Little brain teaser

I did this by avalanching a normal npn and a 1206 case size LED. The idea, however, is from Jim Williams application note #72, appendix B, figure B1.

With the LED in parallel to the 50 Ohms resistor, I have pulses that are shorter than 12ns and quite suitable for testing PMTs and amplifiers. The Voltage across the NPN has go up to 180 volts in order to avalanche the NPN (Cannot remember the type anymore, but I remember that I had to try out some different types).

K> I was thinking of making a small light source,

now.

, however, is from Jim Williams application note #72, appendix B, figure B1= .

shorter than 12ns and quite suitable for testing PMTs and amplifiers. The V= oltage across the NPN has go up to 180 volts in order to avalanche the NPN = (Cannot remember the type anymore, but I remember that I had to try out som= e different types).

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from

ideas now.

IIRR another approach is to reverse-bias the LED into avalanche - a current flowing the wrong way still produces light, and the avalanche process turns on the light quickly. A ferrite bead inductor - perhaps with a catching diode, though you could probably tune the curcuit to make it more or less dead-beat - could turn it off just as quickly.

I came across this approach a long time ago in a paper describing a constant fraction discriminator and how it was tested.

I cited the work in a paper I published in 1978

and the reference is to Lo C C and Leskovar B IEEE Transactions Nuclear Sceince NS-21 93-105 (1974), who seem to have claimed to have generated a 200psec wide light pulse this way.

-- Bill Sloman, Nijmegen

now.

LEDs are generally quite slow, especially at low drive current. There are some IR ones that are quicker (50ish MHz), at least if you drive them hard, but most PMTs aren't too sensitive at 850 nm. You might get lucky and find a quick one in the visible, but you'd have to get a bunch and measure them.

They're very linear as you say, but only above a certain toe current, often in the 1-10 uA range. There are usually lots of trap states that decay nonradiatively, so you have to saturate those before you get the nice linear behaviour.

Their optical quality is also low--like a cheap flashlight, and for the same reason. That leads to some ripple in their angular patterns, which may make life difficult. A small integrating sphere (e.g. a ping-pong ball painted white on the outside, with drill holes at ~60 degrees to each other might do a good job--shine the LED through one hole and the light will come out the other one. (With a low quality sphere like that, you want the patch of light from the first bounce to be out of the field of view of the detector.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

now.

What about a laser diode operated sub-threshold? Is that any faster? Since Jan only wants low amplitude anyway.

VCSELs are designed for fast modulation AFAIK, they are not all that expensive,

--

John Devereux

ideas now.

Sure, but in general they don't have the nice linear behaviour of LEDs. You can make a diode laser into a superluminescent diode (SLD) by gouging up the back facet with a scriber and then running it at higher current, but it's still sensitive to back-reflections and may not be that linear either.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

now.

LEDs have a lot of capacitance, so won't be linear if current-driven at low currents. Different LEDs have very different capacitances, too. Lasers aren't very linear either.

We have some LED linearity curves and capacitance measurements around here somewhere...

You could pulse an LED pretty hard, at constant drive, and attenuate optically/geometrically.

LED output drifts with temperature, too. Photodiodes are much more linear and stable, and can be very fast, so for hig precision monitor the LED output with a P and feed back electronically or manually.

Get Phil's book!

John

rom

deas now.

As a 'crazy' idea, how about a laser hitting a spinning (or maybe just oscillating) mirror. With some slits in front of the detector, and by changing the 'lever' arm length, you can get all sorts of pulse lenghts. (I haven't tried plugging in any numbers.) It should also be pretty easy to calibrate. (Given the laser intensity, angular frequency, slit width, arm length.)

George H.

now.

Whatever you do, avoid a mistake that is made over and over again: It is not sufficient to just sink in a constant current and then turn it off. There needs to be a device that clears out the diode after the pulse. You may also have to goose it on turn-on, depending how fast it has to be.

And I'd use something better than a 555.

--
Regards, Joerg

http://www.analogconsultants.com/

now.

Check out the resonant cavity LEDs, 3ns rise/fall times quite possible. Use optical attenuation to get the low level and drive them with enough current to get to the linear operating area. I'm using constant current NPN sink and short circuit the LED with a PNP to turn it on/off fast. For UV LEDs a diode or two in series of the turn-off transistor will reduce the voltage wiggling at NPN collector to smooth the current even more.

What we use have -0.6%/K temperature coefficient for optical power, so they're mounted to largish aluminum block and actual power output is measured with a photodiode with a slower pulse every now and then.

--
Mikko

now.

If one cares to measure PMT speed, the light source must be at least

10 times faster..try a Huggins Lamp. (Note: that was the term i was given..we used mercury wetted reed relays discharging a open-end coax)

On a sunny day (Fri, 16 Dec 2011 03:58:37 -0800 (PST)) it happened Bill Sloman wrote in :

Would the LED survive? Worth a try though. Thank Bill

On a sunny day (Fri, 16 Dec 2011 07:32:36 -0800) it happened John Larkin wrote in :

Yes, 'get the book', mm I also may want an other book on gamma spectrometry, an other 80 UK pounds... These days with the web we should not need books and share scientific knowledge for free, like how the nuculear b*mb works - and in that context I want to make one remark as follow-up to a previous discussion: it seems to me that if I had to design something that could NOT be compressed in a fast way, then I would chose a sphere. Maybe they published that hollow sphere (and by golly they even fill it with what was it ? tritium-, just as disinformation so people would try that first, and the failed bang would be an alert as to where to send the next drone, but this hollow sphere story is parroted in so many media it MUST be true no? (?), so gun type OK, or 2 sliding half spheres pushed to cover each other, but compress a sphere? Not in my book. But then I am no expert on high explosives, but with explosives that can do that who need nuculear? - anyways, yes, optical feedback is a way, maybe some laser diode from an old DVD burner... I have now so much feedback on this from all you guys that I can experiment much into the next year. Thank you all for the great ideas and help. Keep them coming!

On a sunny day (Fri, 16 Dec 2011 08:41:52 -0500) it happened Phil Hobbs wrote in :

Interesting, yes, at very short pulses all sorts of mystery things happpen. I do not really need shorter than 100 nS, as I found a paper that states that the crystals I use generate no shorter light pulses than that (but some longer). Thanks, pingpong ball is easy to come by :-)

On a sunny day (Fri, 16 Dec 2011 21:11:04 -0800) it happened Robert Baer wrote in :

I looked it up on the web, that seems to be a mercury type gas discharge lamp. Stroboscope?

On a sunny day (Fri, 16 Dec 2011 12:37:22 +0100) it happened Tobias Kahre wrote in :

however, is from Jim Williams application note #72,

shorter than 12ns and quite suitable for testing PMTs

avalanche the NPN (Cannot remember the type

Thank you Tobias, that is a nice circuit, and I will give that a try with some blue LEDs I have.

On a sunny day (Fri, 16 Dec 2011 09:06:33 -0800 (PST)) it happened George Herold wrote in :

Now there is a cool idea, yes, a mechanical solution :-) It could be really fast, I like it!

On a sunny day (Fri, 16 Dec 2011 10:07:02 -0800) it happened Joerg wrote in :

Yes, remove the charge, will need that. Thanks.

On a sunny day (Fri, 16 Dec 2011 21:39:25 +0200) it happened Mikko OH2HVJ wrote in :

Yes this shorting was also pointed out by Joerg. I will look up resonant cavity LEDs. Thank you. I had considered optical attenuation, like neutral density filters perhaps. I was thinking that a bit of extra distance may help too, like perhaps in a long black (on the inside) pipe.

now.

The 3 pack of flashlights sold by Cree and Costco for $20 for three operate at 150 lumens on the high speed mode.

They run at about 300 Hz as I can watch my bike tire tread strobe with it. That circuit is in the back of the flashlight, in the switch circuit.

These LED flashlights are driven with pulsed DC.

Good starting place would be to see what waveform they are driving it with. They also have a flash setting which pulses it at about 5Hz