Author Topic: Will your scope LIE ??  (Read 5523 times)

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Offline Fank1

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Will your scope LIE ??
« on: April 06, 2014, 03:01:39 am »
Same signal on 3 different scopes.
Which do you think are right ?
 

Offline Skimask

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Re: Will your scope LIE ??
« Reply #1 on: April 06, 2014, 04:04:40 am »
Same probes?
Same wall outlet?

Apples and oranges...
I didn't take it apart.
I turned it on.

The only stupid question is, well, most of them...

Save a fuse...Blow an electrician.
 

Online T3sl4co1l

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Re: Will your scope LIE ??
« Reply #2 on: April 06, 2014, 06:14:33 am »
Probably holding the probe upside down, all the electrons will fall out. :P

Also fair to note, we have no reference as to where the trace is centered in the two analog examples  :-/O
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Offline AndyC_772

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Re: Will your scope LIE ??
« Reply #3 on: April 06, 2014, 07:05:06 am »
This is exactly why simple digital 'scopes with no intensity grading make such poor diagnostic tools compared to their analogue (or better quality digital) counterparts. The process of sampling a signal and displaying it as a single pixel, which is either on or off and which gives no indication of how often that pixel is 'hit', tends to exaggerate noise and infrequent events.

Sometimes that's useful, of course, and it's one of the major benefits that all digital scopes have over analogue. Their limited dynamic range in terms of brightness makes artefacts visible which would be too dim to see on an analogue display.

Here, though, it's giving the impression that the signal is much noisier than it really is.

Examples of how different acquisition modes dramatically affect the appearance of a waveform on screen:
https://www.eevblog.com/forum/testgear/oscilloscope-specs-which-are-really-important/msg181637/#msg181637

Offline miguelvp

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Re: Will your scope LIE ??
« Reply #4 on: April 06, 2014, 07:45:18 am »
Yes and now,

The digital scope shows that the discrete signal you are measuring is noisy.
The analog one is sampling the same discrete signal but the output is not showing it because it averages the noise.

So what is better? both combined which is an intensity grading digital scope. phosphor is slow and your eyes interpolate, even to your camera phosphor is slow.

To your eyes, even the digital scope that is showing the extra noise your eye won't see as much detail as the picture you took of it. If you want proof then dust off your old VCR and pause it. noise everywhere, does your eyes see that much noise when playing it? nope. And actually that's an analog signal!
 

Offline AndyC_772

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Re: Will your scope LIE ??
« Reply #5 on: April 06, 2014, 08:27:06 am »
I'm not sure that 'averages' is a word I'd use.

Think of your signal as being like a lottery, and every time you take a sample of it to measure its voltage - the outcome of the lottery - that's like buying a ticket. If the measured level is at or very close to the mean dc level, that's a losing ticket, and if the measured level deviates from that level by more than a couple of pixels' worth, then that's a 'win'.

The display on a digital scope typically refreshes at about 50-60 Hz. Within that time, it's taken millions of samples and combined them together to give a picture of what happened to the signal since the last frame.

Unless the lottery is rigged, buying literally millions of tickets virtually guarantees you a win at some point. So the fat trace on the digital scope shows that it IS possible to win the lottery you're studying - which is absolutely true. But because there's no intensity grading, all it's showing is that some tickets were winners and some were losers. Important information has been lost.

On the analogue scopes, the brightness of any given point depends on the proportion of time the beam spends pointing at that spot. What it's showing is that, just like a real-life lottery, it is overwhelmingly likely that you will lose.

Both statements are true, of course. You could win, but that outcome is extraordinarily unlikely.

Offline EEVblog

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Re: Will your scope LIE ??
« Reply #6 on: April 06, 2014, 08:35:56 am »
Ah, the old analog oscilloscope has less noise than a digital myth.
Assuming all scope are the same bandwidth, and the probing is exactly the same:
That digital scope has no intensity grading, and even if it does, it does a much better job at capturing and showing the noise present. That is what digital scopes do, they capture and hold so you can see stuff. The ones like the 1052E without intensity grading are the "best" in this respect. Or you could say "worst" depending upon your point of view.
The point is that the noise is present on the analog scope too, you just can't see it, and sometimes that's a bad thing.
 

Offline cyr

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Re: Will your scope LIE ??
« Reply #7 on: April 06, 2014, 08:52:02 am »
Unless the lottery is rigged, buying literally millions of tickets virtually guarantees you a win at some point. So the fat trace on the digital scope shows that it IS possible to win the lottery you're studying - which is absolutely true. But because there's no intensity grading, all it's showing is that some tickets were winners and some were losers. Important information has been lost.

In both the low-end DSO case and the analog scope case information has been lost, which is more important (the worst "lie") depends.

The DSO (even more so with peak-detect sampling) will show you the worst case, and the analog scope will tend to hide rare events completely while giving you a good impression of the typical case.

A good DSO will give you both, and let you choose what information is presented to you.
 

Offline David Hess

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Re: Will your scope LIE ??
« Reply #8 on: April 06, 2014, 01:02:24 pm »
Without a graded index display the DSO throws out the statistical distribution so only the peak to peak noise is retained including the peak to peak quantization noise which may be considerable.  The examples using analog oscilloscopes are not setup properly for displaying and measuring noise.  The vertical sensitivity should be set much higher.

Here is some food for thought about measuring noise or at least RMS noise with an analog oscilloscope:


 

Offline Fank1

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Re: Will your scope LIE ??
« Reply #9 on: April 06, 2014, 05:25:32 pm »
Thanks Dave, a very good explanation.
 


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