Experiments in Laser Comms DX Notebook

26/12/99: First attempts at building test receiver:

I needed to be able to characterise the photodiode amplifier IC that I wanted to use in my early experiments (OPT210), so I built up a circuit on a round piece of PCB (dead bug style) which I mounted into the end cap of a 1 metre length of 80mm PVC pipe.  The idea was that it would be completely light-tight and LEDs could be mounted into an end cap at the other end to allow testing of signals in a dark environment.

The photodiode was run with a Rf of 70 Megohms (seven 10 Megohms resistors in series). The pipe turned out to be light-tight (no increase in current when illuminated with a strong light) but the signal was swamped in 50Hz hum from the mains when connected to the PC Soundblaster card.  Grounding the PCB reduced the hum but it was still too high.   Only when I took a laptop out under the back verandah and ran it on batteries and grounded the PCB directly to an earth stake did the level of hum reduce to the level of the photodetector noise.   This was not near good enough.  Also, narrowband analysis of the signal showed other components probably from the laptop itself. These problems led to an attempt to better shield the photodetector from hum and computer noise.

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