Musings about communication in the 900 nanometer band...

[Nan and Sandy Sanders]

Look at http://www.k3pgp.org/ 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.
         Sandy
         WB5MMB

----- Original Message -----
From: "Mike O'Dell" <mo at ccr.org>
To: "Tacos" <tacos at amrad.org>
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
device
> is to drive it in pulse-mode driver - high current with very low duty
cycle.
> 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
pulse-mode
> 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
rep-rate.
>
> 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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