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

[Tim Toast]

One more thought on this conversion to RF thing. The
ultimate in low noise mixing might be using the
photodiode itself as the mixer. 
Instead of injecting a local oscillator signal into
the circuit somewhere, you could modulate a regular
LED with the oscillator signal then shine it on the
photodiode from the side. Then the two beams will
combine and mix there and the output of the preamp
will have the RF signal if it (and the photodiode) has
enough bandwidth to handle it. 
Maybe this setup could be very simple - just a
resistor, the diode and a small capacitor or gimmick
for the output.
An LED can be modulated with an RF sine wave easy
enough. And the brightness can be varied easily to
give the proper levels for maximum mixing on the
photodiode. This same setup is used with
superheterodyne lightwave comms although it probably
uses lasers instead of LED's normally.
I also like the idea of using a photodiode to control
the frequency of an oscillator. The higher the
frequency, the more deviation of the carrier i would
think. So a VHF or UHF LC oscillator possibly followed
by a buffer to remove any residual AM seems perfect
for that. 



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