Author Topic: Why is the PHA minimum 500 mV and why is the minimum Voltage linked to the gain?  (Read 200 times)

Philipp Poeml

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Hi,

I was always wondering about the questions in the subject.
Is there a reason for a minimum of 500 mV in the PHA?
And why is it linked to the gain? From gain 800 or so the minimum Voltage also slowly incerases.
This seems to be a firmware thing, because it applies to PeakSight and PfE. If, for example I am at 1500 gain, and I type 500 mV in PeakSight for the lower limit, the setting is being set to 800 instead (not 800, I am not in front of the machine, I do not know exactly right now). Same for PfE. If I set the PHA at gain 1500 to 0.5 V I can see in PeakSight that instead it is set to 0.8 V. If I press the "read" button in PfE after that, it also does read the 0.8 V.

Thanks
Philipp
« Last Edit: September 23, 2019, 12:42:10 pm by Philipp Poeml »

Probeman

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I was always wondering about the questions in the subject.
Is there a reason for a minimum of 500 mV in the PHA?
And why is it linked to the gain? From gain 800 or so the minimum Voltage also slowly incerases.
This seems to be a firmware thing, because it applies to PeakSight and PfE. If, for example I am at 1500 gain, and I type 500 mV in PeakSight for the lower limit, the setting is being set to 800 instead (not 800, I am not in front of the machine, I do not know exactly right now). Same for PfE. If I set the PHA at gain 1500 to 0.5 V I can see in PeakSight that instead it is set to 0.8 V. If I press the "read" button in PfE after that, it also does read the 0.8 V.

This is just too weird!  I'd never noticed this before! I guess because you have to click the PHA button in PeakSight to see the baseline and window values and we always set our PHAs from Probe for EPMA.

It definitely appears to be some firmware issue because there's nothing in PFE that forces this value to around 0.5 volts. It will warn you if you specify a baseline less than 0.2 volts, but you'd get a warning at least.
The only stupid question is the one not asked!

Philipp Poeml

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I assume this is the same for all SX100s. Is that so? Did you try on yours?

Probeman

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I assume this is the same for all SX100s. Is that so? Did you try on yours?

Yes, I did try it on our SX100. On sp2 the baseline won't go below 0.531 volts.  I had never noticed this before. Too bad, getting Cameca to change this firmware behavior will require a lifetime of effort.  I wonder what their reasoning was/is?
The only stupid question is the one not asked!

Philipp Poeml

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That's exactly my question. What is the reason?
On our machine the lower limit is 500 mV and it will increase with increasing gain.
Anybody knows?

Karsten Goemann

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Remember this old thread:
https://probesoftware.com/smf/index.php?topic=217.msg1048#msg1048

The lower baseline limit depends on the gain. It's a linear relationship.

Cameca introduced this for the newer SX100s and it was carried over to the SX5s. Our vintage 2003 SX100 at UTAS, serial #846 (with the "old" boards in the config) didn't have it.

The baseline is to cut off noise, isn't it? Maybe with increasing gain that noise peak gets broader?

Probeman

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That makes perfect sense, I had forgotten about this.  Increasing the gain does of course broaden the peaks (gain = multiplication), but I'd still rather have control over it myself.

How else can I show students the noise peak?   :)
The only stupid question is the one not asked!

Philipp Poeml

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Yes, ok for the noise peak. So they are automatically increasing the baseline to get rid of a wider noise peak. Interestingly such a noise peak never shows up in the PHA scans. I guess that means the method works.
Unfortunately like this it is never possible to get the complete Ar escape peak.

Karsten Goemann

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So even if you manually increase the gain (instead of using PHA Auto) you can't prevent the Ar escape peak from being cut off at the baseline because the baseline limit keeps going up? Have you tried adjusting the bias instead? Do you have enough separation between escape and main PHA peak to completely cut it off? Depends a lot on the x-ray line I guess.

In that sense the older SX100s had an advantage because they had a fixed lower baseline limit of 0.56V no matter which gain was used.