Also can see that without Sinc there is not so much wobbling in corners and edges upside and downside from trig level. Also this is normal ok. This is one reason why it is good that oscilloscope can always turn to true sample dots without any interpolation (poormans partially emulated RIS mode what is good for some kind of waveforms where Sinc is not best and vice versa.) (of course separate trigger engine interpolation is working if or when it exists independent of displayed wfm interpolation) endependent of what example Keysight say or advertise or produce.
The wobbling (1) is because even with an input signal completely within the Nyquist bandwidth, the non-linearity in the digitizer mixes the signal and sample frequencies producing mixing products above the Nyquist frequency and then sin(x)/x reconstruction fails because essentially there are multiple solutions. Just the aliasing of the input signal by itself will not do that (2) although I think it would screw up digital triggering. It can be thought of as looking as a lightly modulated AM signal.
This is why I much prefer ETS which raises the sampling rate so high that the mixing produces no aliasing and as a bonus, there is none of that pre-shoot and post-shoot nonsense. Averaging helps if you have nothing better but not as much as ETS.
Your later example on the Siglent shows it better and it is exactly what I see on ancient DSOs with sin(x)/x interpolation and no ETS. HP/Agilent/Keysight likes to make fun of Tektronix's interleaved digitizers which tend to suffer more from this due to poorer linearity. Even Rigol seems pretty good in this respect except for that 100 to 200 MHz overload problem in the analog signal conditioning.
(1) I've been calling this wobulation. If you see it, then your have exceeded the capabilities of your DSO.
Yes but ETS is not real time scope at all. ETS can do even with 1Hz sampling speed for reconstruct 1GHz signal... if we look only alone fully repetitive signals... (who need scope only for these) ETS is ok.
When we need real time scope what can solve signals with full BW using one shot... we need also accept some interpolation methods produced not so nice things (exept if we use very high samplerate ADC vs BW what do not need interpolation at all or just tiny vectors but who want spend 10Gsa ADC and huge memory to 50M scope.. ). Btw, I have not seen so much corners wobbling if signal really meet Sinc rules. But then we need also rtemember that Sinc in usual scope can not go up to fNyquist. Not at all. Usually need stay under 0.8*fNyq for minimize sinewave amplitudce wobbling and other waveforms harmonics what ADC can see need also keep least under fNyq (of course amount they affect depends harmonics levels.) if not we see edges corners wobbling. It is one displayed form of aliasing. As we know oscilloscopes Sinc can not be ideal... no one can wait until this calculus is ready. Because samples position related to signal is random then example with square wave etc sample position may be optimal or less optimal. Averaging sequential acquisitions of course helps but.. anly for long time continuous repetitive waveforms what can solve using nearly what ever. Sinc interpolation is filter and it is not perfect Sinc even when it draw via true sample points. If we separate one harmonic what is near fNyq this harmonic start show amplitude wobbling (bit like AM) and depending this Sinc reconstruction performance things (many compromises) also these are good to keep under around 0.8*fNyg. If we need very perfect result all what ADC can see need keep even more below fNyq exept if we start compute lot of sequantial samples ... up to fNyg we need infinite..
But then, some modern advanced scopes may have more inbulid DSP operations for reduce and "makeup" these problems, example Gibbs... do that finally reconstruction is more perfect.
Then also there is other thing... in digital trigger "engine". Its own interpolation methods between true samples for fine adjust wfm trigger position between two samples.
And then, if scope is using
decimated samplerates when slow t/div and memory start reject "samplerate" what is pushed to acquisition memory and from there collected (how ever it is done) to display memory. There, inside trigger sub system, it still can use example true ADC stream before decimation.
Example Siglent do it. So it is possible to run in SARI mode (Sequential Acquisitions Random Interleave).
It can reconstruct signal far over fNyquist if run dots mode with decimated samplerates. Perhaps even Siglent do not know there is this mode implemented. I have not seen any advertisement...
Here 3 images.
1. original signal with full samplerate and interpolations off.. just only true dots (if your browser/computer run these full speed also speed is around same what can see on scope screen irl.
Here sample dots are dense in every acq and also then lot of acqs overlaid and randomly interleaved so no need any interpolations.
2. Samplerate dropped low. Decimated samplerate 50MSa/s (fNyq 25MHz) what is far below signal first harmonic aka base frequency 45MHz. This signal of course have lot of harmonics...example 5. is 225MHz
Display total mess and can not trig of course. There can see running some alias freq products like example 5MHz
3. Same as 2 but now... dots mode. And this is fun. Acq memory samplerate 50MSa/s (fNyq 25MHz)
It can reconstruct and show 45MHz 4ns risetime square. 50MSa/s! And this oscilloscope do not have ETS mode at all!
Just not advertised SARI mode. (yes also some other scope may have this)
Of course if this can do other way and with full ADC speed then it can be like LeCroy RIS mode or emulate it.
With further develop and finishing this can be useful when scope is working in decimated samplerates.
Data points time interval in acquisition memory is 20ns (one point 2div)
What happen. Acquisition and trig position detected between memory data points and quite accurate. Moved to display memory.
As soon as possible next acquistion positioned (note that signal position to samples is time random) and moved to display memory together with previous... now dots are in random time position... then next and next and next until display moved to TFT and whole new turn...
In this case with current acquistion speed is quite low and it can do around 13 - 15 acquisitions for one image where now these
Sequential
Acquistions are
Randomly
Interleaved (due to fact that signal and ADC is not in time sync).
If like it can call SARI mode. How many sequential acquistions (note1) is interleaved (overlaid) depends current wfm/s speed. This is strong evidence that Siglent full digital side trigger engine use ADC true samples for trigger and fine position when operate under true ADC speed (1GSa or 500MSa/s).
Note1. These all sequential acquisitions can also find in waveform history buffer separately and time stamped (time is not true real time due to lack of RTC what can set and what keep time - what a shame they did not implement battery back up RTC)
Just with this setting every last 3912 acquistion with true raw ADC data is stored to history buffer fifo for analyze with scope and if want look with Sinc, Vector or dots mode because both interpolations are fully post processed.
This feature have many limits but it can use and is also useful in some special cases when user is bit experienced with it.
It may work also in some other same kind of oscilloscope where user can select true Dots mode without interpolation.