[Laser] Simple optical beacon experiment in San Diego

[Charles Pooley]

KY1K <ky1k at pivot.net> wrote:
  
>I've never understood the factors that determine the bandwidth of the 
>PGP detector, except of course for the contribution of the photodiode 
>itself. The photodiode effects are relative of course, a larger area 
>PD always lowers the bandwidth...
   
  At low to moderate frequencies, the BW is mainly determined by the PD capacitance and load resistance.  A very surprising thing is the BW of small photovoltaic solar cells.  Often over 100 kHz.  One type I have is about 1/2 by 3/4 inch and has about 300 kHz BW with a 100 ohm load.
   
  Another proposal ( I will be trying this and some similar things in a while ) is to make a PD-to-RF converter, so a radio receiver can be used for most of the gain, and, of course, use of a carrier to eliminater low frequency background effects.
   
  Since a PD with a dc bias is mainly a capacitor, it could be part of a high Q tank for the load, and 1st part of selectivity.  Q > 200 should be easy.
   
  Just about any of the high power leds can be modulated to 10's of MHz (white leds also: the blue output modulates while the longer wavelength phosphor portion does not).
   
  So, how about modulating the diode at 1 MHz, receive with an am radio?  Or 100 MHz and an fm radio?
   
  Charles Pooley KD6HKU