Experiments in Laser Comms DX Notebook
30/12/99: Improved receiver with shielding:
This time an Rf value of only 10 Megohms
was used as the first receiver was very microphonic with the seven
resistors in mid air. A small salmon tin was used as a
capping shield over the photodiode (although it is not very light-tight
at the moment). The arrangement completely eliminated
audible hum when connecting up to the PC desktop Soundblaster
card. However, recording a WAV file and analysing it using
my narrowband software (8 megasamples) showed that even with the
photodiode unpowered there was still signals coming from (presumably)
the PC. The output of the signal was also too low. I
needed to increase the level into the Soundblaster to mask as much
internal noise as possible.
The spectrum was lowpass filtered at 2000Hz after acquisition. The low frequency rolloff is produced by the 470n/10K coupling. Note the small peak on the rising edge of the low frequency side. This is at 100Hz.