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Date: Sat, 27 Nov 2004 16:18:05 +0100
From: Alberto di Bene <dibene@usa.net>
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Subject: LF: Re: MSK etc and stability
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Hi Alan, Jim, Andy, Bill, Wolf, etc. etc.

  just a few wild thougths on the subject, which haven't been thoroughly 
analyzed, but
this is what the collective mind of this forum is meant for, isn't it 
?   :-)
Firstly, I like the idea of Andy, of a GSPSK (Gentle Sloping PSK), which 
should
substantially reduce the amplitude variations of the signal, when 
subject to phase
modulation. Any volunteer for a mathematical analysis (or a simulation) 
of the crest
factor as a function of the "gentleness" of the phase transitions ?

Then it comes the issue of frequency stability and precison, which are 
two different items.
While stability is quite important, let's see if there is a way to relax 
the demand for
absolute precision.  I was thinking of a scheme more or less like the 
following, for the
137 kHz scenario, for demodulating a BPSK transmission.
Suppose your hardware has a fixed LO at 135 kHz, and a couple of 
quadrature mixers.
Then the entire 137 kHz band is converted into the range 700 - 2800 Hz, 
whose I & Q
components are fed to the ubiquitous sound card.
A PC program can now mix down, with a tunable NCO, that range to zero IF 
and apply
a complex FIR lowpass with a BW of just a few Hz. What do you have now ?
A rotating vector (ideally it should not rotate, if the zero IF is 
actually zero),
whose phase is instantaneously defined by  atan(Q/I). If you do a long
term averaging of the speed of variation of this phase, you can then 
more or less easily
measure if the instantaneous phase is leading or lagging this averaged 
value.
This is just the recovered 0 or 1 bit of  the phase modulation applied 
to your carrier.
As long as you stay inside the BW of the above FIR filter, the absolute 
value of the variation
of the phase with time (i.e. the frequency) is of no importance, as you 
are interested only
in the deviations from the mean of this value. So you have a tuning 
tolerance of a few Hz.

Of course there are a lot of details to be worked out carefully, like 
the adaptivness of the
computation of the mean value, and a precoding scheme that would more or 
less guarantee
that no DC component is present in the modulation signal, to avoid undue 
shifts of the
averaged phase speed value, but these are items not impossible to cope with.

As said, just a wild idea. If there are valid reasons to trash it, 
please speak up, so I won't
waste any more time thinking at it.   Thanks.

73  Alberto  I2PHD
P.S.  BTW, with the above scheme, sampling rate errors of the sound card 
would become
of no importance.