[Laser] discrete vs op amp sensitivity

[Art]

Yes, I can believe that Andy, I think I saw your results last year.

Did you have a large surface area PD, or a small one. With this type 
of preamp, you probably can't use a large area PD unless you really 
intend to work at 10 to 15 Hz, or less::>

I'm really curious now......whether the Burr-Brown opt-101/301 is 
more sensitive or the pgp front end is.........

Regards,

Art



At 05:47 PM 1/14/2007, you wrote:
>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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