[Laser] Daytime dx plus a bunch of questions

[steve kavanagh]

Salut et merci, Yves.

It's been a long time since we discussed the 2SK170.  So the data sheet is not quite right calling that curve the noise figure !  I actually ordered the 2K170's to use with a BPW34 so I will probably stick to that plan.....I just got diverted away to the LED detector.

I was suspicious something odd was happening in my modelling...and I have now figured out that the apparent unimportance of the 66 Megohm resistance was due to evaluating performance based on a wideband noise measurement, which is dominated by low frequency noise, due to the detector capacitance.  But even with a broadband audio amplifier, my brain will reject the low frequency noise quite well.  So what matters is only the noise near the signal frequency (~500 Hz).  When I changed to a narrow band noise measurement in my model, things were more as one would expect....that resistor costs me 10 dB or more of sensitivity at night.  In the daytime it is another matter...I think the source impedance of the photodiode drops below 66 Megohms, since the receiver noise gets quieter in strong light.  The noise from the 66 Megohm resistor is then no longer important.  But maybe a reverse biased photodiode would work better.  

So I am starting to try to understand KA7OEI's Version 3....since daytime operation does matter to me.  But I may build the existing circuit anyway, to get something simple going that works to some degree both day and night, and then try to improve on it.

73,
Steve VE3SMA