[Laser] discrete vs op amp sensitivity

[steve kavanagh]

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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