OboR frontend

Used frontend is transimpedance opamp with big feedback resistor.

transimp Transimpedance photovoltaic opamp principle


On the simplified schematics you can see the principle of the transimpedance opamp frontend. It is voltage to voltage converter.

The bigger feedback resistor the better sensitivity.

This principle schematic is working with symetric power source ( +9V to -9V) in photovoltaic mode.





Full schematics of oboR frontend

obor rx
This one works with asymetric power source (+9 to +15V). Feedback resistor is substitute with resistors network. For full sensitivity use all the feedback resistors except R17, R18 and R25. In this mode the frontend is wild animal so it could oscilate. Than I recomend bypass 6 or 12 feedback resitors with 0.5pF capacitor. I did not find TLC272 in SMD so I substitute it for LM358 which is not best but it works.

For daylight use baypass feedback network with one 1Meg resistor.

This frontend is in photoconductive mode - anode of BPW34 is connected to power ground. You can improove the sensitivity with photovoltaic mode. It is enough just cut the anode from the ground and connect it to the virtual ground for opamp - pin3. This contact was made with this improovement.

Af signal is amplified with an ordinary LM386.

This frontend is able to receive modulated wave - CW and AM.

On PCB there is lot of space for tests. Please let me know how it works with your optics :)


obor rx
OboR Frontend                    

Schmematics

PCB

Parts placement