[Laser] Re: Laser Digest, Vol 2, Issue 5

[wa4qal at ix.netcom.com]

One of the problems with most designs is that the dynamic 
range is so small in the optical receiver front end that ambient
daylight saturates the front end.  And, once the front end has
been saturated, it doesn't matter how much more signal power
you pump into it, you won't get any signal out.  

Obviously, the first thing to do is to design the optical part of the
system to exclude as much ambient light as possible.  This usually
means long light shades (e.g., PVC pipe spray painted black inside
to limit the acceptance angle of incoming radiation, and to minimize
reflections).  Additionally, it may mean focusing devices to further
limit the acceptance angle, and to increase the signal power to 
ambient illumination.

Spectral filters can help, too.  Note that spectral filters don't have to
be exceedingly expensive.  Sure, the custom designed ones with 
100 Angstrom bandwidths are worth their weight in gold (or more!).
But, something as simple as a theater lamp gel can attenuate some
ambient light without reducing the signal light excessively.  And, they're
pretty cheap, too.

After you've done all you can to minimize the ambient light in the 
signal path, then it's time to start working on the electronics.  One
approach that I've used is to transformer couple the optical detector
(PIN photodiode) to the high gain, high impedance front end (JFET IC).
The transformer coupling removes the DC component of the signal
while still allowing any AC component through.  It may not be perfect,
but I've used it to communicate across a well lighted (fluorescent) 
conference room.  Anyway, it's another design to think about.

Dave
WA4QAL

-----Original Message-----

Date: Wed, 09 Jun 2004 16:49:57 -0400
From: "Andrew T. Flowers, K0SM" <aflowers at frontiernet.net>
Subject: [Laser] daylight receiver
To: laser at mailman.qth.net
Message-ID: <40C777F5.5080802 at frontiernet.net>
Content-Type: text/plain; charset=us-ascii; format=flowed

Does anyone have a design for a good daylight receiver?  My K3PGP front 
end won't hear much in daylight.  I'm not looking for something that is 
super sensitive--just good enough for a LOS QSO on a sunny afternoon.  I 
was thinking about putting the RX in a black box and using a very small 
lens, thus limiting the amount of total light hitting the surface.  I 
may also try using a low-value resitor on my OPT310 RX to widenthe BW 
and decrease the gain.  Right now it makes a pretty good oscillator if 
you hit it with the laser directly :-)

( and yes, I'm too poor to to afford narrowband filters)

Andy K0SM/2