On Apr 11, 10:59=A0pm, 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=A0pm, 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 curre=nt> >> >> > 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 to=p> >> >> > is the voltage step to the LED (through 100 ohms)... AC coupled t=o 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 pic=ture> >> >> > 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 th=e> >> >> > LED during the step. But I was reading Photodetectors by S. Donat=i,> >> >> > 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.) Th=e> >> >> > second smaller set are those generated in the doped region. If th=ey> >> >> > are within a diffusion length of the depletion region then these =have> >> >> > a chance of adding to the photocurrent. Donati says that these ha=ve 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 ex=pect> >> >> > 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) reverse=d> >> >> > biased at ~12V. > > >> >> > George H. > > >> >> Crank up the bias till the diode is fully depleted, and see. > > >> >> 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://electroopti=cal.net-Hidequoted text -> > >> >> - Show quoted text - > > >> >Thanks Phil, > > >> >I can try more bias. =A0Is 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 tur=n-> >> >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, =A0I noticed that.. Thanks to Grant too. > > > I didn't save the screeen shot on my flash. =A0(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? =A0I've never looked at the step response > >from our laser diode, but we've modulated it up to ~150 MHz. =A0There'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. =A0At 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. =A0There'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. =A0Does the light output go > >up as an LED gets hotter? =A0If 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 -
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 12, 20112011-04-12
Reply by ●April 12, 20112011-04-12
In article <4DA3B786.6010406@electrooptical.net>, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> George Herold wrote: > > On Apr 11, 3:54 pm, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> wrote: > >> George Herold 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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Is there any hidden connection between the insistence of these LEDs on continuing to emit light long after they've been turned off, and the insistence of the participants in this thread in continuing to re-emit screenfuls and screenfuls of previous posts, before finally adding a one-sentence comment at the end?!?!?!
Reply by ●April 12, 20112011-04-12
George Herold wrote:> On Apr 11, 10:23 pm, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> George Herold wrote: >>> On Apr 11, 3:54 pm, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>> George Herold 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 >> >>>>>> -- >>>>>> 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-Hidequoted text - >> >>>>>> - Show quoted text - >> >>>>> 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. >> >>>>> George H. >> >>>> The blue from the LED will mostly be getting absorbed in the epi, so you >>>> won't see much of it. Diffusion out of the epi may be what you're >>>> seeing on the other LEDs too. Try a 900 or 940 nm LED and see if that >>>> makes a difference. >> >>>> Re full depletion: >>>> I'd crank it right up to 30V, with an RC to protect it from breakdown. >>>> You can probably go significantly higher than that. Most diodes speed >>>> up amazingly when they're fully depleted--you may see the bandwidth >>>> double between 27 and 30V. Donati has a curve somewhere. (Great book, >>>> Donati, but a bit unevenly edited. Still, a sterling effort for an E2L >>>> writer.) >> >>>> 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.net-Hide quoted text - >> >>>> - Show quoted text - >> >>> OK 30 Volt bias made no difference in the step response. (I added in >>> two 9V batteries, I put 100 ohms in series, 0.1uF) The lack of a >>> change in the fast response time at 30 V may not be surprising, >>> there's a lot of excess stray capacitance in the circuit. (7 pf or >>> so.) So I may not have noticed. >> >>> I'm going to try an IR LED. (I thought those were too slow >>> though. ... I'll find out.) >> >>> George H. >> >> Interesting. >> >> The fastest LEDs I know of are around 900 nm. They aren't exactly fast, >> about 30 MHz. Have a look at the SFH4550 (12 ns, 850 nm) and VSLB3940 >> (15 ns, 940 nm) >> >> LEDs have long tails on their falling edges due to the relatively long >> carrier lifetime. That rising edge was when the light turned on, right? >> Because the falling edges always look like that. > > Yeah the rising edge, but I just looked and the falling edge looks > identical! Is there a reason the rising and falling edges should look > different? > > I've been trying to convince myself it's just an LED effect. Perhaps > a temperature change, or the above carrier lifetime in the LED? The > reason I don't think it's a Photodiode effect is that when I look at > the high frequency (BW ~ 1.5 MHz) noise with a DC light source and > several hundred mV of bias 'current' across the feedback resistor, > (noise signal dominated by shot noise) I see exactly the same amount > of noise regardless of the light source. > > George H. > > >> >> 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.net- Hide quoted text - >> >> - Show quoted text - >There's a tail on the falling edge of the output of a LED, at least when you hit it really hard. In the high-level injection regime, the light output goes like the carrier density squared, so it drops very fast initially, but then as the carrier density drops, it slows down to the usual minority carrier lifetime, which is somewhere between 20 and 200 ns. I'm sure that's why LED speed is always specified with some really gross amount of drive. 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
AES wrote:> In article<4DA3B786.6010406@electrooptical.net>, > Phil Hobbs<pcdhSpamMeSenseless@electrooptical.net> wrote: >> George Herold wrote: >>> On Apr 11, 3:54 pm, Phil Hobbs >>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>> George Herold 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 > . . . . . . . . > . . . . . . . . > . . . . . . . . > . . . . . . . . > > Is there any hidden connection between the insistence of these LEDs on > continuing to emit light long after they've been turned off, and the > insistence of the participants in this thread in continuing to re-emit > screenfuls and screenfuls of previous posts, before finally adding a > one-sentence comment at the end?!?!?!Modern newsreaders often run on machines with LED backlights? ;) 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 Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold <gherold@teachspin.com> 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 >> >> >> >> -- >> >> >> 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-Hidequoted text - >> >> >> >> - Show quoted text - >> >> >> >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)What's the LED driver circuit like?> >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
Reply by ●April 12, 20112011-04-12
On a sunny day (Tue, 12 Apr 2011 10:06:32 -0400) it happened Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in <4DA45C68.8050709@electrooptical.net>:>AES wrote: >> In article<4DA3B786.6010406@electrooptical.net>, >> Phil Hobbs<pcdhSpamMeSenseless@electrooptical.net> wrote: >>> George Herold wrote: >>>> On Apr 11, 3:54 pm, Phil Hobbs >>>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>>> George Herold 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 >> . . . . . . . . >> . . . . . . . . >> . . . . . . . . >> . . . . . . . . >> >> Is there any hidden connection between the insistence of these LEDs on >> continuing to emit light long after they've been turned off, and the >> insistence of the participants in this thread in continuing to re-emit >> screenfuls and screenfuls of previous posts, before finally adding a >> one-sentence comment at the end?!?!?! > >Modern newsreaders often run on machines with LED backlights? ;)Samsung, the idiots, these days gives customers the choice between LCD and LED TVs.
Reply by ●April 12, 20112011-04-12
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 >> >>>>>> -- >>>>>> 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-Hidequoted text - >> >>>>>> - Show quoted text - >> >>>>> 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 -- 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, 9:50=A0am, George Herold <gher...@teachspin.com> wrote:> On Apr 11, 10:59=A0pm, 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=A0pm, 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 steppi=ng> > >> >> > the current into a LED such that the step doubles the photo cur=rent> > >> >> > from the PD. (I m not starting with the LED off, but on slightl=y.)> > >> >> > 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 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. > > > >> >> > 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. Don=ati,> > >> >> > 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 thes=e 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 wha=t 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 lig=ht.)> > >> >> > So here is the pulse response from a 635 nm red and 594 nm Ambe=r> > >> >> > LED. > > > >> >> >http://img52.imageshack.us/i/tek0026.png/ > > >> >> > The red LED is the trace that is a bit higher. I m looking to s=ee 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) rever=sed> > >> >> > biased at ~12V. > > > >> >> > George H. > > > >> >> Crank up the bias till the diode is fully depleted, and see. > > > >> >> 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://electroop=tical.net-Hidequotedtext -> > > >> >> - Show quoted text - > > > >> >Thanks Phil, > > > >> >I can try more bias. =A0Is there any way to know it's fully deplete=d?> > >> >(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 t=urn-> > >> >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, =A0I noticed that.. Thanks to Grant too. > > > > I didn't save the screeen shot on my flash. =A0(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 respons=e> > >> 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? =A0I've never looked at the step response > > >from our laser diode, but we've modulated it up to ~150 MHz. =A0There'=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. =A0At 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 t=he> > >> 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. =A0There's a T=IA> > >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. =A0Does the light output =go> > >up as an LED gets hotter? =A0If it's linear with delta T, I can't chan=ge> > >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, =A0If it's due to more current because of a drop in forward > voltage, then I should see that in the current. =A0(Which I don't... > sigh) > > OK let me try a totally different idea. =A0Can the stray capacitance of > 'stuff' have a long tail like that. =A0Charges moving around on pieces > of plastic/fiber glass or something? > > I could air wire the circuit and see what that does... > > George H. > >I just tried a laser diode (lamda=3D 785nm). Exactly the same tail... It's some circuit thing? No, A current pulse through a resistor gives a nice square output.... Grrr. I'm going to order a blue LED and set this aside for a bit. (Unless someone has some idea?) Thanks for all the input, George H.> > > > > John- Hide quoted text - > > > - Show quoted text -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
Reply by ●April 12, 20112011-04-12
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 >>> >>>>>>> -- >>>>>>> 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-Hidequoted text - >>> >>>>>>> - Show quoted text - >>> >>>>>> 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. 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
George Herold <gherold@teachspin.com> writes:> 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. >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 quickly (GHz), -- John Devereux
