Dear Wolf, LF Group,
-----Original Message-----
...Jim wrote:
I beleive Wolf has used a type of PSK where the phase of the carrier
is varied linearly from 0 to pi radians,
or pi to 0 radians, for a 0 to 1 or 1 to 0 bit transition.
Yes that's true, but the transition did not use 100% of the symbol time
(but 10..20%) to stay compatible with the "PSK31" standard decoders. So
this was not (yet) comparable to MSK.
OK, I did not realise that - I guess the effect would be to reduce the
amount of power in the quadrature component significantly, but also to
increase the amplitudes of the high-order sidebands significantly. But I am
pretty sure the tests I did a couple of years ago did use a phase transition
period of 100% of the bit period - this seemed to work quite happily with
the "coherent" BPSK decoder. I don't see any obvious reason why the extra
quadrature component in the variable-PSK signal should prevent the standard
PSK31 demodulator from working with "transition period = bit period", but
then I don't know a lot about how the demodulator actually does work, so I
could well be wrong!
I have modified G3PLX's QPSK-mode into Offset-QPSK by delaying the
Q-component in the modulator by a half symbol time, and in the
I-component of the demodulator to compensate this. By doing so, there
are no more 180°-phase transitions in the signal any longer (only +/-
90°), so the amplitude only drops by 3dB. Still not perfectly suited for
class C amplifiers, but the requirements for a "linear" TX are not as
tough as for AM-shaped QPSK or BPSK. No tests done with any of the
"Novel" (G)MSK decoders yet, but at least I am beginning to understand
what the tricky parts are.
Will be interested to see what you come up with. BTW, thanks for the CW QSO
earlier today!
Cheers, Jim Moritz
73 de M0BMU
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