Author Topic: Cameca digital noise issues  (Read 3975 times)

Probeman

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Re: Cameca digital noise issues
« Reply #15 on: May 11, 2021, 12:06:35 PM »
I wish I had a photo of our electron imaging monitor from a couple of years ago, because we were seeing a variable "digital hash" on the Cameca dedicated electron imaging monitor that looked like a choppy static in the lower half of the displayed image. Fortunately this digital noise was not in the saved image data, but it did make live imaging a hassle sometimes.

However, we've noticed that in the last year or two this "digital hash" seems to have disappeared and the only thing we've done to the instrument around that time was to re-solder the power resistors for the stage motors which were running so hot that they desoldered themselves.

Our instrument engineer's solution (after resoldering the connections) was to add some extra cooling fans to the front of the stage as seen here:

https://probesoftware.com/smf/index.php?topic=639.msg8574#msg8574

I can't prove a casual relationship between the two, but seeing the comments above about adding extra grounding to the stage motors makes me think it is possible.  So if you are seeing noise issues in your images, you might want to check those solder joints!
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sem-geologist

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Re: Cameca digital noise issues
« Reply #16 on: May 12, 2021, 02:40:58 AM »
Probeman,
That is not the same digital hash, because our (non variable, periodic) digital hash is visible only on FEG instruments and it is caused by physical periodic misalignment of the beam. W or LaB6-produced beam is to broad to be sensitive enough for such kind of very small misalignment. The general design stayed the same from SX100, and so the ground leak could be not noticed previously as it was completely insignificant, and from design point of view could be simply ignored. FEG produces very narrow beam, and even very small fluctuation in ground level influence the beam alignment. I understand that fixing the resistor on stage could fix your problem, as old Video board (the one which produces RGB signal for physical monitor) bridges 0V_gnd from 5V supply (power for digital) with analog gnd very close to the chip which produces analog VGA (actually it is XGA). I can't discuss exact point as that would require breaching confidentiality of schematics. But basically, due to such design, physical monitor (RGB signal, if to be more precise) is able to catch any severe case ground fluctuations.

BTW, You need those vibration-introducing workaround fans as your engineer (and previously someone who had initially soldered these boards) had ignored the datasheet recommendations of those resistors. Power resistors can heat up up to 400 C (at hottest spot, depends from type and rating of the resistor), datasheets clearly recommend HMP (high melting point) solder which have  ~300 C melting temperature (compared to 183/190°C Pb40Sn60). By HMP I mean the good old-time Pb93.5 Sn5 Ag1.5, not the 99.9Sn (~260C) RoHS-compliant crap. I believe You still can buy that in the states and UK (I had to smuggle that into stupid EU for fixing such resistors, happy that there still are some good-willed citizens in UK). HMP soldering needs carefulness as if contaminated with Sn will lower the melting point, it is best to have separate soldering tip for that. We had some issues with power resistors there and here. I.e. one of our scanning boards was producing crap scanning, but only when warmed up. After through inspection I found out that power resistor had un-soldered itself from the one side and was vibrating (touching and releasing the desoldered terminal). The problem was two-fold: 1) clearly it was soldered with Pb40Sn60 (the lowest melting temperature for Sn based solders), 2) the resistor was mounted not symmetrical - one side was much closer to solder joint, and thus had desoldered. resoldering that with HMP fixed issue.

P.S. just to be clear. Considering your configuration, I am not advertising new single baleyage-visu-display board vs old three separate boards, as old ones is much easier to repair as for present availability of electronic parts (saying that as someone situated in hellhole like EU, for someone situated in states with plenty of old-part suppliers that is a no-brainer). Also new one does not introduce any additional functionality. Considering WDS board - that is very good upgrade, not from OS upgrade point (Which I can't comprehend – we are using windows 7, peaksight 5 and we have old WDS board on our SX100 - no problems), but from eliminated paralysable behavior at very high count rate; and if you are into using a lot of differential PHA mode, new WDS electronics have separate ADC for every spectrometer instead of sharing single ADC for few spectrometers.
« Last Edit: May 12, 2021, 03:49:40 AM by sem-geologist »

Probeman

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Re: Cameca digital noise issues
« Reply #17 on: May 12, 2021, 07:58:05 AM »
That is not the same digital hash, because our (non variable, periodic) digital hash is visible only on FEG instruments and it is caused by physical periodic misalignment of the beam.

SG: I am not claiming it is the "same digital hash".  This is a general topic on noise in Cameca images (e.g., see the original post). I was merely replying to earlier posts in this thread from a couple of years ago that discussed this other noise issue.

BTW, You need those vibration-introducing workaround fans as your engineer (and previously someone who had initially soldered these boards) had ignored the datasheet recommendations of those resistors. Power resistors can heat up up to 400 C (at hottest

These stage motor boards that desoldered are original Cameca parts. And yes, when our instrument engineer (he does not work for Cameca) repaired them, I remember he mentioned using high temp solder.
« Last Edit: May 12, 2021, 08:32:49 AM by Probeman »
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