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

[TWOSIG at aol.com]

I was thinking about how to set up a PSK31 demonstration over a light  beam.  
Since PSK31 is a very linear modulation mode, primarily for narrow  bandwidth 
that is not of primary concern to a light beam channel, it may be  easier to 
impliment with LED than with laser.  The circuit that I use for  AM with LEDs 
should be able to take the audio output from a sound card as well  as from a 
microphone.  I am not happy with the receive system that I  use with the AM 
demo, the receiver from Ramsey pulse width modulation laser  system.  It uses a 
phototransistor as the sensor, and I do not consider it  easy to build, unless 
you buy the Ramsey kit, or to modify for different  needs.
 
I thought of building a pre-amp, such as the K3PGP design, and use that  with 
additional amplification to drive a sound card input.  Then I  remembered a 
system using FM voice that was used by N6IZW.  Instead of  building an FM 
decoder for the 30 KHz signal, it was run into a mixer along with  a 145 MHz 
signal, then decoded with a 2M radio set for 145.030 MHz.  I  realize that the two 
systems are very different.  The nature of the FM  signal would make it immune 
to rather large amplitude noise in the up converter  system.  Phase Shift 
Keying should have some of that noise immunity, it  might not work as well as the 
FM system did.
 
Here is what I am wondering if will work:  Start with an oscillator  with a 
TV color burst crystal because they are cheap and easy to obtain.   Feed that 
to the emitter resistor of a transistor with a photodiode in the base  circuit. 
 The output should be available from the collector using a  "gimmick" or 
small coupling capacitor made by twisting two insulated wires  together, to the 
antenna input terminals of a transceiver or communications  receiver.
 
The receiver gain and selectivity should be able to provide a signal to the  
input of a sound card that is a good representation of the signal photons on 
the  photodiode.  With a relatively strong signal, this might even work with  
something like the PSK-80 "Warbler" ( our radio club bought several of those  
kits a while back ).
 
Now for the question of how practical this system might be for real  
communication, rather than just the demonstrations that I typically work  on.  It 
seems to me that they system moves the problem of signal  amplification to the RF 
and IF systems in the receiver.  Is there any  advantage to doing this 
compared to a simple audio preamp and amplifier?  I  have read that the noise in the 
first amplifier stage is critical, so is there a  signal to noise advantage to 
feeding the photodiode current to the mixer, an RF  stage instead of to an 
audio amplifier?
 
 
>From a practical, well at least a semi-practical, stand point, the system  
would seem to be a way to interface modes that are common on HF or VHF radios,  
by using those radios and the equipment that attaches to them.  Assuming  you 
already have the interface for the radio equipment, then any of them  will use 
the one common light to RF interface rather than new and  separate interfaces 
for them.  There might also be some things that are  easier to do with an 
existing radio than with "base band circuitry", like vary  the IF bandwidth.
 
 
James
N5GUI