[Laser] Simple optical beacon experiment in San Diego

[KY1K]

Hi Kerry,

I've never understood the factors that determine the bandwidth of the 
PGP detector, except of course for the contribution of the photodiode 
itself. The photodiode effects are relative of course, a larger area 
PD always lowers the bandwidth, regardless of the electronics it's 
connected to.

I think it would be a good idea for you to determine the approximate 
frequency response of the setup your using. You can block the 
photodiode and simply measure the distribution of the output spectrum 
using Spectrum Lab. This will be the approximate (overall) frequency 
response of the receiver SYSTEM.

I know John was surprised to find his preamp had very high response 
to 20 Hz with a large active area photodiode, which I think was 1 cm 
square. When he discovered the poor response at 700 Hz, he quickly 
changed to a much lower frequencies and was stunned to see the 
extreme sensitivity of the detector at the lower frequencies.

Rather than going to a higher (700 Hz) modulation frequency, you 
might find it more to your advantage to decrease the transmitters 
modulation frequency to 30 Hz....depending on the measured frequency 
response of your actual receiver.

If you find the frequency response of your receiver to favor 
frequencies less than 60 Hz, you can greatly improve the dynamic 
range of your system (sound card) by building a simple low pass 
filter, and rolling off everything above 60 Hz-taking a large source 
of QRM out at the same time.

With Spectrum Lab, you should be able to detect the 30 Hz carrier 
easily, even in the presence of massive 60 and 120 Hz QRM. If you 
really need to hear the signal with your ears, Spectrum Lab will 
upconvert the 30 Hz signal to 700 Hz and you can build notch filters 
(in software) to take out the 60 and 120 Hz QRM! Enable the speaker 
output, and presto-Spectrum Lab allows you to listen with your ear.

Kerry, are you a member of the Spectrum Lab mailing list on yahoo? To 
join, send a blank message to:

SpectrumLab-subscribe at yahoogroups.com

Regards,

Art