[Laser] Cloud bounce new optical receiver.

[F1AVYopto at aol.com]

Hello  all
After many comparing measurements we have optimised the K3PGP RX  type 
especially for cloud bounce.The K3PGP RX is a no feed  back front end and the photo 
response shows a decremented gain versus frequency  with no define flat bandw
idth area.
For example, the native schematic with a  simple photodiode BPW34 and a 
MPF102 FET gives -18 dB gain reducing when the  frequency changes from 10 Hz to 2 
KHz.
However gain and noise density follow  the same curve profiles up to 100 Hz 
so the signal/noise stays quite good even  at more than 3 KHz.
The extra gain at less than 100 Hz enhances strongly the  scintillation and 
is a problem for L.O.S. voice com but not in  N.L.O.S.   
>From 100Hz to DC most of FETs give a strong 1/F noise  enhancement.
New audio FETs can give ultra low noise in VLF.  
The  new front end schematic remains extremely simple.
Only some resistors values  change, an output emitter follower stage is added 
but the essential of noise  reduction comes from the transistors choice.
All these transistors have an  equivalent input noise lower than 1 nV / 
s.r.Hertz from 10Hz to 10  KHz.
Because the 30 pF FET input capacitance, this RX is particularly well  
matched for the 50 to 300 pF PN or PIN silicon photodiodes and for the low speed  
VLF FFTDSP communication programs like JASON, WOLF-GUI or K0SM Laser  Scatter.
All the tests have been done to 75 Hz with a very low current  modulated LED 
beacon (without optics on receivers).
An 20 dB signal/noise  with Spectran (at 0,084 Hz resolution) with this 
receiver gives less than 5 dB  signal/noise on a VK7MJ RX in the same conditions 
(at 75 Hz).
The schematic  is in Photo Album "F1AVY F8DO cloud bounce" on 
Optical_DX at yahoogroups or can be  loaded at:
http://pageperso.aol.fr/F1AVYopto/opto/ultimepgp7.jpg
The  2SK170 and the SSM2210 are from CONRAD and RADIOSPARE in France.
73 Yves  F1AVY