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.
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)
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