PMT Philips XP2020 damaged

[ChrisRadium2023]

why 1M instead of the usual 50 Ohm? why 1nF more than 10nF ? Before changing the cap on the USB sound card as the Thremino doc suggests I wanted to see by myself the effect of such capacitor on the signal transmission ; so i tried on the pmt output first as its easy to see on the oscilloscope :)

[ChrisRadium2023]

hi,

when you say : "Are you referring to the coupling capacitor?

If so I recommend a 1M load with 1 nF coupling." : At the moment I'm using a 50 Ohm resistance load and a 10nF coupling cap.

Because I saw a 1uF on the chip of the USB sound card I tried something alike on the pmt output instead of my usual 10nF to see how it'd react with the pulses and I can see it create a great "resistance" (not sure about the term in this context) that absorbs most of the pulses energy. I think it explains why my spectrum were in fact just noise and very low energy that I stretched using the theremino gain thinking my spectrum was shrunk.

I will try with a 1M as you recommend but could you explain the choice of 1M instead of 50 Ohm here ?
I'm considering maybe to change the cap on the USB sound card as it was recommended in Theremino file but I'm wondering if it would be easier if I simply stretched the pulse instead to a point when the frequency could pass easily the 1uF filter of the mic input of the sound card.

How can I stretch the pulses easily without using an opAmp? WHat would be the best period for the pulse? I saw something like 100 to 150uS (am I correct ?)

thanks

[Sesselmann]

Chris,

Yes that's correct 1M load 1nF 3kV coupling cap, pulse length of 100 µs will work well with Theremino or PRA.

When you are using a sound card for sampling you want a rounded pulse, not the typical sharp exponential decay, because you are not trying to capture the full energy peak in the same way as some of the hardware MCA's.

Instead we are normalising the pulse and measuring the height between the lowest and highest sample in the wave.

Steven

[ChrisRadium2023]

Hi steven,

I've tried different values of load resistor , and the major difference between the 50 Ohm and the 1M is the period of the pulse that gets much longer (close to 50uS in my case).
But still I cant get any spectrum....

and for a good reason : none of the cheap chinese usb Sound card are suitable for spectrometry (i've burned down a few in my numerous experiments) for the simple reason that they generate distorsion and noise. There is a periodic voltage spike (hence the pig peak at the begining of my spectrum) plus the 50Hz frequency of the main power supply of the pc so the base ligne moves constantly.

In a previous post you mentioned the tool software "In Tune" and I used it......and its catastrophic....yuk.... Enven when I unplug the signal lead from the pmt the typical noise of the usb sound card is visible....

My HV source isnt filtered yet properly but i'm getting there. So All the things that I first thought were spectrum are in fact just noise from the sound card and some of the pulses that passes through the coupling capacitor of the sound card (wich has too high impedance for the signal I send as the shape isnt adequate yet. But still , with a better quality sound card it should be easier and better.

I tried too to record with Audacity and strangely enough the pulses are much better that with In Tune but the filtering action of the sound card is still present and the dynamic range very short.

I'm wainting for some electronic components to builld my preamp to arrive....

to be continued.