On Apr 12, 4:17=A0pm, John Devereux <j...@devereux.me.uk> wrote:> George Herold <gher...@teachspin.com> writes: > > On Apr 12, 11:12=A0am, John Devereux <j...@devereux.me.uk> wrote: > >> George Herold <gher...@teachspin.com> writes: > >> > I=92 looking at the step response of a photodiode (PD). =A0I=92m ste=pping> >> > the current into a LED such that the step doubles the photo current > >> > from the PD. =A0(I=92m not starting with the LED off, but on slightl=y.)> >> > Here=92s a =91scope shot. =A0The bottom trace in the PD response. ==A0(The top> >> > is the voltage step to the LED (through 100 ohms)... AC coupled to g=et> >> > rid of a big DC offset.) > > >> >http://img189.imageshack.us/i/tek0024.png/ > > >> > There is this long tail on the step response. =A0Here=92s the same p=icture> >> > with the timebase slowed down. > > >> >http://img812.imageshack.us/i/tek0025.png/ > > >> > Looks something like a 2-2.5 us tail on the step. > > [...] > > > > > > > > >> >http://img52.imageshack.us/i/tek0026.png/ > >> > The red LED is the trace that is a bit higher. =A0I=92m looking to s=ee if> >> > I can dig up any blue or white LED=92s. > > >> > Any other ideas how I might determine if that's what I'm seeing... > > >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed > >> > biased at ~12V. > > >> Hi George, > > >> You may find VCSELs easier to use than laster diodes. They can usually > >> be safely driven at constant current and have ~circular beams, some of > >> them have built-in collimator lenses too, They are noisy compared to > >> LEDs if that is an issue, but are designed to be modulated very quickl=y> >> (GHz), > > > Thanks John, I've got lotsa laser diodes. =A0VCSELs sound expensive. > > Who sells them? > > They're quite affordable, comparable to laser diodes really depending on > type of course. > > I bought some from Roithner <http://www.roithner-laser.com/> (European > company, they have a huge range of laser diodes and leds). Looks like > cheapest is $9.25 for PM85-D1POU (2mW 850nm with dome lens, 2 degree > beam). > > Just remembered there are also "resonant cavity" leds, similar pricing, > these are also designed for fast modulation (3ns rise/fall times on the > one I am looking at now). Don't know really what their noise is like but > being "leds" I would guess they are better than laser diodes. > > -- > > John Devereux- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -Thanks for the link John D, There's so many kinds of solid state light emitting devices it's hard to keep track of them all. (I think I know 3/4's of the acronyms, and maybe 1/4 of the under-lying physics.) FP laser diodes? Where did you find the prices? George H.
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 12, 20112011-04-12
Reply by ●April 12, 20112011-04-12
John wrote:> On 4/11/2011 9:46 PM, Phil Hobbs wrote: > >>> >>> What would happen if they were reverse biased by about a volt or so at >>> turn-off time? Would that help? >>> >>> John >> >> >> It might. Trying to speed up a LED is a bit like lipstick on a pig, of >> course. > > > > Great! Now I have to clean coffee spray off my monitor screen. > > John >Better than Skoal on your screen! Jamie
Reply by ●April 12, 20112011-04-12
John Larkin wrote:> On Tue, 12 Apr 2011 13:45:20 -0500, wiljan@nospam.pobox.com (Will > Janoschka) wrote: > >> On Tue, 12 Apr 2011 16:21:14, George Herold<gherold@teachspin.com> >> wrote: >> >>> On Apr 12, 10:19�am, John Larkin >>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold >>>> >>> <Big snip> >>> >>>> >>>>> Thanks John, �If it's due to more current because of a drop in forward >>>>> voltage, then I should see that in the current. �(Which I don't... >>>>> sigh) >>>> >>>> What's the LED driver circuit like? >>> >>> I've used a few different configurations... all give the same result. >>> Coax from the signal generator, daisy chains to the 'scope and on to >>> the the LED. There I had 50 ohms to ground and in parallel 500 ohms >>> feeding the LED to ground. Originally there was a few inches of wire >>> between the 500 ohm resistor and the LED, but I later tightened it up >>> and put everything right at the LED. I later just feed the LED >>> through a 50 ohm resistor. >>> >>> >>> The PD circuit is standard TIA >>> >>> http://img651.imageshack.us/i/tia2.png/ >>> >>> I'm going to write to OSI (the PD maker.) I wanted to be sure I >>> wasn't doing something stupid beofre contacting them... >>> >>> George H. >>>> >>>> >>>> >>>>> OK let me try a totally different idea. �Can the stray capacitance of >>>>> 'stuff' have a long tail like that. �Charges moving around on pieces >>>>> of plastic/fiber glass or something? >>>> >>>> Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' >>>> behavior, but your waveforms don't look like that. >>>> >>>> Post (or email me) your LED driver and pd circuits, and I'll see if I >>>> can spot anything. >>>> >>>> >>>> >>>>> I could air wire the circuit and see what that does... >>>> >>>> Time constants in the microseconds probably aren't strays. >>>> >>>> John- Hide quoted text - >>>> >>>> - Show quoted text - >>> >> >> Do you have any kind of a chopper or shutter that you could >> use to isolate the time constant(s) of the PD frome those of the >> LED/ laser? > > It would be interesting to see how fast an edge you could create with > a mechanical chopper. Nanoseconds would be tough. > > John >Mechanical streak cameras are surprisingly fast--you spin a polygon on an air bearing (or a magnetic bearing in a vacuum) and look at the scanned beam from some distance. You can do 12 facets x 100k rpm x 10m ~ 2500 km/s. (*) Nanoseconds are _easy_. They did stuff like that back in the 40s and 50s for bomb development. Cheers Phil Hobbs (*) There's an additional factor of 4 pi in there--2 pi for the angular velocity and another 2 for the law of reflection (the beam scans twice as fast as the polygon turns). -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Reply by ●April 12, 20112011-04-12
George Herold wrote:> On Apr 12, 3:56 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> George Herold wrote: >>> On Apr 12, 10:45 am, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>> Phil Hobbs wrote: >>>>> George Herold wrote: >>>>>> On Apr 11, 10:59 pm, John Larkin >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> On Mon, 11 Apr 2011 16:35:20 -0700 (PDT), George Herold >> >>>>>>> <gher...@teachspin.com> wrote: >>>>>>>> On Apr 11, 7:04 pm, John Larkin >>>>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>>>>> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold >> >>>>>>>>> <gher...@teachspin.com> wrote: >>>>>>>>>> On Apr 11, 3:32 pm, Phil Hobbs >>>>>>>>>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>>>>>>>>> George Herold wrote: >>>>>>>>>>>> I looking at the step response of a photodiode (PD). I m stepping >>>>>>>>>>>> the current into a LED such that the step doubles the photo current >>>>>>>>>>>> from the PD. (I m not starting with the LED off, but on slightly.) >>>>>>>>>>>> Here s a scope shot. The bottom trace in the PD response. (The top >>>>>>>>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled >>>>>>>>>>>> to get >>>>>>>>>>>> rid of a big DC offset.) >> >>>>>>>>>>>> http://img189.imageshack.us/i/tek0024.png/ >> >>>>>>>>>>>> There is this long tail on the step response. Here s the same >>>>>>>>>>>> picture >>>>>>>>>>>> with the timebase slowed down. >> >>>>>>>>>>>> http://img812.imageshack.us/i/tek0025.png/ >> >>>>>>>>>>>> Looks something like a 2-2.5 us tail on the step. >> >>>>>>>>>>>> What s it caused by? I first thought it was perhaps heating of the >>>>>>>>>>>> LED during the step. But I was reading Photodetectors by S. Donati, >>>>>>>>>>>> over the weekend. In Chapter 5 he talks about two sets of photo- >>>>>>>>>>>> generated carriers the majority are made in the depletion layer, >>>>>>>>>>>> and >>>>>>>>>>>> travel with the drift speed, (times in the nano-second range.) The >>>>>>>>>>>> second smaller set are those generated in the doped region. If they >>>>>>>>>>>> are within a diffusion length of the depletion region then these >>>>>>>>>>>> have >>>>>>>>>>>> a chance of adding to the photocurrent. Donati says that these >>>>>>>>>>>> have a >>>>>>>>>>>> time constant in the microsecond range! I wonder if this is what >>>>>>>>>>>> I am >>>>>>>>>>>> seeing? >> >>>>>>>>>>>> I looked at the step response with a different color LED. (you >>>>>>>>>>>> expect >>>>>>>>>>>> more absorption in the doped region with shorter wavelength light.) >>>>>>>>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber >>>>>>>>>>>> LED. >> >>>>>>>>>>>> http://img52.imageshack.us/i/tek0026.png/ >>>>>>>>>>>> The red LED is the trace that is a bit higher. I m looking to >>>>>>>>>>>> see if >>>>>>>>>>>> I can dig up any blue or white LED s. >> >>>>>>>>>>>> Any other ideas how I might determine if that's what I'm seeing... >> >>>>>>>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed >>>>>>>>>>>> biased at ~12V. >> >>>>>>>>>>>> George H. >> >>>>>>>>>>> Crank up the bias till the diode is fully depleted, and see. >> >>>>>>>>>>> Cheers >> >>>>>>>>>>> Phil Hobbs >> >>>>>>>>>> Thanks Phil, >> >>>>>>>>>> I can try more bias. Is there any way to know it's fully depleted? >>>>>>>>>> (Or do I just take it to the maximum as specified by the maker. (30 >>>>>>>>>> volts in this case.) >> >>>>>>>>>> The white LED showed more of the long tail... but less of a sharp >>>>>>>>>> turn- >>>>>>>>>> on, I wonder if it's just the LED time response that I'm seeing. >> >>>>>>>>> A white LED has a phosphor that is likely slow. >> >>>>>>>> Yeah, I noticed that.. Thanks to Grant too. >> >>>>>>>> I didn't save the screeen shot on my flash. (I should hit the save >>>>>>>> button more often.) >>>>>>>> IR led's are slow too. >> >>>>>>>>> Try a laser diode. They are (usually) fast. >> >>>>>>>>> I did encounter some laser diodes that had a weird slow step response >>>>>>>>> when used at low pulse rates. They behaved, electrically and >>>>>>>>> optically, as if there were an inductor in series with the diode. At >>>>>>>>> high rep-rates, 1 MHz maybe, the effect disappeared. >> >>>>>>>> Bring out the big guns ehh? I've never looked at the step response >>>>>>> >from our laser diode, but we've modulated it up to ~150 MHz. There's >>>>>>>> a reported change in the modulation sensitivity, at the microsecond >>>>>>>> range and longer its due to heating, at shorter times it drops by an >>>>>>>> order of magnitude and is due to (index of refraction?) carrier >>>>>>>> density effects. At very high frequency you can get an enhancement >>>>>>>> due to relaxation oscialltion in the LD... about 6 GHz, in the LD's I >>>>>>>> know. >> >>>>>>>>> Try looking at the voltage across the LED, and see if it's a clean >>>>>>>>> step. >> >>>>>>>>> What's the signal path from the photodiode to the scope? Since all the >>>>>>>>> LEDs seem to have similar waveforms, maybe the detector is the >>>>>>>>> problem. >>>>>>>> Yeah sorry I didn't save all the crummy 'scope shots. >>>>>>>> But I'm certainly open to some silly measurement error. There's a TIA >>>>>>>> with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, with >>>>>>>> 3.3pF parallel capacitance (plus some extra stray... call it ~2 pF, >>>>>>>> perhaps a bit less). >> >>>>>>>> I think I'm seeing a LED temperature effect. Does the light output go >>>>>>>> up as an LED gets hotter? If it's linear with delta T, I can't change >>>>>>>> drive level and make it go away. >> >>>>>>> LEDs are less efficient when they're hot. But the forward drop goes >>>>>>> down. So there is usually some source resistance where the effects >>>>>>> cancel. >> >>>>>> Thanks John, If it's due to more current because of a drop in forward >>>>>> voltage, then I should see that in the current. (Which I don't... >>>>>> sigh) >> >>>>>> OK let me try a totally different idea. Can the stray capacitance of >>>>>> 'stuff' have a long tail like that. Charges moving around on pieces >>>>>> of plastic/fiber glass or something? >> >>>>>> I could air wire the circuit and see what that does... >> >>>>>> George H. >> >>>>>>> John- Hide quoted text - >> >>>>>>> - Show quoted text - >> >>>>> How about posting the schematic? If the edges are symmetrical, it's >>>>> probably the TIA--the LED would be cooling down when it wasn't emitting >>>>> light, so you wouldn't see the thermal tail. >> >>>>> Op amps have really terrible supply rejection when it comes to fast >>>>> transients--depending on the feedback, you can easily get _gain_ from >>>>> the supply pin to the output. (PSR is specified with respect to the >>>>> amplifier _inputs_.) >> >>>>> Another possibility is phase funnies in the open loop gain. Composite >>>>> amps are notorious for that sort of thing. The long-time settling >>>>> behaviour is dominated by the lowest-frequency pole or zero in the open >>>>> loop transfer function--lead-lag networks can cause this sort of thing >>>>> even though the closed-loop response looks nice on a spectrum analyzer. >> >>>>> Try touching the summing junction with your finger and see what happens >>>>> to the step response. >> >>>>> Cheers >> >>>>> Phil Hobbs >> >>>> Rereading, it looks like you have something like a 20 pF summing >>>> junction capacitance and a 2 pF feedback cap. That means you have an >>>> open-loop pole/zero pair separated by a factor of 10 in frequency, up >>>> near the GBW of the op amp. That will lead to some entertaining >>>> settling behaviour, which at this point would be my best guess as to >>>> what you're seeing. >> >>> Yeah perhaps more like 3.3 pF across the feedback. >> >>> The pulse response through a 'big' resistor looks just fine, (no PD >>> or LED). I went back and added the 'missing' 12 pF to ground, just in >>> case...(That's the removed PD capacitance, the strays and opamp C are >>> still there.) >> >>> Hey this might be some 'real' physics and not just a circuit screw- >>> up. >> >>> George H. >> >> Well, it does look like the photodiode, which suggests that the >> aforementioned diffusion tail mechanism is doing it. I'm surprised >> that jacking up the bias didn't fix it. It isn't usually as bad as that. >> >> Cheers >> >> Phil Hobbs> > Yeah, but I'm still confused. I tried it with an adjustable bais > (from 0-12V) late today. There was the expected change in the short > time response. But none in the long tail. I had a nice chat with > someone at OSI. (sent pic's and schematics to them.). I'll have to > try a different PD. > > George H.They may have a really thick N+ diffusion on the back. That's one common method of making ohmic contacts--you dope the Si so heavily that it becomes a metal (i.e. lots of electrons sloshing round in the conduction band). Better diodes keep that layer as thin as possible, because it hurts the PD performance. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Reply by ●April 12, 20112011-04-12
On 4/12/2011 7:10 PM, Jamie wrote:> John wrote: >> On 4/11/2011 9:46 PM, Phil Hobbs wrote: >> >>>> >>>> What would happen if they were reverse biased by about a volt or so at >>>> turn-off time? Would that help? >>>> >>>> John >>> >>> >>> It might. Trying to speed up a LED is a bit like lipstick on a pig, of >>> course. >> >> >> >> Great! Now I have to clean coffee spray off my monitor screen. >> >> John >> > Better than Skoal on your screen! > > JamieSo true. I'm glad I gave up tobacco years ago. Life is much easier, screen wise anyway. John
Reply by ●April 12, 20112011-04-12
On Apr 12, 8:11=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> John Larkin wrote: > > On Tue, 12 Apr 2011 13:45:20 -0500, wil...@nospam.pobox.com (Will > > Janoschka) wrote: > > >> On Tue, 12 Apr 2011 16:21:14, George Herold<gher...@teachspin.com> > >> wrote: > > >>> On Apr 12, 10:19 am, John Larkin > >>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0wrote: > >>>> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold > > >>> <Big snip> > > >>>>> Thanks John, If it's due to more current because of a drop in forwa=rd> >>>>> voltage, then I should see that in the current. (Which I don't... > >>>>> sigh) > > >>>> What's the LED driver circuit like? > > >>> I've used a few different configurations... all give the same result. > >>> Coax from the signal generator, daisy chains to the 'scope and on to > >>> the the LED. =A0There I had 50 ohms to ground and in parallel 500 ohm=s> >>> feeding the LED to ground. =A0Originally there was a few inches of wi=re> >>> between the 500 ohm resistor and the LED, but I later tightened it up > >>> and put everything right at the LED. =A0I later just feed the LED > >>> through a 50 ohm resistor. > > >>> The PD circuit is standard TIA > > >>>http://img651.imageshack.us/i/tia2.png/ > > >>> I'm going to write to OSI (the PD maker.) =A0I wanted to be sure I > >>> wasn't doing something stupid beofre contacting them... > > >>> George H. > > >>>>> OK let me try a totally different idea. Can the stray capacitance o=f> >>>>> 'stuff' have a long tail like that. Charges moving around on pieces > >>>>> of plastic/fiber glass or something? > > >>>> Probably not the tail you're seeing. PCB 'capacitors' can have 'hook='> >>>> behavior, but your waveforms don't look like that. > > >>>> Post (or email me) your LED driver and pd circuits, and I'll see if =I> >>>> can spot anything. > > >>>>> I could air wire the circuit and see what that does... > > >>>> Time constants in the microseconds probably aren't strays. > > >>>> John- Hide quoted text - > > >>>> - Show quoted text - > > >> Do you have any kind of a chopper or shutter that you could > >> use to isolate =A0the time constant(s) of the PD frome those of the > >> LED/ laser? > > > It would be interesting to see how fast an edge you could create with > > a mechanical chopper. Nanoseconds would be tough. > > > John > > Mechanical streak cameras are surprisingly fast--you spin a polygon on > an air bearing (or a magnetic bearing in a vacuum) and look at the > scanned beam from some distance. =A0You can do 12 facets x 100k rpm x 10m > ~ 2500 km/s. (*) Nanoseconds are _easy_. =A0They did stuff like that back > in the 40s and 50s for bomb development. > > Cheers > > Phil Hobbs > > (*) There's an additional factor of 4 pi in there--2 pi for the angular > velocity and another 2 for the law of reflection (the beam scans twice > as fast as the polygon turns). > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > > email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.ne=t- Hide quoted text -> > - Show quoted text -I was wondering if you can wiggle something faster than you can rotate it? Well maybe just easier to wiggle. I think I've seen the optical lever trick done multiple times... 'no smoke but lots of mirrors' George H.
Reply by ●April 12, 20112011-04-12
George Herold wrote:> On Apr 12, 8:11 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> John Larkin wrote: >>> On Tue, 12 Apr 2011 13:45:20 -0500, wil...@nospam.pobox.com (Will >>> Janoschka) wrote: >> >>>> On Tue, 12 Apr 2011 16:21:14, George Herold<gher...@teachspin.com> >>>> wrote: >> >>>>> On Apr 12, 10:19 am, John Larkin >>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold >> >>>>> <Big snip> >> >>>>>>> Thanks John, If it's due to more current because of a drop in forward >>>>>>> voltage, then I should see that in the current. (Which I don't... >>>>>>> sigh) >> >>>>>> What's the LED driver circuit like? >> >>>>> I've used a few different configurations... all give the same result. >>>>> Coax from the signal generator, daisy chains to the 'scope and on to >>>>> the the LED. There I had 50 ohms to ground and in parallel 500 ohms >>>>> feeding the LED to ground. Originally there was a few inches of wire >>>>> between the 500 ohm resistor and the LED, but I later tightened it up >>>>> and put everything right at the LED. I later just feed the LED >>>>> through a 50 ohm resistor. >> >>>>> The PD circuit is standard TIA >> >>>>> http://img651.imageshack.us/i/tia2.png/ >> >>>>> I'm going to write to OSI (the PD maker.) I wanted to be sure I >>>>> wasn't doing something stupid beofre contacting them... >> >>>>> George H. >> >>>>>>> OK let me try a totally different idea. Can the stray capacitance of >>>>>>> 'stuff' have a long tail like that. Charges moving around on pieces >>>>>>> of plastic/fiber glass or something? >> >>>>>> Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' >>>>>> behavior, but your waveforms don't look like that. >> >>>>>> Post (or email me) your LED driver and pd circuits, and I'll see if I >>>>>> can spot anything. >> >>>>>>> I could air wire the circuit and see what that does... >> >>>>>> Time constants in the microseconds probably aren't strays. >> >>>>>> John- Hide quoted text - >> >>>>>> - Show quoted text - >> >>>> Do you have any kind of a chopper or shutter that you could >>>> use to isolate the time constant(s) of the PD frome those of the >>>> LED/ laser? >> >>> It would be interesting to see how fast an edge you could create with >>> a mechanical chopper. Nanoseconds would be tough. >> >>> John >> >> Mechanical streak cameras are surprisingly fast--you spin a polygon on >> an air bearing (or a magnetic bearing in a vacuum) and look at the >> scanned beam from some distance. You can do 12 facets x 100k rpm x 10m >> ~ 2500 km/s. (*) Nanoseconds are _easy_. They did stuff like that back >> in the 40s and 50s for bomb development. >> >> Cheers >> >> Phil Hobbs >> >> (*) There's an additional factor of 4 pi in there--2 pi for the angular >> velocity and another 2 for the law of reflection (the beam scans twice >> as fast as the polygon turns). >> >> -- >> Dr Philip C D Hobbs >> Principal >> ElectroOptical Innovations >> 55 Orchard Rd >> Briarcliff Manor NY 10510 >> 845-480-2058 >> >> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net- Hide quoted text - >> >> - Show quoted text - > > I was wondering if you can wiggle something faster than you can rotate > it? > > Well maybe just easier to wiggle. > > I think I've seen the optical lever trick done multiple times... > 'no smoke but lots of mirrors' > > George H.Streak tubes can do 1 ps/div--Hamamatsu still makes them, I think, but they seem to have discontinued their streak cameras. You need a microchannel plate to get enough photons to see at that rate. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Reply by ●April 12, 20112011-04-12
On Apr 12, 8:14=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> George Herold wrote: > > On Apr 12, 3:56 pm, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> =A0wrote: > >> George Herold wrote: > >>> On Apr 12, 10:45 am, Phil Hobbs > >>> <pcdhSpamMeSensel...@electrooptical.net> =A0 =A0wrote: > >>>> Phil Hobbs wrote: > >>>>> George Herold wrote: > >>>>>> On Apr 11, 10:59 pm, John Larkin > >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0 =A0wrote: > >>>>>>> On Mon, 11 Apr 2011 16:35:20 -0700 (PDT), George Herold > > >>>>>>> <gher...@teachspin.com> =A0 =A0wrote: > >>>>>>>> On Apr 11, 7:04 pm, John Larkin > >>>>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0 =A0wrote: > >>>>>>>>> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold > > >>>>>>>>> <gher...@teachspin.com> =A0 =A0wrote: > >>>>>>>>>> On Apr 11, 3:32 pm, Phil Hobbs > >>>>>>>>>> <pcdhSpamMeSensel...@electrooptical.net> =A0 =A0wrote: > >>>>>>>>>>> George Herold wrote: > >>>>>>>>>>>> I looking at the step response of a photodiode (PD). I m ste=pping> >>>>>>>>>>>> the current into a LED such that the step doubles the photo =current> >>>>>>>>>>>> from the PD. (I m not starting with the LED off, but on slig=htly.)> >>>>>>>>>>>> Here s a scope shot. The bottom trace in the PD response. (T=he top> >>>>>>>>>>>> is the voltage step to the LED (through 100 ohms)... AC coup=led> >>>>>>>>>>>> to get > >>>>>>>>>>>> rid of a big DC offset.) > > >>>>>>>>>>>>http://img189.imageshack.us/i/tek0024.png/ > > >>>>>>>>>>>> There is this long tail on the step response. Here s the sam=e> >>>>>>>>>>>> picture > >>>>>>>>>>>> with the timebase slowed down. > > >>>>>>>>>>>>http://img812.imageshack.us/i/tek0025.png/ > > >>>>>>>>>>>> Looks something like a 2-2.5 us tail on the step. > > >>>>>>>>>>>> What s it caused by? I first thought it was perhaps heating =of the> >>>>>>>>>>>> LED during the step. But I was reading Photodetectors by S. =Donati,> >>>>>>>>>>>> over the weekend. In Chapter 5 he talks about two sets of ph=oto-> >>>>>>>>>>>> generated carriers the majority are made in the depletion la=yer,> >>>>>>>>>>>> and > >>>>>>>>>>>> travel with the drift speed, (times in the nano-second range=.) The> >>>>>>>>>>>> second smaller set are those generated in the doped region. =If they> >>>>>>>>>>>> are within a diffusion length of the depletion region then t=hese> >>>>>>>>>>>> have > >>>>>>>>>>>> a chance of adding to the photocurrent. Donati says that the=se> >>>>>>>>>>>> have a > >>>>>>>>>>>> time constant in the microsecond range! I wonder if this is =what> >>>>>>>>>>>> I am > >>>>>>>>>>>> seeing? > > >>>>>>>>>>>> I looked at the step response with a different color LED. (y=ou> >>>>>>>>>>>> expect > >>>>>>>>>>>> more absorption in the doped region with shorter wavelength =light.)> >>>>>>>>>>>> So here is the pulse response from a 635 nm red and 594 nm A=mber> >>>>>>>>>>>> LED. > > >>>>>>>>>>>>http://img52.imageshack.us/i/tek0026.png/ > >>>>>>>>>>>> The red LED is the trace that is a bit higher. I m looking t=o> >>>>>>>>>>>> see if > >>>>>>>>>>>> I can dig up any blue or white LED s. > > >>>>>>>>>>>> Any other ideas how I might determine if that's what I'm see=ing...> > >>>>>>>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) re=versed> >>>>>>>>>>>> biased at ~12V. > > >>>>>>>>>>>> George H. > > >>>>>>>>>>> Crank up the bias till the diode is fully depleted, and see. > > >>>>>>>>>>> Cheers > > >>>>>>>>>>> Phil Hobbs > > >>>>>>>>>> Thanks Phil, > > >>>>>>>>>> I can try more bias. Is there any way to know it's fully deple=ted?> >>>>>>>>>> (Or do I just take it to the maximum as specified by the maker=. (30> >>>>>>>>>> volts in this case.) > > >>>>>>>>>> The white LED showed more of the long tail... but less of a sh=arp> >>>>>>>>>> turn- > >>>>>>>>>> on, I wonder if it's just the LED time response that I'm seein=g.> > >>>>>>>>> A white LED has a phosphor that is likely slow. > > >>>>>>>> Yeah, I noticed that.. Thanks to Grant too. > > >>>>>>>> I didn't save the screeen shot on my flash. (I should hit the sa=ve> >>>>>>>> button more often.) > >>>>>>>> IR led's are slow too. > > >>>>>>>>> Try a laser diode. They are (usually) fast. > > >>>>>>>>> I did encounter some laser diodes that had a weird slow step re=sponse> >>>>>>>>> when used at low pulse rates. They behaved, electrically and > >>>>>>>>> optically, as if there were an inductor in series with the diod=e. At> >>>>>>>>> high rep-rates, 1 MHz maybe, the effect disappeared. > > >>>>>>>> Bring out the big guns ehh? I've never looked at the step respon=se> >>>>>>> >from our laser diode, but we've modulated it up to ~150 MHz. The=re's> >>>>>>>> a reported change in the modulation sensitivity, at the microsec=ond> >>>>>>>> range and longer its due to heating, at shorter times it drops b=y an> >>>>>>>> order of magnitude and is due to (index of refraction?) carrier > >>>>>>>> density effects. At very high frequency you can get an enhanceme=nt> >>>>>>>> due to relaxation oscialltion in the LD... about 6 GHz, in the L=D's I> >>>>>>>> know. > > >>>>>>>>> Try looking at the voltage across the LED, and see if it's a cl=ean> >>>>>>>>> step. > > >>>>>>>>> What's the signal path from the photodiode to the scope? Since =all the> >>>>>>>>> LEDs seem to have similar waveforms, maybe the detector is the > >>>>>>>>> problem. > >>>>>>>> Yeah sorry I didn't save all the crummy 'scope shots. > >>>>>>>> But I'm certainly open to some silly measurement error. There's =a TIA> >>>>>>>> with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, =with> >>>>>>>> 3.3pF parallel capacitance (plus some extra stray... call it ~2 =pF,> >>>>>>>> perhaps a bit less). > > >>>>>>>> I think I'm seeing a LED temperature effect. Does the light outp=ut go> >>>>>>>> up as an LED gets hotter? If it's linear with delta T, I can't c=hange> >>>>>>>> drive level and make it go away. > > >>>>>>> LEDs are less efficient when they're hot. But the forward drop go=es> >>>>>>> down. So there is usually some source resistance where the effect=s> >>>>>>> cancel. > > >>>>>> Thanks John, If it's due to more current because of a drop in forw=ard> >>>>>> voltage, then I should see that in the current. (Which I don't... > >>>>>> sigh) > > >>>>>> OK let me try a totally different idea. Can the stray capacitance =of> >>>>>> 'stuff' have a long tail like that. Charges moving around on piece=s> >>>>>> of plastic/fiber glass or something? > > >>>>>> I could air wire the circuit and see what that does... > > >>>>>> George H. > > >>>>>>> John- Hide quoted text - > > >>>>>>> - Show quoted text - > > >>>>> How about posting the schematic? If the edges are symmetrical, it's > >>>>> probably the TIA--the LED would be cooling down when it wasn't emit=ting> >>>>> light, so you wouldn't see the thermal tail. > > >>>>> Op amps have really terrible supply rejection when it comes to fast > >>>>> transients--depending on the feedback, you can easily get _gain_ fr=om> >>>>> the supply pin to the output. (PSR is specified with respect to the > >>>>> amplifier _inputs_.) > > >>>>> Another possibility is phase funnies in the open loop gain. Composi=te> >>>>> amps are notorious for that sort of thing. The long-time settling > >>>>> behaviour is dominated by the lowest-frequency pole or zero in the =open> >>>>> loop transfer function--lead-lag networks can cause this sort of th=ing> >>>>> even though the closed-loop response looks nice on a spectrum analy=zer.> > >>>>> Try touching the summing junction with your finger and see what hap=pens> >>>>> to the step response. > > >>>>> Cheers > > >>>>> Phil Hobbs > > >>>> Rereading, it looks like you have something like a 20 pF summing > >>>> junction capacitance and a 2 pF feedback cap. =A0That means you have=an> >>>> open-loop pole/zero pair separated by a factor of 10 in frequency, u=p> >>>> near the GBW of the op amp. =A0That will lead to some entertaining > >>>> settling behaviour, which at this point would be my best guess as to > >>>> what you're seeing. > > >>> Yeah perhaps more like 3.3 pF across the feedback. > > >>> The pulse response through a 'big' resistor looks just fine, =A0(no P=D> >>> or LED). =A0I went back and added the 'missing' 12 pF to ground, just=in> >>> case...(That's the removed PD capacitance, the strays and opamp C are > >>> still there.) > > >>> Hey this might be some 'real' physics and not just a circuit screw- > >>> up. > > >>> George H. > > >> Well, it does look like the photodiode, which suggests that the > >> aforementioned diffusion tail mechanism is doing it. =A0 =A0I'm surpri=sed> >> that jacking up the bias didn't fix it. =A0 =A0It isn't usually as bad=as that.> > >> Cheers > > >> Phil Hobbs > > > Yeah, but I'm still confused. =A0I tried it with an adjustable bais > > (from 0-12V) late today. There was the expected change in the short > > time response. =A0But none in the long tail. =A0I had a nice chat with > > someone at OSI. (sent pic's and schematics to them.). =A0I'll have to > > try a different PD. > > > George H. > > They may have a really thick N+ diffusion on the back. =A0That's one > common method of making ohmic contacts--you dope the Si so heavily that > it becomes a metal (i.e. lots of electrons sloshing round in the > conduction band). =A0Better diodes keep that layer as thin as possible, > because it hurts the PD performance. > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > > email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.ne=t- Hide quoted text -> > - Show quoted text -Ohh, the back side. I was thinking about the front p layer. Well then shorter wavelengths should be 'better'. Dang! I hope it's not the PD. It'd be a lot easier to replace the opamp! Did I mention I really like the metal to-5? package the PD comes in. About the same diameter as a 5mm LED and T1-3/4 light bulb. George H.>
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 20:31:59 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>George Herold wrote: >> On Apr 12, 8:11 pm, Phil Hobbs >> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>> John Larkin wrote: >>>> On Tue, 12 Apr 2011 13:45:20 -0500, wil...@nospam.pobox.com (Will >>>> Janoschka) wrote: >>> >>>>> On Tue, 12 Apr 2011 16:21:14, George Herold<gher...@teachspin.com> >>>>> wrote: >>> >>>>>> On Apr 12, 10:19 am, John Larkin >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold >>> >>>>>> <Big snip> >>> >>>>>>>> Thanks John, If it's due to more current because of a drop in forward >>>>>>>> voltage, then I should see that in the current. (Which I don't... >>>>>>>> sigh) >>> >>>>>>> What's the LED driver circuit like? >>> >>>>>> I've used a few different configurations... all give the same result. >>>>>> Coax from the signal generator, daisy chains to the 'scope and on to >>>>>> the the LED. There I had 50 ohms to ground and in parallel 500 ohms >>>>>> feeding the LED to ground. Originally there was a few inches of wire >>>>>> between the 500 ohm resistor and the LED, but I later tightened it up >>>>>> and put everything right at the LED. I later just feed the LED >>>>>> through a 50 ohm resistor. >>> >>>>>> The PD circuit is standard TIA >>> >>>>>> http://img651.imageshack.us/i/tia2.png/ >>> >>>>>> I'm going to write to OSI (the PD maker.) I wanted to be sure I >>>>>> wasn't doing something stupid beofre contacting them... >>> >>>>>> George H. >>> >>>>>>>> OK let me try a totally different idea. Can the stray capacitance of >>>>>>>> 'stuff' have a long tail like that. Charges moving around on pieces >>>>>>>> of plastic/fiber glass or something? >>> >>>>>>> Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' >>>>>>> behavior, but your waveforms don't look like that. >>> >>>>>>> Post (or email me) your LED driver and pd circuits, and I'll see if I >>>>>>> can spot anything. >>> >>>>>>>> I could air wire the circuit and see what that does... >>> >>>>>>> Time constants in the microseconds probably aren't strays. >>> >>>>>>> John- Hide quoted text - >>> >>>>>>> - Show quoted text - >>> >>>>> Do you have any kind of a chopper or shutter that you could >>>>> use to isolate the time constant(s) of the PD frome those of the >>>>> LED/ laser? >>> >>>> It would be interesting to see how fast an edge you could create with >>>> a mechanical chopper. Nanoseconds would be tough. >>> >>>> John >>> >>> Mechanical streak cameras are surprisingly fast--you spin a polygon on >>> an air bearing (or a magnetic bearing in a vacuum) and look at the >>> scanned beam from some distance. You can do 12 facets x 100k rpm x 10m >>> ~ 2500 km/s. (*) Nanoseconds are _easy_. They did stuff like that back >>> in the 40s and 50s for bomb development. >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> (*) There's an additional factor of 4 pi in there--2 pi for the angular >>> velocity and another 2 for the law of reflection (the beam scans twice >>> as fast as the polygon turns). >>> >>> -- >>> Dr Philip C D Hobbs >>> Principal >>> ElectroOptical Innovations >>> 55 Orchard Rd >>> Briarcliff Manor NY 10510 >>> 845-480-2058 >>> >>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net- Hide quoted text - >>> >>> - Show quoted text - >> >> I was wondering if you can wiggle something faster than you can rotate >> it? >> >> Well maybe just easier to wiggle. >> >> I think I've seen the optical lever trick done multiple times... >> 'no smoke but lots of mirrors' >> >> George H. > >Streak tubes can do 1 ps/div--Hamamatsu still makes them, I think, but >they seem to have discontinued their streak cameras. You need a >microchannel plate to get enough photons to see at that rate. > >Cheers > >Phil HobbsThey did an optical-input equivalent-time sampling oscilloscope. It had a photocathode, deflection plates, and a PMT, all in the same jug. Vaguely like an image dissector tube. They tried to demo it for me in Hamamatsu, but they couldn't get it to trigger. I don't think it ever went commercial. Which reminds me of a very strange idea I had for detecting you-know-whats. John
Reply by ●April 12, 20112011-04-12
John Larkin wrote:> On Tue, 12 Apr 2011 20:31:59 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> George Herold wrote: >>> On Apr 12, 8:11 pm, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>> John Larkin wrote: >>>>> On Tue, 12 Apr 2011 13:45:20 -0500, wil...@nospam.pobox.com (Will >>>>> Janoschka) wrote: >>>> >>>>>> On Tue, 12 Apr 2011 16:21:14, George Herold<gher...@teachspin.com> >>>>>> wrote: >>>> >>>>>>> On Apr 12, 10:19 am, John Larkin >>>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>>>> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold >>>> >>>>>>> <Big snip> >>>> >>>>>>>>> Thanks John, If it's due to more current because of a drop in forward >>>>>>>>> voltage, then I should see that in the current. (Which I don't... >>>>>>>>> sigh) >>>> >>>>>>>> What's the LED driver circuit like? >>>> >>>>>>> I've used a few different configurations... all give the same result. >>>>>>> Coax from the signal generator, daisy chains to the 'scope and on to >>>>>>> the the LED. There I had 50 ohms to ground and in parallel 500 ohms >>>>>>> feeding the LED to ground. Originally there was a few inches of wire >>>>>>> between the 500 ohm resistor and the LED, but I later tightened it up >>>>>>> and put everything right at the LED. I later just feed the LED >>>>>>> through a 50 ohm resistor. >>>> >>>>>>> The PD circuit is standard TIA >>>> >>>>>>> http://img651.imageshack.us/i/tia2.png/ >>>> >>>>>>> I'm going to write to OSI (the PD maker.) I wanted to be sure I >>>>>>> wasn't doing something stupid beofre contacting them... >>>> >>>>>>> George H. >>>> >>>>>>>>> OK let me try a totally different idea. Can the stray capacitance of >>>>>>>>> 'stuff' have a long tail like that. Charges moving around on pieces >>>>>>>>> of plastic/fiber glass or something? >>>> >>>>>>>> Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' >>>>>>>> behavior, but your waveforms don't look like that. >>>> >>>>>>>> Post (or email me) your LED driver and pd circuits, and I'll see if I >>>>>>>> can spot anything. >>>> >>>>>>>>> I could air wire the circuit and see what that does... >>>> >>>>>>>> Time constants in the microseconds probably aren't strays. >>>> >>>>>>>> John- Hide quoted text - >>>> >>>>>>>> - Show quoted text - >>>> >>>>>> Do you have any kind of a chopper or shutter that you could >>>>>> use to isolate the time constant(s) of the PD frome those of the >>>>>> LED/ laser? >>>> >>>>> It would be interesting to see how fast an edge you could create with >>>>> a mechanical chopper. Nanoseconds would be tough. >>>> >>>>> John >>>> >>>> Mechanical streak cameras are surprisingly fast--you spin a polygon on >>>> an air bearing (or a magnetic bearing in a vacuum) and look at the >>>> scanned beam from some distance. You can do 12 facets x 100k rpm x 10m >>>> ~ 2500 km/s. (*) Nanoseconds are _easy_. They did stuff like that back >>>> in the 40s and 50s for bomb development. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>>> (*) There's an additional factor of 4 pi in there--2 pi for the angular >>>> velocity and another 2 for the law of reflection (the beam scans twice >>>> as fast as the polygon turns). >>>> >>>> -- >>>> Dr Philip C D Hobbs >>>> Principal >>>> ElectroOptical Innovations >>>> 55 Orchard Rd >>>> Briarcliff Manor NY 10510 >>>> 845-480-2058 >>>> >>>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net- Hide quoted text - >>>> >>>> - Show quoted text - >>> >>> I was wondering if you can wiggle something faster than you can rotate >>> it? >>> >>> Well maybe just easier to wiggle. >>> >>> I think I've seen the optical lever trick done multiple times... >>> 'no smoke but lots of mirrors' >>> >>> George H. >> >> Streak tubes can do 1 ps/div--Hamamatsu still makes them, I think, but >> they seem to have discontinued their streak cameras. You need a >> microchannel plate to get enough photons to see at that rate. >> >> Cheers >> >> Phil Hobbs > > They did an optical-input equivalent-time sampling oscilloscope. It > had a photocathode, deflection plates, and a PMT, all in the same jug. > Vaguely like an image dissector tube. They tried to demo it for me in > Hamamatsu, but they couldn't get it to trigger. I don't think it ever > went commercial. > > Which reminds me of a very strange idea I had for detecting > you-know-whats. > > John >I put in a requisition for one of those things for my antenna-coupled tunnel junction project, but never got the money. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
