[Laser] Retro-Reflector test

[Chuck Hast]

On 5/11/07, Art <KY1K at verizon.net> wrote:
>
> >Wouldn't a high frequency fm-modulated carrier solve the problem? It
> >also would give you more flexibility. Perhaps you would like to
> >transmitt some tv-signals later...
>
> No! Using an FM modulation scheme means your detector has to be
> dumbed down so that it admits the carrier frequency plus the width of
> the modulating signal (sidebands). The NEP varies (is reduced) by the
> square of the overall bandwidth-very bad!
>
> For instance........
>
> Say I choose NBFM and my sub carrier is at 50 Khz. I need the
> receiver to pass dc to 100 Khz. With my OOK, I need only 3 KHz
> bandwidth total. If I use the OOK as a 'reference' or baseline
> sensitivity, my 100 Khz FM system will be 33.3 times 33.3 (or 1109)
> times less sensitive! This is a 30 db worse sensitivity!!!! It
> doesn't take a rocket scientist to figure out WHY wideband and/or FM
> modulated carriers are usually used with a fiber optic link.
>
Exactly... That is why I am staying with OOK for now. It is easy cheap
and will get me down the road...

> If you need daylight operation, it might be a viable trade off-that's
> how IR LED remote controls work (in conjunction with a visible
> wavelength reject filter).
>
> I tried using my baycom tnc's years ago on laser. Despite the fact
> that the software was free, the modems only cost 20 dollars, they are
> powered by the computer comm port and they are very small.....the
> retries cannot be set to transmit often enough. When trying to aim
> the laser, you spend 90 percent of your time 'waiting' for the thing
> to transmit. Even though the settings are adjustable in software in
> the PC, it still is NG-the software author put limits in the software
> to prevent the thing from transmitting often enough to be useful.
>
> No doubt this is due to the nature of the beast-when the unit is
> designed to be used on a shared channel, it should never be allowed
> to transmit constantly-or nearly constantly.
>
> Maybe newer tnc's have over-rides?
>
> Today we have awesome packet decoding and encoding ability available
> by using the soundcard-I'm not sure a tnc is the best answer.
>

I am using a PacComm TNC (a stand alone packet controller) I used to
work for them and know the TNC's well inside and out. I run the laser FDX
and that way I can send it flags, and data whenever I want. I also set the
timers so short that you are never waiting for anything... This is the first
step, the next step will be to move to high speed links (10mb/s) I have
some sample laser driver chips for just such testing, I am using the TNC's
because I have several of them and I can run them at speeds of up to 128
kb/s with no problems. By way the Baycom that I recall was not a true
packet controller, it was a bit banger and the packet piece was all done in
software. Indeed the data was bitbanged over the RTS/CTS lines as the
TXD and RXD lines were never used as far as I can remember. You should
have been able to go into the packet drivers and change the timing in order
to get the high speed retry rates you were looking for.

I actually use the TNC as not only a data source/sink, I also use it as
a signal generator as I can get signals as high as 10Mhz out of the thing,
for wave form testing. Once I have a final circuit I will publish it
so all of those
who have old TNC's laying around can put them to use again if they want.


-- 
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."