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Trying to display bandwith via Math on Siglent SDS2k+/2kHD/800X HD
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gf:

--- Quote from: Martin72 on March 20, 2024, 08:53:13 pm ---With your settings* I´ll get these results, depending on the timebase..

--- End quote ---

100ns and 50ns look basically OK, the others not.
[ In order to see more details, vertical scale adjustment would be nice, say 5dB/div and ~150db (?) reference level. ]

EDIT: When I look at the levels of the peaks, then they seem to suffer from the same problem as Performa01's plot.


--- Quote ---I could try it again with the fast pulse (approx. 400ps risetime, approx. 200khz) from the batronix demoboard, but I don't believe that this will change anything.

--- End quote ---

The lower the frequency, the denser the spacing of the teeth of the comb spectrum. But that needs more FFT points, too.
Do you mean a square wave, or a train of narrow impulses? (if the latter, what's the pulse width?)
Martin72:
Afaik its a square wave, will look at it tomorrow.

I wonder why I don't get the same result with the same settings.
However, there are 2 things that are different with my attempts than with Performa01.
First of all the signal, which comes from the 40ps/10Mhz bodnar pulser.
Secondly, and you can definitely think this through, the scope model.
Performa01 has a "real" 824X HD, I have an 804X HD, which was turned into an 824 by a "trick".
That shouldn't really matter.
Actually.
gf:
Look at Performa01's square wave frequency -> 20x lower.

EDIT: When I look at the levels of your peaks, then they seem to suffer from the same problem as Performa01's plot.
Martin72:
Today with a much lower frequency (197kHz, the demo board can send different frequencies to the fast schmitt-trigger), this looks quite different - apparently it does not seem to "work" with "high" frequencies/smaller time bases.



The following is interesting:
I wanted to make the other signals visible for the screenshot, which were hidden by the marker window and turned the timebase to the left outside zero, nothing else changed...

Performa01:

--- Quote from: Martin72 on March 21, 2024, 08:18:45 pm ---TThe following is interesting:
I wanted to make the other signals visible for the screenshot, which were hidden by the marker window and turned the timebase to the left outside zero, nothing else changed...

--- End quote ---
What you see is probably the effect of windowing.

I hate it to say, but for this kind of special measuement, the Rectangle window is the right choice. Try it and the position-dependance should be gone.

In general you need to keep in mind that the FFT doesn't process the entire screen width. In your example it is 8192 out of 10000 points, hence ~82%. But there are many scenarios where it's just 51.2%.

Therefore, whenever you want to analyze a single event like e.g. a transition, be aware that the FFT might be completely blind for the right half of the screen. As a consequence, do the following:

1. place the event between 20-30% of the screen width.
2. to get identical resuts independent of the position, use the Rectangle window.

EDIT: to get a smooth line, try a faster time base, like 200 ns/div.
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