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

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The Electrician:

--- 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.

Here's what I get with a sine wave at the probe tip.  The scope didn't live up to the data sheet's promise: "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."

Fungus:

--- Quote from: EEVblog on September 14, 2022, 04:04:04 am ---Technically not true, but a good rule of thumb. It depends on the front end bandwidth filter shape and the interpolation used. IIRC there is a Tek paper somewhere that goes through the math for x2.4 is the minimum or something for a gausian shape front end.

--- End quote ---

I searched the Tek web site and found a couple of places where it says "2.5x":

eg. https://www.tek.com/en/documents/application-note/real-time-versus-equivalent-time-sampling

(see appendix for math)

This one says:


--- Code: ---Oscilloscope Sample Rate > 2.5x highest frequency component of signal (For sin(x)/x interpolation)

Oscilloscope Sample Rate > 10x highest frequency component of signal (For linear interpolation)

--- End code ---

So that explains the "x10" rule that the old fogies occasionally mutter around here when they see Rigols/Siglents - they were using 'scopes with linear interpolation(!)  :)

gf:

--- Quote from: The Electrician on September 14, 2022, 04:27:28 am ---... "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."

--- End quote ---

What was the timebase and sample rate for the screenshot?

Maybe the automatic interpolation is only available at a couple of fastest timebase settings, where the sample rate is already at maximum, so that the resolution in samples/div cannot be increased any more by increasing the sample rate.

(That's at least what my cheap Chinese scope does.)

wasedadoc:

--- Quote from: Someone on September 14, 2022, 02:07:51 am ---2.5x is a massive gross simplification that looks ok in most cases for a single frequency sine wave.

--- End quote ---
If the specific combination of sampling rate and reconstruction filter gives an acceptable result on that single frequency sine wave it will also give an acceptable result on any lower frequency single sine wave.  Then by the superposition principle it will also give an acceptable result on any combination of such sine waves.  That is any waveform which does not contain frequencies above that sampling rate divided by 2.5.

The 2x is the theoretical limit for perfect reconstruction of the original. As the sampling rate is decreased down towards 2x the reconstruction filter requirements for perfect reconstruction become more and more demanding. In practice a sampling rate of 2.5x is a good compromise between practically realisable filter complexity and accuracy of reconstruction.

The Nyquist limit (half sampling frequency) applies when the input signal can contain any frequencies up to that limit.  There are situations when the input signal spectrum is not continuous. For example has "gaps" such as monochrome TV signals where the energy is largely confined around integer multiples of the TV line frequency.  (Those gaps were used by the NTSC composite colour system by arranging that most of the energy of the modulated colour subcarrier is confined around n+0.5 multiples of the TV line frequency.)  In the case of non-continuous spectrum signals it is quite possible to employ sub-Nyquist sampling with the alias components folding back into the vacant areas.  And it is possible to unfold them later and reconstruct the original signal. 

nctnico:

--- 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).

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