[Laser] led transceiver

[C. Turner]

Hello Stuart,

Sounds good on your tests.  As noted on the web pages, taking the 
"audio" from the output of the first amplifier will result in a "flat" 
response (not really flat, though as the photodiode rolls it off...) but 
removing the lowpass capacitor across the op amp will also work fine:  
The circuit could be further-tweaked to allow reasonable performance 
higher up, still.

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.  The downside is that 
the gear is a bit more complex and range is reduced (which explains why 
I've not used it recently nor mentioned it on the web page) but for 
relatively short distances with ample signal it's nice to switch to an 
"armchair" mode.

I don't know if you have made contact with Barry, G8AGN in Sheffield to 
the south of you, but he's also working on a somewhat similar project - 
although likely closer to "baseband" - that is, using direct audio on 
the LED.

Again, keep up the good work!

73,

Clint


> Message: 1
> Date: Thu, 21 Oct 2010 18:03:49 -0400
> From: stuart.wisher at talktalk.net
> Subject: [Laser] led transceiver
> To: laser at mailman.qth.net
> Message-ID:<8CD3F82F920EC7C-EE4-8297 at FRR1-L29./f>
> Content-Type: text/plain; charset="us-ascii"
>
>
> Thanks for comments from Sam G4DDK who suggests putting something in the RadCom GHz bands column, good idea; and Clint KA7OEI to whom I owe a debt of gratitude for his excellent front end design on which mine is totally based. Without the feedback capacitor it keeps going up to 100kHz so there is much space to play with subcarriers.
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> The transverter is based on a MC1496 balanced mixer, an op-amp to boost the output on transmit to really drive the Golden Dragon LED hard using  a MOSFET with a DC bias on it so the LED runs at 100mA and peaks up to around 1A on SSB. I use a 8.2 ohm resistor in series with the LED on FM and just 1 ohm on SSB at your own risk! (I have blown a 1A fuse and the LED is still with us). A logic IC and a 2N2222 looks after the ptt line.
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> Stuart G8CYW
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