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

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EEVblog:
The Tek x2.5 reference with the math explained for Sinx/x:
https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-sampling

nctnico:

--- Quote from: EEVblog on September 14, 2022, 12:02:37 pm ---
--- Quote from: Someone on September 14, 2022, 10:21:23 am ---Cool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise with unspecified criteria. What is actually important? amplitude accuracy? relative phase? waveform shape?

--- End quote ---

Remember that the input wave shape gets changed by the input bandwidth response of the scope front end. Different scopes and models have different input antialiasing filters and responses.
This is why the shape of the input filters matters in these dicsussions. Gaussian response is usually assumed unless otherwise specificed.

--- End quote ---
Gaussian response is long gone. The typical anti-aliasing filters are much steeper. If you want a Gaussian response, then you'll need to set the bandwidth limiter. IMHO this is one of the reasons higher end scopes have multiple bandwidth settings so the user can choose to have maximum bandwidth or a slow (first order) roll-off.

tggzzz:

--- Quote from: nctnico on September 14, 2022, 02:40:01 pm ---
--- Quote from: EEVblog on September 14, 2022, 12:02:37 pm ---
--- Quote from: Someone on September 14, 2022, 10:21:23 am ---Cool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise with unspecified criteria. What is actually important? amplitude accuracy? relative phase? waveform shape?

--- End quote ---

Remember that the input wave shape gets changed by the input bandwidth response of the scope front end. Different scopes and models have different input antialiasing filters and responses.
This is why the shape of the input filters matters in these dicsussions. Gaussian response is usually assumed unless otherwise specificed.

--- End quote ---
Gaussian response is long gone. The typical anti-aliasing filters are much steeper. If you want a Gaussian response, then you'll need to set the bandwidth limiter. IMHO this is one of the reasons higher end scopes have multiple bandwidth settings so the user can choose to have maximum bandwidth or a slow (first order) roll-off.

--- End quote ---

Personally I like having the option to turn off interpolation and simply see dots representing samples.

The Electrician:

--- Quote from: nctnico on September 14, 2022, 09:57:24 am ---
--- Quote from: The Electrician on September 14, 2022, 04:27:28 am ---
--- Quote from: nctnico on September 13, 2022, 10:25:21 pm ---
--- 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.

--- End quote ---

Post a link to said datasheet, please.

--- End quote ---
I'm pretty sure you can find that yourself. And yes, sin x/x is supported on the DSO5054 so either you found the 'sweet' spot where it doesn't work or your settings are incompatible. I don't recall my MSO7104A ever showing a signal like yours (I checked some of my old screenshots to be sure).

--- End quote ---

The only data sheet I can find is: https://www.keysight.com/us/en/assets/7018-08449/data-sheets/5989-6110.pdf

It doesn't say "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."  Why is there no mention of an exception where this doesn't work; and I certainly wouldn't call a failure to perform as claimed a "sweet spot".

The data sheet I found says on page 13: "Sinx/x interpolation (single shot BW = sample rate/4 or bandwidth of oscilloscope, whichever is less) with vectors on and in real-time mode".  It doesn't say a word about linear interpolation, which I've demonstrated that the actual oscilloscope can do.  If the scope can do both, why don't they mention both?  Perhaps the mention of "Sinx/x interpolation" was something they intended to do when the data sheet was written, but they chose not to include in the actual hardware when it came time meet a deadline and get something to market.

If you don't post a link to a data sheet that says "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.", your credibility is zero.  You're claiming something you haven't backed up.

Then there is the user manual: https://www.keysight.com/us/en/assets/9018-01896/user-manuals/9018-01896.pdf

Which says on page 187: "When enabled, Vectors draws a line between consecutive waveform data points."  No mention anywhere of sin x/x.

And there is the performance of an actual DSO5054 which I show in Reply #80

Fungus:

--- Quote from: tggzzz on September 14, 2022, 02:59:07 pm ---Personally I like having the option to turn off interpolation and simply see dots representing samples.

--- End quote ---

Some people even turn the screen persistence way up then enable dot mode and pretend they can see the real signal building up on screen.  :)

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