[Laser] another lunar experiment

F1AVYopto at aol.com F1AVYopto at aol.com
Fri May 9 06:38:52 EDT 2008


>To compare that with my xenon strobe
>transmitter, I am pushing  the small tube to its limits to
>get barely 3 kw peak and 3 watts average  power to hit the
>moon.

>Which one would have the strongest  signal i wonder??
>1 killowatt CW vs. 3 kilowatts in pulses, assuming  the
>mirror is 100% modulated at the same frequency as the
>strobe  (10 Hz say)?? 

Tim
The most important is the energy you transmit and  proportionally you receive.
If you use the FREQUENCY DOMAIN to detect your  signal by FFTDSP, a one kW 
average power (square signal modulated at 50% duty  cycle) will be incomparably 
better your 3 kW peak 3 watts average power to hit  the moon.
Many reasons for that:
The number of photons you will receive is  in the energy ratio by time unit.
To be sensitive the FREQUENCY DOMAIN FFTDSP must accumulate in  high 
resolution during long time. 
T duration short  pulses must be amplified in a 1/2T bandwidth that means 
enormous extra  noise.
A short time low rate pulsed signal gives very numerous harmonic  frequencies.
With the FFTDSP FREQUENCY DOMAIN the harmonic FFTDSP spectral  lines will be 
very weak because the energy is divided in  each of them.  
To detect low rate short pulses, you must use a TIME  DOMAIN receiver with 
ultra high accuracy synchronic time window, adapted  filters, correlation, 
convolution... many complex electronic processes.
For  example an ultra short pulse can be detected via an APD in Geiger mode 
which is  over biased just at the expected arrival pulse time and during the 
same duration  the pulse width.
It needs very accurate and difficult time controls....
73  Yves F1AVY



   


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