Crazy PMT issue - Page 1
Forum section: electronics / projects

author: jmelson
author_rank: Super Contributor
timestamp: September 10, 2021, 06:09:08 pm
content: I have a crazy issue with a PMT (Photomultiplier Tube). I have a standard sort of voltage divider in the tube base with capacitors across the last 3 resistors in the chain. These are 10000 pf C0G SMT caps, rated for 250 V. I had no source-end termination on the coax cable (RG178) and had a bit of reflection from the amplifier. I tried to fix that by adjusting the input impedance of the amp. But, I got a bipolar wiggle, indicating the amp might be showing a capacitive input impedance. So, I added a parallel termination resistor at the coax connector on the tube base. That totally fixed the reflection, but then looking closer, there is a high frequency oscillation after a pulse. The pulses are coming from a para-Terphenyl organic scintillator crystal that gives a VERY fast, bright light pulse.So, the total pulse is less than 10 ns wide, with a peak of about 3 V giving 60 mA of anode current. So, putting this straignt into the scope, I can clearly see where the reflection from the scope is if I turn off the 50 Ohm termination, it is about 37 ns after the pulse, a reasonable round-trip time for the cable. But, with the scope properly terminated, we see a damped ringing right after the pulse. The first ring peak seems to be buried in the peak, in fact. The period of the ringing is about 4.7 ns. The only thing I can come up with is the capacitors in the voltage divider are losing charge, the volltage between dynodes is collapsing, and then when they charge up again, another packet of electrons come through.This base uses a positive power supply, so there is a 4700 pF 3 KV ceramic cap between the anode and the 50 Ohm output cable. The 50 Ohm parallel terminating resistor is on the cable side of that cap.Any PMT experts out there who have seen anything like this? We are going to try paralleling the caps on Monday to see if that helps. If the amplitude or frequency changes, then we will know we are on the right track.Thanks,Jon

author: TurboTom
author_rank: Super Contributor
timestamp: September 11, 2021, 07:54:13 am
content: 60mA of peak anode current seems a lot to me. What PMT are you using? Unless you've got a PMT designed and specified for high pulse currents, you should try to stay below 10mA if you need a half-way linear pulse response.Your setup may be prone to afterpusing, are you aware of thePMT technical handbook by Hamamatsu? I'll highly recommend to have a look at it, it's very useful if you are working with PMTs.

author: jonpaul
author_rank: Super Contributor
timestamp: September 11, 2021, 09:13:58 am
content: Agree that 60 mA is very high current.What is the mfg/model of the PMT?What is the cable type and length PMT><scope? Have you probed the divider and dynodes during the poulses? What scope is used? Term 50 Ohms?A picture is worth 1e6 words.Suggest to take the schematic or sketch of entire setup, and scope shots or photos of the pulse, and include in your response. That will save us all a lot of time and unnneded notes.Kind Regards,jon

author: jmelson
author_rank: Super Contributor
timestamp: September 11, 2021, 03:23:06 pm
content: Quote from: TurboTom on September 11, 2021, 07:54:13 amRight, I was horrified when I realized how much anode current It was drawing. Tube is Hamamatsu R1450.Quoteit really doesn't look like afterpulsing, it looks so sinusoidal and regular. I think it is the output charge dumping the reserve charge on the caps on the last 3 dynode stages. Thanks for the link!Jon

author: TurboTom
author_rank: Super Contributor
timestamp: September 11, 2021, 06:55:48 pm
content: Since yoou clearly narrowed down the source of the ringing to the PMT plus its supply circuitry (i.e. scope directly connected and both sides of the cable properly terminated), you may want to "play" with the other parameters. If you look atHamamatsu's datasheet for the P1450, you will find that a pulse response is specified for an output voltage of 130mV into 50 Ohms which resembles a pulse peak current of 2.6mA. Moreover, pulse linearity is specified for 4 and 8 mA with 2 and 5%, respectively. This shows that you are driving the tube far beyond of what it's been meant to be operated at.The first thing I'ld try is reducing the overall supply voltage in order to operate the tube at a lower gain and observe its output pulse shape. It's well possible that this is all that's required to solve the problem. Only after that isn't successful, I'ld try to narrow the problem dow to other causes, looking at the individual dynode voltages at the last (capacitively shunted) dynodes, which would require quite some elaborate probing techniques. If you look at the total transition time of the PMT of 19ns, your observed ringing time of 4.7ns well matches an effect originating form the last three dynodes, taking into account that the propagation time from cathode to the first dynode may be twice the inter-dynode time (19ns/12*3 = 4.75ns).If the cause is actually a capacitive coupling effect between the last dynodes, you may try placing the shunting caps between the corresponding dynode and ground (or anode supply potential since you're running the setup with a positive supply -- in this case, you've got to make sure the supply is very well bypassed to output ground close to the tube) and not beween the individual dynodes. This would require caps of higher voltage rating for D8 and D9, though.

author: jmelson
author_rank: Super Contributor
timestamp: September 13, 2021, 08:22:52 pm
content: Quote from: jonpaul on September 11, 2021, 09:13:58 amHere's a pic, with the scope directly on the end of a 50 Ohm cable, and a 51 Ohm parallel terminating resistor at the connector on the tube base, to catch any reflection off the scope. This is without attempting to fix the caps on the last dynode stages. I had 3 caps across the last 3 dynode stages, and then doubled all those caps. Then I added a cap across the previous stage, and that all seemed to help. I guess it cut that oscillation down to half amplitude.Thanks,Jon

author: jmelson
author_rank: Super Contributor
timestamp: September 23, 2021, 02:19:45 am
content: Well, finally I have made progress. Terminating the anode of the tube at the sending end was a disaster.Something about doubling the anode peak current caused instability. Space charge, collapse of voltage on the dynodes - whatever. So I had to take that out. But, then, the original issue was less than optimal termination going into the amplifier board. That had a long trace that sent the signal to two amps, so that one could amplify the signal without inversion, and one could amplify WITH inversion. The long trace had a lot of capacitance to ground and caused a bipolar wiggle. So, I added a ferrite bead in series with the signal right at the input connector to the amp, and 100 nH seems to do the trick! Almost completely eliminates the bump from the capacitive discontinuity.Jon

author: jmelson
author_rank: Super Contributor
timestamp: September 24, 2021, 04:44:37 pm
content: And, here's some scope photos of the original problem I was trying to solve, and the final fix. The issue was capacitance of the long traces on the board, and the fix was to add a little series inductance on the end of the cable feeding into the board.