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Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)

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Mechatrommer:

--- Quote from: nctnico on October 27, 2023, 04:43:31 pm ---
--- Quote from: 2N3055 on October 27, 2023, 03:29:04 pm ---16 MHz square wave (with 30ps edges) will have harmonics into gigahertz range. Any of these harmonics above nyquist will alias.

--- End quote ---
In theory yes, but given the spacing between harmonics and their levels, the chance of harmonics aliasing back as a visual artifact are not very high as you have to consider the anti-aliasing filter in a DSO. Especially when dealing with a square wave which has mostly odd harmonics which are spaced at twice the fundamental frequency.

--- End quote ---
a perfect square wave is just that... from 0 suddenly 1 (or whatever Vmax it is, dont be pedantic), sampling rate is just that... sampling at certain dt interval. too slow sampling rate, the machine will miss the details during the step. plotting straight line point to point will just looks like trapezoid, or slowly risetime square, or at worst a bunch of triangles. add signal reconstruction/interpolation to it, the imperfect one, thats where the devil is where people misunderstand it  "aliasing", in fact it is just a digital post processing artifacts. maybe people got deluded due to they think in frequency domain, not in time domain, where the samples capturing happens. as long as "fundamentals" is not exceeding nyquist limit, there will be no aliasing. granted the displayed squares will be distorted here and there, but not aliased, maybe due to jittery square wave on circuit, or just that.... artifacts from imperfect interpolation. even if it is indeed an aliasing, if harmonics amplitudes are not too much, it could be unimportant in digital circuit, otherwise the dut square wave got problems, too much oscillations and all, in this case, to verify we must go up the DSO ladder to 10-100GSps class. on the subject of adding LP filter for the sake of antialiasing, the true signal's shape will get hidden in the process, this is what low end DSO is about, we wont know if the signal is good or oscillates in real life.fwiw.

wasedadoc:

--- Quote from: TimFox on October 27, 2023, 05:27:37 pm ---Of course, the output waveform of a "fast" square wave (e.g., the output of a good binary flip-flop) passed through an elementary low-pass filter (e.g., a simple one-pole R-C circuit) should not exhibit the Gibbs phenomenon on a high-bandwidth analog oscilloscope or a fast-enough sampling digital oscilloscope.

--- End quote ---
True, but a more complex LC filter with linear phase response can be physically constructed which will show the phenomenon.

Mechatrommer:

--- Quote from: wasedadoc on October 27, 2023, 05:33:58 pm ---
--- Quote from: TimFox on October 27, 2023, 05:27:37 pm ---Of course, the output waveform of a "fast" square wave (e.g., the output of a good binary flip-flop) passed through an elementary low-pass filter (e.g., a simple one-pole R-C circuit) should not exhibit the Gibbs phenomenon on a high-bandwidth analog oscilloscope or a fast-enough sampling digital oscilloscope.

--- End quote ---
True, but a more complex LC filter with linear phase response can be physically constructed which will show the phenomenon.

--- End quote ---
do siglent and rigol enough room at front end for that? or do they have that actually already? experienced people should already spotted that from tear down photos. and why the effect disappears on screen at max sampling rate? and reappears on much lower sampling rate?

mawyatt:

--- Quote from: TimFox on October 27, 2023, 05:27:37 pm ---Of course, the output waveform of a "fast" square wave (e.g., the output of a good binary flip-flop) passed through an elementary low-pass filter (e.g., a simple one-pole R-C circuit) should not exhibit the Gibbs phenomenon on a high-bandwidth analog oscilloscope or a fast-enough sampling digital oscilloscope.

--- End quote ---

Will throw this out for discussion, altho deviating from the DHO800, but seems lots of folks are involved, so maybe worthwhile.

Think it can be stated that ANY system described by continuous domain IIR Response of ANY kind, be that low pass, band pass, high pass, ANY combinations of poles and zeros, or whatever can NOT exhibit the Gibbs effect on an ideal squarewave.

This is stated without any proof (we left the DSP world long long ago at it's early beginnings).

Will be interesting to see what some of the True DSP gurus have to say!!!

BTW this is not to start/continue an argument, but to maybe add some clarity to the on going discussions.

Best,

mawyatt:

--- Quote from: wasedadoc on October 27, 2023, 05:33:58 pm ---
--- Quote from: TimFox on October 27, 2023, 05:27:37 pm ---Of course, the output waveform of a "fast" square wave (e.g., the output of a good binary flip-flop) passed through an elementary low-pass filter (e.g., a simple one-pole R-C circuit) should not exhibit the Gibbs phenomenon on a high-bandwidth analog oscilloscope or a fast-enough sampling digital oscilloscope.

--- End quote ---
True, but a more complex LC filter with linear phase response can be physically constructed which will show the phenomenon.

--- End quote ---

Please elaborate on such, see above.

Very interested in seeing if this is possible with an IIR system.

Best,

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