[Laser] led transceiver

[Martin Farrell]

C. Turner wrote
>I've used a similar experimental system for testing higher-frequency 
>subcarriers, using a diode-ring mixer (like an SRA-1) on receive (with 
>the "audio" from the receiver going into the "IF" port, using a 
>"crystal can" oscillator for an LO) and using CMOS switches (several 
>74HC4066's or similar) on the "upconverter" with audio applied to 
>produce DSB - although SSB/AM/FM RF could be applied as well.

Hello - another long time lurker puts his head above the parapet!

When I was playing with radio over light systems several years ago, I 
used a high Q 455kHz IF coil and a reverse biassed BPW34 exciting the 
tuned primary. Then I took a lo-Z link straight to a receiver (Kenwood 
R5000 IIRC) or buffered the tuned circuit with an FET source follower 
and then to the RX.
The transmitters were LEDs or laser pointers modulated with a PIC CW 
sender and a Weaver SSB Tx using a ceramic resonator as oscillator.

I reasoned that I could tune out the photodiode capacitance and the coil 
DC resistance would take care of ambient and 100Hz light. It seemed to 
work OK but I never had the patience to extend the range beyond a few 
Km. I gave a 'daylight' demonstration at the Hereford radio club in 
about 1999 but I haven't used it since.

I'd like to know from the experts what fundamental penalties this system 
suffers in comparison to a photovoltaic K3PGP-type receiver with a 
similar BW? Is it more than just the increased pd leakage shot noise?

Regards
-- 
Martin G8ASG