Products > Test Equipment
Sub: Rigol's DHO800 Oscilloscope (Gibbs Effect & Aliasing Misunderstanding)
bdunham7:
--- Quote from: Mechatrommer on November 11, 2023, 03:26:09 am ---Your car doesnt get bumpy before hitting the bump, otherwise thats paranormal.
--- End quote ---
As I pointed out earlier, the causality argument is a trap, a red herring, a.. well, whatever. There is no theoretical causality issue if the display on the screen doesn't appear until long (relatively speaking) until the event is over. In the case of just the analog filter, this means there could be 'pre-ringing' to the extent that the portion of the signal that represents the bulk of the step is delayed in the filter while other components get through faster. This can be demonstrated with with real components, although I don't have an all-analog example that would give you nice pronounced and symmetrical Gibbs ears.
gf:
--- Quote from: Mechatrommer on November 11, 2023, 03:26:09 am ---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.
--- End quote ---
Minimum phase filters don't pre-shoot. The traditional all-pole filters like Bessel, Butterworth, Chebycheff type1 are minimum phase, IIRC. But not every IIR (or analog) filter is mininimum phase. If you linearize the phase response of a (say) Butterworth filter with additional allpass filters you can even approximate a linear phase response in the analog domain.
See also https://cdn.teledynelecroy.com/files/whitepapers/group_delay-designcon2006.pdf
iMo:
As it has been discussed earlier, the pre-ringing has nothing to do with hardware.
The scope samples a couple of samples, and in order to draw a "continuous solid yellow signal" on the LCD display it uses sinc interpolation in math (it creates additional LCD points based on the "sinc" function). You may see it on previous picture in the replay #220 (the Bodnar 312.5MSa/s) where there were only 20 samples taken (3ns from sample to sample, 60ns wide screen, therefore 20 points to interpolate) and the graphical function based on the "sinc" interpolated those 20points into a "continuous picture" you see on the screen. Therefore you see the "pre-ringing". With more points (samples) taken you do not see the pre-ringing.
You cannot get "pre-ringing" when the electrical signal has not entered the filter.
Thus the "sinc" here has nothing to do with a ringing signal, "filtering in hw" or "dsp". It is a graphical interpolation method/tool they have chosen to create continuous visual lines on the LCD screen. You may interpolate number of bananas exported each month within a year and you will get the same pre-ringing and post-ringing with the sinc interpolator when having 12 numbers..
PS: below the picture, the display even says "16points" taken in total, 3ns apart over the entire screen width (info on top)..
Look at the nodes where the wave changes its phase - aprox six times (20ns/3), 3ns apart, mind the time scale below - there are the "6 real physical points sampled", all yellow between them has been "created artificially by the graphical Sinc interpolation".
The ringing before and after the rising edge is made by the sinc graphical interpolation such you get a solid line instead of only 16 points placed on the LCD screen..
Mechatrommer:
--- Quote from: wasedadoc on November 10, 2023, 10:38:40 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 ---
Can you plot the phase versus frequency?
--- End quote ---
i just got home, see attached. not sure what it tells. but anything larger than 40MHz is more than 180deg phase?
bdunham7:
Here is a rather lame attempt to throw something together with assorted junk within reach on my messy bench. First screenshot is a square wave from an FY6900 through a Mini Circuits BLP70+. Second is the same except a small modification was made. Obviously I'd have to tweak my modification to get nicer looking symmetric results.
Edit: I did tweak the modification a bit to look 'nicer', but I obviously still don't have a symmetrical Gibbs ears appearance. Note that even 'real' Gibbs ears are not always perfectly symmetric.
https://www.minicircuits.com/pdfs/BLP-70+.pdf
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