[Laser] Re: PSK31 via light - up-convert to RF vs audio amp

[Tim Toast]

I bet you wouldn't need any further amplification of a
PGP preamp output in the demo version at least. It
seems to have plenty of output to feed a soundcard
directly. 
The one transistor photo-RF converter you described
sounds like the light would modulate the oscillator
kind of weakly, although maybe it would still have a
usable signal. This would be an AM modulated signal to
the radio i'm guessing.
Perhaps there is a way to add an oscillator/mixer to
the PGP RX without having to modify it too much. So
you could have a 3.58 mhz RF output in addition to the
baseband output. Then you could compare the two
signals conveniently. 
In any of these mixer type circuits, i would expect a
small amount of additional noise over the plain
baseband amplifier approach. But maybe there's an
advantage nevertheless in having all or most of the
amplification in the radio receiver. 


PSK seems to be a lot more rugged than the ideal
situation when it's a pure sinewave-ish tone. I think
i read the loss in signal gain or whathaveyou is only
2 or 3 dB?? due to it not being perfectly formed (the
raw digital output of that OR gate) I've seen mildly
over-modulated PSK signals on the ham bands that were
still readable just fine. Granted, they were probably
not raw squarewaves. But being an FM mode inherently
at least in part, i don't see why a PSK signal would
suffer much damage from being fully 'limited' or
squared up. I assume that 2 or 3 dB is lost forever,
but on the receive end, the signal can be cleaned up a
little with a narrow filter before being sent to the
computer's sound card if needed. I'm not sure how much
this does in restoring the original PSK waveform, but
it would do some good i bet.



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