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

[Kerry Banke]

After following the discussions on signal to noise versus frequency for the 
K3PGP type front end, I finally broke down and performed a crude test 
on  own receiver last evening.  The receiver setup is a 4" glass lens with 
8" FL into an SFH-203 PIN diode ( 1 mm2, 11 pF)  detector followed by an 
MPF102 with no bias resistor, MPSA18 Bipolar transistor  stage and an LM386 
amplifier. The coupling capacitors are large to extend the low frequency 
response. I placed a cover over the lens of the receiver with an LED 
sticking through the cover. All lights were turned off as well as the 
entire receiver was covered to create as close to total darkness as I can 
manage.  The LED was driven by an HP 3325 function generator in square wave 
mode through a 6.8K resistor.  The audio output was connected to the sound 
card input of an old Toshiba laptop ( Satellite 440) running Spectrum 
Lab.  I set the function generator output level  for about  a 10 dB signal 
to noise at 500 Hz and then varied the frequency from 10 Hz to 2 KHz.  My 
results with this crude setup showed essentially no change in signal to 
noise from 10 Hz through 1 KHz with the signal to noise decreasing to 5 dB 
at 2 KHz.
  - Kerry N6IZW -