[Laser] discrete vs op amp sensitivity

[Art]

Hi Steve,

I wish John was here to discuss his preamp.

But, I know he used a large area photodiode with it and that he found 
by experimentation that the gain of his premap was poor with applied 
frequency's of 800 Hz and that it peaked dramatically around 20 Hz. 
As far as I know, he never looked at the output of the preamp vs the 
frequency in dark conditions.

Have a look at his website:

http://www.k3pgp.org/frontend2.htm

At the bottom of the page is a capture of a non line of sight LED 
source at 5 miles. John noted that he could just barely detect the 
remote source at 800 Hz, but that at 20 Hz, the source was always 
detectable. The plot of transmit frequency vs relative output shown 
there confirms that the PGP detector has a relatively narrow bandwidth.

I'm not sure this is a problem with a computer to capture the signal, 
but if human ears are used, one might need a receiver with a wider bandwidth.

Regards,

Art



At 11:20 AM 1/14/2007, you 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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