[Laser] AM versus PAM
F1AVYopto at aol.com
F1AVYopto at aol.com
Thu Feb 23 01:09:51 EST 2006
James
I understand your project.
The simplest way you can try needs no complex filters and is very simple.
I use it in 1978 for audio communication with a 2W RCA pulsed laser.
The link was 5 Km long in daylight without optical filter and without
noise.
It needs only a fast front end to receive the pulses spectrum without to
much integration.
This front end is followed by an adjustable threshold detector that extracts
only the request peaks.
The extracted peaks trig a simple one shoot that give enlarged pulses.
With the FM pulse modulation frequency, the one shoot time is chosen for 1/2
duty cycle at the TX frequency rate.
The audio modulation change the pulses density so a simple RC filter
translate the one shoot pulses to a good audio modulation via a first order
integration.
In 1978 my schematics was very simple and archaic with discrete components.
The FM modulation used 8 KHz pulses rate.
The TX modulator was a simple two transistors multivibator with current
bases modulation.
The collector peaks from one of them trigged a thyristor to pulse the laser
(or the power LED)
Today a few ICs should give very simplest schematics.
If you wont I can send you directly the schematics I used.
May be you could adapt them to a new version.
The boxcar averager is a device that extracts repetitive pulses via
programmable time windows.
If you have for example one 10 nS pulse each second in your receiver (from
the moon..HI..),
If you know the arrival time probability, you can catch the pulses in a
temporal window (with a FET gate for example).
It is possible to add or average each stored energy during window.
The noise not correlate with itself but the expected signal adds itself like
the square root of the cycle’s number.
The S/N improvement can be fantastic...
73 Yves.
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