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Siglent SDS2000X Plus Bandwidth & Aliasing Application Note

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Performa01:

--- Quote from: nctnico on January 05, 2022, 04:07:23 pm ---On page 16 you seem to show the difference between sin x/x reconstruction (top) and dot mode (bottom) to show the effect of aliasing. But IMHO this isn't correct. If you have signal distortion due to aliasing, then the dot mode would also show the same signal. The effect you are showing in the top picture looks like the Gibbs effect to me.

--- End quote ---
Yes, you are right. Aliasing should be barely visible, because the strongest signal components above Nyquist around 540 MHz should already be 33 dB down.


--- Quote from: nctnico on January 05, 2022, 04:07:23 pm ---Edit: it also seems to me that the sin x/x is broken from the picture on page 25 and/or the triggering is wrong. A 200MHz square wave should show as a sine-ish wave. At 1Gs/s the limit for sin x/x to stop working is over 400MHz (1Gs/s / 2.5) and 200MHz is far below that limit. The effect shown has nothing to do with aliasing or rise time!

--- End quote ---
I’ve only touched it in my article, maybe I should have been more clear. The DSO shows a signal that cannot exist in the real world. This is because with this particular signal that has a strong -10 dBc spectral component at 600 MHz, the aliasing is so strong that also the internal trigger signal path is affected, as can be clearly seen in the vector display mode screenshot on page 25. Consequently, the trigger point itself gets jittery and the individual samples are misaligned. In other words, the samples have lost their common reference on the time axis.

So yes, triggering is “wrong” – and this can happen whenever the input bandwidth is too wide for the sample rate and the input signal has transition times that generate strong harmonics above Nyquist, even when the fundamental frequency is well within the nominal bandwidth.

Performa01:

--- Quote from: bdunham7 on January 05, 2022, 06:24:47 pm ---
--- Quote from: Performa01 on January 05, 2022, 05:59:10 pm ---There is no such thing as an SDS2504X Plus:

--- End quote ---

What does your scope identify itself as in the system menu? 

--- End quote ---
Unsurprisingly it says: "SDS2354X Plus". The model name in the screenshots also comes from the scope itself.

Performa01:

--- Quote from: bdunham7 on January 05, 2022, 06:53:21 pm ---And to add to 2N3055's second point, I'd like to see the signal on p26 with the settings the same except the trigger slope set to falling or alternate.

--- End quote ---
Clever thinking! ;)

knudch:

--- Quote from: Performa01 on January 05, 2022, 07:08:11 pm ---
--- Quote from: bdunham7 on January 05, 2022, 06:24:47 pm ---
--- Quote from: Performa01 on January 05, 2022, 05:59:10 pm ---There is no such thing as an SDS2504X Plus:

--- End quote ---

What does your scope identify itself as in the system menu? 

--- End quote ---
Unsurprisingly it says: "SDS2354X Plus". The model name in the screenshots also comes from the scope itself.

--- End quote ---

My SDS2104X+ with "features" say SDS2504X plus in system menu

Performa01:

--- Quote from: knudch on January 05, 2022, 08:18:37 pm ---My SDS2104X+ with "features" say SDS2504X plus in system menu

--- End quote ---
Well, quite obviously mine is not an SDS2104X Plus. it says "SDS2354 X Plus" on the front cover.
An SDS2504X might exist in China, but in the rest of the world it is just a hint that the user might be an EEVblog member ;)

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