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| Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding) |
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| nctnico:
--- 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. |
| TimFox:
Also, the odd harmonics in a symmetric square wave fall off as 1/n in amplitude (voltage), so they eventually fall below the noise in the system. |
| 2N3055:
--- 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 --- Problem is that they will all get compressed in same energy "blob" and will add up. As they get folded down in frequency domain. Equivalent in time domain is, if you can imagine a small differential triangle formed by fast edge and edge of risetime of scope. Area of that triangle gets folded on top of the rising edge with a overshoot pulse that has period of 1/risetime.... If energy enters the ADC it will show somewhere.. |
| mawyatt:
One might be temped to create the "Gibbs ears" by means of a fast edge squarewave, then low pass filtering the waveform to reduce/eliminate higher order harmonics to create the "ears" as shown in earlier computer generated visualizations of adding harmonics to create a squarewave approximation. If we can agree the analog scope does not "self-inflict" the Gibbs effect, then view on the analog scope and DSO at various settings. This exercise is left to the reader!!! Best, |
| TimFox:
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. |
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