Products > Test Equipment
is it true, oscilloscope must reach at least 4x observed freq?
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nctnico:

--- Quote from: The Electrician on September 13, 2022, 09:10:22 pm ---The Agilent DSO5054 does not have sin x/x interpolation.

--- End quote ---
It has. It says in the datasheet but not in the manual. It switches automatically between linear and sin x/x mode when linear interpolation is not going to work to produce a waveform that resembles the signal at the probe tip. BTW: I used to own an MSO7000A which is basically the same hardware platform as your DSO5054 so I have some hands-on experience with these models.
Someone:

--- Quote from: BillyO on September 13, 2022, 01:57:39 pm ---I wonder if the OPs question is well answered here?  I would be good if he could return and try to answer the outstanding issues with his first question.

He may not know the difference between BW and SR.
--- End quote ---
Yes, the thread title and opening post are vague enough that the usual suspects are making some big argument out of thin air:
"perfect band limited signal in infinite time capture can be perfect"
vs
"real world signal with real world oscilloscope does not work that way"

Even "sin(x)/x" is not a fully specified filter, as many (most?) implementations are truncating/windowing the extents.

2.5x is a magic number associated with "sharp" yet still practical antialiasing filters that has been widely misunderstood. It all depends on the input (antialiasing) and output (reconstruction) filters in the specific implementation/use-case....   remembering that most scopes dont have those sharp filters and use gaussian/bessel filters or something close to that for which the "rule of thumb" is 4x or 5x sample rate to bandwidth (attached below since Keysight have started personalinfo-walling all their documents).
Someone:

--- Quote from: The Electrician on September 13, 2022, 03:54:41 am ---
--- Quote from: Someone on September 13, 2022, 03:15:08 am ---
--- Quote from: Fungus on September 13, 2022, 02:59:00 am ---
--- Quote from: Someone on September 13, 2022, 02:33:56 am ---and to be more clear, 2.5x is a figure that provides some non-specific limit to that effect under typical band limiting/anitaliasing/reconstruction and does not ensure errors are avoided.
--- End quote ---
I don't think you can get major distortions at 2.5x with a band-limited signal, but that's still an "if". How would you know?
--- End quote ---
Because I do this stuff for a living... you can keep putting out non-specific/vague figures but they are just that, something which might be true in some non-specified situation.

There are easily found examples of 2.5x being inadequate.

--- End quote ---

Please give us some examples where 2.5x is inadequate.  :)
--- End quote ---
Please provide the band limited signal source that has zero energy above the measurement frequency and I'm sure it would be difficult. But real world signals are not perfectly band limited....
This can go around and around as many times as you like. It's an entirely different argument which has derailed the thread immediately into complete nonsense.

What are the errors that you are trying to avoid/limit? what is the accuracy of the measurement (and what is the measurement) you are trying to make? What are the antialiasing and reconstruction filters being used?

With that sort of information you could start to answer what the actual bandwidth to sample rate is required. Its not some simple every situation works at 2.5x or everything is fine once you go past 5x, it depends on the specifics.
Fungus:

--- Quote from: Someone on September 14, 2022, 01:11:17 am ---
--- Quote from: The Electrician on September 13, 2022, 03:54:41 am ---Please give us some examples where 2.5x is inadequate.  :)
--- End quote ---
Please provide the band limited signal source that has zero energy above the measurement frequency and I'm sure it would be difficult.

--- End quote ---

I'll take that as a no.


--- Quote from: Someone on September 14, 2022, 01:11:17 am ---Its not some simple every situation works at 2.5x or everything is fine

--- End quote ---

No, but in the real world there are some very simple cases that fail below 2.5x. 2.5x is a good starting point.

FWIW: I was observing the AM modulation effect below 2.5x using a hardware reconstruction filter.
BillyO:

--- Quote from: Someone on September 14, 2022, 01:04:26 am ---
--- Quote from: BillyO on September 13, 2022, 01:57:39 pm ---I wonder if the OPs question is well answered here?  I would be good if he could return and try to answer the outstanding issues with his first question.

He may not know the difference between BW and SR.
--- End quote ---
Yes, the thread title and opening post are vague enough that the usual suspects are making some big argument out of thin air:
"perfect band limited signal in infinite time capture can be perfect"
vs
"real world signal with real world oscilloscope does not work that way"

Even "sin(x)/x" is not a fully specified filter, as many (most?) implementations are truncating/windowing the extents.

--- End quote ---

That's what I see too.  If the OP is a newbie to all this and does not even know the difference between BW and SR, how is this discussion gong to help them?  More than likely it will send them away.  There seems to be a certain amount of elitism on this site, and I can understand that, but if you are not willing to address a new members basic questions with patience then maybe go off to the the "I know everything better than you" forum and hash out the nuances there.

We need more people in this field rather than out of it and contemplating f'd up concepts of ignorance such as flat earth and anti-science.  The experts are the ones we need supporting STEM and helping people elevate themselves and not alienating those taking baby steps.  This is something I struggle with too, so..

Sorry for the rant.
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