Products > Test Equipment
Test Equipment Anonymous (TEA) group therapy thread
med6753:
--- Quote from: Specmaster on June 01, 2018, 11:18:55 am ---
Quote from: med6753 on Today at 11:56:39 am
Let me throw something out for discussion....
I have a fixed (5Khz, .5V p-p) Sine wave that I use as a quick check of scope function. It has a very small glitch on it. On the 2465 it shows up clearly, even with the scope set to 20Mhz bandwidth limit. (Pix 1). Now look at the Siglent DSO. (Pix 2) Do you see it? Yes, because you KNOW it's there. If you didn't know it you wouldn't have a clue. Granted, the Siglent is an entry level DSO with a somewhat limited sample rate. So maybe I'm comparing apples to oranges. But to me this gives the advantage to the CRO. Basically infinite sample rate to see glitches.
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To be fair to the Siglent, I think it has picked up the glitch but it is lost in the background noise. I believe it can be seen on the 3rd graticle line from the right and because CROs generally have a far lower noise level it is clearly visible.
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Yep, that's it on the 3rd graticle from the upper right. But if I didn't show the 2465 first would you know that's a glitch? My point is no, you wouldn't because it is lost in the noise. But I'll try single shot and see what happens...if I can figure this damn DSO out. This is my first DSO. Up until about 2 years ago I really knew nothing about them.
tggzzz:
--- Quote from: bd139 on June 01, 2018, 10:15:59 am ---Please note I only said the interface of the 54600. The actual implementation is a different matter. Things have changed a lot since 1994. As with all gear you're looking at the abstraction of a problem, not the problem itself so you need to understand what happens between one and the other.
Perhaps that is a good place for the analogue scope; when you need to intimately understand the difference between input and eyeballs without delving into sampling theorem. You can model an analogue scope as a low pass filter.
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There are, apparently, still as many "mere implementation" problems in low-end DSOs as there were 20 years ago.
The problem with the 54621 wasn't with the sampling theorem but with the "helpful" reconstrution done on the screen - it made the signal look far far worse than it was in reality. Turning off the reconstruction and the signal "magically improved" back to reality.
Of course all that shenanigans was (and is) buried deep in the menuing structure, so you would only find it if you knew what you were looking for. Beginners certainly wouldn't and (from experience) most engineers wouldn't either.
tggzzz:
--- Quote from: Specmaster on June 01, 2018, 11:18:55 am ---
Quote from: med6753 on Today at 11:56:39 am
Let me throw something out for discussion....
I have a fixed (5Khz, .5V p-p) Sine wave that I use as a quick check of scope function. It has a very small glitch on it. On the 2465 it shows up clearly, even with the scope set to 20Mhz bandwidth limit. (Pix 1). Now look at the Siglent DSO. (Pix 2) Do you see it? Yes, because you KNOW it's there. If you didn't know it you wouldn't have a clue. Granted, the Siglent is an entry level DSO with a somewhat limited sample rate. So maybe I'm comparing apples to oranges. But to me this gives the advantage to the CRO. Basically infinite sample rate to see glitches.
--- End quote ---
To be fair to the Siglent, I think it has picked up the glitch but it is lost in the background noise. I believe it can be seen on the 3rd graticle line from the right and because CROs generally have a far lower noise level it is clearly visible.
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Has the "quote" button stopped working in your browser? I can't be bothered to guess what you/med6573 are trying to say!
tggzzz:
--- Quote from: med6753 on June 01, 2018, 10:56:39 am ---I have a fixed (5Khz, .5V p-p) Sine wave that I use as a quick check of scope function. It has a very small glitch on it. On the 2465 it shows up clearly, even with the scope set to 20Mhz bandwidth limit. (Pix 1). Now look at the Siglent DSO. (Pix 2) Do you see it? Yes, because you KNOW it's there. If you didn't know it you wouldn't have a clue. Granted, the Siglent is an entry level DSO with a somewhat limited sample rate. So maybe I'm comparing apples to oranges. But to me this gives the advantage to the CRO. Basically infinite sample rate to see glitches.
--- End quote ---
Indeed, but the problem isn't the sampling rate per se.
The problems are centred around the screen's resolution and the DAC's resolution.
Try delving deep into the menuing system to see if there is something like "peak display" or "high resolution" mode.
N.B. that's a problem in itself for teachers: each DSO can do similar things in several different ways - but they all call it something different and it is located somewhere different in the menuing system.
bd139:
14 bit DAC does help on this. The very low resolution of the 8 bit scopes doesn’t give you a whole lot of dynamic range. Also when they apply averaging across several sweeps the DAC errors which are quite significant tend to mask smaller problems and glitches. One of the reasons I was looking at the higher end Keysight scopes actually.
Yes. I think it is important that teachers use the same kit across the board in these circumstances and actually understand it. When I was at university we just got some of the earlier Hp digital scopes and the students knew more about them and the limitations than the lab and tutors did. While people bitch about the longevity of the TI83 calculator, it’s the same problem.
Also it takes you months to learn and understand a piece of equipment before it becomes an extension of your mind and body. A lot of mistakes are made by not second guessing your assumptions about it up front.
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