[Laser] Results form a crude signal to noise versus frequency check on my...

[F1AVYopto at aol.com]

"Last evening I had  time to try the test suggested by Yves in the email 
below.  I set  Spectrum Lab as he described and set the signal generator 
driving the LED  source for a 5 dB signal to noise.  I  found the frequency  
response of my SFH-203 diode based receiver to still be  essentially  flat 
from 10 Hz to 1000 Hz and down maybe 2 dB at 1300 Hz which is  about  the 
high frequency limit for Spectrum Lab set up in this  configuration. It 
appears that with this particular receiver setup, the  operating frequency 
can be anywhere in the 10-1000 Hz range without  significant change.
- Kerry N6IZW -"

Hi Kerry
Your signal to  noise versus frequency check is very interesting because it 
gives a good ratio  base to calculate the K3PGP RX response directly with the 
PIN photodiodes  parameters at low level.
If the SFH-203 PIN gives 1 KHz bandwidth with 11 pF  and R shunt near 20 
GOhms.(value from Idark 1 nA for -20 V reverse) it means  that :
With a BPW34 70 pF and 5 GOhms BW could be (1 KHz / 7) x (20/5) =  about 570 
Hz
With a BPW33 630 pF and 200 GOhms BW could be (1 KHz / 60) x  (20/200) = 
about 16 Hz
These BWs stay well above the theoretical calculations  values.
I think it is because the optical flux during the test  stays above the 
sensitivity limits.
In these conditions it is probable  the photodiodes real Rshunt are lower 
than there dark values.
Thank you for  this check that confirms the SFH-203 is a very good choice.
I did not still find anything about  the RT5-2354DTS-C2 photodiodes...
73 Yves.