Fast switch with picosecond falltime - Page 1
Forum section: electronics / projects

author: bradfield
author_rank: Newbie
timestamp: June 11, 2016, 08:45:42 pm
content: Hello forums,consider this application:I have a chunk of fluorescent material, whereby the fluorescence has a livetime of about 15 nanoseconds.The fluorescence is excited by a highpower LED. To measure the fluorescence I can use a phototransistor or photodiode with amplifying circuit. Since the livetime is very short, The LED needs to be turned off very fast, so that the measurement can take place timely.The LED is run by a current source of 1 Amp. How can I switch off the LED within a time of subnanoseconds? And how can I start the data aquisition of the photodiode shortly after that?The first idea that I got from this post:http://www.jensign.com/opto/ledlaserdrivers/is to put the LED in parallel to the avalanche transistor. By the time when the transistor starts tunneling, the LED shuts up.

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 09:18:44 pm
content: HiI guess the first point is a matter of units. A rise time of 100 ps is not all that crazy. A rise time of 1 ps is quite a challenge.In most areas of research, a 15 ns "thing" would be quite happy with rise and fall in the 750 ps range. That would keep it below 10% of the total. Measuring the actual performance would get you added accuracy past that.What sort of performance do you need?Is a $100,000 solution ok? Does it need to be under $100 to fit the budget? That's very roughly the sort of range you cover as you go from 1 ns to 1 ps for rise time on a 1A switch.Bob

author: bradfield
author_rank: Newbie
timestamp: June 11, 2016, 09:25:51 pm
content: I think 750ps are awesome. We should stay below 200 dollars.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 11, 2016, 09:28:00 pm
content: Build the current source to withstand a short circuit, then use a relay to short the LED? That would work if super accurate timing is not required.

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 09:31:45 pm
content: HiSo here's a ..errr ... stupid way to do it:You can fairly easily get buffer chips that are in the < 1 ns rise time range. They will put out < 50 ma, but they are nice and fast. Parallel up 20 or so of them and you have 1 A at a "maybe fast enough" rise time. It will cost more for your time sorting through chip data sheets than it will for the chips themselves.Bob

author: Siwastaja
author_rank: Super Contributor
timestamp: June 11, 2016, 09:38:15 pm
content: Current-limiting series resistor (even if it's a current source; there probably is some capacitance) and a power GaN FET shorting out the LED, physically placed right next to the LED (I mean, like a few millimeters max) to minimize the inductance.Syncing:Use two identical photodetectors, one directly coupled to the LED only, one measuring the fluorescent material. Use a light-absorbing filter (Neutral Density filter, for example) on the LED so that the light intensity on both detectors is similar; intensity may affect the response time. Then, you can measure the time difference between the two detectors, ruling out many sources of error.BTW, do you think that the LED itself is fast enough? I don't know, that's why I'm asking. Actually shorting it out is good against the LED capacitance that normally limits the speed, but it may not be enough, or there may be other mechanism that slow the LED down in addition to the electrical capacitance.

author: richnormand
author_rank: Unknown
timestamp: June 11, 2016, 09:49:37 pm
content: The avalanche transistor (or a chain of transistors in series, like a an electro-optic laser Q switch) or GaN or GaAs switch is most likely the better option. Getting it to work fast is not the issue but getting the signals in and out of the package usually is.If the LED is current limited or has a resistor in series you might consider buffering it and use a small mercury reed switch to short circuit it. You will have to keep the wires very short and have a good RF geometry. This assume you can have a (very) long LED light exposure to saturate your sample and only want a very sharp excitation light cutoff to measure the fluro lifetime afterwards. Triggering on the light output may be the only option as I do not know the time jitter with the mercury reed switches. But: they are cheap.Final though if a spark from a sparkgap be (with lots of UV for excitation) be powerful to excite your material and fast enough?Do you know the rise and fall time of your photo diode setup and LED to start with? Some PMTs are also very fast and sensitive given the appropriate setup as you might have to give up some excitation power to get faster on/off action.

author: bradfield
author_rank: Newbie
timestamp: June 11, 2016, 11:22:21 pm
content: I did not find anything about the rise and fall times of the LED in the datasheets. But I found a photodiode with a risetime of a nanosecond.Is it possible to decrease the falltime of the LED by applying a reverse voltage?Concerning the sparkgap: I need 480-500nm radiation (nitrogen would work), but I have no idea how fast the spark will decay.What are PMTs?QuoteWhat do you mean by that?I am wondering how the professional implementations of pulsed fluorometers work? It can be done, somehow.EDIT:The only way to get the LED rise/fall quickly is to overshoot/undershoot it. This can be done with pulse shaping. How can I incorporate a pulse shaping with the avalanche transistor method?

author: richnormand
author_rank: Unknown
timestamp: June 12, 2016, 12:45:17 am
content: Quote from: bradfield on June 11, 2016, 11:22:21 pmGetting the signal in/out was the challenge when we were using fast (20 to 100 picosec) photodiodes. The specs were there, but getting the signals out of the pcb stripline and properly matched to the outside world without smudging the signal was always an issue.PMTs: photomultiplier tubes. In some cases we ended up using special high speed PMT tubes (Hamamatsu and RCA in the old days). The trade off was that the photodiodes surface area had to be very small to get the speed needed and sometime the PMT had better performance and 10^6 gain to collect a useful signal.I'll look for a circuit to drive photo-radar lasers that I have somewhere. It was using the concept of overshooting the transitions for on/off. It might be applicable to LEDs too. You had to get it right with the circuit and the layout impedance to work well (ie: many blown lasers....). Basically it was based on inducing a reactive ring at the transition with the next ring below the laser threshold. With lasers it was also temperature dependent.Turning diode lasers on/off in less than a nanosec might end up being easier than with an LED due to the large threshold for lasing to initiate.In the meantime you might find these useful:http://www.reed.edu/physics/courses/Physics332.s16/pdf/LED%20Luminescence%20Decay.pdflast post on page one:https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/Look at last post here, driving LED?:http://www.overunityresearch.com/index.php?topic=1556.150http://www.highlandtechnology.com/DSS/T160DS.shtmlhttps://www.lasercomponents.com/de/?embedded=1&file=fileadmin/user_upload/home/Datasheets/lc/applikationsreport/drive-electronics-pulsed-laser-diodes.pdf&no_cache=1

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 02:15:08 am
content: Hi"Getting it in and out":Consider that you are working with a microwave circuit here. Losses and the like need to be under control at least into the UHF region. If they aren't going to be the dominant effect, the layout and connections need to be good into the low microwave region (say 3 GHz). You have a cascade setup of many parts and elements. Each one knocks down the bandwidth a bit ....Bob

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2016, 02:15:52 am
content: The bond wire in the LED is going to keep current going for a bit if you short circuit it with a RF mercury relay, not ns range but 100s of ps for a nh of stray inductance. A switchable current source build with fast RF transistors and some TVS could provide a couple of 10s of V of push back, cutting that time down by up to an order of magnitude.

author: Someone
author_rank: Super Contributor
timestamp: June 12, 2016, 03:22:05 am
content: 1ns FWHM is bleeding edge performance for pulsing an LED, to get sharper edges you either move to lasers or optical techniques.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2016, 03:27:22 am
content: How long does the LED have to be on before it is abruptly shut off?If an on duration of 10s of nanoseconds is acceptable, then I would try driving the LED with a charge line. This would work especially well using an avalanche transistor for switching.

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2016, 03:45:50 am
content: Quote from: Someone on June 12, 2016, 03:22:05 amQuestion is what limits it ... if it's bondwire inductance alone then you can go faster simply by increasing the driving voltages.I realized it is of course possible to use a mercury wetted relay to apply "breaking" voltages. If you put a Schottky in series with LED and put the relay on a tap in between the diodes and the current source you can use the relay to jam some negative voltage (relative to the LED anode) into the circuit.

author: Someone
author_rank: Super Contributor
timestamp: June 12, 2016, 05:20:20 am
content: Quote from: Marco on June 12, 2016, 03:45:50 amThe lead/bondwire inductance and junction capacitance are non-trivial but there are also carrier lifetimes that fundamentally limit it, there has been a lot of research to end up with multi gigabit optical networks.

author: ap
author_rank: Frequent Contributor
timestamp: June 12, 2016, 10:34:11 am
content: I read in an other topic in this forum that certain LEDs use internal fluorescent material to generate the colour required, i think in this thread they talked about white. While i did not further dig into this, i wanted to make you aware, and maybe using a diode laser is the safer way to go, cheap these days

author: Twoflower
author_rank: Frequent Contributor
timestamp: June 12, 2016, 11:03:17 am
content: I think the spark gap idea is worth a second view. As Air-Gap flash they're used for high speed photography. Have a look athttps://en.wikipedia.org/wiki/Air-gap_flashand I found an article (including schematics and measurements):https://web.archive.org/web/20130719151245/http://web.mit.edu/Edgerton/www/pdf/Article.pdf. But keep in mind that this uses a bit higher voltages than LEDs

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2016, 01:10:56 pm
content: The only way to get the gap to extinguish that fast is to have a sub ns decaying high voltage pulse and the efficiency will be pathetic, so you need a lot more power.I'd just try to make a differential pair current switcher out of some RF BJTs and an ADCMP580 at 100 mA first and see how far you get with a LED. At 100 mA the ADCMP580 can drive the pair directly, easy to "breadboard" (on a bit of copper ground plane). For 1A the circuit would get a lot more involved and/or the transistors would get a lot more expensive. Maybe by some fluke the UV LEDs you picked switch off fast.You coulduse a resistor and inductor in parallel with the LEDto sweep out carriers (I don't have access to the paper, I just saw the image on google). So for instance instead of switching 100 mA you switch 200 mA, half of which going through say a 100 nH inductor in series with a 33 Ohm resistor (the resistor sets the inductor current together with LED Vf). The inductor dumps it's energy into the LED as a reverse current when the switch goes off and sweeps out the carriers. This will require some experimentation and quite likely destroyed LEDs.

author: richnormand
author_rank: Unknown
timestamp: June 12, 2016, 03:03:12 pm
content: Quote from: ap on June 12, 2016, 10:34:11 amThis is an astute observation. If OP is using a "white" LED they are usually driven by a blue LED with a doped silicone lens over it that contains various phosphors or doping compounds that emit a broad spectrum. At the end of the day you might end up measuring the fluo decay time of the LED dopants and not your sample.... Also, if that is the case, even if you have a good fast driver the off time would always be the fluo decay time of the LED dopants....Using a pulsed diode laser is most likely the way to go as long as the laser wavelength is shorter than the fluo wavelength he/she is trying to measure. Otherwise there wont be much excitation of the sample.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 12, 2016, 04:13:33 pm
content: This is really a job for Phase Flourometry. Which is a technique that geatly reduces the need for ultra-high speed electronics.Steve

author: dexters_lab
author_rank: Unknown
timestamp: June 12, 2016, 05:06:58 pm
content: Quote from: Twoflower on June 12, 2016, 11:03:17 ami have an air-gap flash that was for schlieren photography, it's minimum time is 500ns so they might not be fast enough anywayit also uses 10kv and argon so i'd say... go with the LED option

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2016, 05:37:50 pm
content: Quote from: richnormand on June 12, 2016, 03:03:12 pmHereis the problem with that.

author: bradfield
author_rank: Newbie
timestamp: June 12, 2016, 07:37:46 pm
content: QuoteBut I am not trying to time-resolve the decay. The only two informations I want to get out are the following:-Is there a fluorescence yes or no?,-and if yes: approx. how strong?So I just need an integrator, which gets triggered by the beginning of the rise of the photosignal and stops integration when the radiation has decayed below some stop value. This should provide a rough measure of energy absorbed by the protein sample.I dont know if there is still a problem with "getting out the signal", but the integrator is directly connected to the diode / amplifier. Everything is happening "onboard".In principle this can be done by continuously radiating the sample with appropriate frequencies, and then somehow only catch the fluorescence. But the fluorescence light is very weak compared to the excitation, and the excitation can not be filterd out sufficiently, so that it will always dominate the fluorescence.Quote from: David Hess on June 12, 2016, 03:27:22 amWhat is a "charge line" doing?

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2016, 08:24:59 pm
content: It's just various forms of pulsing ... but the pulsing isn't really an issue. Within your budget the ADCM580 and some RF transistors is the easiest way to generate a really fast step edge. The problem is having an blue light emitter which can actually be turned off fast enough.Laser diodes can, but laser diodes in the blue region are 4 figures.LEDs need complex driving to get near a ns (ie. carrier sweep out) and might not ever get below a ns. On the other hand, if you luck out and you stumble on some blue LED which can be driven hard enough to do it there's a paper in it.

author: bradfield
author_rank: Newbie
timestamp: June 12, 2016, 09:29:20 pm
content: @MarcoWhat is the idea with the differential pair current switcher? Please could you elaborate a bit more on your attempt?

author: uncle_bob
author_rank: Unknown
timestamp: June 12, 2016, 10:08:06 pm
content: Quote from: bradfield on June 12, 2016, 09:29:20 pmHiPossibly to be better able to elaborate it properly:How much of a background do you have in high speed logic / high power microwave / RF pcb layout ?Bob

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2016, 07:47:10 am
content: Quote from: David Hess on June 12, 2016, 03:27:22 amWhat is a "charge line" doing?A charge line is an open transmission line like a coaxial cable which is charged and then a switch discharges it into the device. In avalanche pulse generators, a charge line can replace the capacitor so instead of producing a narrow fast rise and fall pulse, the output is a wide pulse with fast rise and fall times. The width of the pulse is twice the length of the charge line.

author: dmg
author_rank: Regular Contributor
timestamp: June 13, 2016, 09:14:31 am
content: Quote from: Marco on June 12, 2016, 08:24:59 pmNo, they're actually dirt cheap these days. 405nm, 445nm and 462nm multimode 1W+ CW emmiters can be had for under $100 a pop (even under $50 for 445's) and 100's of mW single mode emmiters can be had for about the same. And I'm talking stuff with a datasheet, sometimes new from factory, sometimes harvested from consumer electronics.Back in 2011 (or thereabouts) I built a DIY 1W 445nm laser pointer. The diode, a multimode emmiter harvested from a casio projector, was $45.

author: Marco
author_rank: Super Contributor
timestamp: June 13, 2016, 11:34:23 am
content: As I said, blue, not violet. He wanted 480 to 500 nm. I assume he's using something like Alexa Fluor®488.That said, I don't really see the need to pulse anything with a laser ... he says the excitation overwhelms the detector, but with laser excitation there is no energy in the fluorescing wavelength at all. Existing machines seem to often use argon ion lasers, so I assume filtering can take care of it.There's some very cheap argon ion laser modules on ebay from the UK at the moment ... ludicrously cheap. Cheap enough it's hard to resist hitting buy now even though I have no use for them :/ Cheap enough that if I link them I doubt OP would be able to buy them(Of course if they don't work you end up with beryllium oxide you have to get rid off.)

author: LaserSteve
author_rank: Super Contributor
timestamp: June 14, 2016, 12:34:27 am
content: I'm trying to figure out why he wants to do this the expensive, way. I just picked up a used unit for my department. It uses a Pockels cell to modulate a xenon lamp or an external laser, adding a little RF in the form of sidebands to the light beam. But why do you need my 350 picosecond Pockels cell? Answer is you DONT!The Pockels cell has a DC bias and the RF if is summed on top of that to create the modulation. There is a reference frequency applied to one side of mixer, and there is a signal from the PMT which is the light passing thru the monochromator that picks out the fluorescence. The PMT signal has the sidebands. A bit of signal processing shows the time shift. A dual lab grade frequency synthesizer that tunes in one hertz steps (PTS D300 , a few are on Fleabay right now) creates the modulation. A direct conversion mixer handles the signal conversion, and the amplitude and phase shift of this LF product signal shows the fluorescence lifetime. Calibration is done by placing five out of ten possible cheap known materials in the cell.Now a little goggling today shows that my 69,000$ new, now zero dollar donated, unit has been upgraded to use low intensity blue and UV leds to replace the xenon lamp, pockels cell, and the argon laser. The LEDs have a DC bias and a RF injection, and now need nowhere near the 10 to 20 watts of RF used to "flip" the Pockels cell.This can be done cheap...I quote the description for Review of Scientific Instruments for (Rev. Sci. Instrum. 49, 39 (1978)Picosecond?resolution fluorescence lifetime measuring system with a cw laser and a radio.E. Roland Menzel1 and Zoran D. Popovic1Quote:A procedure for measurement of fluorescence lifetimes with picosecond time resolution is described. A cw laser beam is modulated with a standing?wave acousto?optic modulator. The modulated beam is split; one part serves as a reference beam, the other part excites the fluorescent sample. The sample flourescence and the reference beam, attenuated and delayed optically to be equal in amplitude and opposite in phase to the fluorescence, are incident onto a single photomultiplier tube. The thus achieved photodetector ac null is monitored either by an AM radio, whose intermediate?frequency signal is displayed on an oscilloscope, or by a spectrum analyzer. With 30?MHz light modulation and the radio, lifetimes could be determined with resolution better than 15 ps. With the spectrum analyzer and 170?MHz light modulation frequency we have achieved 4?ps lifetime resolution. Correction for photomultiplier transit time versus incident wavelength is made.End Quote..Those guys used a DX160 and a fast PMT.. Which if you've ever used a Radio Shack DX160, you'd be amazed that it was that stable...------------------------------------------------------------------------Some Googling:A portable time-domain LED fluorimeter for nanosecond fluorescence lifetime measurements,Rev. Sci. Instrum. 85, 055003 (2014)The above Uses a ring oscillator made of 4049 inverters to drive the led.... It uses the pulse edges to drive a simple transistor circuit using 4 RF transistors to drive the 405 nm Led, and a PIC /PIC32MX795F512L-80I/PT for all the signal processing...----------------------------------------------------------------------------You can easily inject 30 Mhz RF to a blue LED with a bias Tee...---------------------------------------------------------------------One other thing, if you pulse a blue LED with a nanosecond pulse, the spectrum shifts to the UV, yes, that's right, fast pulses from avalanche drivers change carrier lifetimes and cause spectral shifts. RF modulation causes far, far, less of a shift...So why try to use a picosecond scope, an expensive modulator or pulsed laser diode, when a blue LED and a simple photo detector with a lock-in amplifier, microcontroller, superheterodyne receiver, or DSP will now do this for a fraction of the cost?---------------------------------------------------------------------------------------------MarcoBTW< the BeO in the cheap air-cooled argon laser is a tough, solid block or rod of ceramic with an added binding ceramic such as 1% Alumina. Only dust in a certain micron size range is potentially super-hazardous, and if your not using a diamond saw or diamond grinder on the BeO, you have no problems...I'm a professional laser user, and we either pay shipping for the surplus BeO to be recycled by the laser maker, or toss them in normal trash disposal, legally. Usually if a tube cracks we just spray it with a water squirter and bag it, and toss it. In twenty years of use, I've only ever cracked two, and that was by dropping one, the other was a fan interlock failure. Cracked tubes don't generate enough of the dust particles to make me worry. Solid tubes cleave in BIG pieces with little or no dust. The stuff is not some horrid alpha emitting isotope causing instant death in two weeks by inhalation. In fact only about 20% of the population is very genetically sensitive and extremely vulnerable to the disease. You need chronic exposure to really set it off. If it were such a problem, why does nearly every college and hospital have air cooled lasers venting to room air?I've been handling BeO for years, and while I respect it, I do not fear using the laser or changing/repumping the tubes.I do fear the 10 Amp 106 volt,, or 35 amp 250 volts of DC that drives the arc, depending on the size of the laser. I rear even more getting an accidental retinal burn...Kind Regards,Laser Steve ....

author: LaserSteve
author_rank: Super Contributor
timestamp: June 14, 2016, 12:52:20 am
content: If you insist on pulsing and a fast scope...http://www.reed.edu/physics/courses/Physics332.s16/pdf/LED%20Luminescence%20Decay.pdfSteve

author: Marco
author_rank: Super Contributor
timestamp: June 14, 2016, 01:03:09 am
content: Quote from: LaserSteve on June 14, 2016, 12:34:27 amI wasn't afraid of the material, I simply thought it was more expensive to dispose of than that.Quote from: LaserSteve on June 14, 2016, 12:52:20 am"However, it is not agood LED driver circuit for a waveform capture system. Inour evaluations we used the circuit to drive a 70000 MCD(millicandela units) blue LED at 2000 Hz. Even with theroom lights off, very little light from the LED was visible tothe eye."A circuit which drives a LED with logic levels through a 33 Ohm current limiting resistor, compared to one which drives it with 100V through a 50 Ohm current limiting resistor ... they felt the need to compare output power of these by eye?PS. I like that they build the avalanche pulser on a bog standard breadboard though.

author: David Hess
author_rank: Super Contributor
timestamp: June 14, 2016, 02:29:12 am
content: Quote from: LaserSteve on June 14, 2016, 12:52:20 amHa! Figure 4a shows the charge line suggestion I made although they only used it for testing avalanche behavior in transistors.They mention the 300 MHz 2440 oscilloscope they used as being too slow but I suspect based on the photograph of their physical implementation using breadboard that they had significant other problems; I do not see even local power supply decoupling. The recorded pulse they show is not consistent with the performance of a 2440. At the end they do discuss the shortcoming of their construction method.Their good results however show that the driving pulse to the LED is not as critical as one might expect and that their construction and performance was completely adequate.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 14, 2016, 01:48:40 pm
content: I selected that document because it was not behind a pay-wall. I work at a academic institution, I have nearly every scientific journal for "free" at my fingertips and can search for ANYTHING... I know the funding agency ultimately pays... But no one bills my cost center for digital library access...So sending links that cost civilians to have to pay 35$ to see a five page PDF is bad karma from my point of view... The Menzel/ Popovic paper has a complete schematic, and if the OP emailed Menzel I would not be surprised if he was given the source code..Besides, the art of making a simple publication useful, is based on looking at the references at the back of the paper..As for using the breadboard...Getting rid of the breadboard would help. My jury rigged avalanche circuit hits ~700 pS FWHM into 50 Ohms, and its just very careful dead bug on a piece of FR4... No shockline there, either.. For TO-92 packaged devices, the loop area of the circuit is the main determining factor for pulse width.. If I made my circuit SMD, I'd probably break 350 pS...Steve

author: Berni
author_rank: Super Contributor
timestamp: June 14, 2016, 02:52:00 pm
content: Transistors can switch pretty fast in certain modes. I built a Jim Williams pulse generator and got 160ps rise times out of it using a jelly bean transistor from my parts bin. I imagine a special RF transistor is faster. But you do want to keep all connection lengths to a minimum. The inductance of a few cm of wire can screw things up properly.Also when making such stuff you probably want to have access to a fast oscilloscope with 1GHz of bandwidth. Both to check how fast your light source is switching and to capture the resulting waveform from the detector (That also needs to be really quick)EDIT:Oh and on the topic of fast pulses i have a HP8082A Pulse Generator that i measured to have a 800ps rise and fall time. Its an ancient analog dinosaur but is still quite useful today as it can create any form of pulse or square wave signal in differential pair up to 250MHz with up to 5V of amplitude at those rise times. Try asking that from a regular signal generator for under 5 grand.

author: Marco
author_rank: Super Contributor
timestamp: June 14, 2016, 07:05:51 pm
content: The pulse doesn't really seem to be the limiting factor in a LED, it's the remaining charge carriers recombining. The ideal drive would be a bipolar pulse, you could probably adapt the avalanche pulser to generate that.Maybe with some ultra-low capacitance TVS to protect the LED during the reverse current (a 0402 LED can probably handle a 1ns 1A pulse, which won't have a TVS build in).

author: uncle_bob
author_rank: Unknown
timestamp: June 14, 2016, 10:49:11 pm
content: Quote from: blueskull on June 14, 2016, 01:54:09 pmHiGive it a try ... it works pretty well.Bob

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2016, 01:41:30 am
content: Quote from: Marco on June 14, 2016, 07:05:51 pmThe paper shows how to do that using a unipolar pulse with a parallel inductor. It mentions empirical testing to get the inductance correct so there is enough reverse bias to sweep the carriers out without destroying the LED.

author: Marco
author_rank: Super Contributor
timestamp: June 15, 2016, 03:06:50 am
content: Sure, but they used relatively wide pulses. I'm not sure if the parallel inductor is the best circuit if you want to excite the LED with really short bipolar pulses. I founda paperwhich actually gets sub-ns pulses out of standard LEDs (I don't really like their circuit either though).I suspect that if you turn a LED on for less time you can turn it off faster too, up to the transit time, since the electrons/holes (slow movers) didn't fill the entire depletion zone yet ... so carrier sweep out more quickly empties it.

author: Berni
author_rank: Super Contributor
timestamp: June 15, 2016, 05:20:15 am
content: Quote from: Marco on June 15, 2016, 03:06:50 amThe circuit they use to reverse the diode for a moment seams like a good idea to me. It provides a predictable reverse pulse so you don't blow your LED due to reverse breakdown.I don't really get how exactly the actual pulse generator circuit works. But it sounds like one could make that using some fast 74xx logic chips.Everyone is concerned about the LED, but how fast are light detectors like photodiodes?

author: Marco
author_rank: Super Contributor
timestamp: June 15, 2016, 09:22:45 am
content: I think for fast low noise they generally use avalanche photodiodes, expensive buggers but I assume he has that solved.How about something like the following? Q1 is set to avalanche on the trigger, using a RF transistor since the LED doesn't really need high voltages (might need to change the trigger circuit a bit for reliable triggering, a bit of negative bias on the base for instance though it would require a large trigger voltage). D2 is a sub-pF TVS. R2/C2 are slightly larger than R1/C1, R3&R4 are significantly larger (they mostly just bring the circuit back to equilibrium). I think this gives you enough free parameters to shape the bipolar pulse, except the rise time of the initial current pulse which is purely down to the avalanche transistor but you want that as fast as possible any way (might want to bypass "part of" R1 with a small cap for some peaking even).

author: LaserSteve
author_rank: Super Contributor
timestamp: June 15, 2016, 01:51:56 pm
content: I'm liking the two circuits in the attached picturesFromAn ultraviolet nanosecond light pulse generator using a light emitting diode for test ofphotodetectors. Tsutomu Araki, Yasumitsu Fujisawa, and Mamoru HashimotoLink:http://scitation.aip.org/content/aip/journal/rsi/68/3/10.1063/1.1147943At least the ZTX-415 is made of Obtainium and is in production..VA is 520 to 630V according to the publication. It would work at much lower voltages but the higher voltage reduced timing jitter walk. Cmax is 60 pf, above that the led is damaged. Cs in the lower circuit is for impedance matching, value ~ 330 pF, and R is about 60 Ohms, not 50, for best matching.The risetime on the shockline circuit was not measured in detail, but the scope picture is very, very, nice.... The shockline pulse is flat-topped, the sort of thing you would want for checking scope bandwidth... However, it is a long pulse, having very desirable sharp edges.I quote: "Because propagation speed of the current in the coaxial cable is 0.2 m/ns, resultant pulse width W in nS, is given as W = 10 D, where D is the cable length in meters"Sadly they do not mention the cable type...From looking at the scope photos, the emission mechanism for the UV shortens the optical pulse considerably, while the visible emission goes long with an afterglow.. So they used a UV filter for the obvious reasons..Two LED, circuits operating with a slight delay between them, and combined with a beamsplitter, were then used to test the recovery time of their PMTs...Their work is based on measurements done with a 12 Ghz scope and a 10 Picosecond sampling scope, so this paper is very valid...LED used is a Model NLPB 520. from Nichia, and the spectrum splits,I Quote:"The 380 nm light originates from the InGaN/AlGaN component, while the 450 nm light comes from thedoped Zn to which the energy is transferred from the InGaN/AlGaN. Under the conventional dc operation below 30 mA,the 450 nm light is dominant, and the intensity of the 380 nmlight is not appreciable. When a current larger than 50 mA isapplied, the intensity of the 450 nm light saturates, but thatof 380 nm light continues to increase. The 380 nm lightappears and fades rapidly, while the 450 nm light exists evenafter extinction of the 380 nm light when applying a fast pulse"Not as fast as things could go, but does not need SRDs made of pure Unicorn horn, the 415 is available from Digikey... I'd do it with logic gate edges, but that's just me...Curiously, led life was limited to ~1.0 x 10^9 shots at 20 Khz, not from overcurrent or overheating, but from shock waves to the Die?...From:An ultraviolet nanosecond light pulse generator using a light emitting diode for test ofphotodetectors.Tsutomu Araki, Yasumitsu Fujisawa, and Mamoru HashimotoReview of Scientific Instruments 68, 1365 (1997); doi: 10.1063/1.1147943Steve

author: Marco
author_rank: Super Contributor
timestamp: June 15, 2016, 04:20:04 pm
content: Quote from: LaserSteve on June 15, 2016, 01:51:56 pmJay Diddy avalanched a BR505 in 100 ps, you don't really need SRDs to go sonic fast ... there's the SiGe comparators too of course, but avalanche so easily goes to massive current without multiple stages of amplification that it's hard to resist.So did they put a divider probe across the diode or use a shunt? I'd not trust that they didn't overcurrent it without seeing a waveform.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 15, 2016, 06:21:53 pm
content: They don't say, but if you own both a Hamamatsu Photonics OOS-01,and a 12 GHz sampling Iwatsu SAS-8130 with SH-4B plug-in you probably know to use a shunt.Steve

author: Marco
author_rank: Super Contributor
timestamp: June 15, 2016, 09:08:00 pm
content: Quote from: LaserSteve on June 15, 2016, 06:21:53 pmMaybe, but on the other hand the value of the voltage and current waveform in explaining the workings of their circuit is plain to see too.Energy wise ~ 1 microjoule of energy, in lets say 500 picoseconds with most of the energy dissipated across the transistors with around 200V drop gives average current of 10A ... hmm, less than I expected with 600V. Guess if they keep the legs on those transistors and the led long enough it's not so bad.