PMT and Amplifier discriminator plus oscilloscope HELP!

Thread Starter

Leo Silver

Joined Apr 27, 2016
46
Hello to everyone this is the first time I am writing. I would really appreciate your help. My issue goes like this:
I have a PMT signal from which I want to trigger an ICCD camera. The thing is that the pulses aren't proper to trigger the camera thus I want to discriminate them and then form the right square pulse to trigger the camera. What I use is an amplifier/discriminator and a gate/delay generator. I get the signal from the pmt and send it to the discriminator first. The thing is I want to monitor the light signal so I split it with a T connector and send it to both oscilloscope and discriminator. But then the signal I observe from the oscilloscope is attenuated. I don't understand why this happens. I suspect some impedance mismatch but I tried several things and I couldn't get my initial signal (without the discriminator connected). Even when the discriminator is turned off the signal will be attenuated. I am getting crazy here. How is this possible? Current leakage?? Can someone please explain to me what could be the issue? Thank you for any help and please dont judge me too much I am already getting all the stress I can handle :)
ps. Probably this happens because I split the current in two ways but why does this occur when I have the discriminator off. Is its impedance still present and draws the current?
 

Thread Starter

Leo Silver

Joined Apr 27, 2016
46
Hello,

Could it be that the signal from the PMT is to small and only a spike?
Then you will have to amplify and stretch the signal.

I have attached a handbook about PMT's.

Bertus
Thank you very much I will have a look :)
It isn't that small when I get it directly in the oscilloscope I would say several V in scope scale. The thing that confuses me is whether the same signal I acquire from the oscilloscope is the same that the discriminator gets or is it the other I get when I connect the pmt directly to the oscilloscope? Cause if it is not the attenuated then I wont know what threshold window I should apply to my signal to choose the right pulse. My pmt signal has many spikes and it will have a dc component if it is not connected to the discriminator but when it is connected it seems that the dc component is vanished and the spikes are also attenuated somehow..
 
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Thread Starter

Leo Silver

Joined Apr 27, 2016
46
The output impedance of the PMT is very high. Use a PMT preamp.
Will this change the impedance of the PMT? The PMT also has a variable resistance now it is set to 1kOhm but it can get up to 51KOhms..I changed it as well but it wouldn't have any effect on the pmt signal..Thank you for your reply and I appreciate any help I can get!
 

MrChips

Joined Oct 2, 2009
21,687
You have to give us a clearer picture of your situation, where you are and what you are doing.
Is this a university or some kind of research establishment?
What technical resources do you have on hand?
What is the PMT measuring?
What is the make and model of the PMT?
What is the HV input? Is it positive or negative HV?

The preamp does not change the output impedance of the PMT.
The preamp has high input impedance which is what the PMT would prefer to see.
The preamp would have 50Ω output impedance which would match RG-58/U coax cable and then your amplifier and discriminator.
Do you have access to preamp, amp, discriminator, NIM crate?
 

AnalogKid

Joined Aug 1, 2013
8,540
PMT
ICCD
Amplifier/discriminator
gate/delay generator
lotsa hardware, zero information

Manufacturer, model number, schematic, overall system wiring diagram - ***anything***?

ak
 

Thread Starter

Leo Silver

Joined Apr 27, 2016
46
OK thank you for your questions. The PMT is a Hamamatsu R955 and it is negatively polarized at -1kV, the discriminator is an old Nuclear Enterprises 4667. The gate and delay generator is an ORTEC 416A. The ICCD needs a -0.3 to 5V pulse to get triggered. The signal that I record with the ICCD is coming from a discharge. The amplifier/discriminator and the gate/delay is inside a NIM crate.Unfortunately I have no access to preamp.
 

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Thread Starter

Leo Silver

Joined Apr 27, 2016
46
Also the signal from the PMT when not connected to the discriminator is around -6V..The discriminator gets as an input a 0 to -1 V range, dc limited to +-3.5V as the manual I have attached mentions. This means that I could damage the discriminator if I use this signal from the PMT right?
 

MrChips

Joined Oct 2, 2009
21,687
Note that you will need a low voltage bipolar supply to power the preamp.
The ORTEC 113 comes with a DE-9 connector/cable which normally is plugged into the back of an amplifier. Some NIM crates have a power connector.
 

Thread Starter

Leo Silver

Joined Apr 27, 2016
46
I will have a look and I may need to get back to you with questions if this is okay. I really want to know what I am doing and the better way for me is through reading and discussion with people who know about these things. Thank you for your assistance!
 

MrChips

Joined Oct 2, 2009
21,687
btw, I work with PMTs and nuclear instrumentation daily at my day job, among the many other things that I have to do on the job. Don't be afraid to ask questions.
 

Thread Starter

Leo Silver

Joined Apr 27, 2016
46
btw, I work with PMTs and nuclear instrumentation daily at my day job, among the many other things that I have to do on the job. Don't be afraid to ask questions.
This is great! I dont know how to thank you. I read that since the PMT output is essentially a current source thus the output impedance is very high and sometimes people back match the output cable with a 50 Ohm resistor connected directly to the tube then we have the 50Ohm coaxial line which is itself terminated by a 50Ohm. Do you think this could work or the only way is the preamp?
 

MrChips

Joined Oct 2, 2009
21,687
When you have a high impedance source driving a low impedance load you are essentially creating a voltage divider that is going to attenuate your signal to almost nothing. Your gain ratio is RLOAD/(RSOURCE + RLOAD). Do the calculation for a 1MΩ source and a 50Ω load.
Your 50Ω load is going to leave you with almost no signal.

That is what a preamp prevents. The preamp has high impedance input and presents a low impedance output, besides the fact that a charge coupled preamp converts current to voltage.
 
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