[Laser] Thoughts on receiver design

Tue, 23 Mar 2004 10:16:48 -0800

Tom - I'll try the known fast detectors and will send out the result.
  - Kerry -

At 09:28 AM 3/23/2004 -0800, you wrote:
>Hi Kerry,
>
>I remember your description of this phenomenon.  I wondered whether the 
>falloff
>might be caused by the capacitance, since each capacitor is frequency 
>biased of
>its very nature.
>
>Tom Upton  AD6N
>
>Kerry Banke wrote:
>
> > The frequency response of the cheap lasers with series resistor is
> > 100MHz+.  I did a demo of a simple laser pointer TV link where I biased the
> > laser through a 100 ohm resistor to a power supply and then capacitively
> > coupled about 0 dBm of channel 3 audio/video modulated RF from a VCR to the
> > laser pointer. The receiver was just a PIN diode connected directly across
> > the TV   RF input connector with the TV set for channel 3. It worked well
> > up to  channel 6 as well but fell off when I jumped to channel 7.  Using a
> > sweep generator showed useable response to 125 MHz but I don't know if it
> > was limited by the Laser or detector. I recently purchased some 1 ns
> > response PIN detectors so will try checking the response again.
> >   - Kerry N6IZW -
> >
> > At 06:01 PM 3/22/2004 -0500, you wrote:
> > >[email protected] wrote:
> > >
> > >>I haven't frequency tested my laser pointers yet, but they
> > >>don't seem to have any trouble responding at 800 Hz, and
> > >>based on the shape of the waveform at that frequency, I
> > >>expect that they'll go a lot higher.  Laser diodes themselves
> > >>can work well up into the GHz range (or even higher!), but
> > >>the circuitry in a laser pointer to regulate the current to the diodes
> > >>usually has some capacitors which tend to limit the maximum
> > >>frequency.  Of course, you could always design your own current regulator
> > >>to get around the frequency limitations (and, you'll have to if you're
> > >>going to use raw
> > >>diodes, rather than laser pointers).
> > >>
> > >>
> > >If you go with the really cheap pointers you will find that the "current
> > >regulator" consists of a 47-ohm chip resistor.  The little laser keychain
> > >units can be had for around $1/ea off of ebay if you buy in quantities of
> > >10 or more.  I they shouldn't have any trouble with being modulated into
> > >the RF range unless there is some weird capacitance in the dye that I
> > >don't know about.
> > >
> > >Andy K0SM/2
> > >
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