Electronics > Beginners
Floating probe! For $2.50
<< < (9/10) > >>
IDEngineer:

--- Quote from: BrianHG on September 28, 2018, 01:12:08 am ---According to these guys, bandwidth tests done, blue really the best at 800mb, then green, then red
--- End quote ---
Hmm, I'm missing something in that article. They say "The maximum data rates at which the BER did not exceed a 2·10−3 forward error correction threshold were ~ 294 (red), ~ 223 (green), and ~ 286 (blue) Mb/s", meaning that red was fastest and green was slowest. Granted these were three dies on an RGB LED, and they were using a modulation scheme to push the bit rate past the raw LED bandwidth, but presuming the same tests were run on all three dies it appears red was the fastest. I honestly *expected* blue to be the fastest... what am I missing?
BrianHG:

--- Quote from: IDEngineer on September 28, 2018, 03:13:41 am ---
--- Quote from: BrianHG on September 28, 2018, 01:12:08 am ---According to these guys, bandwidth tests done, blue really the best at 800mb, then green, then red
--- End quote ---
Hmm, I'm missing something in that article. They say "The maximum data rates at which the BER did not exceed a 2·10−3 forward error correction threshold were ~ 294 (red), ~ 223 (green), and ~ 286 (blue) Mb/s", meaning that red was fastest and green was slowest. Granted these were three dies on an RGB LED, and they were using a modulation scheme to push the bit rate past the raw LED bandwidth, but presuming the same tests were run on all three dies it appears red was the fastest. I honestly *expected* blue to be the fastest... what am I missing?

--- End quote ---
That's because they are trying to communicate with overall white light so your office LED lighting is the network and people wont be working in an all blue lit environment.  Look at the first table with the axis " Demonstrated rate [Mbps] ".  You see on that table, the red is the slowest while the blue is 800Mbps all on it's own.  Get rid of all other light colors in you office and illuminate it pure blue, and you will get 800Mbps all in one led, one wavelength of light.

If you want your employees the luxury of white lighting, mixing 3 colors to always achieve a single brightness and color of white, WHILE TRANSMITTING DATA, then the figures they quote for their trick to keep one brightness of white with one color temperature are the maximum speeds they can achieve.

Pure white LEDs are slower due to them being blue LEDs with a yellow phosphor which has an inherent afterglow to it slowing down the maximum Mbps.
StillTrying:
I joined a string of LEDs and with 5mA through the string, measured the uAs produced by a SFH213 in contact with the LED, they're the highest blue and green uAs I've seen.
uA
235 SB Blue
225 SB Green
140 Warm White
080  Medium White
070  SB Red
050  SB Orange
026  Std Red
025  old IR

With the SB blue or SB green the BW now seems to limited by what the 3140's output/top of LED can do, if I scope the 3140's output, the voltage shape is a close match to the received light shape.
BrianHG:

--- Quote from: StillTrying on September 29, 2018, 12:22:44 pm ---I joined a string of LEDs and with 5mA through the string,
--- End quote ---
What doe this mean?
Are you driving all these LEDs in series?  Parallel?
Different LEDs have different capacitance and current rise and fall times based on it's internal switching time.  Wouldn't testing 1 led at a time yield better results.
StillTrying:
"What does this mean?
Are you driving all these LEDs in series?  Parallel?"

5mA DC from a PSU through the series string, and a bare SFH213 connected directly to DMM to measure the max. head on uA photo current from each 5mm water clear LED, 3 times around the string and averaged, so not exact uA, (depends on the exact head on alignment), the 5mA might have changed a bit, but the order of sensitivity was the same for each of the 3 times the string was measured.

I'm surprised no one asks what I'm using to receive the light, most of the time I'm using the PD +1 transistor from the light bulb thread, I'd add the imperfect 1 TR receive on to the schem in the previous post #42 when I find it.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod