Electronics > Projects, Designs, and Technical Stuff
Looking for Cheap, Ultra Fast LEDs (2-3ns)
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OwO:
Those rise and fall times in that datasheet are complete bollocks. There is no test circuit, nothing about the drive conditions, nothing about how I_F is defined. For all you know they could be driving with a hard low-impedance voltage pulse source and only counting final steady state current (which experience says they probably are). It's the same story with MOSFET rise/fall times. A better way to characterize speed is to drive with a defined impedance source (e.g. 50ohms), defined power level and plot the optical response vs frequency. Then you can look at the -3dB bandwidth and say how fast the LED is.
supperman:
Wow, a lot to catch up on guys.. thanks..

I have my own circuit.. cost me about $0.35 right now + LED. It pre-biases the LED (keeps it off but warm). Gives a very large positive or negative (adjustable) pule on high / low transitions wile keeping currents low on continuous on stages.
I measure voltage and current to verify. I have a very fast photo-diode to see optical response. There is plenty of room for error here.. but I'm not a test lab.. just looking for a readable signal.
For the most part the rise time on spec sheets is well defined.. they typically list the drive current and since it is 10-90% in most cases.. they don't worry about pre-biasing it. Usually a fast Mosfet acting as a switch.

I'm using it to send data across about a 1 cm air gap.. and I use around 10+ of them to get the data I need. Not easy.. since we are talking about serious data over very cheap components.

I always have to remind myself that light travels about one foot at 1ns.. to get some perspective..

Also - running this with an FPGA which is limited to about 600 Mbit per pin so taking it down to say a single laser would add all sorts of cost to serialize multiple channels.. etc.. so I think I'm happy with LEDs giving me around 400 Mbit... I think it should be possible..


dmendesf:
Is this an isolated scope/probe? I can't imagine something else that would need isolation and transfer gigabytes per second...
supperman:

--- Quote from: dmendesf on January 26, 2020, 05:39:18 pm ---Is this an isolated scope/probe? I can't imagine something else that would need isolation and transfer gigabytes per second...

--- End quote ---

No. Consumer. Video... can't say more than that...
Marco:
Did you see the speedup circuit in the paper? That should fit in your budget and might eek out some extra performance from indicator LEDs if necessary.
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