Electronics > Projects, Designs, and Technical Stuff
Pole-zero cancellation and preamplifiers
ChristofferB:
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!
KE5FX:
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?
duak:
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!
ChristofferB:
--- Quote from: KE5FX on May 08, 2020, 01:25:48 am ---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?
--- End quote ---
Actually 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!
pardo-bsso:
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?
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