[Laser] Re : Daytime dx plus a bunch of questions

[f1avyopto at aol.com]

Hi steve
You are right
In daylight the photodiode internal Rshunt becomes so low that the FET gate resistor effect becomes rather neglectible...
The daylight photo current becomes the main noise source (shot noise).
The optical bandwidth filtering matched to the LED color or spectral band and a very small optical field from the receiver (eg. very long focal length) are two keys of a good SNR in daylight ...   

 73 
Yves

 

 

-----E-mail d'origine-----
De : steve kavanagh <skavanagh73 at yahoo.ca>
A: laser <laser at mailman.qth.net>
Cc: f1avy <f1avy at yahoo.fr>
Envoyé le : Vendredi, 9 Décembre 2011 3:32
Sujet : Re: [Laser] Daytime dx plus a bunch of questions


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
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