21 Nov 2011

More 481THz lightbeam progress

670nm receive head and converter to 80m

Today I did some further light beam experiments, this time using a 25kHz modulation signal on the light beam and receiving the signal on my FT817 with this head/converter unit above. It consists of a BPW34 photo-detector feeding a cascode FET/transistor stage into an emitter follower and SBL1 mixer to convert the signal to around 3.584MHz. With a current into the TX LED of just 10uA (a very dim glow from the red LED) the signal was 20dB S/N in 0.67Hz bandwidth on Spectran at a distance of 25cms without optics. If my calculations are correct, this means a range of around 100m could be obtained even with this miniscule power if 100mm lenses were added at each end to give some 27dB gain at each end. Using the same TX LED at 10mA (1000 times the current) then the range is already in the many km region, and this is without using power LEDs. This is encouraging progress. Tomorrow I want to repeat the test with the same LED as the TX as the detector.


Anonymous said...


This would make a good article for Practical Wireless. Not Radio but still Wireless communication.


TiƩgui said...

Hello Roger

why don't you use the BPW 34 at 950 nm where its sensitivity is maximum.
Tomorrow I'll start some experiments as yours with my students, they will have to design an infrared TX/RX... will see what they can do :)

73, from Thierry.