[Laser] Ramsey transmit to CdS

[J. Forster]

TWOSIG@aol.com wrote:

> Hmmmmm.   I don't quite understand the point you are making.   If I was
> sending a PWM signal at  2.4 GHz and my neighbor HAM is sending another at 2.4005
> GHz, I would expect to be able to separate the two signals.
>
> With the detection scheme used on the Ramsey LBC6, if I am using a red laser
> and my neighbor is using a green one, I cannot tell the difference between the
> two.  For that matter, I cannot tell either from a light beam that uses an
> amplitude modulated current to control the brightness of a neon bulb.

If you want to separate the laser colors, use an optical BP filter.  That's the
carrier.

> Even if you use LEDs of the same color, the system response the same to one
> that is PWM with pulses at 20 KHz,  at 50 KHz, at 500 KHz.

Those would be sub-carriers.

> And if you use the
> same percentage of modulation, you can make it "look" like an LED driven with
> an audio signal with a DC bias current.

The problem is that your detector, the CdS, looks like a LP filter with a light level
dependent cutoff of a few Hz to a few 100s Hz. You can't get 20 HKz through it. The
data shows the long CdS response times.

> I guess that I am really looking for a better detector system.  This works.
> It is (sorta) inexpensive.  And it is what I have.  I would like to know about
> more.

Use a back-biased photodiode. Even a solar cell would work better.-J

> Thank you for your comments.
>
> James
> N5GUI
>
> Hi
> That is exactly how PWM is used ... classically.  If you truly have a PWM
> light source, and the PWM sampling frequency is above 8 khz or so, usually
> 16 or 20 khz, then the 0-4 khz band contains an exact copy of the sampled
> audio source.  The photo detector should have a BW of at least 3-4 khz for
> audio.
> John Matz KB9II

It's called 'the sampling theorem' You must sample at twice the maximum information
rate.-J