[Laser] Simple optical beacon experiment in San Diego

[KY1K]

Thanks for the info Yves, I had not seen this before, not sure why. I 
know Andy monitors the list, so not sure how I could have missed it.

 From the diagram you referred to, it appears the response of the PGP 
detector is down 6db at less than 100 Hz, although the range is a 
little compressed, it's hard to identify the exact 6 db down point. 
But, clearly it favors frequencies well below 100 Hz.

I do not know which photodiode Andy used in his test, it isn't 
specified on his website.

John, K3PGP had a very large active area PD and found his peak 
response was at 20 Hz, so 100 Hz shown in Andy's response curve 
probably means he had a more typical (smaller) photodiode active 
area. Note that John believed the actual response peak was even lower 
due to an unfavorable coupling capacitor value in his sound card. 
Since sound cards of that era were generally used for playing sounds 
on the computer and listening to sound files-so values under 20 Hz 
weren't a priority::>

John did an on the air test showing his frequency response, here is 
the text from his web page:

----------Begin quoted text.

I was prompted to try experimenting with much lower modulation 
frequencies of lasers for communications than the traditional 800 Hz 
when I started to become aware that my optical receiver appeared to 
be MUCH more sensitive to naturally occurring low frequency 
phenomenon. Several people (most notably KY1K) also suggested that it 
might be possible to detect much weaker optical signals if the 
modulation frequency was lowered.

To see what might be possible I set up a test using the lowest 
amplitude 800 Hz laser signal that I could detect then started 
dropping the modulation frequency in 100 Hz steps. This produced the 
following graph:

----------End quoted text.

The graph can be seen at:

http://www.k3pgp.org/Notebook/Lflaser/lflaser.htm

The graph of the relative frequency response is about 2/3 down from 
the top of the page.

Based on John and Andy's data, my guess is that Kerry should modulate 
his Luxeon at 20 Hz or so, and roll off the response above that by 
using a low pass filter before applying it to the sound card input.

Regards,

Art



At 04:01 PM 3/14/2006, you wrote:
>About the PGP RX  response:
>Andy K0SM did a very good work about the effective PGP RX  response.
>A typical dark night conditions response curve can be found at:
>http://mail.rochester.edu/~af006m/dark.gif
>This noise density curve is nearly  the same that the gain curve.
>In fact the RX sensitivity is mostly  determinate by the equivalent value of
>the internal Rshunt PIN photodiode  resistor seen in parallel with the
>amplifier input resistor and the diode  polarisation resistor if there is one.
>The best sensitive PIN photodiode we  can find today is the HAMAMATSU
>S2386-18K with a 7. 10-16 W/Hz^1/2Hz NEP.
>But  the HAMAMATSU S2386-18K has a typical 20 GOhms Rshunt with a 100 pF
>capacitance  at -1V reverse.
>The FET PN4117A that gives good results on the PGP RX  has about the same
>gate leakage resistor value.
>A simple 1/2.pi.RC  calculation gives the -3 dB cut off frequency of this
>ultra sensitive PGP RX at  less than 2 Hz!
>However the gain has not the same curve that the S/N at  ULF and with a very
>lowest gain this RX stays very sensitive to 20 Hz but not  much more!
>73 Yves.
>
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