900 switches when all 4 ch on and MSO
I see. Yeah that's kinda expected.
A hacked 800 doesn't do it, it stays on 312.5.
you havent activated the LA in your video. if that D (with number 1-16) button on the bottom screen becomes green and you have many horizontal green lines on the screen, then your sample rate will become halved again iirc (156.25MSps)
It however starts to fail well below Nyquist (which is 156.25 MHz at this sampling rate): the displayed signal begins to have lower frequency noise at about 100 Mhz and it becomes next to useless at ~130 MHz.
sin(x)/x will start to go wrong at sample rate / 2.5
1) to be precise, anything more or equal to Sample rate / 2 will not be reconstructed correctly, i havent found literature saying exactly 2.5, i guess 2.5 figure is just a safer bet from scope manufacturers for Sinc interpolation to be correct, or the right BW limit roll off for anything Sr / 2 and higher to be suppressed to an acceptable level. iirc even signal with Sr / 2.1 can be reconstructed correctly, given the condition...
"thou shalt not have any harmonics (higher frequency elements) equals to or more than the sample rate / 2, not even the slightest" which comes to the second statements.
2) we usually try to prove Sinc failure with
fundamentals or pure sine signal very close to Sr / 2 but infact if front end BW is not
brickwalled at sample rate / X ... (X > 2, eq 2.1, 2.2, 2.5 etc) Sinc will still produce "not exactly the original" signal on screen due to presence of non zero harmonics above nyquist limit, even if
fundamentals is way below the limit. its just its not so obvious when harmonics magnitudes are small. a good example is low frequency square wave signal but very high rise/fall time, on larger time scale it seems ok, until we zoom closely into the rise/fall time portion (also read as
"artefacts").
so please bear in mind Sinc inaccuracy doesnt only happens at
fundamentals near nyquist limit, what matters is the highest frequency content presence, the bigger the magnitude the worst the reconstruction will be. thats why some people swear by the "properly BW limited" scope.
unfortunately they sometime wreak havoc from reality, if your circuit is already BW limited (no oscillation) then it should be fine... but if you expect your circuit to contain higher harmonics at significantly high magnitudes and you choose to probe with <= Sr / 2 anyway, then imho its you that have problem, not the scope. otoh built in properly BW limited scope will not be able to sense circuit misbehavior above Sr / 2, everything will just look fine, until the problem manifests/propagates itself on another portion of the circuit. fwiw.