[Laser] Thoughts on receiver design

[Andrew T. Flowers, K0SM]

Phil,

I'll address your questions one by one as best as I can.  I don't claim 
to be an expert in all of this, but I'll try to dump upon you what 
little knowledge I can:

>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?
>
I know that my PGP front end gets saturated in daylight.  I imagine that 
a fairly bright laser would be hearable, but it's not all that great.  I 
wonder how the 310 handles?  Also, a bandpass filter would be good--one 
wide enought to tolerate the several-nm drift from the diodes.  As far 
as darkness goes, with the PGP front end and a 4" glass lens I can hear 
stuff on 670nm that I can't see.  I'm told the 310 is the same way.

>
>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?
>
Remember that you will want to keep noise figure and detector size as 
your main criteria.  Unfortunately this are inversely related. 
 Obviously you need a large enough detector to receive all of the light 
from the lens.  If you are using telescope quality glass lenses I 
imagine this won't be too difficult.  Just remember that if you choose a 
tiny detector you will have to get it in JUST the right place.  I have 
the PDB-V107 here.  It has just the right detector size for a 1ft^2 
fresnel lens, which doesn't produce as sharp of an image.  The price is 
certainly right.  Remember, you can always upgrade the diode! While you 
are ordering, they have some 10mw laser diodes for around $15, which is 
a good deal as those devices go.  If you break it you won't cry too much.

>Since I plan on using a tight bandpass filter I'd like
>to stay near a "standard" (if there is one) to be
>compatible with others rigs. G0MRF uses 500Hz. My ear
>prefers a bit higher frequency like 700Hz though.
>There may be a electronic limit as to how fast the
>LASER diode can be keyed as well. Any other comments?
>
Remember that you will be competing with the harmonics from manmade 
light.  In most cases this means you will have 120Hz buzz and it's 
harmonics to deal with (or 100Hz in EU, the lights flash on both the + 
and - parts of the cycle).  You don't want your tone to be too close to 
one of those harmonics otherwise your ear will have a very difficult 
time copying...remember what it's like to copy the DX station on HF when 
some LID keys down on him, zero-beat....I have some audio recordings and 
some spectrograms of downtown Rochester/NY scattering off of the clouds 
on my website, which may give you some ideas of the interference you'll 
have to compete with.

If you feel the need to make a narrow bandpass filter, you will probably 
want it to be switchable.  There may be folks who want to talk back in 
AM voice, or even PWM from the ramsey kits that are floating around. 
 For those you will want a standard 300-3000 KHz passband.  All of these 
things use a simple AM detector and there is no problem with 
compatibility provided you can switch out the narrow filter!

As for the limit of the diode, most of them are good will into the 100's 
of MHz if not GHZ--if you are using a laser pen, there may be a 
capacitor to ground that will limit the frequency to the audio 
range--still not a problem for the MCW stuff.  I know someone on the 
list has used RF modulators to send video using CH 3 or 4 as a 
subcarrier.  I'm thinking that would be a cool setup for field day :-)

My website is at http://mail.rochester.edu/~af006m/index.html , but it's 
been down since Saturday morning, along with the school mail server.  I 
fear the worst, but fortunately all that stuff is backed up here.  There 
are lots of nasty viruses out there right now--good time to make sure 
windows is up to date!  In the meantime I can send my stuff along as an 
attachement if anyone would like.

Andy K0SM/2