[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.  




More information about the Laser mailing list