Electronics > Projects, Designs, and Technical Stuff

Looking for Cheap, Ultra Fast LEDs (2-3ns)

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TheUnnamedNewbie:

--- Quote from: supperman on January 27, 2020, 10:02:44 pm ---
--- Quote from: dmendesf on January 27, 2020, 09:49:17 pm ---He was talking about increasing the data rate without increasing the simbol rate by coding more than one bit per simbol. Suppose instead of on / off you had 4 different brighteness levels... Then you could double the data rate without needing a faster diode. There are other coding schemes to achieve this.

--- End quote ---

Ah.. Understood! Not possible in this design since the brightness will vary based on external forces (i.e. LED and sensor will not always be aligned perfectly) I think I can safely clip on "on / off" but can get no more detail out of it.

--- End quote ---

Usually you would solve this with a low-pass filter. Using encoding like 8b/10b or 64b/66b, you can actually remove DC from any datastream (and thus you have a signal that has no spectral content below a certain frequency). You then block that out from your signal with a low-pass filter (usually in the form of a DC block). Since environmental changes will be low frequency (even if your device was spinning at 1000 RPM, it would be a low-frequency change with respect to your >100 Mbit/s signal) it would be stopped by this DC block.

supperman:

--- Quote from: TheUnnamedNewbie on January 28, 2020, 07:57:13 am ---Usually you would solve this with a low-pass filter. Using encoding like 8b/10b or 64b/66b, you can actually remove DC from any datastream (and thus you have a signal that has no spectral content below a certain frequency). You then block that out from your signal with a low-pass filter (usually in the form of a DC block). Since environmental changes will be low frequency (even if your device was spinning at 1000 RPM, it would be a low-frequency change with respect to your >100 Mbit/s signal) it would be stopped by this DC block.

--- End quote ---

Ok.. this does sound interesting! Of course these transitions are low frequency.. and I did not consider them as such. You got me thinking in a new way.. so thank you. Let me dig a little deeper and perhaps get back to you with questions. Any links would be helpful if you have them.

I'm still rather new to this forum.. but I have to say.. it has been a positive experience.

skylar:

--- Quote from: supperman on January 27, 2020, 10:17:23 pm ---Sorry - in my mind I thought you meant optical transceiver..  which of course I would still need.. at a high speed rate. I don't know of anything cheap here.. $10 for the transmitter, $10 for the receiver? Another $6 for 2nd SERDES.. optics? stuff adds up very quick. And debugging 4gbit signals.. am limited to 1ghz scope.

I'm not saying no. perhaps you have links to come components here that simplify things? I just don't know of a transmitter or receiver that directly takes/sends a differential signal at that speed that does not cost an arm and a leg... and if I'm building one I think I may be in a world of hurt at that data rate..

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As you say, you would need an (or 2x) optical transceiver for your air gap. I luckily have gotten these as scrap or cheap used components as the SANs out there go to the higher data rates and the 4/8Gb transceivers are no longer as dear.

supperman:

--- Quote from: TheUnnamedNewbie on January 28, 2020, 07:57:13 am ---Usually you would solve this with a low-pass filter. Using encoding like 8b/10b or 64b/66b, you can actually remove DC from any datastream (and thus you have a signal that has no spectral content below a certain frequency). You then block that out from your signal with a low-pass filter (usually in the form of a DC block). Since environmental changes will be low frequency (even if your device was spinning at 1000 RPM, it would be a low-frequency change with respect to your >100 Mbit/s signal) it would be stopped by this DC block.

--- End quote ---

I guess the first question is how would you modulate the signal.. with AtoD and DtoA converters? I can see avoiding DtoA via some smart ways of powering the LED.. but on the receiver side it seems unavoidable? Sounds expensive at that data rate... or is there some smart way of using cheap compareitors ?

Marco:
Fast ADCs are ridiculously overpriced ... maybe PPM/PWM can get you a little more out of a single channel.

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