Optical RX schematic

Schematic of the G3XBM optical RX

Several people have asked for the schematic of the optical receiver used in the recent "over the horizon" red LED 481THz tests. Here it is. The BPW34 photodiode does not have any bias and is connected directly (in the air) to the gate of the FET. I have tried reverse biasing but it made little noticable difference to sensitivity. The cascode "head" is from KA7OEI and this is followed by an op-amp buffer and gain stage which feeds the PC. There is an additional stage of gain to drive a small crystal earpiece. I am sure the circuit can be further simplified and optimised but it has allowed me to copy a baseband scatter signal at 8.63km by ear and is as sensitive as any receiver I have yet tried.