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How to modulate a S...

How to modulate a SMD mounted LED with DMT signal for VLC?  


Active Member
Joined: 2 months ago
Posts: 8
16/01/2020 3:12 am  
I am building a VLC system. A DMT signal (300MHz) from AWG directly modulates my LED. The LED is of SMD type mounted on an MC-PCB. I have transferred the DMT signal from AWG onto a 50-ohm trace on another PCB (via SMA jacks). 
How can I connect the signal from trace with SMD LED terminals? The trace is located don another PCB and LED is mounted on a separate MC-PCB. Can I use a simple copper wire to connect the trace and the LED terminals? For RF applications coaxial are preferable. Should I use a coaxial cable for the connection between the LED and trace?

Jimmy Davis
Active Member
Joined: 3 months ago
Posts: 14
16/01/2020 10:43 am  
Here come one good and one bad news:  The bad first:
The maximum output voltage of the generator is 0.5 V peak to peak = +/-0.25 V (with DC component = 0 V) into 50 ohm = +/-5 mA.  The LEDs need a DC bias voltage in the range of 2.0 V to 4.5 V, with a current of a few 100 mA.  So you cannot drive the LEDs directly by the output signal of the generator.
The good news: If you place a suitable amplifier (transistor plus the necessary passive components) very near the two pads of one LED, you don't have to worry about reflections.  I would try a simple (not optimal) circuit like the one in the attached scan.  However, if this would be your first "homemade" PCB I would advise you to ask someone in the lab for support.
And, still being doubtful about the switching speed of these LEDs, I would start the tests using a very low frequency (e. g. 50 Hz). At frequencies below 100 Hz it might be necessary to use a greater capacitor at the input (e. g. 100 myF at 12.5 Hz).
I hope this is helpful. Good luck!