[Laser] Thoughts on receiver design

[laser@mailman.qth.net]

>Date: Sun, 21 Mar 2004 11:55:53 -0800 (PST)
>From: Phil Lefever <kb0nes@yahoo.com>
>To: laser@mailman.qth.net
>Subject: [Laser] Thoughts on receiver design
>Reply-To: laser@mailman.qth.net
>
>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've played with my own design, which is documented
at:

http://bellsouthpwp.net/k/e/ke4kwr1/Laser%20Demo/Laser_Demo.html

I use a PIN photodiode as the receiver, but I transformer
couple it into the receiver preamp to minimize problems
with saturation effects from ambient daylight.

I haven't range or noise tested this design, but, since most
of my work has been done on short ranges with large amounts
of ambient light, it's proved adequate.

>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 <5mw Red LASER
>diodes.
>
>One final question, is there any consensus at to
>modulation frequency?

The designs I've found seem to favor 800 Hz.  I'm a little
surprised that 1 KHz wasn't chosen.  As another poster
mentioned, you have to beware of the 120Hz harmonics.
Fluorescent, Mercury Vapour, and Sodium Vapour lamps
produce some rather intense noise at these frequencies,
although by the time you get up to the 800Hz range, it's
dropped off a bit (and, 800 Hz falls in a gap, as does 1 KHz).

>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?

If you make the bandpass filter adjustable, such as I have 
done in my prototype, then you can accomodate any 
frequency from about 500 Hz up to about 2500 Hz.

I haven't frequency tested my laser pointers yet, but they
don't seem to have any trouble responding at 800 Hz, and
based on the shape of the waveform at that frequency, I
expect that they'll go a lot higher.  Laser diodes themselves
can work well up into the GHz range (or even higher!), but
the circuitry in a laser pointer to regulate the current to the 
diodes usually has some capacitors which tend to limit the 
maximum frequency.  Of course, you could always design 
your own current regulator to get around the frequency 
limitations (and, you'll have to if you're going to use raw
diodes, rather than laser pointers).

>73 & hope to _see_ you on the band ;)
>
>Phil, KB0NES
>EN34jt
>

Dave
WA4QAL