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

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Mechatrommer:
right! playing with sim tonight to design 3rd order elliptic filter (analog/physical) filter (about 60MHz cutoff)... no gibbs effect on sim. so there must be a special one to build a physical filter that can cause "gibbs like" effect. fwiw..

wasedadoc:

--- Quote from: Mechatrommer on November 10, 2023, 07:08:21 pm ---right! playing with sim tonight to design 3rd order elliptic filter (analog/physical) filter (about 60MHz cutoff)... no gibbs effect on sim. so there must be a special one to build a physical filter that can cause "gibbs like" effect. fwiw..

--- End quote ---
Can you plot the phase versus frequency?

gf:

--- Quote from: Mechatrommer on November 10, 2023, 07:08:21 pm ---right! playing with sim tonight to design 3rd order elliptic filter (analog/physical) filter (about 60MHz cutoff)... no gibbs effect on sim. so there must be a special one to build a physical filter that can cause "gibbs like" effect. fwiw..

--- End quote ---

I do see overshoot/ringing caused by attenuation of higher harmonics. Why do you classify it as "no Gibbs"? Symmetry is not the criterion. Gibbs ringing introduced by a filter when applied to an ideal square wave can be either symmetric or asymmetric, depending on the filter's phase response.

The only difference between the filters that produce the step responses in figure1 and figure2 is their phase response. Both filters have the same magnitude response (figure3), i.e. attenuation of harmonics is the same. Figure1 is minimum phase and figure2 is linear phase. A causal, stable IIR cannot have linear phase, though.

nctnico:
I still don't agree with attributing ringing due to low pass filtering a non-perfect square (IOW: a square wave as it exists in the real world) wave to Gibbs effect. The way I read the mathematical description is that Gibbs ears occur because you can't create a perfect square wave from adding sine waves (a Fourier series) which in turn is because in the mathematical world a square wave is not a continuous function. You could call Gibbs ears a 'residu'. However, a discontinuous square wave can exist in the digital world after sampling a square wave or other waveforms with  steep edges in relation to the samplerate. When the sampled signal is brought back to the real world using sin x/x reconstruction (for example), you'll get Gibbs ears as a displayed artefact.

Mechatrommer:

--- Quote from: gf on November 10, 2023, 11:45:22 pm ---
--- Quote from: Mechatrommer on November 10, 2023, 07:08:21 pm ---right! playing with sim tonight to design 3rd order elliptic filter (analog/physical) filter (about 60MHz cutoff)... no gibbs effect on sim. so there must be a special one to build a physical filter that can cause "gibbs like" effect. fwiw..

--- End quote ---

I do see overshoot/ringing caused by attenuation of higher harmonics. Why do you classify it as "no Gibbs"? Symmetry is not the criterion. Gibbs ringing introduced by a filter when applied to an ideal square wave can be either symmetric or asymmetric, depending on the filter's phase response.

The only difference between the filters that produce the step responses in figure1 and figure2 is their phase response. Both filters have the same magnitude response (figure3), i.e. attenuation of harmonics is the same. Figure1 is minimum phase and figure2 is linear phase. A causal, stable IIR cannot have linear phase, though.

--- End quote ---
i only focus on pre ringing part, the unrealistic one. the post ringing overshoots are normal, so i will not question that part. Not just in electrical signal, but also normal in physical control system n mechanical. Your car doesnt get bumpy before hitting the bump, otherwise thats paranormal.

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