# Oscilloscope measuring of PMT in photon counting mode

I have a setup as drawn below: DC source of 15V connected to high voltage (HV) source of 1500V connected to photomultiplier tube Hamamatsu, R1-R3 are resistors builtin the socket of PMT (there are actualy 10 of them and some capacitors...). R4 is load resistance to convert PMT current to voltage. I am measuring voltage with oscilloscope. Oscilloscope is powered from isolation transformer. Input DC voltage is also powered from power supply with transformer, so the grounds should be isolated.

My problem is shown below on images: if I connect oscilloscope probes in one polarity I get expected result (photon excited pulses), image 1. But if I change polarity I get weird ripples on 13kHz, image 2. I believe it is the ripple from HV power supply. But I don't understand why am I measuring it. If I change probe polarity I would expect just the change of polarity of measured signal (peaks up, not down).

This makes me puzzled most: it is the same when I use batteries as 15V DC instead of regulated power supply.

Why do I get these results? I want to count pulses but it seems that when I try to do so with ADC and Arduino powered from USB I am counting just the ripples or some similar noice, not the peaks. Is it problem with some ground loops? How can I avoid this? Using instrumental amplifier with R4 in negative feedback loop but with similar result.

simulate this circuit – Schematic created using CircuitLab

• What happens when you put the isolation transformer on the power supply side?
– mng
Commented Jan 7, 2015 at 20:09
• Good question, I forgot to mention it: when I use batteries instead of 15V DC power supply it is the same. That's what makes me puzzled the most. Commented Jan 7, 2015 at 20:37

## 2 Answers

I'm not sure what your 13 kHz ripple is, but it seems more like power supply ripple than a ground loop. If your HV supply is a positive supply, then the PMT 'ground' is going to be ripple-y. The floating ground lead on your scope presumably has a lot of parasitic capacitance, since it goes to the chassis and everywhere inside. When the ground lead is connected to PMT ground, there's not much capacitance on the anode, but when the ground lead is on the anode, there's a big chunk of capacitance to charge and discharge with the ripple. That's my speculation.

In general, what you want when dealing with pulses is a "pulse discriminator", which lets you put conditions on the height, length, etc. of the pulses you're interested in. Since you're pulse counting, you probably don't need anything fancy, a simple comparator will do, but it will need to have sufficient negative input range. You can build a simple one using an op-amp, but you can get ones off the shelf that are faster and with logic level output. Alternatively you could use an inverting amplifier topology to flip the signal, but you will still need negative input range and sufficiently high bandwidth to pass pulse waveforms through.

Edit: One more thing, isn't the max voltage on the R1166 1250V?

• Thanks for comment. To the R1166 voltage: yes, I found in specs that it is 1250V. But I got two of them dissassembled from some medical device already hardwired to 1500V DC source. I have no experience with PMTs. Is it a big problem? Commented Jan 8, 2015 at 7:23
• It's probably not a big deal. The performance will be different from the spec, but you probably don't care about that.
– mng
Commented Jan 8, 2015 at 19:31
• Why If your HV supply is a positive supply, then the PMT 'ground' is going to be ripple-y? Are there any advantages to negative HVPS?
– Pana
Commented Feb 14, 2021 at 15:20
• @Pana, generally HV is made by a flyback transformer or capacitor multiplier, or maybe both. So there will be ripple on the 'hot' end. Since electrons want to go more positive, if you use a +HV supply with a PMT, it would be connected with the supply ground to the first dynode, and the +HV to the PMT "ground". So using a rippley supply as what amounts to the ground reference voltage is not ideal.
– mng
Commented Mar 5, 2021 at 9:08

Well you can't run 15V DC through a transformer and get 1500V on the output. I'd guess there is an oscillator in your HV supply and a cockroft-walton HV section. That might be the source of your 12 kHz signal. Without seeing your setup there must still be some ground/common connection between the 'scope and PMT.

BTW I'd reduce R4 to 50 or 100 ohms... then your ~50 us pulses will turn into ~50 ns pulses and you will have a much higher maximum count rate.

• Sure there is not just transformer in HV source, I just could not find better schematics :-) Values of resistors are given by E974-13 I have not experimented with resistors values yet. I have a set of PMT (Hamamatsu R1166-05) and socket. Thanks for the point. Commented Jan 7, 2015 at 19:16
• Even if there is some common point (but it should not, 15V DC is from the transformer powered power supply and oscilloscope is also powered from isolation transformer) how can I measure with different polarity? Commented Jan 7, 2015 at 19:21
• You get to call (and connect) one point ground. Connect my 'sc Commented Jan 8, 2015 at 1:51
• A comment about R4: It's purpose usually is to prevent the anode from charging to a high voltage possibly harmful for humans and electronics behind (though more for electronics, the capacity is low). It's a good idea to put 50 Ohm into the output line, use a 50 Ohm cable, and also put the scope to 50 Ohm mode. This way, the load is 100 Ohms and you measure only half the output voltage, but it's a first step to match impedances. Once I worked with PMTs and 1ns pulses. This helped a lot to improve the signal. Commented Apr 9, 2015 at 11:42