Hi Matt, so you really want to do this? Why 100MHz?
I would use a lower frequency, the commercial units
do. Maybe 5MHz?

What distance and resolution did you want to achieve?
:o)
-Roman



Matt Bennett wrote:
>
> Richard Ottosen wrote:
> >
> > With respect to risetime verses bandwidth:
> > An approximation is Risetime = 1/Frequency * 0.35.
> > This gives 70MHz bandwidth for a 5ns risetime. Not trivial but a lot
> > easier than 280MHz. There are a lot of voltage and current feedback
> > opamps available for these frequencies and risetimes. Not terribly
> > expensive either.
> >
>
> Thanks- I guess I did remember the equation wrong.  This also brings the
> frequency well within what I would expect from some fast TTL... now the
> big question is, how do I modulate the LED and recieve it that fast?  I
> remember reading in a scope manual about how two consecutive rise times
> affect each other- something like a 10ns rise time into an amplifier
> with a 10ns rise time gives about a 14ns rise time? (I seem to remember
> a sqrt(2) in there).  I guess the LED needs about a 100MHz BW from the
> LED to get the 5ns rise time out of a 70MHz reciever.
>
> So, how about it... anyone have circuits for transmitting/recieving
> light modulated at about 100MHz?  Gigabit Ethernet does it at over 1GHz,
> and OC192 approaches 10 GHz optically, so there should be a way.
>
> Matt
>
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