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is it true, oscilloscope must reach at least 4x observed freq?
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Fungus:

--- Quote from: nctnico on September 13, 2022, 04:25:16 pm ---IMHO switching to linear interpolation isn't a good solution either. It is entirely possible that the signal you are looking at still meets Nyquist. The oscilloscope simply doesn't know that.

--- End quote ---

Yep.

The correct thing to do would be to put a (discreet) warning message on screen, not to try and "out-think" the user.

A user might know what they're doing or they might be trying to demonstrate/explore the limits of signal theory.
2N3055:

--- Quote from: switchabl on September 13, 2022, 03:32:42 pm ---If the scope has a 200 MHz front-end and the sampling rate is set to 10 kSa/s like in the example, it is not mathematically correct (unless the user makes sure the signal itself is < 5kHz BW or it does decimation). Keysight scopes e.g. switch to linear interpolation in such a scenario which I think is a sane default. At the very least it gives you an immediate visual cue that you are sub-sampling. I guess the only mathematically correct option would be dot mode?

EDIT: I am not sure if the Siglent behaviour is a Lecroy-ism or if this is actually more widespread. It seems strange to me.

--- End quote ---


On first image, scope is set for 500 us/div (7 ms total on screen) and 2kS/s. That is only 14 points on the screen, although scope shows memory setting of 14 kpoints.
On other images timebase and memory depth is show the same but sample rate is different?
How was variable sample rate with fixed memory length and same timebase achieved? SDS1104X-E has manual sample rate setting? I don't have it, so I'm asking for clarification and manual is not saying...

Sample screen shown are certainly not how scope would behave by default.
Scope is by default trying to keep sample rate as high as it can. Also that is smallest memory settings, only 14kpoints of 14Mpoints available, so 1000 times worse than possible.
Also it has both linear and sin(X)/X interpolation, and point mode that does no interpolation whatsoever. And would probably show this signal nicely.

I can demonstrate wrong signals with any scope if I deliberately set it wrong...

I understand that post was meant to demonstrate sampling, but this is not scope normal behaviour. Robert had to specifically set it to show these effects.
Fungus:

--- Quote from: 2N3055 on September 13, 2022, 04:35:56 pm ---On first image, scope is set for 500 us/div (7 ms total on screen) and 2kS/s. That is only 14 points on the screen, although scope shows memory setting of 14 kpoints.

--- End quote ---

Does that mean it could zoom out a bit?  >:D
2N3055:

--- Quote from: Fungus on September 13, 2022, 04:39:11 pm ---
--- Quote from: 2N3055 on September 13, 2022, 04:35:56 pm ---On first image, scope is set for 500 us/div (7 ms total on screen) and 2kS/s. That is only 14 points on the screen, although scope shows memory setting of 14 kpoints.

--- End quote ---

Does that mean it could zoom out a bit?  >:D

--- End quote ---

No it probably means he captured this at extremely long timebases like 1sec/div and then changed timebase back to 500us/div with stopped acquisition to provoke this effect. Like I said, definitely not how you would use the scope..
While i understand he did it to explain sampling, without explaining what was done, people here started discussing how scope is performing poorly. Any scope would do this even Keysights mentioned if driven this way..
switchabl:

--- Quote from: nctnico on September 13, 2022, 04:25:16 pm ---IMHO switching to linear interpolation isn't a good solution either. It is entirely possible that the signal you are looking at still meets Nyquist at a lower sampling rate. The oscilloscope simply doesn't know that. This is a typical case where the user needs to know what he/she is doing.

--- End quote ---

It is not ideal but I think it makes sense as a default. The oscillscope can't know, so it shouldn't guess but instead use the safest option. I am not saying you shouldn't be able to override it if you want to. If you have a large touch-screen, it might be nice to have a little warning symbol pop up that you can tap to change the setting or something. I don't know if anyone actually has that.

I mean if you actually forced a super low sample-rate manually, I guess you had it coming and are probably getting what you wanted. But if just did a single-shot at a long timebase, the sample-rate dropped automatically and then you zoom in, chances are you didn't want to see massive ringing that isn't actually there.


--- Quote from: 2N3055 on September 13, 2022, 04:53:54 pm ---While i understand he did it to explain sampling, without explaining what was done, people here started discussing how scope is performing poorly. Any scope would do this even Keysights mentioned if driven this way..

--- End quote ---

No, I realize what he did. I was just surprised it doesn't turn off sinc-interpolation when you do that (I am sure you could do that manually). With the Keysights this actually happens more often because they don't have a lot of sample memory, so I know they switch to linear interpolation and don't do this.
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