be smoothly ramped by setting a "hardkey" flag to zero.  

The configuration file is here:
 http://df6nm.bplaced.net/VLF/fec_tests/vlf_tx_pll_fastpsk.zip

For EbNaut transmissions, you will need to use Paul's software and prepare a file "psk.txt" (containing zeroes, ones and spaces, similar to the provided template), and set up start times etc. in scheduled actions. You can also produce a simple 2-second test pattern by activating the second line in periodic actions. As before, the output frequency is defined by the second test signal generator, which provides the GPS-controlled reference.

Hope this may be useful,
Markus (DF6NM) 

-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mo, 28 Nov 2016 7:40 pm
Betreff: Re: VLF: EbNaut transmissions on lower frequencies?, pre-tests: 6.47kHz

Thanks Paul,

There could be another reason, see attachment. So far the phase is
inverted within SpecLab, not by e.g. keying a relay by the serial
interface. Due to the PLL and time delay in the sound card chain this
takes some time. With a short symbol length of 12 seconds the effect may
become relevant. A phase turn of 180 deg is done by varying the
frequency for a certain time as you can see.
So maybe this costs a dB of SNR here?

For a 7 second symbol length in the 75 character message i will need a
different technique, taking 'no' time for the phase inversion.

Another thing could be the moving sunrise / sunset time on our path. We
already saw a wider phase variation in the 20 character message than in
the 7 day 'TEST' message before. Maybe it is even worse now.

Am 16.11.2016 19:48, schrieb Paul Nicholson:
>
> For 1uW ERP, DK7FC to Todmorden
>
> http://abelian.org/vlf/tmp/1479317903_9356.png

That plot must have been valid 2 weeks ago. What does it show now? Maybe
it is a good idea to start 08:15 UTC instead of 08:00 UTC...
Could it (it = the move of the positive green slope towards later times)
be the reason for the higher phase variations that we can see?

73, Stefan