[Laser] A lot but very little

[Kerry Banke]

Chuck - I'm assuming the response is being limited by the 
receiver.  I have been driving the low cost laser pointers ( 
internally only a resistor and laser diode) with a good frequency 
response of at least 500 MHz and I suspect that is still limited by 
my receiver.  If you want to see what is doable from a frequency 
response standpoint  then I'd go to a 50 ohm system but then you need 
some form of modulation to pass through RFamplifiers. With the system 
you have , you will probably need to reduce the input resistance to 
improve the frequency response but of course that will reduce your 
sensitivity.  For a frequency response  test I drive an LED with a 
signal generator and look at the receiver output vs frequency ( for 
responses in the low  MHz range). For higher frequencies I use the 
laser pointer.  The K3PGP front end is great for low frequency ( 2 
KHz or lower depending on the PIN diode), weak signal work but you 
may be better off with a transimpedance amp design for higher speed data.
  - Kerry N6IZW -

At 06:18 PM 12/26/2006, you wrote:
>Folks,
>I know that most of the thrust of the list is to low signal level 
>long distance
>work, but I am working in the high speed high signal level area, I have
>been looking, I find a lot of info about low speed low signal level systems
>but so far not too much luck with the high speed links that I would like to
>build. I have looked at Sam's at k3pgp, and the site down under. Along
>with many others, but still not too much into. I wonder if any of you have
>any other recommendations?
>
>I have been able to move data at up to 9k6 b/s but I can do much better
>than that with RF.
>
>I have also been able to get my lasers to generate data at up to that
>speed but when I try to go much higher it appears that something is
>slowing things down. The test transmit end is a laser pointer, I opened
>it up and found that it is a regulated one, I went in and used a VN10
>to key the laser, but still can not get it to switch faster than about 9k6.
>
>It appears that the rise time is good, but the shut off time is ramped
>down. But that maybe a artifact of my reciever, but again I am not
>sure, I have tried a switched LED, and it appears to  track the led
>while it is switched at up to 38k4.
>
>Any good recommendations for power users?
>
>--
>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."
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