Author Topic: EEVblog #983 - A Shocking Oscilloscope Problem ! aka Whack Triggering ...  (Read 19998 times)

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Offline jonovid

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Quote
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There are almost always ways around it but it would be nice if the way around was build in the scope. (if it cost very little)
The fact that no one has is testament to the fact that this is a complete non-issue in real world use.

microphonics noise tolerance specifications should be listed by the oscilloscope manufacturer?  :-// IMO
so the informed engineer can make decisions as to which oscilloscope to purchase.
if high microphonics noise tolerances are required.   
« Last Edit: April 06, 2017, 07:15:52 am by jonovid »
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Offline mikeselectricstuff

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Offline FrankBuss

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More from Daniel :
https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2017/04/05/the-piezoelectric-effect-and-oscilloscopes

Good advice in the article for those who live in Australia: "If you are extra concerned about this or work on the back of a kangaroo, try using an equipment cart or table that has built-in suspension."
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Offline gnif

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Offline Nobody2

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Wow, I knew that capacitors can be susceptible by mechanical stresses, but I would have never ever guessed that measurements could be influenced just by lightly tapping the equipment.
 

Offline David Hess

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The impulse in all cases appears to be coming from the BNC itself and I ran other tests using different adapters and coaxial patch cords which seemed to show the same thing.

Yes I tried tapping a open BNC connector on the other end of the patch cord, same thing. An even tapping an unterminated coax cable with no connector on the far end does the same. I fact it seems tapping the coax has a more severe effect then tapping the connector. So yes, no escape  :o

I had considerable difficulty testing BNCs without the effect of the cables which was almost always greater.

Maybe it's just me but I was taught that when you are measuring small signals you don't touch anything during the measurement. Not only is this "not a big issue in everyday use" as Dave says but IHMO it's not an issue at all in any proper use of an oscilloscope.  Really!

If you know about the effect, and that is the point of the videos. I thought that was obvious, but it seems not, as quite a few have complained the videos were pointless.
There would be a hell of a lot of people who would have no idea it was even possible.
And BTW, it's not uncommon to accidentally bump your scope probe etc (which are very susceptible), and I have seen accidental impulses in real use. Luckily I knew what caused it, others might think it's a real signal, or an accidental trigger could send you up the garden path etc.

It is a real problem in some applications.  I have occasionally had it when making single shot acquisitions where I have to set everything up, arm the oscilloscope, and then trigger the circuit or device under test.  Usually I end up with a few false triggers while I arrange everything or I give up and instead change the test setup to use a trigger source with low sensitivity.

I never considered it a mystery; connectors, cables, and probes suffer from piezoelectric and triboelectric effects.  What I am shocked about is the other examples Dave showed where areas other than the connectors, cables, and probes were sensitive and sometimes much more so; that is just poor and sloppy design.  The touch screen example is particularly egregious.

 

Offline mtdoc

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I have occasionally had it when making single shot acquisitions where I have to set everything up, arm the oscilloscope, and then trigger the circuit or device under test.  Usually I end up with a few false triggers while I arrange everything or I give up and instead change the test setup to use a trigger source with low sensitivity.

I never considered it a mystery; connectors, cables, and probes suffer from piezoelectric and triboelectric effects.  What I am shocked about is the other examples Dave showed where areas other than the connectors, cables, and probes were sensitive and sometimes much more so; that is just poor and sloppy design.  The touch screen example is particularly egregious.

I suspect we've all seen it when setting up measurements, moving connectors, probes, etc, even if we didn't know the cause. My point is that it does not or at least should not impact any measurements, unless someone is being a complete boob and bumping their equipment during measurements. Yes,it's good to know about this issue - just as another reminder of why good measurement practices are important when looking at small or sensitive signals.  But again, I think if this had much of a real world impact it woold be common knowledge and scope makers would be addressing  it and marketing this.

As far as the touch screen - i have yet to see any instance where it was triggered with a touch - only instances where it was tapped hard enough to make a sound and when a probe was connected, only when it was stabbed hard with a plastic stick.
 

Offline joeqsmith

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Looks like LeCroy did their homework pretty well, I can see while without the probe connected, you tapped the screen quite hard and its not affected.  :-+
That LeCroy is heavy. Transferring energy from the screen to the PCB is much harder in that case. Attach a chunk of led to R&S and you will get similar result.

Steel frame, aluminum case, 68000 VME chassis, this one is heavy.  Pictures showing the interconnect that I mention in the video.

https://www.youtube.com/watch?v=xQ5gyEaCS3U&feature=youtu.be
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline KedasProbe

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Required label when you are doing measurements:  ;D
Not everything that counts can be measured. Not everything that can be measured counts.
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Offline bitwelder

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Would covering the capacitors in the front-end stage with a blob of silicon glue, celastic or other similar material have any effect in reducing the sensibility to external vibrations?
 

Offline joeqsmith

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Only seemed fitting to show something made to operate in a bit more harsh environment.  I'm sure both of these scopes saw much worse over their life in a garage.  My plan is to use them for this same purpose.

« Last Edit: April 10, 2017, 08:54:35 pm by joeqsmith »
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Online EEVblog

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« Last Edit: April 10, 2017, 12:05:55 pm by EEVblog »
 

Offline watersb

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Of course some peoples may use open inputs for hunting ghosts but this can do also with empty matchbox depending how ...

Wait, what? An empty matchbox works?

Returning this Owon scope...

I was doing it wrong! Thanks for the tip!
 


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