[Laser] Thoughts on receiver design

Tue, 23 Mar 2004 09:28:57 -0800

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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