On Tue, 12 Apr 2011 21:40:11, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> 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 >Yes nanoseconds are tough. I come from the optics side but, I also know about the electrical stuff. With the proper analysis of the apertures. the risetime can be calculated even with a curved chopper, the difference is the photodiode/photoconductor response, that depend on temperature, pressure, and the odor of your farts. That does not make the photothingies useless.
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 12, 20112011-04-12
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 20:11:25 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: [snip]>> > >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).Yep. My first job as a technician in MIT Building 20 was designing and building a dual 400W amplifier to drive a two-phase motor for a smear camera at 30K RPM ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Remember: Once you go over the hill, you pick up speed
Reply by ●April 12, 20112011-04-12
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in message news:flv9q69p4kj3glv8q5v230vgjvgnfv0ke0@4ax.com...> On Tue, 12 Apr 2011 20:11:25 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > [snip] >>> >> >>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). > > Yep. My first job as a technician in MIT Building 20 was designing > and building a dual 400W amplifier to drive a two-phase motor for a > smear camera at 30K RPM > > ...Jim ThompsonThat would have been no small task back then. Now they are less than $100. tm
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 22:34:24, George Herold <gherold@teachspin.com> wrote:> On Apr 12, 2:45�pm, 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?- Hide quoted text - > > > > - Show quoted text - > > Yeah, I was thinking about sweeping a laser across the PD face, with a > rotating mirror or something. But then it's real optics and not just > sticking an LED against a PD. > > George H.What is it that you want to measure? Please note 'that that' measurement will have nothing to do with other photodiodes!
Reply by ●April 13, 20112011-04-13
On Tue, 12 Apr 2011 15:34:24 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Apr 12, 2:45�pm, 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?- Hide quoted text - >> >> - Show quoted text - > >Yeah, I was thinking about sweeping a laser across the PD face, with a >rotating mirror or something. But then it's real optics and not just >sticking an LED against a PD. > >George H.The trick would be to make a clean step, namely a fast rise and a long, flat top. I suppose you could measure the impulse response of a pd and math that to the step response, making some assumptions. John
Reply by ●April 13, 20112011-04-13
On Tue, 12 Apr 2011 09:53:30 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Apr 12, 11:12�am, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:>> >> It's your receiver circuit. >> >> Try just the photodiode and a resistor. Signal average on the scope if >> you need to. >> >> John- Hide quoted text - >> >> - Show quoted text - > >Oh, I did that Friday. The PD is reversed biased 12 V, then through >10k ohm with a buffer looking at the top of the 10k. > >Here's the fast response... slower than without the TIA. > >http://img535.imageshack.us/i/tek0018.png/ > >And here's the slow time.... The long tail remains! > >http://img859.imageshack.us/i/tek0019.png/ > >George H.Oh. In that case, equalize it out. John
Reply by ●April 13, 20112011-04-13
On Tue, 12 Apr 2011 09:56:13 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Apr 12, 11:12�am, John Devereux <j...@devereux.me.uk> wrote: >> George Herold <gher...@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- Hide quoted text - >> >> - Show quoted text - > >Thanks John, I've got lotsa laser diodes. VCSELs sound expensive. >Who sells them? > >George H.We buy VCSELS from Lasermate and Appointech. The connectorized, aligned fiber-coupled ones are around $30, so I'd imagine the TO18s would be a lot cheaper. We buy 850, 1310, and 1500 nm [1], and they typically put a couple of milliwatts into a fiber. Risetimes are brutal, like 100 ps. The Optek VCSELS have been, well, disappointing. Their photodiodes are OK. John [1] the 850s are VCSELS for sure; not positive about the longer wavelengths.
Reply by ●April 13, 20112011-04-13
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> writes:> On Tue, 12 Apr 2011 21:17:38 +0100, John Devereux > <john@devereux.me.uk> wrote: > >>George Herold <gherold@teachspin.com> writes: >> >>> On Apr 12, 11:12 am, John Devereux <j...@devereux.me.uk> wrote: >>>> George Herold <gher...@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. >> >>[...] >> >>>> >>>> >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), >>> >>> Thanks John, I've got lotsa laser diodes. VCSELs 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). > > Nice. Any hassles with end-use certificates or do they just ship > whatever you order asap?Not sure what you mean but never had any hassle with anything like that. Then again I never tried to buy the $42000 15kW laser diode either. I'd probably blow it up :) Shipping is normally a few days (from them in Austria to me in the UK). They do seem to stock the things I have bought. Shipping and other charges can be a bit expensive for "hobbyist" uses IME. -- John Devereux
Reply by ●April 13, 20112011-04-13
George Herold <gherold@teachspin.com> writes:> On Apr 12, 4:17 pm, John Devereux <j...@devereux.me.uk> wrote: >> George Herold <gher...@teachspin.com> writes: >> > On Apr 12, 11:12 am, John Devereux <j...@devereux.me.uk> wrote: >> >> 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), >> >> > Thanks John, I've got lotsa laser diodes. VCSELs 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?Fabry-Perot. "Something to do with the mirrors I expect" :) Hey, googles first link is that britney spears semiconductor physics site. I remember that from 10 years ago! <http://britneyspears.ac/physics/fplasers/fplasers.htm>> Where did you find the prices?Top left link on home page - you get a PDF price list of most of their range then you have to search through it. Their website is a bit primitive I'm afraid, but unmatched as far as range of available opto devices is concerned IMO. UV lasers! VCSELS, Resonant Cavity LEDs. "Superluminescent Diodes". "Quantum Cascade Lasers" - sounds like something off the Starship Enterprise. -- John Devereux
Reply by ●April 13, 20112011-04-13
On a sunny day (Tue, 12 Apr 2011 18:19:57 -0700) it happened John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in <s8u9q65tqrct8ub9hq8ni7cbne9pq1jcbp@4ax.com>:> >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.And that is?
