Looking for Cheap, Ultra Fast LEDs (2-3ns) - Page 1
Forum section: electronics / projects

author: supperman
author_rank: Regular Contributor
timestamp: January 24, 2020, 05:24:16 pm
content: I have found it very difficult to find LEDs with ultra-fast rise and fall times. The best I can find are by Vishay.http://www.vishay.com/docs/84581/vsly3943.pdfThey have the right price point ($0.22 in low volume - DigiKey) and I can overdrive them to 2ns rise / fall with some fancy circuitry. But they are not ideal. I would rather go towards the visual range (650-850nm). And I would love to save money on the drive circuit by having a faster LED. I could also use a larger field of view.The problem is it is very difficult to even search for these specs...Any tips on how to search and find LEDs by rise and fall time? Only IR LEDs seem to list it on data sheets and most websites don't let you search on it (some luck with mouser). Low current is preferred because it is cheaper to overdrive them.any tips greatly appreciated..

author: Marco
author_rank: Super Contributor
timestamp: January 24, 2020, 05:44:17 pm
content: Is the application really so price sensitive you can't use a laser diode?If it has to be a LED I'd try to get the smallest blue LED you can find, probably 0201, and giving them a spin. Reducing size seems pretty much a necessity to speed up LEDs.

author: supperman
author_rank: Regular Contributor
timestamp: January 24, 2020, 06:42:29 pm
content: Quote from: Marco on January 24, 2020, 05:44:17 pmI'm moving a lot of data in a unique configuration.. so I will need at least 10+ sources.. so cost per unit is a factor. Also, cost of logic goes up as well as data rates per unit go up. I would prefer to stick with LEDs.. unless you know of a laser diode for $0.50 or so..

author: Marco
author_rank: Super Contributor
timestamp: January 24, 2020, 10:57:56 pm
content: Quote from: supperman on January 24, 2020, 06:42:29 pmChina must have some sources, because they are selling modules with lenses for ~10 cents a piece.PS. oh, I guess thereally cheap onescan't really easily be PCB mounted. Cheapest TO18's seem more in 80 cent range.

author: moffy
author_rank: Super Contributor
timestamp: January 25, 2020, 11:03:00 pm
content: This is a nice article about driving a blue LED with 1-2ns rise times:https://arxiv.org/ftp/arxiv/papers/1011/1011.1954.pdfMaybe some of the Broadcom HLMP devices would be suitable.

author: supperman
author_rank: Regular Contributor
timestamp: January 26, 2020, 01:18:51 am
content: Quote from: Marco on January 24, 2020, 10:57:56 pmThanks for the link. I did look at them.. but this is going to be a consumer product and I can't pick components without companies or data sheets... I think these lasers are mean for key chains.

author: supperman
author_rank: Regular Contributor
timestamp: January 26, 2020, 01:24:55 am
content: Quote from: moffy on January 25, 2020, 11:03:00 pmThanks for that article.I had read it during my research. They don't mention the LED (from what I remember) and it is not real world (i.e. PSpice only). In the end it is about driving the LED with an OpAmp.. and at that speed and current that is super expensive for the OpAmp. My circuit will pre-load (keep the LED off but warm) then spike it positive or negative on a change in on/off state for under 1ns. The circuit is about $0.35 right now + LED.Mainly just looking for actual LEDs that are fast.. in the 650-950nm range.. 800ish preferred.

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2020, 01:52:17 am
content: I think if you want visible light you're just going to have to bite the bullet and experiment, I don't think anyone has ever tried the latest smallest SMD LEDs yet and it's certainly not a datasheet feature. The 940 nm LEDs are probably used for POF, so they mention rise/fall time ... but no one is doing that with visible, so at best the datasheet might mention junction capacitance.You might get optical speed up by applying a bit of negative voltage too.

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2020, 04:16:35 am
content: Quote from: moffy on January 25, 2020, 11:03:00 pm"High-Speed LED Driver for ns-Pulse Switching of High-Current LEDs" is a bit more valid, actually measures optical rise/fall with ways to speed it up.They use a capacitor to give an initial high current kick to speed up the optical rise time and a parallel inductor to provide negative current during turn off to speed up optical fall time.

author: james_s
author_rank: Super Contributor
timestamp: January 26, 2020, 04:43:57 am
content: Does the size of the LED even matter? I thought the smallest SMT LEDs had pretty much the same size die as larger ones and ordinary 3mm and 5mm LEDs just in a smaller package?

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2020, 05:06:25 am
content: At least the bond wires will be shorter if nothing else.

author: moffy
author_rank: Super Contributor
timestamp: January 26, 2020, 06:38:48 am
content: Quote from: supperman on January 26, 2020, 01:24:55 amThey do verify their PSICE model with actual measurements, at least voltage rise and fall. The opamp is just providing a very fast, high current pulse (there is no feedback except for voltage gain). You just need a push/pull output stage to source/sink the current. Lots of ways to do that cheaply.The LED is mentioned in the appendix: The datasheet for the HLMP-CB15-P0000. It is referenced along with the datasheet for the opamp. Unfortunately it is obsolete but you should be able to find an equivalent.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 26, 2020, 06:47:09 am
content: Interesting topic, what do you want to do with these leds? If you found good sources in china please share

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2020, 06:53:53 am
content: Quote from: moffy on January 26, 2020, 06:38:48 amThe problem is that until the entire junction is filled/swept-out the current doesn't equal the optical output, so electrical response isn't all that relevant.

author: OwO
author_rank: Super Contributor
timestamp: January 26, 2020, 07:05:35 am
content: Ordinary blue LEDs (usually GaN based) will all go to a few hundred MHz easily from my tests. What you want to do is bias the LED to half the operating current and measure the small signal impedance, like so:Then tune out the reactive portion and drive it with a wideband MMIC.

author: OwO
author_rank: Super Contributor
timestamp: January 26, 2020, 07:13:22 am
content: 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.

author: supperman
author_rank: Regular Contributor
timestamp: January 26, 2020, 03:52:42 pm
content: 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..

author: dmendesf
author_rank: Frequent Contributor
timestamp: January 26, 2020, 05:39:18 pm
content: Is this an isolated scope/probe? I can't imagine something else that would need isolation and transfer gigabytes per second...

author: supperman
author_rank: Regular Contributor
timestamp: January 26, 2020, 09:16:01 pm
content: Quote from: dmendesf on January 26, 2020, 05:39:18 pmNo. Consumer. Video... can't say more than that...

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2020, 10:08:30 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: January 26, 2020, 11:22:50 pm
content: Quote from: supperman on January 26, 2020, 03:52:42 pmI think you could do a 4Mb/s channel for $0.50, but I think there's no chance of 400Mb/s.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 27, 2020, 06:18:42 am
content: What's the receiving circuit? are they LEDs too?

author: Marco
author_rank: Super Contributor
timestamp: January 27, 2020, 06:32:57 am
content: Presumably SFH2701.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 27, 2020, 03:19:36 pm
content: This sounds a lot like an x-y problem.Incidentally, getting lots (>>gigabit) data from A-to-B for cheap is the field I'm working in, so perhaps I could help with some outside-of-the-box ideas if you can give more information/limits you are facing. What volume are you looking at?Depending on what output type you have, you might be able to do something with larger symbol size? instead of OOK, consider 4 or even 8 ASK to lower symbolrate?

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 04:39:49 pm
content: Quote from: Marco on January 27, 2020, 06:32:57 am

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 04:43:56 pm
content: Quote from: StillTrying on January 26, 2020, 11:22:50 pmNot sure where you are coming from. I'm running at 3ns switching times now.. and am looking for help finding other LEDs to test. I am of course open to other circuits as well.. but most of what I see online are inferior at this point.

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 04:48:59 pm
content: Quote from: Marco on January 26, 2020, 10:08:30 pmYes, I have seen this and decided to go a different way. I find that it is limited in how I can tweak it.. and it does not run efficiently for continuous data. (As I think they even point out in their paper). I don't use capacitors or inductors.. I use separate circuits to drive pos and neg spikes and have complete control over duration and current on them.when you look at LED data sheets you can quickly see that continuous drive current may be 70ma.. but they can handle spikes up to an AMP. The above circuit can't deliver that.

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 05:52:14 pm
content: Quote from: TheUnnamedNewbie on January 27, 2020, 03:19:36 pmI'm not sure I know what you are talking about? Encoding data for error correction? I'm not there yet.. just doing single channel bit-rate speed tests. I'll take the problems as they come.. but distances are short and I'll layout all paths with matched lengths so I'm hoping that data will be reasonably in sync and that I don't need to worry. It is video so 100% lossless is also not required.. and I'm hoping to sit on top of some protocol used by the FPGA to serialize and combine..

author: skylar
author_rank: Contributor
timestamp: January 27, 2020, 08:06:40 pm
content: Quote from: supperman on January 26, 2020, 03:52:42 pmIs the 1cm air gap critical or just part of this particular LED implementation?A single SERDES pair and an AC coupled differential pair is about the same cost as 10x of your LED circuits and is good for our isolation needs and certainly meets your speed requirements. For better isolation, the guts of a GigE SFP provides the optical link in our stuff (get a FC 4Gb since you need the speed, they even go up to >16Gb these days). Cost wise you could probably buy one and split the Tx and Rx side onto your own boards with only a little tweaking. Overall it does probably double your parts cost, but its proven tech :/

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 08:37:52 pm
content: Quote from: skylar on January 27, 2020, 08:06:40 pmYes, there is unfortunately a physical requirement - air gap AND about 10+ units... so I have to work with that.Also.. if I could use a single link.. now we are talking about an FPGA with 2x 5 gbit ports - or a dedicated chip to do the same.. and that certainly blows the budget, right? I have not gotten FPGA quotes with high speed ports.. but adding them seems to triple the price at least at retail.

author: skylar
author_rank: Contributor
timestamp: January 27, 2020, 09:10:14 pm
content: Quote from: supperman on January 27, 2020, 08:37:52 pmI was talking about a dedicated chip, the SERDES (serial/de-serializer) I find in your ~10bit 4Gb use case are about $6/EA and let you use the same 10x 400Mb ports on your FPGA. Since you mentioned its a video application, there's even dedicated 16bit RGB video SERDES like the Renesas ISL76322ARZ that popped up in my search, not sure what you are doing with your FPGA but that's an interesting option to just shuttle video data around that may not need an FPGA?

author: dmendesf
author_rank: Frequent Contributor
timestamp: January 27, 2020, 09:49:17 pm
content: 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.

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 10:02:44 pm
content: Quote from: dmendesf on January 27, 2020, 09:49:17 pmAh.. 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.

author: supperman
author_rank: Regular Contributor
timestamp: January 27, 2020, 10:17:23 pm
content: Quote from: skylar on January 27, 2020, 09:10:14 pmSorry - 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..This is how I always come back to the LED design(and for the people paying attention.. yes, potentially there is a way to use a single transmit/receive rather than 10+. It is either or)

author: StillTrying
author_rank: Super Contributor
timestamp: January 28, 2020, 01:12:29 am
content: Quote from: supperman on January 27, 2020, 04:43:56 pmIf you're putting a digital 100Mb/s in one end and recovering a 100Mb/s digital output from the other I've no idea how you're doing that with a few $0.10 components!QuoteI think if any one here knows much about switching normal LEDsand photodiodesat these speeds they're keeping it to themselves.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 28, 2020, 07:57:13 am
content: Quote from: supperman on January 27, 2020, 10:02:44 pmUsually 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.

author: supperman
author_rank: Regular Contributor
timestamp: January 28, 2020, 03:40:57 pm
content: Quote from: TheUnnamedNewbie on January 28, 2020, 07:57:13 amOk.. 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.

author: skylar
author_rank: Contributor
timestamp: January 28, 2020, 04:29:23 pm
content: Quote from: supperman on January 27, 2020, 10:17:23 pmAs 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.

author: supperman
author_rank: Regular Contributor
timestamp: January 28, 2020, 05:19:26 pm
content: Quote from: TheUnnamedNewbie on January 28, 2020, 07:57:13 amI 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 ?

author: Marco
author_rank: Super Contributor
timestamp: January 28, 2020, 07:11:51 pm
content: Fast ADCs are ridiculously overpriced ... maybe PPM/PWM can get you a little more out of a single channel.

author: technix
author_rank: Super Contributor
timestamp: January 28, 2020, 07:26:28 pm
content: What for though? Is it ultra-fast display? Fiber optics?If it is the former, I have nothing. If it is the latter, maybe you can use a ready-made module (TOSLINK transmitter for slower ones, SFP+ module for something that is real fast) and just focus on the digital logic part. If your source chip has serdes, it is likely you can get SFP+ would work with that.

author: supperman
author_rank: Regular Contributor
timestamp: January 28, 2020, 09:00:41 pm
content: Quote from: Marco on January 28, 2020, 07:11:51 pmAgree.. dug a little deeper.. like $3.5 per channel at 8 bit and 100msample. One could probably get 2-3 bits per sample of data sampling both amplitude and frequency shifts (amplitude at the crossing point) on a carrier wave.... so 400mbit+ but it really busts the budget.

author: supperman
author_rank: Regular Contributor
timestamp: January 28, 2020, 09:02:49 pm
content: Quote from: technix on January 28, 2020, 07:26:28 pmThis is for video transmission in a very specialized "air gaped" setup. I would just call it video.. not super fast or anything.. more than 1080p, less than 4k.. uncompressed.

author: dmendesf
author_rank: Frequent Contributor
timestamp: January 29, 2020, 12:44:52 am
content: If you want to quantize in 4 steps (to double your data rate) you could try using the embedded LVDS receivers in the receiving FPGA as comparators. Crude but cheap. Of course this won't scale much more than that.

author: Marco
author_rank: Super Contributor
timestamp: January 29, 2020, 01:01:45 am
content: Quote from: StillTrying on January 28, 2020, 01:12:29 amThe standard fast'ish cheap photodiode is SFH2701 ... the Chinese must have low cost faster photodiodes though, they sell Fiber->CATV converters for a couple bucks.PS. actually they seem to be cheap ROSAs, not loose photodiodes. Less useful.

author: supperman
author_rank: Regular Contributor
timestamp: January 29, 2020, 01:25:24 am
content: Quote from: Marco on January 29, 2020, 01:01:45 amYes, that is the one I'm using. 2ns rise and fall for $0.22 at DigiKey in volume..

author: technix
author_rank: Super Contributor
timestamp: January 29, 2020, 05:19:52 am
content: Quote from: supperman on January 28, 2020, 09:02:49 pmIf it is video, maybe HDBaseT over 10Gbps fiber?

author: StillTrying
author_rank: Super Contributor
timestamp: February 01, 2020, 08:28:22 pm
content: Has the OP managed to make a $0.50 400Mb/s opto isolator yet.

author: supperman
author_rank: Regular Contributor
timestamp: February 02, 2020, 01:41:17 am
content: Quote from: StillTrying on February 01, 2020, 08:28:22 pmI'm around 200mbit and around $1 for sender and receiver components. Still some stability issues. I'm optimistic to raise this speed up into the target range. Mostly I'm dealing with radiation.. so it is as much about PCB design as anything else. This of course does not count the two FPGAs on either end that have some useful i/o capabilities.Again, I'm not here to debate if what I'm already doing can be done or not. I was asking about other LED components that may work better than what I got.. and it has lead to some interesting conversations.

author: StillTrying
author_rank: Super Contributor
timestamp: February 02, 2020, 07:23:34 am
content: Quote from: supperman on February 02, 2020, 01:41:17 amI hope that wasn't a surprise!QuoteWe don't need to know why/what the 1cm air gap is for, and you've given reasons why some solutions aren't useful, but without knowing what/if anything is useful I think some ofuswill be finding the conversations just frustrating.

author: supperman
author_rank: Regular Contributor
timestamp: February 02, 2020, 04:58:07 pm
content: Quote from: StillTrying on February 02, 2020, 07:23:34 amI'm new to this... so yes! In a delightful / dreadful way it was a surprise to see radiation be stronger than the signal depending on the layout.Quote from: StillTrying on February 02, 2020, 07:23:34 amI can see that this can be frustrating and I'm sorry. Also sorry if I came across as a bit aggravated.. don't mean to be. Clearly you have experience in this space that exceeds mine.. My only advantage is that I MUST make it work.. so I will.I need to get a video signal to another part of the design that can not have a physical connection to the signals origin for mechanical reasons. It is not like a slip-ring.. but perhaps it is best if you think of it as such.

author: m98
author_rank: Frequent Contributor
timestamp: February 02, 2020, 05:44:35 pm
content: Quote from: supperman on February 02, 2020, 04:58:07 pmMaybe take a look at the ST60A2G0.

author: supperman
author_rank: Regular Contributor
timestamp: February 02, 2020, 06:22:08 pm
content: Quote from: m98 on February 02, 2020, 05:44:35 pmThat looks Fantastic!! I can't find any pricing.. or distribution. Any idea on price or how to buy?

author: supperman
author_rank: Regular Contributor
timestamp: February 02, 2020, 06:46:32 pm
content: I reached out to ST.. will post here when I hear back. I would do $5-6 per chip.. what are the chances?Does anyone think I'll run into extra issues during emissions approval if using a chip like this? 60Ghz probably does not "travel" too far.

author: StillTrying
author_rank: Super Contributor
timestamp: February 03, 2020, 02:28:14 am
content: Quote from: supperman on February 02, 2020, 04:58:07 pmOh dear! In reply#26 you seemed to be hinting at driving 100s of mA <<3ns edges to speed up the LEDs switching, I imagined multiple channels switching at exactly the same time only 1cm from the fast photo diode amps! But don't worry, I think everyone finds out the effects of fast edges the hard way.In my simple LED switching experiments a 5mm UV seemed to have faster rise and fall times than most, but each edge ended in a horrible curve so I didn't try any further.I don't know how you're turning the small photo diode currents back into a 400Mb/s logic level, but I know it's a lot harder than 'just use a 2ns photo diode'.The new? ST60A2G0 looks like it could be a good solution.I don't know much about high speed stuff at all.

author: supperman
author_rank: Regular Contributor
timestamp: February 06, 2020, 09:15:01 pm
content: WOW.. got some feedback on my request for info on the ST60A2G0.They say it is "Classified"! What??? The press release has it positioned as a low cost chip for consumer products..They seem to actually mean that is is Classified.. as in security clearance required rather than DNA.They did tell me to call a number which I did. But I think this is a dead end.

author: dmendesf
author_rank: Frequent Contributor
timestamp: February 06, 2020, 11:43:16 pm
content: There's a "databrief" of this device. Seems the antennas are not embedded in the BGA. (I've seen others that do... ). This implies in RF voodoo without instrumentation (who can measure anything at 60GHz?) So the manufacturer must go "hand to hand" to make it work/pass compliance. This sort of thing can only pay itself if you order large orders... So they don't want to talk with you.

author: StillTrying
author_rank: Super Contributor
timestamp: February 07, 2020, 12:27:54 pm
content: I think what's classified about the ST60A2G0 is that they probably haven't managed to mass produce any yet.