[Laser] Thoughts on receiver design

[Phil Lefever]

>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=
=20
>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=20
>should be
>able to build the entire receiver system on one module.

True, I tend to favor 700Hz personally, but I think I will end up with
a variable BPF so that I could possibly work others with different Mod
frequencies.

>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=20
>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.

I already have a buddy of mine that has been writing some DSP software
for fun. He wants to use the computer to possibly modulate the LASER
so we could run FM,AM,SSB or digital audio across the beam too. I told
him to let me master MCW first ;) I will have a jack on the receiver to
keep him happy though!

>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=20
>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.

I don't think that pointing will be an issue at all. Provided I have a
LOS path and the air is fairly clear I will easily see the other setup
out to 20+ miles. Point a flashlight my way and I will be aimed at the
other station visually. Reverse the process and the pointing is done
except for a little peaking on the actual signal. It doesn't hurt to
have a SOLID mount with motors that move it by fractions of an arc
minute either ;)

Also I have some spiffy filters from the astronomy hobby that should
provide some interesting filtering. They pass 670nm with only a couple
percent of attenuation but they will provide a deep notch on Mercury
and Sodium vapor light emissions.

>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.

I have already started building them. I will end up using the OPT301
sensor. Using a small 2"x3"x1" LMB box I will mount the sensor and
an initial gain stage with minimal bandpass filtering. Outputs will
feed either the computer or a box with a variable BPF and a LM386
and speaker. The sensor will be mounted in the center of a 1 1/4"
eyepiece barrel attached to the side of the box. Positioning the
sensor at the same point in the barrel as the field stop of an
eyepiece allows me to focus and center visually with the eyepiece,
then pull it and drop in the receiver. The transmitter will ride
in a 3rd box on top the scope. I have a couple of really accurate
X/Y slow motion controls that will be used to bore sight the LASER
with the optical axis of the scopes.

I just need to find the time to build and use the setup. I think
what I have in mind should work well for the available LOS paths
that are available in my flat part of the world...

73

Phil


-------------------------------------------------------------------------
Phil Lefever    KB=D8NES                          Twin Cities Repeater Club
kb0nes@tcrc.org                                 Burnsville,MN