[Laser] serial port PDM and PWM

[Chuck Hast]

On 12/7/06, Art <KY1K at verizon.net> wrote:

>
> OK, I am sorry-I jumped ship without telling you, and I can see where
> it's confusing.
>
> The hardware transceivers I was talking about are IRDA trasnceivers, not USB.
>
> The USB comments I made are still valid, but the small self contained
> transceivers are IRDA, not USB.
>
I have wondered about replacing the IRDA LED with a IR laser at the IRDA
RX filter freq. I know that the data rate is much improved, and I think most
of them look like a serial port to the PC so they are probably pretty easy to
interface, just need to get some, Probably best to get one that allows for
outboard RX and TX so that I can replace the LED. Also probably want the
RX somewhat removed from the transceiver IC too.


>
>
> OK, one issue you might not have considered much was beam steering.
> Believe it or not, those lasers DON'T travel in a straight line. If
> you set up a test range with a 1 milliradian (for example) laser at 1
> kM and set 4 other identical receivers 1 meter away on each axis
> (relative to the center), you can watch the atmosphere steer the
> transmitter laser, even over a moderately short path. It's for this
> reason (I believe) that the very expensive and high throughput 2 way
> data links for laser ALWAYS use a 2 watt laser. If they could
> sacrifice some reliability, they could achieve very good speed with
> 10 or 20 milliwatts.
>
Yes, once I get a single beam, single RX system up and running I was
thinking about putting multiple RX points out so that the beam movement
will put the beam on at least one RX at any given time, and then use some
form of diversity RX to select the good RX, probably just look at signal
strength and the one with the best gets voted.

That is another thing I want to play with and learn about is just how much
that beam can move around and then build for it. Should not be that big
of a deal to put multipe RX heads on the thing and select the best S/N.

> I wonder though....Are you going to use baseband or will you modulate
> this on a carrier to achieve daylight immunity? If you are
> considering doing baseband, you should take a look at Burr-Brown's Ap
> note on using an integrator to cancel constant state daytime light
> levels. Without using a second detector or a second photodiode, you
> can effectively achieve daylight operation without dumbing down the
> receiver sensitivity (which is the common method of achieving
> daylight immunity).
>

I was going to do OOK initially and then look at what I can do to improve
background noise rejection. I will look at the Burr-Brown app note.

Tnx for the observations, and will keep you all posted as I move forward
with this. I may have to use it as the IP link to the device as the county is
not real happy with non-county devices on their network in the building we
will have the Tampa switch located in.


-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."