Musings about communication in the 900 nanometer band...

Nan and Sandy Sanders esanders at
Sat Jul 8 15:21:06 CDT 2006

Look at lots of good info. Notice the part on how the
most sensitive receivers work better at very low data rates.
Note the diagrams on the first receiver construction page have a coding
error. Edit the URL for the gifs and replace %5C with / and you will be able
to see the diagrams.

----- Original Message -----
From: "Mike O'Dell" <mo at>
To: "Tacos" <tacos at>
Sent: Saturday, July 08, 2006 11:40 AM
Subject: Musings about communication in the 900 nanometer band...

> i read through several of the sections of the web site
> describing their experiments and they argue (pretty convincingly)
> that "AM will always win in weak-signal reception."
> however, the discussion of modulating the Luxeon took a somewhat
> narrow view of the problem.
> from my reading, the way to get the brightest light from a Luxeon-style
> is to drive it in pulse-mode driver - high current with very low duty
> the average optical power is equivalent but the peak power is much higher.
> (the electrical consumption can actually be lower in some cases.)
> it makes one wonder whether revisiting FM might not be worthwhile.
> simply FM the pulse rate with the audio. i believe off-the-shelf
> LED drivers (designed for this new generation of LEDs) run with
> 50-100KHz rep rates.  there's a limit to the deviation you'd
> want to run to keep the duty cycle (and hence device dissipation)
> under control, but i think that's workable although i haven't
> done the sums to compute the modulation index vs pulse rep-rate curve
> vs device dissipation curves.
> this also questions whether the limitations of an avalanche PIN diode
> detector would be as problematic.  when biased with relatively HV
> (80-100VDC) a PIN diode operates much like a photomultiplier, but
> it is very noisy, non-linear, and the response isn't terribly fast.
> however, the optical input is a narrow pulse so the PIN acts as
> a low-pass filter - it should smooth the pulse train into something
> much closer to a sinusoid at the pulse rep-rate fundamental.
> so treat the PIN as a source for a low-level FM carrier at the pulse
> amplify, bandpass filter at the center frequency, amplify to clipping
> (ie, a limiter stage) and then demod with a PLL to recover the audio.
> for the center frequencies of interest, the bandpass filter could be
> implemented as a state-variable filter with fast op-amps so the frequency
> and Q would be tunable to make it easier to experiment with the rep-rate.
> (and getting a state-variable filter to clip is not hard (grin) so you
> might subsume the limiter function in that stage as well.)
> this analysis is probably completely bogus because a PIN turns out to be
> way too slow or some of the other sums don't work out right when you
really do them.
> so i offer it as no more than an amusing speculation.
>      -mo
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