PMT Noise

Thread Starter

Ravaner

Joined Apr 7, 2008
12
Hello. I use a PMT with a load of 50R directly connected to scope. In complete obscurity (0 mV) or with moderate light excitation output signal (60 mV) doesn't show any significant noise. But ... with very low lighting, close to complete darkness, a very important noise appears (amplitude 40 mV) Can someone explain this situation ?
 

Audioguru

Joined Dec 20, 2007
11,251
I have an ABC and my neighbour has an XYZ.
You use a PMT.
You don't know what I am talking about and I don't know what you are talking about.

Why talk in Letters?? Why not name things??
 

joeyd999

Joined Jun 6, 2011
4,291
Hello. I use a PMT with a load of 50R directly connected to scope. In complete obscurity (0 mV) or with moderate light excitation output signal (60 mV) doesn't show any significant noise. But ... with very low lighting, close to complete darkness, a very important noise appears (amplitude 40 mV) Can someone explain this situation ?
Probably shot noise. PMTs count individual photons. Photons come at random intervals, causing 'white' noise. You can expect a 'crest factor' of about 6 (p-p noise is about 6x RMS noise). The S/N is proportional to the square root of the number of photons vs. time. No light, no noise. Little bit of light, lots of noise. Increase the light, and the noise drops.

If shot noise is your problem, the only solution is to integrate your signal over a period of time, thus 'counting' more photons for each measurement.
 

Thread Starter

Ravaner

Joined Apr 7, 2008
12
Thanks for reply. Seems to be the right answer. As I have to digitize this signal at a frequency of about 1 MHz I don't know how to "clean" this shot noise.
 

joeyd999

Joined Jun 6, 2011
4,291
Thanks for reply. Seems to be the right answer. As I have to digitize this signal at a frequency of about 1 MHz I don't know how to "clean" this shot noise.
You cannot clean it. It is a physical phenomena that simply exists. There are three ways around it: 1) more light, 2) filter continuously (will affect your rise time, 3) integrate the signal for a long(er) period of time.
 

DickCappels

Joined Aug 21, 2008
6,087
Make sure your electrode supplies are clean -that's often a source of noise. Also, the tube may be reacting to the occasional nuclear particle, which can be shielded (You DID electrically shield the tube, didn't you?). Also, many tubes only put out microamps; if you are getting milliamps, you might be damaging the tube -check the datasheet if you can. Sorry, its been a long time since I used one.
 

joeyd999

Joined Jun 6, 2011
4,291
Make sure your electrode supplies are clean -that's often a source of noise. Also, the tube may be reacting to the occasional nuclear particle, which can be shielded (You DID electrically shield the tube, didn't you?). Also, many tubes only put out microamps; if you are getting milliamps, you might be damaging the tube -check the datasheet if you can. Sorry, its been a long time since I used one.
They are also susceptible to magnetic fields as well. They are sometimes shielded with mu-metal.
 

Thread Starter

Ravaner

Joined Apr 7, 2008
12
Thanks for all your replies. I put a 56R load and voltage indicates now few µA anode current. As the PMT has a lateral window, seems almost impossible to place a µ metal shield (bending, cutting ... will damage magnetic properties). But I will try to insert at least an electrical Faraday shield. PS : all that is placed in a very confined volume under high vacuum
 
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