[Laser] Thoughts on receiver design

[laser@mailman.qth.net]

Phil,

Best wishes on your experiments with light communications.

Some simple thoughts from a tinkerer and relative newcomer.

If you are planning to use the receiver for only your own transmitter, you 
can optimize it to suit your needs.  You stated that you only intend to work 
MCW, then I would suggest you use a frequency that is easy for you to hear or is 
easy for your equipment to pickout.  If you are used to operating CW, then 
maybe the way to go is a modulation tone you are comfortable with.  You should be 
able to build the entire receiver system on one module.  


On the other hand, if you would like to try to receive PSK or slow CW or 
voice, then think about dividing the sensor and pre-amp into one module and have a 
second module for the filter and audio stages.  If you want to connect to a 
computer for processing, then the output of the filter may be enough to drive 
the computer sound card.  The sensor will need some filtration before you 
connect to a computer.

For the range you are suggesting, and with an 8 inch telescope optical 
system, your receiver may not need a sharp filter.  You should have a strong signal 
with very little noise, so a simple audio amplifier may suffice.  The sensor 
may have a bandwidth of megahertz, but if you are only going to drive a 
speaker, you may not know, or care.  I think that your bigest problem will be 
pointing the system.

Earlier to this group, I have described the 555 timer circuit that I used on 
my first experiments with a laser pointer.  Since then, I have gotten several 
inexpensive pointers that I cut the barrel from them to access the ciruit 
board. ( They run on 4.5 volts max with a 62 ohm chip resistor for current 
limiting instead of 3 volts with electronic power stabilization. )  The circuit I 
used before only needed adjustment of the drive current.  If you would like a 
schematic I can send it to you.  The receiver was crude, an audio amp driven by a 
capacitor coupled CdS photo resistor in front of a 4 inch magnifier.

Since those days I tried the Ramsey LBC6 kit.  Nice. Lots of room for 
improvement, but the receiver works with my old MCW transmitter and its transmitter 
works with my photo resistor - amp rig.  At least at the range for public 
demonstations.  Guess it helps to be lucky and have lots of signal.

Have you figured out how you will mount the photo sensor in the telescope?  I 
would like to know how it works out.  I might get some help from an astronomy 
club if I can mount a sensor in a standard 1.25 inch eyepiece barrel so it 
won't scare away the astronomers.

73


James 
N5GUI



In a message dated 3/21/2004 1:57:12 PM Central Standard Time, 
kb0nes@yahoo.com writes:
Hi all,

I'm finally getting around to building my LASER setup.
I am trying to nail down an adequate receiver design
for use with MCW using standard 670nm LASER diodes. My
receivers will be used at prime focus of 8" and larger
telescopes. My design goal is line of sight
communication over paths of 20+ miles primarily at
night.

I have been looking at the K3PGP and G0MRF designs.

The G0MRF design looks interesting and easy to build
based on the OPT301 sensor. The downside is the high
cost (~$45) of the sensor itself. The OPT101 is a lot
more cost effective although it appears to not be as
sensitive/low noise. Thoughts?

If I go the route of the K3PGP design what is the best
PIN diode sensor to choose? I have been looking into
the Photonics Detectors PDB-V107 as a possibility.
What are other people using with the 'PGP front end
design?

My transmitter will be modeled after the G0MRF design
initially starting with standard 


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