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| BrianHG:
--- Quote from: StillTrying on September 19, 2018, 04:10:04 pm ---A super bright white ~1MHz looks the easiest ATM, it might have 250ns rise and fall times but it reaches the flat top of the 1us pulse before the red, and is 10-100X brighter. A super bright green, red or cool white might be faster, but I don't have any loose ones to test - yet. --- End quote --- Super bright white is a blue led with yellow phosphor to make it look white. The super bright blue should be the fastest as it has no phosphor and it's a fundamental color generated by the die directly. A true super bright 405nm violet led or UV ones should be fastest. |
| StillTrying:
"I'll probably test the linearity of the super bright white around 3mA - 30mA soon. I'm expecting the open loop linearity to be very good, and the BW ~1MHz" At 8.5mA +/-5mA the super bright white LEDs light output seems very linear. :) Max. input(and output) is 200mVpp, with the ca3140 10%-90% rise/fall times ~250ns, -1dB at 280kHz so -3dB is going to be <1MHz. :'( Green trace is the light output. |
| BrianHG:
--- Quote from: StillTrying on September 25, 2018, 06:39:49 pm ---"I'll probably test the linearity of the super bright white around 3mA - 30mA soon. I'm expecting the open loop linearity to be very good, and the BW ~1MHz" At 8.5mA +/-5mA the super bright white LEDs light output seems very linear. :) Max. input(and output) is 200mVpp, with the ca3140 10%-90% rise/fall times ~250ns, -1dB at 280kHz so -3dB is going to be <1MHz. :'( Green trace is the light output. --- End quote --- What does the throughput of a triangle wave look like? |
| StillTrying:
"What does the throughput of a triangle wave look like?" I don't know, my simple AF SG doesn't do triangular, but as there's already a CMOS 555 on the other end of the breadboard, I might produce and capture some rough triangular before swapping the SB white. I'd expect the linearity to be OK, but the different rise and fall times of the SB white to become visible around 10kHz. The first start of clipping on the bottom of sines was the 3140 struggling to pull down against the 1k2 pull-up (which was there help it!), rather than the LED current going <3mA. By simply removing the 1k2 and letting the 3140 supply all the LED current the dynamic range before clipping increased ~240mV to ~350mVpp, and the speed, with a slight increase of the 8.5mA the SB white BW must now be close to 1MHz, probably about the best a white LED is going to get at these low LED currents. With a -ve supply availiable, an NE5532's higher output current and BW might be quite good there, the voltage gain needed is very low because the voltage across the LED only varies ~100mV + the 20R. SuperBright Green, Orange and Blue's have arrived, it might be a day or 2 before I test them. |
| BrianHG:
According to these guys, bandwidth tests done, blue really the best at 800mb, then green, then red: http://www.spie.org/newsroom/3941-optical-wireless-network-built-on-white-light-leds-reaches-800mb/s The problem with red is that there are so many different formulations, from 620nm through 670nm, some slow, some fast. You can at least count on super bright blue always being the same formulation. |
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