Pole-zero cancellation and preamplifiers - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 07, 2020, 11:53:09 pm
content: Hi!I'm hoping you can help me with this, I'm going absolutely crazy.I'm building a setup for gamma spectroscopy and other particle detection stuff.Pretty basic in the world of particle/nuclear physics, but my setup consists of a scintillation probe (scintillator+PMT), high voltage bias tee, preamp (home built), and a 'spectroscopy amplifier'.The spectroscopy amplifier is a pulse shaping amplifier, taking the long-tailed sharp rise pulse from the PMT (preamp) and making a short, gaussian pulse from that.Here's the problem: I get a bipolar pulse out of my spectroscopy amplifier. see attached.Here are the observations:- I'm 90% the spec. amplifier (Ortec 572) is functioning, I have 2 identical and a third and they all behave identically.- PMT and divider is a finished bicron module.- Bias tee for the PMT is copied from the one the PMT came with, and HV PSU is good too.-The preamp is home made. A so-called parasitic capacitance amplifier. Loosely based on the block diagram in:http://courses.washington.edu/phys433/equipment/Ortec_113.pdf- I have tried all combinations of 50 ohm terminations on in/out of all cables. No difference in undershoot whatsoever.- AC or DC coupling of the preamp doesn't change anything.- Spectroscopy amp. input impedance is 500 ohms.- Spectroscopy amp. pole-zero potentiometer changes nothing.What can I be doing wrong? I've built 3 different preamps all with this problem. I'm stumped. Should my preamp have its own P/Z cancellation network and a buffer before going to the spectroscopy amplifier? Have i messed up the impedance matching grossly?I hope you can help! Thanks!

author: KE5FX
author_rank: Super Contributor
timestamp: May 08, 2020, 01:25:48 am
content: Are you actually driving 100 ohm cable from the preamp with the 100-ohm source termination resistor? Try reducing that resistor to 40 or 50 ohms (basically the cable Zo minus the rated output impedance of the opamp.)Also, in your sketch, do those waveforms appear with or without the next stage connected? In other words, are you using a tee connector to sample the actual signal, or are you disconnecting the various stages and hooking them up directly to your scope?

author: duak
author_rank: Super Contributor
timestamp: May 08, 2020, 04:19:55 am
content: I don't know anything special about these devices but I don't think the problem is in the cables.This link has a good explanation for the various circuit elements and their effects:http://www.dnp.fmph.uniba.sk/~kollar/je_w/el2.htm.Look at the PZC section as it discusses undershoot.Good Luck with your project!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 08, 2020, 01:49:19 pm
content: Quote from: KE5FX on May 08, 2020, 01:25:48 amActually it's a 50 ohm cable, and so is all my terminations. I've replaced the series resistor with 47 ohm and upped the output cap to 10 uF. Didn't change anything.And irregardless of modules are connected or not, I see the same undershoot.What bothers me most is nothing I do seem to impact the undershoot. I can't even make it worse! This gives me the feeling I'm looking in the wrong place..thanks for the advice though!

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: May 08, 2020, 04:18:26 pm
content: What's going on at the far end of the last cable?Have you tried terminating it with something like, 22 ohms, 50 (or closest) and something higher, like 220 and see how the pulse changes?

author: Tom45
author_rank: Frequent Contributor
timestamp: May 08, 2020, 04:44:27 pm
content: Perhaps it is a probe ground artifact.What probe are you using and how long is the probe's ground lead? Are you using a high impedance probe/scope input or 50 ohm connection and scope input?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 08, 2020, 09:28:27 pm
content: It's a Bicron 1M.25/1.5L-X probe. It used to have a grounding wire tab that broke off so you might be on to something. I just tried bolting grounding leads from it, the bias tee box and the preamp box to a central point on my main electronics rack (NIM bin) where all the pulse process modules are. 50 cm at .75 mm2. Didn't change anything unfortunately.could it be an issue of too much anode current to the PMT? the HV current limiting resistor is only 100K, you often see multi megohm ones. It IS the one the probe came with, however. but the probe was used as X-ray detector so probably a pretty high count rate, almost like a current.

author: jmelson
author_rank: Super Contributor
timestamp: May 08, 2020, 09:34:30 pm
content: The problem is the spectroscopy amp has a differentiating pole. You need to add a zero that exactly compensates for this pole. But, in general terms, you have a differentiation function in the amplifier, so you need to add an integration after the amp. So, if the amp acts like a series cap-parallel resistor,then add a series resistor-parallel cap to ground to correct for it. It may be easier to just cut and try a few components than try to calculate the RC needed.But, doesn't the spectroscopy amp HAVE a built-in pole-zero adjustment? Most do. Look for a pot labeled P/Z.Jon

author: jmelson
author_rank: Super Contributor
timestamp: May 08, 2020, 09:36:59 pm
content: Yup, right in the middle of the unit -- a tiny screwdriver-adjustable pot marked P/Z adjust.Jon

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 08, 2020, 10:23:53 pm
content: That's the strange thing. The p/z adjustment does nothing to the undershoot. And this is the case on 3 spectroscopy amplifiers.

author: jmelson
author_rank: Super Contributor
timestamp: May 09, 2020, 02:31:41 am
content: Quote from: ChristofferB on May 08, 2020, 10:23:53 pmWell, the shaping time is selectable by a switch, does that do anything? You don't indicate the time scale in your first drawing, so i don't know if yourscintillator tail shape is compatible with the 572 shaping amp. It might be too fast or too slow, and beyond the range of the P/Z circuit on the 572.Jon

author: dmendesf
author_rank: Frequent Contributor
timestamp: May 09, 2020, 11:18:18 am
content: I know nothing about spectroscopy but as long as your signal is AC coupled then the output NEEDs to have a mean of zero (so it goes positive and negative). I'm just justifying the behavior here. No idea about how to fix, but integrating the output seems like a good idea.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 14, 2020, 01:32:49 pm
content: Sorry to revive an old thread, but I wanted to share with you a PMT preamplifier circuit that almost eliminates the issue completely.The topology is swapped from a 'parasitic capacitance' charge amp to a standard CSA. Works a charm. The network inbetween the two op amps can be used to tweak pulse shape slightly.I'm quite pleased, see attached schematic.Thanks for the input!