[Laser] discrete vs op amp sensitivity

[Andrew T. Flowers, K0SM]

Steve,

I've done some tests with my PGP recievers and found that the LP cutoff 
was at least below that of the soundcard--which rolls off around 15 Hz 
or so.  I can certainly get a recording for you if you are interested.  
I rememberthe noise rolling off at 1/f starting at essentially DC.  I 
think Yves has a picture of this somewhere.

Andy K0SM


steve kavanagh wrote:

>It seems to me that the K3PGP (night) preamp must
>normally be operating above its low pass cutoff
>frequency.  Has anyone ever tried measuring the
>baseband frequency response by varying the laser
>modulation frequency ?  Or looked at the spectrum of
>the receiver output noise ?   Even with my circuit,
>which is kind of similar (but AC coupled and with 66
>megohms from gate to ground) I see a bandwidth of only
>about 1 kHz or so when observing the noise output of
>the receiver under dark conditions.  This is with
>quite a small area detector - all six sections of a
>CD-player detector in parallel.
>
>I agree with Yves that the cound card noise level
>should not be a major factor as long as there is
>sufficient gain in the preamp that the preamp noise
>was dominant.  The higher the preamp gain the lower
>the overall dynamic range.  But this would not seem to
>me to be much of an issue for most optical
>communications purposes (unless perhaps in an urban
>area with a large area detector).  For an HF receiver,
>on the other hand it is very important to minimize the
>gain in order to achieve the necessary dynamic range,
>so the sound card noise would be important.
>
>Choice of a better FET and the possibility of
>paralleling them is interesting.  I really should sit
>down and do some calculations so I can understand this
>stuff better.  But, unless one is using quite a large
>area detector with high capacitance, won't paralleling
>FETs result in a significant decrease in bandwidth due
>to the extra preamp input capacitance ?
>
>73,
>Steve VE3SMA
>
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