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

--- Quote from: Mechatrommer on November 03, 2023, 01:09:55 pm ---, ie capture at 4GSa/s. how amazing people can miss it! ;D


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As you can see not everyone...
Mechatrommer:

--- Quote from: rf-loop on November 03, 2023, 01:27:34 pm ---
--- Quote from: Mechatrommer on November 03, 2023, 01:09:55 pm ---, ie capture at 4GSa/s. how amazing most people can miss it! ;D

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As you can see not everyone...
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there corrected for me. i know you in arena that keep rf work again and again ;) usually people less talk because doing more work... cheers.
nctnico:

--- Quote from: Mechatrommer on November 03, 2023, 01:09:55 pm ---
--- Quote from: gf on November 03, 2023, 11:07:40 am ---
--- Quote from: Mechatrommer on November 02, 2023, 05:32:00 am ---
--- Quote from: Martin72 on November 01, 2023, 10:43:10 pm ---Why is the bandwidth highest at 2GSa/s....

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i guess because it produces cleaner less jittery Gibbs (Sinc interpolation) at 2GSa/s (200pts/screen 500ps/pts)? what surprises us is why the properly bw limited scope can still produce gibbs artifact at 10x sampling rate ::)

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Keep in mind that any filter in the signal path with a sharp enough cut-off can introduce overshoot/ringing near edges, and if the filter happens to be a linear phase filter, then this ringing is symmetric (pre + post). This is not limited to sinc reconstruction filters, and it is not limited to undersampled signals.

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show me in any rigol DSO, or any other "better" competitive DSO from teardown photos that have this "sharp enough cut-off can introduce overshoot/ringing near edges". one member here proved that filter exists, but not for DSO. even if it is, it will not fit from teardown photos evident, we dont see fancy inductors capacitors 3x 5x parellel networks over there.

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If you look carefully at Dave's teardowns you'll see quite a few scopes have LC filters in front of the ADC to previous aliasing. However, these filters are designed so their phase response doesn't cause (excessive) ringing. For obvious reasons as the designer of a DSO wants to introduce as little 'distortion' to the visible signal as possible.
Martin72:

--- Quote from: rf-loop on November 03, 2023, 08:04:36 am ---Think about it.

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I'll read through this first before I start thinking(link to a pdf):

https://cdn.teledynelecroy.com/files/whitepapers/wp_interpolation_102203.pdf

Maybe that will help me think. ;)
gf:

--- Quote from: Martin72 on November 04, 2023, 12:57:37 pm ---https://cdn.teledynelecroy.com/files/whitepapers/wp_interpolation_102203.pdf

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The Simple Test described in this paper was also my reconstruction validity criterion when I wrote


--- Quote ---And since the trace is pretty clean, I'm quite confident that it doesn't suffer from aliasing (at least not significantly), i.e. very likely it was properly bandwidth limited before sampling.

OTOH, I'm pretty confident, that the signal in Bodnar_Quad.png (1GSa/s) was not properly bandwidth limited before sampling and that the apparent (let me call it) "reconstruction jitter" is caused by aliasing.

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However, there is admittedly a catch: If the sampled signal happens to be filtered (e.g. if a bandwidth limit is applied), then the check described in the paper indicates whether the filtered signal was reconstructed properly. It does not indicate whether a decent reconstruction would have been possible from the unfiltered samples.



--- Quote from: Mechatrommer on November 03, 2023, 01:09:55 pm ---
--- Quote from: gf on November 03, 2023, 11:07:40 am ---In Bodnar_Dual.png (2GSa/s) I do not think that the pre- and post-ringing comes from an interpolation filter.

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nope! until evident provided. i've provided a few, its your turn... here's the challenge below if you miss it (on any brand scope thats capable both Sinc ON and vector/dot (Sinc OFF) mode. its like nobody understand this thread and i'm just talking to the air :palm: in fact the evident that there is no pre-ringing is just right above the 2GSa/s image, ie capture at 4GSa/s. how amazing people can miss it! ;D

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Well, let's assume it comes from the interpolation filter. Why would you design an interpolation filter that cuts off 50% of the recoverable bandwidth? Sure, in practice you have to sacrifice a little, but why so much? Maybe I am missing some information to understand the intention :-// As a result, the interpolation filter does not only reconstruct, but additionally it acts as a bandwidth limiter. And the latter, of course, can still introduce ringing, even if the reconstruction alone at (almost) full bandwidth would possibly not do that.

EDIT: Of course, we do not know if we are dealing with two separate filters here (one for bandwidth limiting, and a second for reconstruction), or a single filter that combines both. At the end, we can only observe the combined effect.
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