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

[Kerry Banke]

I see two  possible issues with this concept based on my own 
experience.  The noise of the photodiode increases with light  level 
so adding a strong enough light LO source to cause non-linear mixing 
might bring up the noise considerably.  Also going for an   RF 
frequency response  requires the diode to now be terminated in a low 
impedance which reduces the sensitivity way below that achievable 
with the audio frequency, very high impedance,   K3PGP design. In the 
end it may be about the same as directly sending RF modulated 
light.   Just my 2 cents worth.
  - Kerry N6IZW -

At 03:17 PM 8/9/2006, you wrote:
> >From toasty256 at yahoo.com :
>
> >One more thought on this conversion to RF thing.  The
> >ultimate in low noise mixing might be using the
> >photodiode  itself as the mixer.
>
>
>Hmmm.  Very interesting thoughts.    If the path was  without phase
>distortion ( like a satellite to satellite link ) you could use 
>a  laser for the "Local
>Oscillator" and get lots of bandwidth.  A path in air  would result in so
>much de-coherence that I do not see how you would get  anything but noise.
>
>As you suggest, an RF modulated light source might result in a  mixed signal
>that a receiver might handle without adding much internal  noise.
>
>A sine wave would probably be less likely to have noise that would  interfere
>with the desired signal.  However, even a asymetric square wave  might also
>serve since the harmonic content of pulsed light would be rejected by  the
>receiver.  Amplitude modulation near the signal frequency would be more  of a
>problem.
>
>
>Thank you for you thoughts.
>
>James
>N5GUI
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