Hello Jim and the group,
After carefully reading Jim's article, and collecting some more material
about MSK, GMSK, etc, I came to the conclusion that there is a bit more
behind (G)MSK than just BPSK with additional quadrature component (90°)
to fill the gap.
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.
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.
After all this high-tech stuff, I will be active on 136kHz using the
good old morse code 136kHz on saturday and sunday, but will look out for
other signals as well ;-)
Regards,
Wolf DL4YHF.
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