I am modulating an OOK signal (0.2 Vpeak to peak, 6 MHz) over LED (Please see fig.1). The signal after modulation is detected by a trans-impedance amplifier. In contrast to the input OOK signal, the output signal's low level amplitude is not flat(fig.2) in other words, the zero pulse duration is reduced. Can any one help me understand why this happens?

 

Sure. The LED cannot respond to a 6 MHz. signal. It looks like the rise time of the LED response is faster than the fall time. This may be due to the persistence of the illumination in the transmitting LED after the driving current is removed. This type of response is typical of most optical circuits.

 

Post your circuit schematic with part values.

 

Here is the block diagram.

 

Here is the block diagram.

I thought I was the profligacy champion when it came to expensive LED flashers. But that rig puts my best efforts to SHAME!

 

Thanks for the diagram . It helps me to understand the errors and how to fix it for the future.



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