Channeltron woes

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ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup » Sat Apr 03, 2021 9:10 pm

Oh and some imagery
IMG_20210402_175739_301.jpg
IMG_20210403_142749_515.jpg
IMG_20210403_224409_318.jpg
IMG_20210403_224409_307.jpg

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Maciek Szymanski
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Re: Channeltron woes

Post by Maciek Szymanski » Sat Apr 03, 2021 9:58 pm

Great! That's the real experimenter's spirit! And nice to see an analog scope.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup » Mon Apr 05, 2021 1:31 pm

I just realized something strange is going on - Why are my output pulses positive?! I've set up the channeltron for negative ions/electrons, and with signal tapped from the anode.

See below for an actual schematic of my setup. If electrons collect on the anode, then the bias should momentarily drop, producing negative pulses. This looks more like "last dynode" pulses.

Can anyone spot what Is going on? I sure can't. the 1M resistor is just a normal 0.25W, perhaps it's too conductive at HV to not just act as a short?

There is large variance in the internal Rcm, it seems.
electrontest.jpg

John Futter
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Re: Channeltron woes

Post by John Futter » Mon Apr 05, 2021 7:12 pm

1 meg is very low and that is your source impedance so terminating the scope to 50 ohms will be a very big attenuator
I would use 10 meg or higher and then use a preamp in your metal box as an impedance convertor ie Jfet frontend and 50 ohm output impedance so you can use the scope termination. This stops cable reflections on your signals

ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup » Mon Apr 05, 2021 7:49 pm

For bias a meg seems pretty standard for electron multipliers, but I could be mistaken.
Here's an article featuring a neg bias VEU-6 for positive ion detection, and to me it looks like they terminate the output directly in 50 ohm, and get 100-200 mV pulses out.

http://www.mathnet.ru/links/da376d78129 ... tf4571.pdf

All of my channeltron preamps has an input impedance of 50 ohm as well, but I haven't tried adding them in yet.

John Futter
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Re: Channeltron woes

Post by John Futter » Tue Apr 06, 2021 9:30 am

Again
an input impedance of 50 ohms is going to attenuate your signal by 20,000 times if it has a source impedance of 1 megohm.
so .2volts out multiplied by 20000 gives 4000 volts coming from your channeltron (I do not think so)

ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup » Tue Apr 06, 2021 2:48 pm

I don't think that's right. There's no voltage drop over the anode resistor, it's purely a current limiting thing. One can also ground it, and take the inverted signal from the last dynode.

http://winspa.de/help101/images/Channeltron.jpg
http://winspa.de/help101/hardware.htm

Here's an example of a bias network for a (different) channeltron. I also plan on using the Ortec VT120's as preamplifier - they have an input impedance of 50 ohm too.

1M is a pretty common anode resistor for PMT's as well. Not sure what's going on.

John Futter
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Re: Channeltron woes

Post by John Futter » Wed Apr 07, 2021 8:31 am

ChristofferBraestrup
your included circuits provided all the clues
the VT 120A preamp is fed via a 10pf cap so for about 100kHz pulse repetition this equals about 3 meg input impedance into the 50 ohm preamp. Yes your preamp is 50 ohms optimized for extremely low noise current input and with a gain of 200
If you use your scope direct no termination you will attenuate by about 3 your signal in peak to peak and have to put up with lots of reflections from a not matched source / load and your high impedance will be slowing up your pulses and your noise will have risen because of 4ktbR in tis case the R

so far I see little wrong with the info I gave you so far

ChristofferBraestrup
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Re: Channeltron woes

Post by ChristofferBraestrup » Wed Apr 07, 2021 2:43 pm

Oh I'm sure you're right, and I'm very grateful for the help!

I will try adding in the preamp and see if there is a difference. Perhaps the 3.6n is too large a coupling cap as well.

The wrong polarity of my pulses could be my scope being too slow to see the original negative component, leaving only some wierd double differentiated tail to be seen. Or perhaps it's just the cable reflections

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