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

[Tim Toast]

I was thinking about the kind of radio receiver this
would be most like. I like the comparison of light and
radio receivers, since both are using electromagnetic
waves -only the frequencies are different. If the
basic photodiode followed by a high gain amp is the
"crystal set", then it seems like the same setup, with
the addition of a local oscillator light source, would
be a "converter" or direct conversion type receiver
-since the LO is added at the diode for mixing before
detection. Although the output of the mixing process
is not audio as in most DC receivers. So i guess
"converter" fits best.

I know this is not really 'true' superhetrodyne in the
sense of two coherent carriers being mixed. It's only
the modulations of the carrier beams that are
intentionally being mixed in this case (the
subcarriers of the light beam carriers). It's true
though that the lightwave carriers are also mixing,
it's just the product of that isn't being used and may
even be absent without some non-linear mixing.  

I think the dividing line between mixing and
modulation is a bit blurry here. I was thinking if the
LO is relatively dim and doesn't saturate the
detector, then the mixing would remain linear. This is
more like the mixing of two microphones in an audio
mixing console. But if the LO does saturate the
detector, then it might be more like a 'chopping'
effect - 100% modulation of the received beam, which
would be a non-linear type mixing. I'm assuming the
non-linear mixing is more efficient than linear.

All this is probably in vain if the LO adds too much
noise by illuminating the detector brightly - or the
lowered sensitivity due to a required low impedance
diode termination for RF response. While a bright LO
may not be needed, there's probably no way around the
low impedance RF thing, sadly. It's like there's a
permanent S/N penalty to pay for going from crystal
set to converter.

http://www.arrl.org/news/stories/2002/12/23/1/Marconi-2-lrg.jpg




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