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Siglent SDS2000X Plus Bandwidth & Aliasing Application Note

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Performa01:
I had to replace the document one more time, see changelog below:


--- Quote from: Performa01 on January 05, 2022, 12:24:02 pm ---EDIT2 20220107: Document revised again:

* Misleading "Safe Signals" and "Conclusion" sections corrected - in half channel mode, the SDS2000X Plus can handle at least the 200 ps rise-time signals without artifacts.

* Quoted Conclusions in this posting adapted according to the revised document.

* Artifacts can be Gibbs phenomenon and/or aliasing, both caused by high frequency portions of the input signal that vastly exceed the Nyquist bandwidth of the DSO.

* Lower bandwidth models most likely do not have a digital filter.

* Version number added to the title page (V1.02).

--- End quote ---

tv84:

--- Quote from: Performa01 on January 05, 2022, 09:00:01 pm ---
--- Quote from: tautech on January 05, 2022, 08:54:56 pm ---Yes, AFAIK SDS2354X+ with the 500 MHz BW SDS2000XP-4BW05 option applied should display SDS2504X+ in the system menu.

--- End quote ---
Do these scopes display the 500 MHz option in the options menu?

--- End quote ---

As I've described in the scope's thread awhile ago:

It seems there was an initial batch of SDS2000X+ that contained an "incorrect" .cfg file (maybe on purpose, at the time...  ::) ).

I find it no surprise that, as some other members here, Performa01 has got one of those. When this was discovered, some members corrected the thing by replacing the .cfg file with the newer one (or simply upgrading the FW with the recent .cfg file).

So, although Siglent doesn't show in its site the model SDS2504X+, the fact is Siglent from long ago started including that designation in the scope's config files (which is what most of the EEVBLOG users see in their system menu). Nitpicking: for me it would be  clearer/simpler if the document stated SDS2504X+ instead of SDS2354X+ with 500 Mhz option.

Attached are the 2 .cfg files (one from the old batch and the newer one) and their respective parsing so that you see their content's little difference.

BTW, nice document!  :clap:

hpw:
@Performa01

Lets do an "HpW G/s Equation" to calculate the required DSO BW so handy for the sales team :-DD

DSO BW as : 20..50 / (5 x of fastest rise of a single slope 10..90%) [1/s]

 this will shows more IMHO or less the findings within the document...

as 500ps = 20 / (5 * 500ps) = 8 G/s

as 1ns    = 20 / (5 * 1ns) = 4 G/s

as 50ps = 20 / (5 * 50ps) = 80 G/s  :-DD

For the probes to use, may 1/2 of the DSO BW

About why 5 times:

1. the fastest slope

2. repeat 1 as second slope

3. & 4 as the top & bottom level time equal to 1 & 2

5. to compensate the other 0..10% and 90..100% time

so to get more or less 5 times the fastest slope as our signal frequency


So lets  :box:

Performa01:
Yes, scope manufacturers tend to recommend the use of an instrument that has only one tenth of the signal rise time.

Of course they do, because it will increase their income, as engineers will approach their bosses and say: "look, what Tek specifies here in this application note! So we need to buy a 100 GHz highest end DSO in order to properly look at our signals with 50 ps rise time".

This could easily backfire though, because bosses might reply: "ooops, do they even have such a beast? If not, we need to look elsewhere! Can you bring the LeCroy and Keysight catalogs?"  :-DD

Of course, it is the same thing everywhere. Buy a measurement solution that is ten times faster and/or ten times more accurate than what your DUT can deliver, and you should be able to trust all your measurements blindingly ... if you know what you're doing and how to avoid systematic errors due to probing or environmental effects, that is.

Yet there are situations, where we need to leave that comfort zone and make do with an instrument, that is only little faster and/or little more accurate than our DUT, or not even that – it is also an art to determine whether an instrument is up to the task. In any case we need to use a calculator (or even spreadsheet) in such cases.

We can calculate the rise time of e.g. the 500 ps pulse that is measured as 540 ps on the SDS6204 by taking the 230 ps rise time specification of the scope and do the math: the result is roundabout 490 ps. Close enough?

Even the result for the 200 ps signal, which is already faster than the scope itself, is still pretty close.

And with regard to the bandwidth, I guess no one can argue against the practice to check the spectrum and then decide that a scope covering all signals down to a certain level is sufficient for a certain application. Of course, if we want the maximum fidelity, we need to cover the dynamic range defined by the ENOB, which might be more than 45 dB on an excellent 8-bit system - and not that much more even in 12-bit high bandwidth systems.

Performa01:
Only now I have found a supplemented version of my document, which for some reason didn't make it to the public back in January 2022. I have now updated the initial posting and the attachment.

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