Pulse distortion in LED modulation

Discussion in 'Wireless & RF Design' started by Kris@UA, Jun 5, 2017.

  1. Kris@UA

    Thread Starter New Member

    Apr 29, 2017
    18
    1
    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?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    12,040
    2,591
    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.
     
  3. crutschow

    Expert

    Mar 14, 2008
    21,728
    6,266
    Post your circuit schematic with part values.
     
  4. Kris@UA

    Thread Starter New Member

    Apr 29, 2017
    18
    1
    Here is the block diagram.
     
    • FIG1.PNG
      FIG1.PNG
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  5. Papabravo

    Expert

    Feb 24, 2006
    12,040
    2,591
    I thought I was the profligacy champion when it came to expensive LED flashers. But that rig puts my best efforts to SHAME!
     
  6. Shah Alam 1

    New Member

    Jun 8, 2017
    2
    0
    Thanks for the diagram . It helps me to understand the errors and how to fix it for the future.



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