[Laser] RTTY over light, anyone tried it?

[James Whitfield]

Paolo

Since you are using an LED for your emitter, perhaps you would consider not 
trying to "square" the drive signal.  The following is based on the circuit 
that I use in the transmitter of my "Talk on a Beam of Light" demonstration.

Assume for the moment that you are going to use a PC sound card system to 
generate signals, even if you intend to transfer them to an MP3 player for 
testing in the field or a beacon.  Take that audio signal and run it through 
a simple op amp stage with variable gain then to an op amp LED driver stage. 
It would take a dual op amp operating with a single 12 volt supply, and use 
a pseudo ground at 6 volts.  The first stage would be capacitively coupled, 
or transformer coupled, to the audio source.  At zero input, the first stage 
output rests at 6 volts.  As audio is fed into the system, adjust the gain 
so that the audio peaks swing within the linear range of the particular op 
amp, perhaps 2 volts on the negative swing and 10 volts on the positive. 
Later you can always boost the gain so that the squaring function becomes 
necessary.

Run the output of the first stage to a resistor divider, the center of which 
is connected to the non-inverting input to the second stage.  The second 
stage output would go to the base of an NPN transistor.  The LED is wired 
from the V+ supply to the collector of the transistor.  The emitter of the 
transistor goes through a sensing resistor to ground.  There is a line from 
the emitter to the inverting input of the second stage.

At no signal, the first stage output is 6 volts, passes through the resistor 
divided to set the stage two input to 3 volts.  It will drive the base of 
the transistor to draw current through the LED and down to the emitter, then 
through the sense resistor.  Select the value for the sense resistor so that 
at 3 volts the current flow will be equal to the rated continuouse current 
of the LED, or less if you choose.  When a signal is supplied, the current 
through the LED will be proportional to the voltage out of the first stage. 
Note that as long as you do not over drive the op amp, the average current, 
and hence the heating of the LED, will remain constant.  A symetrical 
waveform will provide equal drive above the average and below on each cycle, 
which simplifies the adjustment to protect the LED from overdrive.

Perhaps you would be able to run comparison of the different digital methods 
when used on an optical link, PSKxx versus RTTY ( FSK ) versus MFSK.   There 
may also be value in comparing different audio frequency "carriers", say 
250, 500, 750, 1000, 1250, 1500 Hz for PSKxx or the other modes.  You may 
find benefits for one mode or frequency range for a given data rate.

Sending an continuous valued rather than squared light signal may allow you 
to try something like MT63 without having to resort to pulse frequency 
modulation or pulse width modulation.

Please keep us posted on how your work is going.

One last thought, had you looked into Laser Scatter by K0SM as a digital 
mode?  I think of the data rate as being too slow for keyboard conversation. 
It might suit your needs.


James
 n5gui


----- Original Message ----- 
From: "Paolo Cravero" <pcravero at as2594.net>


> So far I have built an MCW transmitter and an OPT202 receiver. The TX is a 
> 10mm red LED, and soon the RX will have a nice telescope mount so that 
> first "beaconed" outdoor tests can take place. No-lens tests took part 
> indoor on 12m paths, both LOS and NLOS.