# Speed of light circuits

A

#### Ancient_Hacker

Jan 1, 1970
0
Let's say I want to measure the speed of light using a LED pulser and
a photomultiplier tube.

A quick web search doesnt turn up anything relevant.

I suppose I need an avalanche driver into a fast LED.

And a fast amplifier on the photomultiplier tube.

And a good sampling head on the 7S11 plugin. Or I'll have to fix the
bad power supply on the 7104.

Now I could guess some ballpark numbers, like a few hundred pF
discharging through a fast transistor. And maybe a MMIC stage or
three to amplify the phototube current.

But of course itprobably takes plenty of optimizing all the little

Anybody been there, done that, and have any proven circuits?

Thanks,.

A_H

P

#### Phil Hobbs

Jan 1, 1970
0
Ancient_Hacker said:
Let's say I want to measure the speed of light using a LED pulser and
a photomultiplier tube.

A quick web search doesnt turn up anything relevant.

I suppose I need an avalanche driver into a fast LED.

And a fast amplifier on the photomultiplier tube.

And a good sampling head on the 7S11 plugin. Or I'll have to fix the
bad power supply on the 7104.

Now I could guess some ballpark numbers, like a few hundred pF
discharging through a fast transistor. And maybe a MMIC stage or
three to amplify the phototube current.

But of course itprobably takes plenty of optimizing all the little

Anybody been there, done that, and have any proven circuits?

Thanks,.

A_H

You'll do a great deal better using CW and a phase measurement. Not as
much fun, though.

Cheers,

Phil Hobbs

J

#### John McMillan

Jan 1, 1970
0
Ancient_Hacker said:
Let's say I want to measure the speed of light using a LED pulser and
a photomultiplier tube.

A quick web search doesnt turn up anything relevant.

I suppose I need an avalanche driver into a fast LED.

The LED pulse should be no slower than the response of the
photomultiplier. say 2ns risetime 5ns FWHM.

I've used a slight modification of the circuit presented
in "A fast timing light pulser for scintillation detectors"
Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment, Volume
241, Issues 2-3, 1 December 1985, Pages 612-613
J. S. Kapustinsky et al.

It works with some blue LEDs (you've got to match the photocathode)
And a fast amplifier on the photomultiplier tube.

Not really. If you use a high gain tube. The pulser will kick
about 10^7 photons per flash. You can run the photomultiplier
straight into the scope.
And a good sampling head on the 7S11 plugin. Or I'll have to fix the
bad power supply on the 7104.

No idea. Sampling scopes plus photomultipliers are normally
a bad idea. You've got 20ns transit time on the photomultiplier
plus ~3ns per metre on the optical pathlength - total delay of ~100ns
for a reasonable size experiment. Just use an oscilloscope with
100MHz bandwidth

Now I could guess some ballpark numbers, like a few hundred pF
discharging through a fast transistor. And maybe a MMIC stage or
three to amplify the phototube current.

But of course itprobably takes plenty of optimizing all the little

Anybody been there, done that, and have any proven circuits?

We did this to calibrate the Antares neutrino telescope
http://antares.in2p3.fr/Publications/conferences/2005/aguilar-icrc05-pape
r-timing.pdf

J

#### John Larkin

Jan 1, 1970
0
Let's say I want to measure the speed of light using a LED pulser and
a photomultiplier tube.

A quick web search doesnt turn up anything relevant.

I suppose I need an avalanche driver into a fast LED.

And a fast amplifier on the photomultiplier tube.

And a good sampling head on the 7S11 plugin. Or I'll have to fix the
bad power supply on the 7104.

Now I could guess some ballpark numbers, like a few hundred pF
discharging through a fast transistor. And maybe a MMIC stage or
three to amplify the phototube current.

But of course itprobably takes plenty of optimizing all the little

Anybody been there, done that, and have any proven circuits?

Thanks,.

A_H

Spark gaps can be interesting. Charge up a low-inductance cap until it
fires a gap. You'll get kilowatts of light with ns risetime. With some
moderate optics and a pmt (fresnel or curved mirror for the
transmitter, small telescope for the pmt) you can get echoes off
clouds.

John

A

#### Ancient_Hacker

Jan 1, 1970
0
Spark gaps can be interesting. Charge up a low-inductance cap until it
fires a gap. You'll get kilowatts of light with ns risetime. With some
moderate optics and a pmt (fresnel or curved mirror for the
transmitter, small telescope for the pmt) you can get echoes off
clouds.

John

Oh, that sounds so COOL! An excuse to play with high voltage!
I'll have to try that as soon as I make it throught the Honey-Do list.

R

#### Robert Baer

Jan 1, 1970
0
Ancient_Hacker said:
Let's say I want to measure the speed of light using a LED pulser and
a photomultiplier tube.

A quick web search doesnt turn up anything relevant.

I suppose I need an avalanche driver into a fast LED.

And a fast amplifier on the photomultiplier tube.

And a good sampling head on the 7S11 plugin. Or I'll have to fix the
bad power supply on the 7104.

Now I could guess some ballpark numbers, like a few hundred pF
discharging through a fast transistor. And maybe a MMIC stage or
three to amplify the phototube current.

But of course itprobably takes plenty of optimizing all the little

Anybody been there, done that, and have any proven circuits?

Thanks,.

A_H
PMTs seemto be in the 7nSec region unless you want to $pend a lot more for (maybe) a X3 improvement... Now if you take that light source and bounce it off the moon (assuming you know the distance) or off a reflector on a rather distant mountain (again assuming you know the distance), then it might be a little more practical. G #### Genome Jan 1, 1970 0 Ancient_Hacker said: Let's say I want to measure the speed of light using a LED pulser and a photomultiplier tube. A quick web search doesnt turn up anything relevant. Anybody been there, done that, and have any proven circuits? Thanks,. A_H Some blokes did it a bit back with a vacuum cleaner and a couple of pint glasses. I'd suggest you are not really an Ancient_Hacker and more like a Lazy_Arse if you can't sort that one out using modern tecknowledgy. DNA J #### John McMillan Jan 1, 1970 0 Robert Baer said: PMTs seemto be in the 7nSec region unless you want to$pend a lot
more for (maybe) a X3 improvement...

I was thinking of the Electron Tubes 9813 and similar (sim. Burle 8575)
which are ~2ns.
OEtech have them for USD 215 http://www.oetech.com/pmt9_12.html

I've got a whole load of these looking for a project...
Now if you take that light source and bounce it off the moon
(assuming you know the distance) or off a reflector on a rather distant
mountain (again assuming you know the distance), then it might be a
little more practical.

No, it is quite feasible in a largish room. I've done it.
I even offered the technology to Ralph S******* as evidence
that the speed of light was actually c - but he wasn't interested....

O

#### [email protected]

Jan 1, 1970
0
There is also the method of a moderate speed spinning mirror, a hene
laser or even a good quality laser pointer, a larger mirror and a
football field. Needs far less scope bandwidth and only a simple diode
detector, but its a phase method and not so satisfying.
You can get the commecial off the shelf kit for ~100$. Just add scope. Steve Roberts C #### colin Jan 1, 1970 0 Ancient_Hacker said: Let's say I want to measure the speed of light using a LED pulser and a photomultiplier tube. A quick web search doesnt turn up anything relevant. I suppose I need an avalanche driver into a fast LED. And a fast amplifier on the photomultiplier tube. And a good sampling head on the 7S11 plugin. Or I'll have to fix the bad power supply on the 7104. Now I could guess some ballpark numbers, like a few hundred pF discharging through a fast transistor. And maybe a MMIC stage or three to amplify the phototube current. But of course itprobably takes plenty of optimizing all the little details to get the best risetimes. Anybody been there, done that, and have any proven circuits? Are you going to measure the average roundtrip and bounce the light back or go for the realy hard part and try to measure it in a single direction ? if so how will you avoid the propgation delay of the cables between trasmitter/sensor/computer from negating your results ? If you use a continuos train of pulses you can use a hp5328b to average the difference between two pulses with sub picosecond resolution, with only moderatly fast tramsitter/sensors. Colin =^.^= J #### John Larkin Jan 1, 1970 0 Are you going to measure the average roundtrip and bounce the light back or go for the realy hard part and try to measure it in a single direction ? if so how will you avoid the propgation delay of the cables between trasmitter/sensor/computer from negating your results ? Don't change the cables! John J #### jasen Jan 1, 1970 0 Let's say I want to measure the speed of light using a LED pulser and a photomultiplier tube. use two tubes one with a short optical path (possibly take from the side of the led) through an attenuator (if neccesary) ajdust the pulse frequency until the phases match. d=N*lambda Bye. Jasen J #### John McMillan Jan 1, 1970 0 John Larkin said: Don't change the cables! John Use pulsed light and do time of flight. Rig some contraption with mirrors and corner cube reflectors. In one configuration the light takes a short path (10cm); in the other it goes to the end of the room and hits a corner cube and comes back (20m). You measure the distance with a tape measure to 1mm. You leave all the cables untouched and don't screw with the pulser or photomultiplier supplies between the two configurations. You measure the time delay to a few hundred picoseconds and you have the speed of light to less than a percent accuracy. Its easy. j R #### Rich Grise Jan 1, 1970 0 There is also the method of a moderate speed spinning mirror, a hene laser or even a good quality laser pointer, a larger mirror and a football field. Needs far less scope bandwidth and only a simple diode detector, but its a phase method and not so satisfying. You can get the commecial off the shelf kit for ~100$. Just add
scope.

Steve Roberts

Instead of a "larger mirror" on the far end, you should use a corner
reflector/retroreflector. Aiming a flat mirror would be a nightmare.

Cheers!
Rich

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