Message-ID: <41B0ECBF.1090900@freenet.de>
Date: Fri, 03 Dec 2004 23:46:23 +0100
From: Wolf DL4YHF <dl4yhf@freenet.de>
User-Agent: Mozilla Thunderbird 0.9 (Windows/20041103)
MIME-Version: 1.0
To: RSGB LF Group <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Variable-PSK theory (warning - goes on a bit!)
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

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.